Material Testing Techniques

 

MATERIAL TESTING  

Presented By: Muhammad Atique Atlas Honda Ltd.

Muhammad Arif Alamgir Group Of Industries.

Mohsin Javed Pakistan Spring Ltd.

Project Advisor: Mr. Muhammad Ali GCT. Railway Road Lhr.

PRESTON UNIVERSITY LAHORE CAMPUS

PRESTON UNIVERSITY, LAHORE CAMPUS

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IN THE NAME OF ALLAH THE MOST BENEFICIENT AND THE MOST MERCIFUL…!

Special Dedications To:

The Holy Prophet Hazrat Muhammad (SAW). Our Parents. Our Teachers.

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PREFACE The objective of Material Testing continues to be the presentation of the entire scope of material testing in industries in a single comprehensive volume. Thus the book starts with a continuum description of ferrous & non-ferrous metals, their major mechanical properties and then considers the major mechanical property tests for these materials as well as the equipment used to perform these tests with their standards. Such as • • • • • • • •

Hardness test Fatigue test Creep test Tensile test Bend test Compression test Ultrasonic test Radiographic test etc.

As an aid to the students, numerous illustrative examples have been included through out the book. We can make our manufacturing more standardized and with quality by following up the different tests standards, mentioned in this book.

Muhammad Atique Muhammad Arif Muhsin Javed

PRESTON UNIVERSITY, LAHORE CAMPUS

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ACKNOWLEDGEMENT All thanks are dully on Almighty Allah, most gracious, most merciful who enabled me to complete this project. I would like to express my thanks for the many useful comments and suggestions by Muhsin Javed and Muhammad Arif who have rendered a great help to me in completion of this project. Their help enabled me to surmount the problems faced from the inception to the completion of this project. It was my heartiest desire to select a project, which has common and vast practical applications. I am especially indebted to our Project Advisor Muhammad Ali Sb being much kind toward me by awarding such opportunity. He is well known for assigning such projects. Which inspire the students to work upon them and create new dimensions. His inspiring guidance , constant encouragement , generous help , advices and remarkable suggestions have been immensely valuable. I have gained tremendously from the vast reservoirs of knowledge, which he possesses and I feel that these few lines of acknowledgement reflect only a fraction of the gratitude.

Muhammad Atique

PRESTON UNIVERSITY, LAHORE CAMPUS

IV

CONTENTS

Chapter 1

Ferrous Metals 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Introduction Carbon steel Alloy steel Stainless steel Tool steel ASLA steel Steel for strength Iron based super alloys

1 1 4 5 8 9 10 11

Chapter 2

Non-Ferrous Metals 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

Introduction Aluminium Beryllium Copper Magnesium Nickel Refractory Metals Titanium Zirconium

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12 12 15 16 19 20 24 26 28

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Chapter 3

Mechanical Properties 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17

Introduction Hardness Brittleness Malleability Ductility Elasticity Toughness Density Strength Stiffness Fatigue Creep Stress Strain Terms for behavior of materials Stress-Strain diagram Hooke’s law

30 30 30 30 31 31 31 31 31 32 32 36 38 39 40 40 43

Chapter 4

Applications To Materials Testing 4.1 4.2 4.3 4.4 4.5 4.6

Introduction Destructive Test Fracture Toughness Test Spark Test Bending Test Hardness Test • Brinell Hardness Test • Knoop Test • Rockwell Test • Shore Test • Vickers Test 4.7 Compression Test 4.8 Fatigue Test 4.9 Flexure Test 4.10 Jominy End-Quench Test 4.11 Impact Test

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45 46 46 49 50 52 53 55 57 58 60 61 62 63 64 66

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4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22

Torsion test Tensile Test Creep Test Charpy Test Izod Test Non-Destructive Test Ultrasonic Testing Liquid Penetrant Testing Radiographic Testing Magnetic Particle Testing Magnetic Flux Leackage Test

67 68 73 75 76 77 78 80 82 83 84

Chapter 5

Material Testing Equipment 5.1 Introduction 5.2 5.3 5.4 5.5 5.6 5.7 5.8

Brinell tester Rockwell tester Riehle tester Barcol tester Ernst tester Universal hardness tester Micro Vickers hardness tester

85 85 86 88 89 92 92 93

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VII

FERROUS METALS

CHAPTER

ONE FERROUS METALS

1.1 INTRODUCTION As the most abundant of all commercial metals, alloys of iron and steel continue to cover a broad range of structural applications. Iron ore constitutes about 5% of the earth's crust and is easy to convert to a useful form. Iron is obtained by fusing the ore to drive off oxygen, sulfur, and other impurities. The ore is melted in a furnace in direct contact with the fuel using limestone as a flux. The limestone combines with impurities and forms a slag, which is easily removed. Adding carbon in small amounts reduces the melting point (2,777°F) of iron. All commercial forms of iron and steel contain carbon, which is an integral part of the metallurgy of iron and steel. Manipulation of atom-to-atom relationships between iron, carbon, and various alloying elements establishes the specific properties of ferrous metals. As atoms transform from one specific arrangement, or crystal lattice, to another, strength, toughness, impact resistance, hardness, ductility, and other properties are altered. The metallurgy of iron and steel is a study of how these atomic rearrangements take place, how they can be controlled, and which properties are affected.

Topics on ferrous Metals: • • • • • • •

Carbon Steel Alloy Steel Stainless Steel Tool Steel HSLA Steel Steel for strength Iron based super alloys

1.2 CARBON STEEL Carbon steel, also called plain carbon steel, is a malleable, iron-based metal containing carbon, small amounts of manganese, and other elements that are inherently present. Steels can either be cast to shape or wrought into various mill forms from which finished parts are formed, machined, forged, stamped, or otherwise shaped. Cast steels are poured to near-final shape in sand molds. The castings are then heat treated to achieve specified properties and machined to required dimensions. Wrought steel undergoes two operations. First, it is either poured into ingots or strand cast. Then, the metal is reheated and hot rolled into the finished, wrought form. Hot-rolled steel is characterized by a scaled

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FERROUS METALS

surface and a decarburized skin. Hot-rolled bars may be subsequently finished in a two-part process. First, acid pickling or shot blasting removes scale. Then, cold drawing through a die and restraightening improves surface properties and strength. Hot-rolled steel may also be cold finished by metal-removal processes such as turning or grinding. Wrought steel can be subsequently heat treated to improve machinability or to adjust mechanical properties. Carbon steels may be specified by chemical composition, mechanical properties, method of deoxidation, or thermal treatment (and the resulting microstructure).

Composition Wrought steels are most often specified by composition. No single element controls the characteristics of a steel; rather, the combined effects of several elements influence hardness, machinability, corrosion resistance, tensile strength, deoxidation of the solidifying metal, and microstructure of the solidified metal. Effects of carbon, the principal hardening and strengthening element in steel, include increased hardness and strength and decreased weldability and ductility. For plain carbon steels, about 0.2 to 0.25% C provides the best machinability. Above and below this level, machinability is generally lower for hot-rolled steels. Standard wrought-steel compositions (for both carbon and alloy steels) are designated by an AISI or SAE four-digit code, the last two digits of which indicate the nominal carbon content. The carbon-steel grades are: • • • •

10xx: Plain carbon 11xx: Resulfurized 12xz: Resulfurized and rephosphorized 15xx: Nonresulfurized, Mn over 1.0%

The letter "L" between the second and third digits indicates a leaded steel; "B" indicates a boron steel. Cast-carbon steels are usually specified by grade, such as A, B, or C. The A grade (also LCA, WCA, AN, AQ, etc.) contains 0.25% C and 0.70% Mn maximum. B-grade steels contain 0.30% C and 1.00% Mn, and the C-grade steels contain 0.25% C and 1.20% Mn. These carbon and manganese contents are designed to provide good strength, toughness, and weldability. Cast carbon steels are specified to ASTM A27, A216, A352, or A487. Microalloying technology has created a new category of steels, positioned both in cost and in performance between carbon steels and the alloy grades. These in-between steels consist of conventional carbon steels to which minute quantities of alloying elements -- usually less than 0.5% -- are added in the steelmaking process to improve mechanical properties. Strength and hardness are increased significantly. Any base-grade steel can be microalloyed, but the technique was first used in sheet steel a number of years ago. More recently, microalloying has been applied to bar products to eliminate the need for heat-treating operations after parts are forged. Automotive and truck applications include connecting rods, blower shafts, stabilizer bars, U-bolts, and universal joints. Other uses are sucker rods for oil wells and anchor bolts for the construction industry.

Mechanical properties Cast and wrought products are often specified to meet distinct mechanical requirements in structural applications where forming and machining are not extensive. When steels are specified by mechanical properties only, the producer is free to adjust the analysis of the steel (within limits) to obtain the required properties. Properties may vary with cross section and part size.

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Mechanical tests are usually specified under one of two conditions: mechanical test requirements and no chemical limits on any element, or mechanical test requirements and chemical limits on one or more elements, provided that such requirements are technologically compatible.

Method of deoxidation Molten steel contains dissolved oxygen -- an important element in the steelmaking reaction. How this oxygen is removed or allowed to escape as the metal solidifies determines some of the properties of the steel. So in many cases, "method of deoxidation" is specified in addition to AISI and SAE chemical compositions. For "killed" steels, elements such as aluminum and silicon may be added to combine chemically with the oxygen, removing most of it from the liquid steel. Killed steels are often specified for hot forging, carburizing, and other processes or applications where maximum uniformity is required. In sheet steel, aging is controlled by killing -- usually with aluminum. Steels intended for use in the as-cast condition are always killed. For this reason, steels for casting are always fully deoxidized. On the other hand, for "rimmed" steels, oxygen (in the form of carbon monoxide) evolves briskly throughout the solidification process. The outer skin of rimmed steels is practically free from carbon and is very ductile. For these reasons, rimmed steels are often specified for cold-forming applications. Rimmed steels are often available in grades with less than 0.25% C and 0.60% Mn. Segregation -- a nonuniform variation in internal characteristics and composition that results when various alloying elements redistribute themselves during solidification -- may be pronounced in rimmed steels. For this reason, they are usually not specified for hot forging or for applications requiring uniformity. "Capped" and "semikilled" steels fall between the rimmed and killed steels in behavior, properties, and degree of oxidation and segregation. Capped steels, for example, are suited for certain cold-forming applications because they have a soft, ductile, surface skin, which is thinner than rimmed-steel skin. For other cold-forming applications, such as cold extrusion, killed steels are more suitable.

Microstructure The microstructure of carbon and alloy steels in the as-rolled or as-cast condition generally consists of ferrite and pearlite. This basic structure can be altered significantly by various heat treatments or by rolling techniques. A spheroidized annealed structure would consist of spheroids of iron and alloy carbides dispersed in a ferrite matrix for low hardness and maximum ductility, as might be required for coldforming operations. Quenching and tempering provide the optimum combination of mechanical properties and toughness obtainable from steel. Grain size can also be an important aspect of the microstructure. Toughness of fine-grained steels is generally greater than that of coarse-grained steels.

Free-machining steels Several free-machining carbon steels are available as castings and as hot-rolled or cold-drawn bar stock and plate. Machinability in steels is improved in several ways, including: •

• • •

Addition of elements such as lead (the "leaded" steels such as 12L13 and 12L14), phosphorus and sulfur (the "rephosphorized, resulfurized" steels such as 1211, 1212, or 1213), sulfur (the "resulfurized only" steels such as 1117, 1118, or 1119), and tellerium, selenium, and bismuth (the "super" free-machining steels) Cold finishing Reducing the level of residual stress (usually by a stress-relieving heat treatment) Adjusting microstructure to optimize machinability

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FERROUS METALS

1.3 ALLOY STEEL Steels that contain specified amounts of alloying elements -- other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus -- are known as alloy steels. Alloying elements are added to change mechanical or physical properties. A steel is considered to be an alloy when the maximum of the range given for the content of alloying elements exceeds one or more of these limits: 1.65% Mn, 0.60% Si, or 0.60% Cu; or when a definite range or minimum amount of any of the following elements is specified or required within the limits recognized for constructional alloy steels: aluminum, chromium (to 3.99%), cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium or other element added to obtain an alloying effect. Technically, then, tool and stainless steels are alloy steels. In this chapter, however, the term alloy steel is reserved for those steels that contain a modest amount of alloying elements and that usually depend on thermal treatment to develop specific properties. With proper heat treatment, for example, tensile strength of certain alloy steels can be raised from about 55,000 psi to nearly 300,000 psi. Subdivisions for most steels in this family include "through-hardenable" and "carburizing" grades (plus several specialty grades such as nitriding steels). Through-hardening grades -- which are heat treated by quenching and tempering -- are used when maximum hardness and strength must extend deep within a part. Carburizing grades are used where a tough core and relatively shallow, hard surface are needed. After a surface-hardening treatment such as carburizing (or nitriding for nitriding alloys), these steels are suitable for parts that must withstand wear as well as high stresses. Cast steels are generally through hardened, not surface treated. Carbon content and alloying elements influence the overall characteristics of both types of alloy steels. Maximum attainable surface hardness depends primarily on carbon content. Maximum hardness and strength in small sections increase as carbon content increases, up to about 0.7%. However, carbon contents greater than 0.3% can increase the possibility of cracking during quenching or welding. Alloying elements primarily influence hardenability. They also influence other mechanical and fabrication properties including toughness and machinability. Lead additions (0.15 to 0.35%) substantially improve machinability of alloy steels by high-speed tool steels. For machining with carbide tools, calcium-treated steels are reported to double or triple tool life in addition to improving surface finish. Few exact rules exist for selecting through-hardening or surface-hardening grades of alloy steels. In most cases, critical parts are field tested to evaluate their performance. Parts with large sections -- heavy forgings, for example -- are often made from alloy steels that have been vacuum degassed. While in a molten state, these steels are exposed to a vacuum which removes hydrogen and, to a lesser degree, oxygen and nitrogen. Alloy steels are often specified when high strength is needed in moderate-to-large sections. Whether tensile or yield strength is the basis of design, thermally treated alloy steels generally offer high strengthto-weight ratios. For applications requiring maximum ductility, alloys with low sulfur levels ( 105 cycles). Under these conditions the stress, on a gross scale, is elastic, but as we shall see shortly the metal deforms plastically in a highly localized way. At higher stresses the fatigue life is progressively decreased, but the gross plastic deformation makes interpretation difficult in terms of stress. For the lowcycle fatigue region (N < 104 or 105 cycles) tests are conducted with controlled cycles of elastic plus plastic strain instead of controlled load or stress cycles. The usual procedure for determining an S-N curve is to test the first specimen at a high stress where failure is expected in a fairly short number of cycles, e.g., at about two-thirds the static tensile strength of the material. The test stress is decreased for each succeeding specimen until one or two specimens do not fail in the specified numbers of cycles, which is usually at least 107 cycles. The highest stress at which a runout (non-failure) is obtained is taken as the fatigue limit. For materials without a fatigue limit the test is usually terminated for practical considerations at a low stress where the life is about 108 or 5x108 cycles. The S-N curve is usually determined with about 8 to 12 specimens.

Statistical Nature of Fatigue A considerable amount of interest has been shown in the statistical analysis of fatigue data and in reasons for the variability in fatigue-test results. Since fatigue life and fatigue limit are statistical quantities, it must be realized that considerable deviation from an average curve determined with only a few specimens is to be expected. It is necessary to think in terms of the probability of a specimen attaining a certain life at a given stress or the probability of failure at a given stress in the vicinity of the fatigue limit. To do this requires the testing of considerably more specimens than in the past so that the statistical parameters for estimating these probabilities can be determined. The basic method for expressing fatigue data should then be a three-dimensional surface representing the relationship between stress, number of cycles to failure, and probability of failure. In determining the fatigue limit of a material, it should be recognized that each specimen has its own fatigue limit, a stress above which it will fail but below which it will not fail, and that this critical stress varies from specimen to specimen for very obscure reasons. It is known that inclusions in steel have an important effect on the fatigue limit and its variability, but even vacuum-melted steel shows appreciable scatter in fatigue limit. The statistical problem of accurately determining the fatigue limit is complicated by the fact that we cannot measure the individual value of the fatigue limit for any given specimen. We can only test a PRESTON UNIVERSITY, LAHORE CAMPUS

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MECHANICAL PROPERTIES

specimen at a particular stress, and if the specimen fails, then the stress was somewhere above the fatigue limit of the specimen. The two statistical methods which are used for making a statistical estimate of the fatigue limit are called probit analysis and the staircase method. The procedures for applying these methods of analysis to the determination of the fatigue limit have been well established.

Effect of Mean Stress on Fatigue Much of the fatigue data in the literature have been determined for conditions of completely reversed cycles of stress, σm = 0. However, conditions are frequently met in engineering practice where the stress situation consists of an alternating stress and a superimposed mean, or steady, stress. There are several possible methods of determining an S-N diagram for a situation where the mean stress is not equal to zero.

Cyclic Stress-Strain Curve Cyclic strain controlled fatigue, as opposed to our previous discussion of cyclic stress controlled fatigue, occurs when the strain amplitude is held constant during cycling. Strain controlled cyclic loading is found in thermal cycling, where a component expands and contracts in response to fluctuations in the operating temperature. In a more general view, the localized plastic strains at a notch subjected to either cyclic stress or strain conditions result in strain controlled conditions near the root of the notch due to the constraint effect of the larger surrounding mass of essentially elastically deformed material. Since plastic deformation is not completely reversible, modifications to the structure occur during cyclic straining and these can result in changes in the stress-strain response. Depending on the initial state a metal may undergo cyclic hardening, cyclic softening, or remain cyclically stable. It is not uncommon for all three behaviors to occur in a given material depending on the initial state of the material and the test conditions. Generally the hysteresis loop stabilizes after about 100 cycles and the material arrives at an equilibrium condition for the imposed strain amplitude. The cyclically stabilized stress-strain curve may be quite different from the stress-strain curve obtained on monotonic static loading. The cyclic stress-strain curve is usually determined by connecting the tips of stable hysteresis loops from constant-strain-amplitude fatigue tests of specimens cycled at different strain amplitudes. Under conditions where saturation of the hysteresis loop is not obtained, the maximum stress amplitude for hardening or the minimum stress amplitude for softening is used. Sometimes the stress is taken at 50 percent of the life to failure. Several shortcut procedures have been developed.

Low-Cycle Fatigue Although historically fatigue studies have been concerned with conditions of service in which failure occurred at more than 104 cycles of stress, there is growing recognition of engineering failures which occur at relatively high stress and low numbers of cycles to failure. This type of fatigue failure must be considered in the design of nuclear pressure vessels, steam turbines, and most other types of power machinery. Low-cycle fatigue conditions frequently are created where the repeated stresses are of thermal origin. Since thermal stresses arise from the thermal expansion of the material, it is easy to see that in this case fatigue results from cyclic strain rather than from cyclic stress.

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3.12 CREEP High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated. A typical creep curve is shown below:

In a creep test a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is then measured over a period of time. The slope of the curve, identified in the above figure, is the strain rate of the test during stage II or the creep rate of the material. Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation.

Stress Rupture Stress rupture testing is similar to creep testing except that the stresses used are higher than in a creep test. Stress rupture testing is always done until failure of the material. In creep testing the main goal is to determine the minimum creep rate in stage II. Once a designer knows the materials will creep and has accounted for this deformation a primary goal is to avoid failure of the component.

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Stress rupture tests are used to determine the time to cause failure. Data is plotted log-log as in the chart above. A straight line is usually obtained at each temperature. This information can then be used to extrapolate time to failure for longer times. Changes in slope of the stress rupture line are due to structural changes in the material. It is significant to be aware of these changes in material behavior, because they could result in large errors when extrapolating the data.

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Failure Analysis High temperature failures is a significant problem. A failure analysis can identify the root cause of your failure to prevent reoccurrence. AMC can provide failure analysis of high temperature failures to identify the root cause of your component failure.

3.13 STRESS Stress is defined as force per unit area. It has the same units as pressure, and in fact pressure is one special variety of stress. However, stress is a much more complex quantity than pressure because it varies both with direction and with the surface it acts on. Stress = Load / cross-sectional area ( N / mm2) •

Compression: Stress that acts to shorten an object.

•

Tension: Stress that acts to lengthen an object.

•

Normal Stress: Stress that acts perpendicular to a surface. Can be either compressional or tensional.

•

Shear: Stress that acts parallel to a surface. It can cause one object to slide over another. It also tends to deform originally rectangular objects into parallelograms. The most general definition is that shear acts to change the angles in an object.

•

Hydrostatic: Stress (usually compressional) that is uniform in all directions. A scuba diver experiences hydrostatic stress. Stress in the earth is nearly hydrostatic. The term for uniform stress in the earth is lithostatic.

•

Directed Stress: Stress that varies with direction. Stress under a stone slab is directed; there is a force in one direction but no counteracting forces perpendicular to it. This is why a person under a thick slab gets squashed but a scuba diver under the same pressure doesn't. The scuba diver feels the same force in all directions.

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3.14 STRAIN You will also be able to find the amount of stretch or elongation the specimen undergoes during tensile testing This can be expressed as an absolute measurement in the change in length or as a relative measurement called "strain". Strain itself can be expressed in two different ways, as "engineering strain" and "true strain". Engineering strain is probably the easiest and the most common expression of strain used. It is the ratio of the change in length to the original length,

. Whereas, the true strain is similar but based on the instantaneous length of the specimen as the test progresses,

, where Li is the instantaneous length and L0 the initial length.

•

Longitudinal or Linear Strain Strain that changes the length of a line without changing its direction. Can be either compressional or tensional.

•

Compression Longitudinal strain that shortens an object.

•

Tension Longitudinal strain that lengthens an object.

•

Shear Strain that changes the angles of an object. Shear causes lines to rotate.

•

Infinitesimal Strain: Strain that is tiny, a few percent or less. Allows a number of useful mathematical simplifications and approximations.

•

Finite Strain: Strain larger than a few percent. Requires a more complicated mathematical treatment than infinitesimal strain.

•

Homogeneous Strain: Uniform strain. Straight lines in the original object remain straight. Parallel lines remain parallel. Circles deform to ellipses. Note that this definition rules out folding, since an originally straight layer has to remain straight.

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•

Inhomogeneous Strain: How real geology behaves. Deformation varies from place to place. Lines may bend and do not necessarily remain parallel.

3.15 Terms for behavior of materials •

Elastic: Material deforms under stress but returns to its original size and shape when the stress is released. There is no permanent deformation. Some elastic strain, like in a rubber band, can be large, but in rocks it is usually small enough to be considered infinitesimal.

•

Brittle: Material deforms by fracturing. Glass is brittle. Rocks are typically brittle at low temperatures and pressures.

•

Ductile: Material deforms without breaking. Metals are ductile. Many materials show both types of behavior. They may deform in a ductile manner if deformed slowly, but fracture if deformed too quickly or too much. Rocks are typically ductile at high temperatures or pressures.

•

Viscous: Materials that deform steadily under stress. Purely viscous materials like liquids deform under even the smallest stress. Rocks may behave like viscous materials under high temperature and pressure.

•

Plastic: Material does not flow until a threshhold stress has been exceeded.

•

Viscoelastic: Combines elastic and viscous behavior. Models of glacio-isostasy frequently assume a viscoelastic earth: the crust flexes elastically and the underlying mantle flows viscously.

3.16 Strain-Stress Diagram A stress-strain curve is a graph derived from measuring load (stress - σ) versus extension (strain - ε) for a sample of a material. The nature of the curve varies from material to material. The following diagrams illustrate the stress-strain behaviour of typical materials in terms of the engineering stress and engineering strain where the stress and strain are calculated based on the original dimensions of the sample and not the instantaneous values. In each case the samples are loaded in tension although in many cases similar behaviour is observed in compression.

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The stress value at the point P is called the limit of proportionality: σp= FP / S0 This behavior conforms to the Hook’s Law: σ = E*δ Where E is a constant, known as Young’s Modulus or Modulus of Elasticity. The value of Young’s Modulus is determined mainly by the nature of the material and is nearly insensitive to the heat treatment and composition. Modulus of elasticity determines stiffness - resistance of a body to elastic deformation caused by an applied force. The line 0E in the strain-stress curve indicates the range of elastic deformation – removal of the load at any point of this part of the curve results in return of the specimen length to its original value. The elastic behavior is characterized by the elasticity limit (stress value at the point E): σel= FE / S0 For the most materials the points P and E coincide and therefore σel=σp. A point where the stress causes sudden deformation without any increase in the force is called yield limit (yield stress, yield strength):

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σy= FY / S0 The highest stress (point YU) , occurring before the sudden deformation is called upper yield limit . The lower stress value, causing the sudden deformation (point YL) is called lower yield limit. The commonly used parameter of yield limit is actually lower yield limit. If the load reaches the yield point the specimen undergoes plastic deformation – it does not return to its original length after removal of the load. Hard steels and non-ferrous metals do not have defined yield limit, therefore a stress, corresponding to a definite deformation (0.1% or 0.2%) is commonly used instead of yield limit. This stress is called proof stress or offset yield limit (offset yield strength): σ0.2%= F0.2% / S0 The method of obtaining the proof stress is shown in the picture.

As the load increase, the specimen continues to undergo plastic deformation and at a certain stress value its cross-section decreases due to “necking” (point S in the strain-stress diagram). At this point the stress reaches the maximum value, which is called ultimate tensile strength (tensile strength): σt= FS / S0 PRESTON UNIVERSITY, LAHORE CAMPUS

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Continuation of the deformation results in breaking the specimen - the point B in the diagram. The actual strain-stress curve is obtained by taking into account the true specimen cross-section instead of the original value. Other important characteristic of metals is ductility - ability of a material to deform under tension without rupture. Two ductility parameters may be obtain from the tensile test: Relative elongation - ratio between the increase of the specimen length before its rupture and its original length: δ = (Lm– L0) / L0 Where Lm– maximum specimen length. Relative reduction of area - ratio between the decrease of the specimen cross-section area before its rupture and its original cross-section area: ψ= (S0– Smin) / S0 Where Smin– minimum specimen cross-section area.

3.17 HOOKE,S LAW

For most tensile testing of materials, you will notice that in the initial portion of the test, the relationship between the applied force, or load, and the elongation the specimen exhibits is linear. In this linear region, the line obeys the relationship defined as "Hooke's Law" where the ratio of stress to strain is a constant, or . E is the slope of the line in this region where stress (σ) is proportional to strain (ε) and is called the "Modulus of Elasticity" or "Young's Modulus".

Modulus of elasticity The modulus of elasticity is a measure of the stiffness of the material, but it only applies in the linear region of the curve. If a specimen is loaded within this linear region, the material will return to its exact same condition if the load is removed. At the point that the curve is no longer linear and deviates from the PRESTON UNIVERSITY, LAHORE CAMPUS

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straight-line relationship, Hooke's Law no longer applies and some permanent deformation occurs in the specimen. This point is called the "elastic, or proportional, limit". From this point on in the tensile test, the material reacts plastically to any further increase in load or stress. It will not return to its original, unstressed condition if the load were removed.

Yield strength A value called "yield strength" of a material is defined as the stress applied to the material at which plastic deformation starts to occur while the material is loaded.

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APPLICATIONS TO MATERIALS TESTING

CHAPTER

FOUR APPLICATIONS TO MATERIALS TESTING

4.1 INTRODUCTION Major branches of engineering depend on the results of mechanical tests for design and/or quality control purposes. Test specimens are prepared for metallic and non-metallic materials in the evaluation of tensile, compression, impact, fracture toughness, fatigue and bend properties. Routine testing of fasteners, chain materials, weld coupons, wire rope, castings, sheet, plate, forgings and other components is done in an expedient manner providing an efficient, quality conscious service. Many fabricators, heat treaters and foundries rely on Bodycote’s definitive mechanical testing services to facilitate early production release or production start. Some materials require more in-depth testing, such as dynamic fracture test, or cryogenic and elevated temperature mechanical properties. Our customers can feel confident that not only routine but also the most diverse test requests will be handles by highly experienced engineers and technicians.

Capabilities include: • • • • • • • • • • •

Bend, Compression, Plastic Strain Ratio, Ring Flaring/Flattening Tensile, Nick Break, Fillet Fracture, Hydrogen Embrittlement Impact, Fracture Toughness Hardness, Jominy Hardenability Creep, Stress Rupture Drop Weight, Dynamic Load Fatigue Component Testing Vibration & Shock Fasteners Tribology/Wear

The test involve comparison of the behavior of test pieces under conditions which ore approximately similar to those conditions in which the metals are used. The tests take a lot of time, sometimes months and in some case several years to give conclusive results, which may be applied for the development of components of machinery, etc. The test for creep, fatigue, stress, corrosion, etc are also made at elevated temperature. Theses tests essentially require long time. The material testing can be divided into two main groups: 1. Destructive 2. Non –Destructive PRESTON UNIVERSITY, LAHORE CAMPUS

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4.2 DESTRUCTIVE TEST In destuctive testing, tests are carried out to the specimen’s failure. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing. Testing of an object is often done in view of future use, which would make destructive testing pointless. However, it can be useful if the result gives information about similar specimens which are not tested. Some types of destructive testing are: • • • • • • • • • • • • • •

Fracture Toughness Test Spark Test Bend Test Hardness Test Compression test Fatigue Test Flexure Test Jominy End-Quench Test Impact Test Torsion test Tensile Test Creep Test Charpy Test Izod Test

4.3 FRACTURA TOUGHNESS TEST Background The resistance to fracture of a material is known as its fracture toughness. Fracture toughness generally depends on temperature, environment, loading rate, the composition of the material and its microstructure, together with geometric effects (constraint). [1] These factors are of particular importance for welded joints, where the metallurgical and geometric effects are complex [2,3] Fracture toughness is a critical input parameter for fracture-mechanics based fitness-for-service assessments. Although fracture toughness can sometimes be obtained from the literature, or materials properties databases, it is preferable to determine this by experiment for the particular material and joint being assessed. Various measures of 'toughness' exist, including the widely used but qualitative Charpy impact test. Although it is possible to correlate Charpy energy with fracture toughness, a large degree of uncertainty is associated with correlations because they are empirical. It is preferable to determine fracture toughness in a rigorous fashion, in terms of K (stress intensity factor), CTOD (crack tip opening displacement), or J (the J integral); see also What is a fracture toughness test? Standards exist for performing fracture mechanics tests, with the most common specimen configuration shown in Fig.1 (the single-edge notch bend SENB specimen). A sharp fatigue notch is inserted in the specimen, which is loaded to failure. The crack driving force is calculated for the failure condition, giving the fracture toughness.

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Fig. Fracture mechanics testing

Various national Standards have been developed for fracture toughness testing: •

The British Standard BS 7448 [4] includes four parts, for testing of metallic materials, including parent materials, weldments, high strain rates (dynamic fracture toughness testing, to be published in 2005) and resistance curves (R-curves for ductile tearing). BS 7448: Part 2 is the first Standard worldwide to apply specifically to weldments.

•

A series of American ASTM Standards cover K, CTOD, J testing (including R-curves), ASTM E1290 (CTOD testing) and ASTM E1820 (K, J & CTOD). None specifically address testing of welds. together with a summary of applicable terminology. [5-8]

•

A series of international (ISO) standards are being developed. ISO 12135 covers all aspects of fracture testing (K, J & CTOD) of plain material. Standards are being prepared on testing of welds (ISO/CD 15653) and stable crack growth in low constraint specimens (ISO/CD 15653). The latter is mainly concerned with testing thin, sheet material.

•

The European Structural Integrity Society (ESIS) has published procedures for R-curve and standard fracture toughness testing of metallic materials. [9-10] Currently, a draft unified testing procedure (ESIS P3-04), which includes weld testing, is being developed. (These are not standards in the usual sense, but rather testing protocols that have been agreed by experts).

Although different standards have historically been published for determining K, CTOD and J, the tests are very similar, and generally all three values can be established from one test. See Are there any differences between fracture toughness tests carried out to BS7448 and E1820?

Test specimens The most widely used fracture toughness test configurations are the single edge notch bend (SENB or three-point bend), and the compact (CT) specimens, as shown in Fig.2. The compact specimen has the advantage that it requires less material, but is more expensive to machine and more complex to test compared with the SENB specimen. Also, special requirements are needed for temperature control (e.g. use of an environmental chamber). SENB specimens are typically immersed in a bath for low temperature tests. Although the compact specimen is loaded in tension, the crack tip conditions are predominantly bending (high constraint). If limited material is available, it is possible to fabricate SENB specimens by PRESTON UNIVERSITY, LAHORE CAMPUS

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welding extension pieces (for the loading arms) to the material sample. (Electron beam welding is typically used, because the weld is narrow and causes little distortion).

Fig. Examples of common fracture toughness test specimen types

Other specimen configurations include centre-cracked tension (CCT) panels, single edge notch tension (SENT) specimens, and shallow-crack tests. These specialised tests are associated with lower levels of constraint, and can be more structurally representative than standard SENB or CT specimens. The position and orientation of the specimen is important. In particular, the location and orientation of the notch is critical, especially for welded joints. Typically, the notch (fatigue pre-crack) is positioned such that a chosen microstructure is sampled. The orientation of the notch is defined with respect to either the weld axis for welded joints, or the rolling direction or forging axis for other components. In standard SENB & C T specimens (see Fig.1), the notch depth is within the range 45-70% of the specimen width, W, giving a lower-bound estimate of fracture toughness, because of the high level of crack tip constraint generated by the specimen design. A notch is machined into the fracture toughness specimen, following which a fatigue crack is grown by applying cyclic loading to the specimen. Specialised high frequency resonance or servo-hydraulic machines are often used for this process. The fracture mechanics test standards include many checks to ensure that results are credible. These include restrictions on the fatigue crack size, position and shape, together with limitations on the maximum allowable fatigue force (this is to ensure that the crack-tip plastic zone produced during fatigue precracking is small in comparison with the plastic zone produced during testing). Many of these checks can only be performed after testing.

Instrumentation and loading During fracture toughness testing, the force applied to the specimen and specimen displacements and loading rate (using load cells and displacement transducers), together with the temperature are recorded. One of the displacements is the crack-mouth opening. This is measured using a clip gauge either attached to knife edges mounted at the crack mouth (see Fig.1) or integral knife edges machined into the notch. These gauges comprise two cantilevered beams on which are positioned four strain gauges. By measuring the elastic strains and calibration it is possible to infer the crack-mouth opening. Fracture toughness tests are performed in universal hydraulic test machines, generally using displacement control. PRESTON UNIVERSITY, LAHORE CAMPUS

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Fracture toughness parameters The following are the fracture toughness parameters commonly obtained from testing: •

K (stress intensity factor) can be considered as a stress-based estimate of fracture toughness. It is derived from a function which depends on the applied force at failure. K depends on geometry (the flaw depth, together with a geometric function, which is given in test standards for each test specimen geometry).

•

CTOD or (crack-tip opening displacement) can be considered as a strain-based estimate of fracture toughness. However, it can be separated into elastic and plastic components. The elastic part of CTOD is derived from the stress intensity factor, K. In some standards, the plastic component of CTOD is obtained by assuming that the specimen rotates about a plastic hinge. The plastic component is derived from the crack mouth opening displacement (measured using a clip gauge). The position of the plastic hinge (defined by r p ) is given in test standards for each specimen type. Alternative methods exist for estimating CTOD, which make no assumption regarding the position of the plastic hinge. These require the determination of J from which CTOD is derived. [6,7] CTOD values determined from formulations assuming a plastic hinge [4] may differ from those determined from J. [6,7]

•

J (the J-integral) is an energy-based estimate of fracture toughness. It can be separated into elastic and plastic components. As with CTOD, the elastic component is based on K, while the plastic component is derived from the plastic area under the force-displacement curve.

4.4 SPARK TEST Spark testing metals is done by noting the type of sparks that issue from a piece of steel that has been put to a grinding wheel, From this one can deduce with some accuracy the type of alloy present (for instance; percentage carbon, vanadium, chromium). Spark characteristics to note are: 1. Colour 2. Length 3. Branching One of the simplest tests is to note how the sparks branch. Higher carbon steels will produce shorter streams of sparks with a large amount of branching, in contrast low carbon steel will produce a longer stream with less branching. Additionally wrought iron will produce practically no branching, and cast iron extremely short stream with excessive branching

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Fig. Spark test

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4.5 BENDING TEST The bend test is a simple and inexpensive qualitative test that can be used to evaluate both the ductility and soundness of a material. It is often used as a quality control test for butt-welded joints, having the advantage of simplicity of both test piece and equipment. No expensive test equipment is needed, test specimens are easily prepared and the test can, if required, be carried out on the shop floor as a quality control test to ensure consistency in production. The bend test uses a coupon that is bent in three point bending to a specified angle. The outside of the bend is extensively plastically deformed so that any defects in, or embrittlement of, the material will be revealed by the premature failure of the coupon. The bend test may be free formed or guided. The guided bend test is where the coupon is wrapped around a former of a specified diameter and is the type of test specified in the welding procedure and welder qualification specifications. For example, it is a requirement in ASME IX, the EN 287 and EN 288 series of specifications and ISO 15614 Part 1. As the guided bend test is the only form of bend test specified in welding qualification specifications it is the only one that will be dealt with in this article. Typical bend test jigs are illustrated in Fig.1(a) and 1(b).

Fig.1(a) shows a guided bend test jig that uses a male and a female former, the commonest form of equipment

Fig.1(b) shows a wrap-around guided bend test machine that works on the same principles as a plumber's pipe bender

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The strain applied to the specimen depends on the diameter of the former around which the coupon is bent and this is related to the thickness of the coupon 't', normally expressed as a multiple of 't' eg 3t, 4t etc. The former diameter is specified in the test standard and varies with the strength and ductility of the material - the bend former diameter for a low ductility material such as a fully hard aluminium alloy may be as large as 8t. An annealed low carbon steel on the other hand may require a former diameter of only 3t. The angle of bend may be 90°, 120° or 180° depending on the specification requirements. On completion of the test the coupon is examined for defects that may have opened up on the tension face. Most specifications regard a defect over 3mm in length as being cause for rejection. For butt weld procedure and welder qualification testing the bend coupons may be oriented transverse or parallel to the welding direction. Below approximately 12mm material thickness transverse specimens are usually tested with the root or face of the weld in tension. Material over 12mm thick is normally tested using the side bend test that tests the full section thickness, Fig.2.

Fig.2

Where the material thickness is too great to permit the full section to be bent the specifications allow a number of narrower specimens to be taken provided that the full material thickness is tested. Conventionally, most welding specifications require two root and two face bend coupons or four side bends to be taken from each butt welded test piece. The transverse face bend specimen will reveal any defects on the face such as excessive undercut or lack of sidewall fusion close to the cap. The transverse root bend is also excellent at revealing lack of root fusion or penetration. The transverse side bend tests the full weld thickness and is particularly good at revealing lack of side-wall fusion and lack of root fusion in double-V butt joints. This specimen orientation is also useful for testing weld cladding where any brittle regions close to the fusion line are readily revealed. Longitudinal bend specimens are machined to include the full weld width, both HAZs and a portion of each parent metal. They may be bent with the face, root or side in tension and are used where there is a difference in mechanical strength between the two parent metals or the parent metal and the weld. The test will readily reveal any transverse defects but it is less good at revealing longitudinally oriented defects such as lack of fusion or penetration. Whilst the bend test is simple and straightforward to perform there are some features that may result in the test being invalid. In cutting the coupon from the test weld the effects of the cutting must not be allowed to affect the result. Thus it is necessary to remove any HAZ from flame cutting or work hardened metal if the sample is sheared. It is normal to machine or grind flat the face and root of a weld bend test coupon to reduce the stress raising effect that these would have. Sharp corners can cause premature failure and should be rounded off to a maximum radius of 3mm. PRESTON UNIVERSITY, LAHORE CAMPUS

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The edges of transverse bend coupons from small diameter tubes will experience very high tensile stresses when the ID is in tension and this can result in tearing at the specimen edges. Weld joints with non-uniform properties such as dissimilar metal joints or where the weld and parent metal strengths are substantially different can result in 'peaking' of the bend coupon. This is when most of the deformation takes place in the weaker of the two materials which therefore experiences excessive localised deformation that may result in premature failure. A dissimilar metal joint where one of the parent metals is very high strength is a good example of where this may occur and similar peaking can be seen in fully hard welded aluminium alloy joints. In these instances the roller bend test illustrated in Fig.1(b) is the best method of performing a bend test as each component of the coupon is strained by a similar amount and peaking is to a great extent eliminated.

4.6 HARDNESS TEST

Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize that hardness is an empirical test and therefore hardness is not a material property. This is because there are several different hardness tests that will each determine a different hardness value for the same piece of material. Therefore, hardness is test method dependent and every test result has to have a label identifying the test method used. Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. All of the hardness tests described in this section involve the use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value.

Why Use a Hardness Test? • • • • •

Easy to perform Quick - 1 to 30 seconds Relatively inexpensive Non-destructive Finished parts can be tested - but not ruined

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• •

Virtually any size and shape can be tested Practical QC device - incoming, outgoing

The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications.

Hardness Scales There are five major hardness scales: • • • • •

Brinell - HB Knoop - HK Rockwell - HR Shore - HS Vickers - HV

Each of these scales involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent. All of the scales are arranged so that the hardness values determined increase as the material gets harder. The hardness values are reported using the proper symbol, HR, HV, HK, etc. indicating the test scale performed.

Five Determining Factors The following five factors can be used to determine the correct hardness test for your application. 1. 2. 3. 4. 5.

Material - grain size, metal, rubber, etc. Approximate Hardness - hardened steel, rubber, etc. Shape - thickness, size, etc. Heat Treatment – through or casehardened, annealed, etc. Production Requirements - sample or 100%

The application guide is designed to help determine the hardness tests that can be used on some typical materials.

•

Brinell Hardness Test

Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The oldest of the hardness test methods in common use today, the Brinell test is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force independent as long as the ball size/test force relationship is the same. In the USA, Brinell testing is typically done on iron and steel castings using a 3000Kg test force and a 10mm diameter carbide ball. Aluminum and other softer alloys are frequently tested using a 500Kg test force and a 10 or 5mm carbide ball. Therefore the typical range of Brinell testing in this country is 500 to 3000kg with 5 or 10mm carbide balls. In Europe Brinell testing is done using a much wider range of forces PRESTON UNIVERSITY, LAHORE CAMPUS

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and ball sizes. It's common in Europe to perform Brinell tests on small parts using a 1mm carbide ball and a test force as low as 1kg. These low load tests are commonly referred to as baby Brinell tests.

Standards Brinell Test methods are defined in the following standards: • •

ASTM E10 ISO 6506

Fig. Brinell hardness tester.

Fig. Microscopic view of impression.

Brinell Test Method All Brinell tests use a carbide ball indenter. The test procedure is as follows:

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• • • • •

The indenter is pressed into the sample by an accurately controlled test force. The force is maintained for a specific dwell time, normally 10 - 15 seconds. After the dwell time is complete, the indenter is removed leaving a round indent in the sample. The size of the indent is determined optically by measuring two diagonals of the round indent using either a portable microscope or one that is integrated with the load application device. The Brinell hardness number is a function of the test force divided by the curved surface area of the indent. The indentation is considered to be spherical with a radius equal to half the diameter of the ball. The average of the two diagonals is used in the following formula to calculate the Brinell hardness.

The Brinell number, which normally ranges from HB 50 to HB 750 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple. A typical Brinell hardness is specified as follows: 356HBW Where 356 is the calculated hardness and the W indicates that a carbide ball was used. Note- Previous standards allowed a steel ball and had an S designation. Steel balls are no longer allowed.

Applications Because of the wide test force range the Brinell test can be used on almost any metallic material. See the application guide. The part size is only limited by the testing instrument's capacity.

Strengths 1. One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same. 2. A wide range of test forces and ball sizes to suit every application. 3. Nondestructive, sample can normally be reused.

Weaknesses 1. The main drawback of the Brinell test is the need to optically measure the indent size. This requires that the test point be finished well enough to make an accurate measurement. 2. Slow. Testing can take 30 seconds not counting the sample preparation time.

•

Knoop Test

Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.

Standards Knoop test methods are defined in ASTM E384. PRESTON UNIVERSITY, LAHORE CAMPUS

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Knoop Test Method Knoop testing is done with a rhombic-based pyramidal diamond indenter that forms an elongated diamond shaped indent.

• • • • •

The indenter is pressed into the sample by an accurately controlled test force. The force is maintained for a specific dwell time, normally 10 - 15 seconds. After the dwell time is complete, the indenter is removed leaving an elongated diamond shaped indent in the sample. The size of the indent is determined optically by measuring the longest diagonal of the diamond shaped indent. The Knoop hardness number is a function of the test force divided by the projected area of the indent. The diagonal is used in the following formula to calculate the Knoop hardness.

HK = Constant x test force / indent diagonal squared The constant is a function of the indenter geometry and the units of force and diagonal. The Knoop number, which normally ranges from HK 60 to HK1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Knoop hardness is specified as follows: 450HV0.5 Where 450 is the calculated hardness and 0.5 is the test force in kg.

Applications Because of the wide test force range, the Knoop test can be used on almost any metallic material. See the application guide. The part size is only limited by the testing instrument's capacity.

Strengths • • • •

The elongated diamond indenter and low test forces allows testing very small parts or material features not capable if being tested any other way. One scale covers the entire hardness range. Test results a mainly test force independent over 100g. A wide range of test forces to suit every application.

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Weaknesses • •

The main drawback of the Knoop test is the need to optically measure the indent size. This requires that the test point be highly polished to be able to see the indent well enough to make an accurate measurement. Slow. Testing can take 30 seconds not counting the sample preparation time.

•

Rockwell Test

Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for a large ball bearing company and he wanted a fast non-destructive way to determine if the heat treatment process they were doing on the bearing races was successful. The only hardness tests he had available at time were Vickers, Brinell and Scleroscope. The Vickers test was too time consuming, Brinell indents were too big for his parts and the Scleroscope was difficult to use, especially on his small parts. To satisfy his needs he invented the Rockwell test method. This simple sequence of test force application proved to be a major advance in the world of hardness testing. It enabled the user to perform an accurate hardness test on a variety of sized parts in just a few seconds. Rockwell test methods are defined in the following standards: • • •

ASTM E18 Metals ISO 6508 Metals ASTM D785 Plastics

Types of the Rockwell Test There are two types of Rockwell tests: 1. Rockwell: the minor load is 10 kgf, the major load is 60, 100, or 150 kgf. 2. Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30, or 45 kgf. In both tests, the indenter may be either a diamond cone or steel ball, depending upon the characteristics of the material being tested.

Fig. Rockwell hardness tester. PRESTON UNIVERSITY, LAHORE CAMPUS

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Rockwell Scales Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads. The hardness number is expressed by the symbol HR and the scale designation. There are 30 different scales. The majority of applications are covered by the Rockwell C and B scales for testing steel, brass, and other metals. However, the increasing use of materials other than steel and brass as well as thin materials necessitates a basic knowledge of the factors that must be considered in choosing the correct scale to ensure an accurate Rockwell test. The choice is not only between the regular hardness test and superficial hardness test, with three different major loads for each, but also between the diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter steel ball indenters. If no specification exists or there is doubt about the suitability of the specified scale, an analysis should be made of the following factors that control scale selection: • • • •

Type of material Specimen thickness Test location Scale limitations

Principal of the Rockwell Test

• • • •

Select image to enlarge The indenter moves down into position on the part surface A minor load is applied and a zero reference position is established The major load is applied for a specified time period (dwell time) beyond zero The major load is released leaving the minor load applied

The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load.

•

Shore Test

The Shore test has been used since 1907 to determine the hardness of a wide variety of rubber and soft plastics. Originally there were only 4 different scales for rubbers. However, now there are 12 scales to allow testing an even wider range of materials from small rubber O rings to very soft foam products. The testers that perform Shore tests have been commonly referred to as Durometers and the results frequently called Durometer hardness. With the exception of the M scale testers, all Durometers can be used either as a portable unit or in an operating stand. This flexibility adds greatly to the usefulness of the Shore scale.

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Standards Shore test methods are defined in the following standards: • • • • •

ASTM D-2240 DIN 53 505 ISO 7619 Part 1 JIS K 6301* ASKER C-SRIS-0101

NOTE: The JIS standard is very similar to the ASTM 2240 standard. However, there are small but important differences.

Shore Test Method The Shore test uses a hardened indenter, an accurately calibrated spring, a depth indicator, and a flat presser foot. The indenter is mounted in the middle of the presser foot and extends 2.5mm from the surface of the foot. In the fully extended position the indicator displays zero. When the indenter is depressed flat even with the presser foot's surface, the indicator displays 100. Therefore, every Shore point is equal to 0.0025mm penetration (M scale is 0.00125mm). In use the unit is placed on the sample so that the presser foot is held firmly against the test surface. The spring pushes the indenter into the sample and the indicator indicates the depth of penetration. The deeper the indentation the softer the material and the lower the indicator reading. The different Shore scales, A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R are created by using 7 different indenter shapes, 5 different springs, 2 different indenter extensions an 2 different presser foot specifications. The A and D scales are by far the most commonly used. The M scale uses a very low force spring and was developed to allow testing very small parts like O rings that can not be tested in the normal A scale. Because different materials respond to the test scales in different ways, there is no correlation between the different scales.

Applications All Durometers except for the M scale units can be used as a portable device. Test stands are recommended for best accuracy and are required for M scale testing because of its increased sensitivity. Some stands have extra weights to make sure that the force on the presser foot is constant from test to test. Normally multiple tests are done on each sample and the average result is used.

Strengths • • • •

Fast, easy to use Inexpensive Wide range of materials can be tested Non destructive, part can normally be used after testing

Weaknesses • • •

Dwell time variables can cause poor readings. Inconsistent force on the presser foot will cause errors. Difficulties keeping the indenter perpendicular to the test surface will cause errors.

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•

The test surface must be large enough to support the presser foot.

•

Vickers Test

The Vickers (HV) test was developed in England is 1925 and was formally known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has two distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000). With the exception of test forces below 200g, Vickers values are generally considered test force independent. In other words, if the material tested is uniform, the Vickers values will be the same if tested using a 500g force or a 50kg force. Below 200g, caution must be used when trying to compare results.

Standards Vickers test methods are defined in the following standards: • • •

ASTM E384 – micro force ranges – 10g to 1kg ASTM E92 – macro force ranges - 1kg to 100kg ISO 6507-1,2,3 – micro and macro ranges

Vickers Test Method

All Vickers ranges use a 136° pyramidal diamond indenter that forms a square indent. • • • • •

The indenter is pressed into the sample by an accurately controlled test force. The force is maintained for a specific dwell time, normally 10 – 15 seconds. After the dwell time is complete, the indenter is removed leaving an indent in the sample that appears square shaped on the surface. The size of the indent is determined optically by measuring the two diagonals of the square indent. The Vickers hardness number is a function of the test force divided by the surface area of the indent. The average of the two diagonals is used in the following formula to calculate the Vickers hardness.

HV = Constant x test force / indent diagonal squared The constant is a function of the indenter geometry and the units of force and diagonal. The Vickers number, which normally ranges from HV 100 to HV1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Vickers hardness is specified as follows: PRESTON UNIVERSITY, LAHORE CAMPUS

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356HV0.5 Where 356 is the calculated hardness and 0.5 is the test force in kg.

Applications Because of the wide test force range, the Vickers test can be used on almost any metallic material. See the application guide. The part size is only limited by the testing instrument's capacity.

Strengths • • •

One scale covers the entire hardness range. A wide range of test forces to suit every application. Nondestructive, sample can normally be used.

Weaknesses • •

The main drawback of the Vickers test is the need to optically measure the indent size. This requires that the test point be highly finished to be able to see the indent well enough to make an accurate measurement. Slow. Testing can take 30 seconds not counting the sample preparation time.

4.7 COMPRESSION TEST

A compression test determines behavior of materials under crushing loads. The specimen is compressed and deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and, for some materials, compressive strength.

Why Perform a Compression Test? The ASM Handbook®, Volume 8, Mechanical Testing and Evaluation states: "Axial compression testing is a useful procedure for measuring the plastic flow behavior and ductile fracture limits of a material. Measuring the plastic flow behavior requires frictionless (homogenous compression) test conditions, while measuring ductile fracture limits takes advantage of the barrel formation and controlled stress and strain conditions at the equator of the barreled surface when compression is carried out with friction. Axial PRESTON UNIVERSITY, LAHORE CAMPUS

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compression testing is also useful for measurement of elastic and compressive fracture properties of brittle materials or low-ductility materials. In any case, the use of specimens having large L/D ratios should be avoided to prevent buckling and shearing modes of deformation1." The image at right shows variation of the strains during a compression test without friction (homogenous compression) and with progressively higher levels of friction and decreasing aspect ratio L/D (shown as h/d)1.

Fig. Modes of Deformation in Compression Testing

The figure illustrates the modes of deformation in compression testing. (a) Buckling, when L/D > 5. (b) Shearing, when L/D > 2.5. (c) Double barreling, when L/D > 2.0 and friction is present at the contact surfaces. (d) Barreling, when L/D < 2.0 and friction is present at the contact surfaces. (e) Homogenous compression, when L/D < 2.0 and no friction is present at the contact surfaces. (f) Compressive instability due to work-softening material1.

Typical Materials The following materials are typically subjected to a compression test. • • • • • •

Concrete Metals Plastics Ceramics Composites Corrugated Cardboard

4.8 FATIGUE TEST The definition of fatigue testing can be thought of as simply applying cyclic loading to your test specimen to understand how it will perform under similar conditions in actual use. The load application can either be a repeated application of a fixed load or simulation of inservice loads. The load application may be repeated PRESTON UNIVERSITY, LAHORE CAMPUS

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millions of times and up to several hundred times per second.

Why Do a Fatigue Test? In many applications, materials are subjected to vibrating or oscillating forces. The behavior of materials under such load conditions differs from the behavior under a static load. Because the material is subjected to repeated load cycles (fatigue) in actual use, designers are faced with predicting fatigue life, which is defined as the total number of cycles to failure under specified loading conditions. Fatigue testing gives much better data to predict the in-service life of materials.

4.9 FLEXURE TEST The flexure test method measures behavior of materials subjected to simple beam loading. It is also called a transverse beam test with some materials. Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted in a stress-strain diagram. Flexural strength is defined as the maximum stress in the outermost fiber. This is calculated at the surface of the specimen on the convex or tension side. Flexural modulus is calculated from the slope of the stress vs. deflection curve. If the curve has no linear region, a secant line is fitted to the curve to determine slope.

Why Perform a Flexure Test? A flexure test produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline. To ensure the primary failure comes from tensile or compression stress the shear stress must be minimized. This is done by controlling the span to depth ratio; the length of the outer span divided by the height (depth) of the specimen. For most materials S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough.

Types of Flexure Tests Flexure testing is often done on relatively flexible materials such as polymers, wood and composites. There are two test types; 3-point flex and 4-point flex. In a 3-point test the area of uniform stress is quite small and concentrated under the center loading point. In a 4-point test, the area of uniform stress exists between the inner span loading points (typically half the outer span length).

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Typical Materials Polymers The 3-point flexure test is the most common for polymers. Specimen deflection is usually measured by the crosshead position. Test results include flexural strength and flexural modulus. Wood and Composites The 4-point flexure test is common for wood and composites. The 4-point test requires a deflectometer to accurately measure specimen deflection at the center of the support span. Test results include flexural strength and flexural modulus. Brittle Materials When a 3-point flexure test is done on a brittle material like ceramic or concrete it is often called modulus of rupture (MOR). This test provides flex strength data only, not stiffness (modulus). The 4-point test can also be used on brittle materials. Alignment of the support and loading anvils is critical with brittle materials. The test fixture for these materials usually has self-aligning anvils.

4.10 JOMINY END-QUENCH TEST One standard procedure that is widely used to measure hardenability of steel is the Jominy end-quench test. In this test water is sprayed on one end of a bar of steel while it is hot. This leads to a one dimensional heat transfer cooling. Except near the surface of the bar the temperature is controlled by that flow along the length of the bar. Moving axially inward from the quenched end of the bar, the temperature and the rate of change of temperature are changing. The temperature is higher and the rate is slower away from the quenched end. If hardness is measured as a function of distance from the end, a hardness profile can be obtained which applies to any part made from the same steel.

Experimental Procedure You will be given two steels (1045) and an alloy steel (4130). Before heating the specimens practice mounting the specimens in the rack and at the proper water flow to spray the ends of the specimen. Mark each specimen noting the hardness on the Rockwell C scale. Check to make sure the collar of the Jominy is secure and put the specimen in the furnace at 1600 degrees F for 45 minutes. While you are waiting to heat the specimens examine the microstructure of the allow steel and carbon steel specimens provided by the instructor. At the end of the austenizing treatment remove one specimen and carefully but rapidly place the specimen in the hold with the water turned on.

Method of Test Fig. 4.

Schematic of Jominy end-

quench test specimen (a) mounted during quenching and (b) after hardness testing. PRESTON UNIVERSITY, LAHORE CAMPUS

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The standard method for the Jominy test is ASTM-A255. The specimen consists of a cylindrical bar with a 1-in diameter and 4-in length and with a 1/16 in flange at one end. The test consists of austenitizing at 5°F above the solvus line on the Fe-C phase which separates γ from γ + α iron. Thereafter the specimen is removed from the furnace and is placed in the hardenability fixture as in Figure 4a. The time spent transferring the specimen from the furnace to the fixture should not be more than 5 sec. The fixture is constructed so that the specimen is held 1/2 inch above the water opening so that a column of water is directed only at the bottom of the bar. The water opening is 1/2 inch in diameter and the flow is previously adjusted to cause the column to rise 2-1/2 inches without the specimen in place. The test piece is held 10 minutes in the fixture under the action of cooling before quenching in cold water. After cooling, shallow flats 0.015 in. deep are ground along the specimen length (Figure 4b). Hardness (Rockwell C scale) measurements are taken for the first 2 ½ in. along each flat; for the first ½ in., hardness readings are taken at 1/16 in. intervals, for the remaining 2 in., hardness readings are taken every 1/8 in. Figure 5 shows the correlation between the hardness and the distance from the quenched end is due to the variation in the cooling rates that result to different microstructures at different distances from the quenched end.

Fig. 5. Correlation of hardenability and continuous cooling information for eutectoid steel.

Using the observed hardness values at different distances from the quenched end, hardenability curves can be plotted. Figure 6 shows typical hardenability curves for commonly used steel alloys.

Fig. 6 Hardenability curves for five different steel alloys PRESTON UNIVERSITY, LAHORE CAMPUS

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4.11 IMPACT TEST

In this test the pendulum is swing up to its starting position (height H ) and then it is allowed to strike the notched specimen, fixed in a vice. The pendulum fractures the specimen, spending a part of its energy. After the fracture the pendulum swings up to a height H. The impact toughness of the specimen is calculated by the formula: a = A/ S Where a-impact toughness, A – the work, required for breaking the specimen ( A = M*g*H0–M*g*H), M - the pendulum mass, S - cross-section area of the specimen at the notch. One of the most popular impact tests is the Charpy Test, schematically presented in the figure below:

Why is Impact Testing Important? Impact resistance is one of the most important properties for a part designer to consider, and without question, the most difficult to quantify. The impact resistance of a part is, in many applications, a critical measure of service life. More importantly these days, it involves the perplexing problem of product safety and liability. One must determine: • •

the impact energies the part can be expected to see in its lifetime, the type of impact that will deliver that energy, and then

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•

select a material that will resist such assaults over the projected life span.

Molded-in stresses, polymer orientation, weak spots (e.g. weld lines or gate areas), and part geometry will affect impact performance. Impact properties also change when additives, e.g. coloring agents, are added to plastics. Further complication is offered by the choice of failure modes: ductile or brittle. Brittle materials take little energy to start a crack, little more to propagate it to a shattering climax. Other materials possess ductility to varying degrees. Highly ductile materials fail by puncture in drop weight testing and require a high energy load to initiate and propagate the crack. Many materials are capable of either ductile or brittle failure, depending upon the type of test and rate and temperature conditions. They possess a ductile/brittle transition that actually shifts according to these variables.

4.12 TORSION TEST As in the macroscopic world, several kinds of tests have to be performed to be able to establish reliable failure criteria. An important experiment in this context is the torsional test, as it allows to validate or expand the criteria gained from tensile tests. The experimental setup is a major challenge. The actuator part has to be able to apply pure torsion by minimizing errors due to bending. The sensor part is able to measure the resulting angle and torque, with a resolution of 0.03o and 0.3 mNm, respectively. A balance records the tensile forces occurring in the specimens during the experiments. Experiments have been performed on both silicon and metallic specimens. The resulting curves are shown in Figure 1 .

Fig. 1 Torque/rotation diagrams for silicon and Ni specimens

A numerical simulation (see Figure 2 ), using finite element techniques, of the experiments in combination with an analytical analysis allows the determination of the governing elastic moduli, which in this case consist of two shear-moduli, as the material behaviour is considered to be transversely isotropic.

Fig. 2 FE-model of LIGA specimen PRESTON UNIVERSITY, LAHORE CAMPUS

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In addition to the determination of elastic moduli, it is important to know the relevant failure criterion in order to have engineering design rules. For Ni specimens, which show an isotropic behaviour, it is possible to show that the von Mises yield criterion is in good agreement with the experiments. The anisotropic NiFe alloys do not show the same behaviour. Their yielding point in torsion lies beyond the point predicted by von Mises, using the data from the tensile test and assuming an isotropic yield criterion.

Why Perform a Torsion Test? Many products and components are subjected to torsional forces during their operation. Products such as biomedical catheter tubing, switches, fasteners, and automotive steering columns are just a few devices subject to such torsional stresses. By testing these products in torsion, manufacturers are able to simulate real life service conditions, check product quality, verify designs, and ensure proper manufacturing techniques.

Types of Torsion Tests Torsion tests can be performed by applying only a rotational motion or by applying both axial (tension or compression) and torsional forces. Types of torsion testing vary from product to product but can usually be classified as failure, proof, or product operation testing. • • • • •

Torsion Only: Applying only torsional loads to the test specimen. Axial-Torsion: Applying both axial (tension or compression) and torsional forces to the test specimen. Failure Testing: Twisting the product, component, or specimen until failure. Failure can be classified as either a physical break or a kink/defect in the specimen. Proof Testing: Applying a torsional load and holding this torque load for a fixed amount of time. Operational Testing: Testing complete assemblies or products such as bottle caps, switches, dial pens, or steering columns to verify that the product performs as expected under torsion loads.

4.13 TENSILE TESTING •

•

As mentioned earlier the tensile test is used to provide information that will be used in design calculations or to demonstrate that a material complies with the requirements of the appropriate specification - it may therefore be either a quantitative OR a qualitative test. The test is made by gripping the ends of a suitably prepared standardised test piece in a tensile test machine and then applying a continually increasing uni-axial load until such time as failure occurs. Test pieces are standardised in order that results are reproducible and comparable as shown in Fig 2.

Fig.2. Standard shape tensile specimens PRESTON UNIVERSITY, LAHORE CAMPUS

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Specimens are said to be proportional when the gauge length, L 0 , is related to the original cross sectional area, A 0 , expressed as L 0 =k A 0 . The constant k is 5.65 in EN specifications and 5 in the ASME codes. These give gauge lengths of approximately 5x specimen diameter and 4x specimen diameter respectively - whilst this difference may not be technically significant it is important when claiming compliance with specifications.

Fig.3. Stress/strain curve • •

•

•

•

Both the load (stress) and the test piece extension (strain) are measured and from this data an engineering stress/strain curve is constructed, Fig.3. From this curve we can determine: a) the tensile strength, also known as the ultimate tensile strength, the load at failure divided by the original cross sectional area where the ultimate tensile strength (U.T.S.), max = P max /A 0 , where P max = maximum load, A 0 = original cross sectional area. In EN specifications this parameter is also identified as 'R m '; b) the yield point (YP), the stress at which deformation changes from elastic to plastic behaviour ie below the yield point unloading the specimen means that it returns to its original length, above the yield point permanent plastic deformation has occurred, YP or y = P yp /A 0 where P yp = load at the yield point. In EN specifications this parameter is also identified as 'R e '; c) By reassembling the broken specimen we can also measure the percentage elongation, El% how much the test piece had stretched at failure where El% = (L f - L 0 /L o ) x100 where Lf = gauge length at fracture and L0 = original gauge length. In EN specifications this parameter is also identified as 'A' ( Fig.4a). d) the percentage reduction of area, how much the specimen has necked or reduced in diameter at the point of failure where R of A% =(A 0 - A f /A 0 ) x 100 where A f = cross sectional area at site of the fracture. In EN specifications this parameter is also identified as 'Z', ( Fig.4b).

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Fig.4

• • • • •

• • •

•

a) Calculation of percentage elongation b) Calculation of percentage reduction of area

Fig (a) and (b) are measures of the strength of the material, (c) and (d) indicate the ductility or ability of the material to deform without fracture. The slope of the elastic portion of the curve, essentially a straight line, will give Young's Modulus of Elasticity, a measure of how much a structure will elastically deform when loaded. A low modulus means that a structure will be flexible, a high modulus a structure that will be stiff and inflexible. To produce the most accurate stress/strain curve an extensometer should be attached to the specimen to measure the elongation of the gauge length. A less accurate method is to measure the movement of the cross-head of the tensile machine. The stress strain curve in Fig.3 shows a material that has a well pronounced yield point but only annealed carbon steel exhibits this sort of behaviour. Metals that are strengthened by alloying, by heat treatment or by cold working do not have a pronounced yield and some other method must be found to determine the 'yield point'. This is done by measuring the proof stress ( offset yield strength in American terminology), the stress required to produce a small specified amount of plastic deformation in the test piece. The proof stress is measured by drawing a line parallel to the elastic portion of the stress/strain curve at a specified strain, this strain being a percentage of the original gauge length, hence 0.2% proof, 1% proof (see Fig.5). For example, 0.2% proof strength would be measured using 0.2mm of permanent deformation in a specimen with a gauge length of 100mm. Proof strength is therefore not a fixed material characteristic, such as the yield point, but will depend upon how much plastic deformation is specified. It is essential therefore when considering proof strengths that the percentage figure is always quoted. Most steel specifications use 0.2% deformation, R P0.2 in the EN specifications. Some materials such as annealed copper, grey iron and plastics do not have a straight line elastic portion on the stress/strain curve. In this case the usual practice, analogous to the method of

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determining proof strength, is to define the 'yield strength' as the stress to produce a specified amount of permanent deformation.

Fig.5. Determination of proof (offset yield) strength

Results generally required from a commercial tensile test: The result generally required from a commercial tensile test are as follow: • • • •

Tensile strength Elongation % Reduction in area Stress at yield point (when this is present)

Tensile strength This is found by dividing the maximum load by the original cross sectional area of the specimen (Note that for ductile materials the maximum load may be greater than the breaking load).

Tensile Strength =

Maximum load ________________________ Original cross-sectional area

Elongation % This is given by expressing the stretch as a percentage of the of the original test length.

Stretch Elongation % = ______________ x 100 Original length PRESTON UNIVERSITY, LAHORE CAMPUS

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Good elongation combined with a reasonable strength indicates a ductile material. Besides these two main properties a specification may stipulate other requirement which may be obtained from the tensile test :

Reduction in area % This is given by: Reduction in area at fracture Reduction in area % = ___________________________ x 100 Original area Original area - Area at fracture x 100 = ____________________________ Original area

A high reduction of area indication that the material will lend itself more readily to cold working

Stress at yield point (when this is present)

Load at yield point Stress at yield point = ___________________ Cross-sectional area

Note that as a stress will be expressed as a load per unit area . for example if a load in Newton’s is divided by an area in square millimeter. the unit stress will be Newton per square millimeter, the units of stress will be Newton per square millimeter.

Example : Determine the results of a test on a BS test piece, 14mm test diameter , 70mm gauge length which gave the following results when tested: • • • •

Load at yield point Maximum load Length between gauge point with broken end put together Diameter at fracture

51 KN 82 KN 90 mm 11.5 mm

Solution: Tensile strength

Maximum Load = __________________ Cross-sectional area

82 x 103 = _____________ = 2 3.14 / 4 x 14 PRESTON UNIVERSITY, LAHORE CAMPUS

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Load at yield point Stress at yield point = _________________ Cross-sectional area 51 x 103 = ______________ = 331 N / mm2 3.14 / 4 x 142

Elongation %

Stretch = ______________ x 100 Original length 90 - 70 x 100 = 28.6 % = _________ 70

Original area - Area at fracture x 100 Reduction in area % = ____________________________ Original area (3.14 / 4 x 142) - (3.14 / 4 x 11.52) x 100 = _________________________________ 3.14 / 4 x 142 = 32.4 %

4.14 CREEP TEST Creep is a phenomenon of slow plastic deformation (elongation) of a metal at high temperature under a constant load.

The creep mechanism: At low stresses the creep is controlled by the diffusion of atoms through the grain boundaries. At higher stresses the creep strain proceeds due to the dislocations movement. The rate of creep is a function of the material, the applied stress value, the temperature, and the time exposure. Considerable creep deformation, causing damage of machines and structures occur at high temperatures (about a half of the melting point measured in the absolute temperature scale). Therefore this phenomenon is taken into account in design and operation of heat exchangers, steam boilers and pipes, jet engines and other loaded equipment, working at high temperatures. Soft metals (lead, tin) may experience creep at room temperature. A typical creep behavior is presented in the diagram:

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Fig. Creep behavior

The initial strain is not time dependent and it is caused mainly by elastic deformation. The first stage creep is characterized by relatively fast plastic deformation occurring at decreasing rate. During this stage resistance creep increases causing decrease the deformation rate. The second stage creep occurs at a constant and relatively low deformation rate. This rate is used in the engineering design. The rate of creep at the second stage depends on both the load (stress) and the temperature. The third stage creep is associated with accelerated strain rate caused by decrease of the cross sectional area of the specimen (necking). This stage is finalized by the specimen fracture. At room temperature creep is negligible at any stress below the yield point. The quantity, which is used in precise design of machines and structures working at elevated temperatures, is creep strength. Creep strength is a stress which causes a definite creep strain after a specified period of time at a given temperature. Creep strength of a material is much lower, than its tensile strength. If a large amount of deformation is tolerated rupture strength is used in design. Rupture strength is a stress which causes a fracture of a metal after a specified period of time at a given temperature. Creep strength and rupture strength are determined in stress-rupture tests conducted in [Tensile test and Strain-Stress Diagram|tensile test]] machines equipped with a furnace providing uniform heating of the tested specimens. This machine records amount of strain at every moment after the test has started and until the specimen failure.

How to Perform a Creep Test? To determine creep properties, a material is subjected to prolonged constant tension or compression loading at constant elevated temperature. Deformation is recorded at specified time intervals and a creep vs. time diagram is plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates the test PRESTON UNIVERSITY, LAHORE CAMPUS

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and the time for rupture is recorded. If specimen does not fracture within the test period, creep recovery may be measured.

How to Determine Stress-Relaxation? To determine stress-relaxation of a material, the specimen is deformed a given amount and decrease in stress is recorded over prolonged period of exposure at constant elevated temperature. The stress-relaxation rate is the slope of the curve at any point.

Typical Applications • • • •

Metal Working Springs Soldered Joints High-Temperature Materials

4.15 CHARPY TEST While most commonly used on metals, it is also used on polymers, ceramics and composites. The Charpy test is most commonly used to evaluate the relative toughness or impact toughness of materials and as such is often used in quality control applications where it is a fast and economical test. It is used more as a comparative test rather than a definitive test.

Fig. The hammer striking energy in the Charpy test is 220 ft*lbf (300 J).

Charpy Test Specimens Charpy test specimens normally measure 55x10x10mm and have a notch machined across one of the larger faces. The notches may be: • •

V-notch – A V-shaped notch, 2mm deep, with 45° angle and 0.25mm radius along the base U-notch or keyhole notch – A 5mm deep notch with 1mm radius at the base of the notch.

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What Does the Charpy Test Involve? The Charpy test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is fixed in place at both ends and the striker impacts the test piece immediately behind a machined notch.

Fig. 1 Schematic of the Charpy impact test.

4.16 IZOD TEST The Izod test is has become the standard testing procedure for comparing the impact resistances of plastics. While being the standard for plastics it is also used on other materials. The Izod test is most commonly used to evaluate the relative toughness or impact toughness of materials and as such is often used in quality control applications where it is a fast and economical test. It is used more as a comparative test rather than a definitive test. This is also in part due to the fact that the values do not relate accurately to the impact strength of moulded parts or actual components under actual operational conditions.

Izod Test Specimens Izod test specimens vary depending on what material is being tested. Metallic samples tend to be square in cross section, while polymeric test specimens are often rectangular, being struck parallel to the long axis of the rectangle. Izod test sample usually have a V-notch cut into them, although specimens with no notch as also used on occasion.

What Does the Izod Test Involve? The Izod test involves striking a suitable test piece with a striker, mounted at the end of a pendulum. The test piece is clamped vertically with the notch facing the striker. The striker swings downwards impacting the test piece at the bottom of its swing.

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Some Izod impact testers are equipped to be able to utilise different sized strikers, which impart different amounts of energy. Often a series of stri8kers may be used to determine the impact energy, starting with small strikers and working up until failure occurs.

Fig. 1 Schematic of the Izod impact test.

Izod Tests at Different Temperatures Tests are often performed at different temperatures to more closely simulate the actual service conditions. In the case of low temperature tests, specimens may are kept in a freezer until their temperature has equilibrated. They are then immediately removed and tested within seconds of removal from the freezer.

4.17 NON-DESTRUCTIVE TEST Nondestructive testing (NDT), also called nondestructive evaluation (NDE) and nondestructive inspection (NDI), is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components and structures. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo blip on the screen). This article is aimed mainly at industrial NDT, but many of the methods described here can be used to test the human body. In fact methods from the medical field have often been adapted for industrial use, as was the case with Phased array ultrasonics and Computed radiography. While destructive testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. Destructive testing is also inappropriate in many circumstances, such as forensic investigation. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively; destructive testing is performed on a sampling of test objects that is drawn randomly for the purpose of characterizing the testing reliability of the nondestructive test.

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The need of NDT It is very difficult to weld or mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential. During the process of molding a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture. During their service lives, many industrial components need regular nondestructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example: Aircraft skins need regular checking to detect cracks; • • • • •

Underground pipelines are subjected to corrosion and stress corrosion cracking; Pipes in industrial plants may be subject to erosion and corrosion from the products they carry; Concrete structures may be weakend if the inner reinforcing steel is corroded; Pressure vessels may develop crcacks in welds; The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important

NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT. Some types of non-destructive tests are: • • • • •

Ultrasonic testing Liquid penetrant testing Radiographic testing Magnetic particle testing Magnetic flux leackage test

4.18 ULTRASONIC TESTING In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. It is also commonly used to determine the thickness of the test object - monitoring pipework corrosion being a good example. Ultrasonic Inspection is often performed on steel and other metals and alloys, though it can be used on concrete and other materials such as composites. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors.

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An example of Ultrasonic Testing (UT) on blade roots of a V2500 IAE aircraft engine. Step 1: The UT probe is placed on the root of the blades to be inspected with the help of a special borescope tool(videoprobe). Step2:Instrumentsettingsareinput. Step 3: The probe is scanned over the blade root. In this case, an indication (peak in the data) through the red line (or gate) indicates a good blade; an indication to the left of that range indicates a crack. Many structures and components are being successfully inspected with Zetec products that use ultrasound technologies, such as: •

Pressure retaining welds in nuclear and fossil power plants: piping welds, reactor pressure vessel welds, and nozzle welds are just a few examples

•

Turbine blade roots and attachments

•

Turbine bores

•

Feeder tubes in CANDU type nuclear power plants

How it works? In ultrasonic testing, a transducer connected to a diagnostic machine is passed over the object being inspected. In reflection (or pulse-echo) mode, the transducer sends pulsed waves through a couplant (such as water or oil) on the surface of the object, and receives the "sound" reflected back to the device. Reflected ultrasound comes from an interface - such as the back wall of the object or from an imperfection. The screen on the calibrated diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance taken for the reflection to return to the transducer. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after travelling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted thus indicating their presence.

Advantages 1. Superior penetrating power, which allows the detection of flaws deep in the part. 2. High sensitivity, permitting the detection of extremely small flaws. 3. Only one surface need to be accessible. PRESTON UNIVERSITY, LAHORE CAMPUS

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4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces. 5. Some capability of estimating the size, orientation, shape and nature of defects. 6. Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity. 7. Capable of portable or highly automated operation.

Disadvantages 1. Manual operation requires careful attention by experienced technicians. 2. Extensive technical knowledge is required for the development of inspection procedures. 3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect. 4. Surface must be prepared by cleaning and removing loose scale, paint, etc. (UT can often be used successfully through paint that is properly bonded to a surface) 5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT). 6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.

4.19 LIQUID PENETRANT TESTING

1. 2. 3. 4.

Section of material with a surface-breaking crack that is not visible to the naked eye. Penetrant is applied to the surface. Excess penetrant is removed. Developer is applied, rendering the crack visible.

Liquid penetrant inspection is a widely applied and low-cost inspection method used to locate surfacebreaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic particle inspection is preferred PRESTON UNIVERSITY, LAHORE CAMPUS

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for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components. Principles LPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surfacebreaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used - fluorescent or nonfluorescent (visible). Mtaerials Penetrants are classified into sensitivity levels. Visible penetrants are typically red in color, and represent the lowest sensitivity. Fluorescent penetrants contain two or more dyes that fluoresce when excited by ultraviolet (UV-A) radiation (also known as black light). Since FPI is performed in a darkened environment, and the excited dyes emit brilliant yellow-green light that contrasts strongly against the dark background, this material is more sensitive to small defects. When selecting a sensitivity level one must consider many factors, including the environment under which the test will be performed, the surface finish of the specimen, and the size of defects sought. One must also assure that the test chemicals are compatible with the sample so that the examination will not cause permanent staining, or degradation. This technique can be quite portable, because in its simplest form the inspection requires only 3 aerosol spray cans, some paper towels, and adequate visible light. Stationary systems with dedicated application, wash, and development stations, are more costly and complicated, but result in better sensitivity and higher sample through-put.

Inspection steps Below are the main steps of Liquid Penetrant Inspection:

1. Precleaning The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor degreasing, or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination.

2. Application of penetrant The penetrant is then applied to the surface of the item being tested. The penetrant is allowed time to soak into any flaws (generally 10 to 30 minutes). The soak time mainly depends upon the material being testing and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature one must be careful not to apply visible red dye penetrant to a sample that may later be inspected with fluorescent penetrant.

3. Excess penetrant removal The excess penetrant is then removed from the surface. Removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic postemulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrant to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can the remove the penetrant from the flaws. This process must be performed under controlled conditions so that PRESTON UNIVERSITY, LAHORE CAMPUS

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all penetrant on the surface is removed (background noise), but penetrant trapped in real defects remains in place.

4. Application of developer After excess penetrant has been removed a white developer is applied to the sample. Several developer types are available, including: non-aqueous wet developer, dry powder, water suspendible, and water soluble. Choice of developer is governed by penetrant compatibility (one can't use water-soluble or suspedible developer with water-washable penetrant), and by inspection conditions. When using nonaqueous wet developer (NAWD) or dry powder the sample must be dried prior to application, while soluble and suspendible developers are applied with the part still wet from the previous step. NAWD is commercially available in aerosol spray cans, and may employ acetone, isopropyl alcohol, or a propellant that is a combination of the two. Developer should form a thin, even coating on the surface. The developer draws penetrant from defects out onto the surface to form a visible indication, a process similar to the action of blotting paper. Any colored stains indicate the positions and types of defects on the surface under inspection.

5. Inspection The inspector will use visible light with adequate intensity (100 foot-candles is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after a 10 minute development time. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye, but this should not be done when using fluorescent penetrant. Also of concern, if one waits too long after development the indications may "bleed out" such that interpretation is hindered.

6. Post cleaning The test surface is often cleaned after inspection and recording of defects (if found), especially if postinspection coating processes are scheduled.

Features The flaws are more visible, because; • • • • • •

The defect indication has a high visual contrast (e.g. red dye against a white developer background, or a bright fluorescent indication against a dark background). The developer draws the penetrant out of the flaw over a wider area than the real flaw, so it looks wider. Limited training is required for the operator — although experience is quite valuable. Low testing costs. Proper cleaning is necessary to assure that surface contaminants have been removed and any defects present are clean and dry. Some cleaning methods have been shown to be detrimental to test sensitivity, so acid etching to remove metal smearing and re-open the defect may be necessary. Penetrant dyes stain cloth, skin and other porous surfaces brought into contact. One should verify compatibility on the test material, especially when considering the testing of plastic components.

4.20 RADIOGRAPHIC TESTING Radiographic Testing (RT), or industrial radiography, is a nondestructive testing (NDT) method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materialS. PRESTON UNIVERSITY, LAHORE CAMPUS

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Either an X-ray machine or a radioactive source (Ir-192, Co-60, or in rare cases Cs-137) can be used as a source of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses neutrons instead of photons to penetrate materials. This can see very different things from X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils. Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

Inspection of welds The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded. The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served. Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult. After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique. Defects such as delaminations and planar cracks are difficult to detect using radiography, which is why penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, chloroform and diiodomethane. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has the advantages of high opacity, ease of penetration, and ease of removal because it evaporates relatively quickly. However, it can cause skin burns.

Safety Industrial radiography appears to have one of the worst safety profiles of the radiation professions, possibly because there are many operators using strong gamma sources (> 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within hospitals.

4.21 MEGNATIC PARTICLE TESTING Magnetic particle inspection processes are non-destructive methods for the detection of defects in ferrous materials. They make use of an externally applied magnetic field or DC current through the material, and PRESTON UNIVERSITY, LAHORE CAMPUS

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the principle that the magnetic susceptibility of a defect is markedly poorer (the magnetic resistance is greater) than that of the surrounding material. The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. Thus the size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of the tiny particles that are attracted to the leakage fields. The most common method of magnetic particle inspection uses finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene). This fluid is referred to as carrier. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. The suspension is sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles. This inspection can be applied to raw material in a steel mill (billets or slabs), in the early stages of manufacturing (forgings, castings), or most commonly to machined parts before they are put into service. It is also very commonly used for inspecting structural parts (e.g. landing gear) that have been in-service for some time to find fatigue cracks.

4.22 MAGNETIC FLUX LEACKAGE TEST Magnetic flux leakage (MFL) is a magnetic method of nondestructive testing that is used to detect corrosion and pitting in steel structures, most commonly pipelines and storage tanks. The basic principle is that a powerful magnet is used to magnetize the steel. At areas where there is corrosion or missing metal, the magnetic field "leaks" from the steel. In an MFL tool, a magnetic detector is placed between the poles of the magnet to detect the leakage field. Analysts interpret the chart recording of the leakage field to identify damaged areas and hopefully to estimate the depth of metal loss. This article currently focuses mainly on the pipeline application of MFL, but links to tank floor examination are provided at the end.

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CHAPTER

FIVE MATERIAL TESTING EQUIPEMENT

5.1 INTRODUCTION Mechanical testing equipment covers devices used for adhesion, compression, drop (shock), tensile, vibration and fatigue testing. The growing importance of quality control and assurance in production has contributed to an increasing demand for mechanical testing equipment with quality-control procedures existing on all production levels of many industrial markets. Mechanical testing for quality control serves two major purposes: product-endurance analysis and product-safety assurance. Understanding the strength and endurance of the product is beneficial to the end-user and to the supplier. Mechanical testing contributes to quality enhancement of a product because it enables manufacturers to test material characteristics before and after the final assembly stage. Because of the diverse nature of mechanical testing equipment, materials and structures of all sizes can be quality tested. Mechanical test method, user interface options, display options, additional output options, and environmental parameters. User interface options for mechanical testing equipment include local interfaces that are analog or digital, computer interfaces, serial or parallel communications, and application software. Display options for mechanical testing equipment include analog meters, digital readouts, and video displays. Additional output options include analog voltage, pulse signal, analog current, and switch or relay. Important environmental parameters to consider for mechanical testing equipment include operating temperature and operating humidity.

5.2 BRINELL TESTER The Brinell hardness tester, shown in figure 1-25, uses a hardened spherical ball, which is forced into the surface of the metal. The ball is 10 millimeters (0.3937 inch) in diameter. A pressure of 3,000 kilograms (6,600 pounds) is used for ferrous metals and 500 kilograms for nonferrous metals. Normally, the load should be applied for 30 seconds. In order to produce equilibrium, this period may be increased to 1 minute for extremely hard steels. The load is applied by means of hydraulic pressure. The hydraulic pressure is built up by a hand pump or an electric motor, depending on the model of tester. A pressure gauge indicates the amount of pressure. There is a release mechanism for relieving the pressure after the test has been made, and a calibrated microscope is provided for measuring the diameter of the impression in millimeters. The machine has various shaped anvils for supporting the specimen and an elevating screw for bringing the specimen in contact with the ball penetrator. There are attachments for special tests. To determine the Brinell hardness number for a metal, the diameter of the impression is first measured, using the calibrated microscope furnished with the tester.

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Table 1-3.-Portion of Conversion Table Furnished with Brinell Tester

the microscope. After measuring the diameter of the impression, the measurement is converted into the Brinell hardness number on the conversion table furnished with the tester. A portion of the conversion table is shown in table 1-3.

Fig. 1-25.-Brinell hardness tester.

5.3 ROCKWELL TESTER The Rockwell hardness tester, shown in figure 1-27, measures the resistance to penetration as does the Brinell tester, but instead of measuring the diameter of the impression, the Rockwell tester measures the depth, and the hardness is indicated directly on a dial attached to the machine. The more shallow the penetration, the higher the hardness number. Two types of penetrators are used with the Rockwell tester–a diamond cone and a hardened steel ball. The load that forces the penetrator into the metal is called the "major load," and is measured in kilograms. The PRESTON UNIVERSITY, LAHORE CAMPUS

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results of each penetrator and load combination are reported on separate scales, designated by letters. The penetrator, the major load, and the scale vary with the kind of metal being tested. For hardened steels, the diamond penetrator is used, the major load is 150 kilograms, and the hardness is read on the C scale. When this reading is recorded, the letter C must precede the number indicated by the pointer. The C-scale setup is used for testing metals ranging in hardness from C-20 to the hardest steel (usually about C-70). If the metal is softer than C-20, the B-scale setup is used. With this setup, the 1/16inch ball is used as a penetrator, the major load is 100 kilograms, and the hardness is read on the B scale. In addition to the C and B scales, there are other setups for special testing. The scales, penetrators, major loads, and dial numbers are listed in table 1-4. The dial numbers in the outer circle are black, and the inner numbers are red.

Fig. 1-27.-Rockwell hardness tester. The Rockwell tester is equipped with a weight pan, and two weights are supplied with the machine. One weight is marked in red. The other weight is marked in black. With no weight in the weight pan, the machine applies a major load of 60 kilograms. If the scale setup calls for a 100-kilogram load, the red weight is placed in the pan. For a 150-kilogram load, the black weight is added to the red weight. The black weight is always used in conjunction with the red weight; it is never used alone. Practically all testing is done with either the B-scale setup or the C-scale setup. For these scales, the colors may be used as a guide in selecting the weight (or weights) and in reading the dial. For the B-scale test, use the red weight and read the red numbers. For a C-scale test, add the black weight to the red weight and read the black numbers.

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In setting up the Rockwell machine, use the diamond penetrator for testing materials that are known to be hard. If in doubt, try the diamond, since the steel ball may be deformed if used for testing hard materials. If the metal tests below C-22, then change to the steel ball. Use the steel ball for all soft materials-those testing less than B-100. Should an overlap occur at the top of the B scale and the bottom of the C scale, use the C-scale setup. Before the major load is applied, the test specimen must be securely locked in place to prevent slipping and to properly seat the anvil and penetrator. To do this, a load of 10 kilograms is applied before the lever is tripped. This preliminary load is called the "minor load." The minor load is 10 kilograms regardless of the scale setup. When the machine is set up properly, it auto-matically applies the 10-kilogram load. The metal to be tested in the Rockwell tester must be ground smooth on two opposite sides and be free of scratches and foreign matter. The surface should be perpendicular to the axis of penetration, and the two opposite ground surfaces should be parallel. If the specimen is tapered, the amount of error will depend on the taper. A curved surface will also cause a slight error in the hardness test. The amount of error depends on the curvature–the smaller the radius of curvature, the greater the error. To eliminate such error, a small flat should be ground on the curved surface if possible.

5.4 RIEHLE TESTER The Riehle hardness tester is a portable unit that is designed for making Rockwell tests comparable to the bench-type machine. The instrument is quite universal in its application, being readily adjustable to a wide range of sizes and shapes that would be difficult, or impossible, to test on a bench-type tester.

Fig. 1-28.-Riehle portable hardness tester. Figure 1-28 shows the tester and its proper use. It may be noted that the adjusting screws and the penetration indicator are set back some distance from the penetrator end of the clamps. This makes it practicable to use the tester on either the outside or inside surface of tubing, as well as on many other applications where the clearance above the penetrator or below the anvil is limited. The indicator brackets are arranged so that it is possible to turn the indicators to any angle for greater convenience in a specific application, or to facilitate its use by a left-handed operator. Adjustment of the lower clamp is made by the small knurled knob below the clamp. The larger diameter knob, extending through the slot in the side of the clamp, is used for actual clamping. Each Riehle tester is supplied with a diamond pene-trator and a 1/16-inch ball penetrator. The ball penetrator should not be used on materials harder than B-100 nor on a load heavier than 100 kilograms. This is to avoid the danger of flattening the ball. PRESTON UNIVERSITY, LAHORE CAMPUS

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The diamond penetrator, when used with a 150-kilogram load, may be used on materials from the hardest down to those giving a reading of C-20. When the expected hardness of a material is completely unknown to the operator, it is advisable to take a preliminary reading on the A scale as a guide in selecting the proper scale to be used.

Testing Procedure The basic procedures for making a test with the Riehle tester are as follows: 1. Apply a minor load of 10 kilograms. 2. Set the penetration indicator to zero. 3. Apply a major load of 60, 100, or 150 kilograms (depending on the scale), and then reduce the load back to the initial 10-kilogram load. 4. Read the hardness directly on the penetration indicator. The hardness reading is based on the measurement of the additional increment of penetration produced by applying a major load after an initial penetration has been produced by the minor load. In reporting a hardness number, the number must be prefixed by the letter indicating the scale on which the reading was obtained. Removal and Replacement of a Penetrator The penetrator is retained in the tester by means of a small knurled clamp screw extending from the top of

Fig. 1-29.-Barcol portable hardness tester.

5.5 BARCOL TESTER The Barcol hardness tester, the upper clamp. To remove a penetrator, there should beat least 2 or 3 inches of space between the upper and lower clamps so that one hand can be placed underneath the upper clamp to catch the penetrator when it is released. Two or three turns of the clamp screw will release the penetrator. The two contact pins that extend through the penetrator on either side of the point are retained in the tester when the penetrator is removed. To replace a penetrator, it must be turned so that the flat side faces the clamp screw, and the locating pin on the penetrator is in line with the slot provided to take the pin. The contact pins should be guided into their respective holes through the penetrator. With the penetrator in place, it should then be clamped securely by turning the clamp screw. Before you make an actual test, one or two preliminary tests should be made to properly seat the penetrator. shown in figure 1-29, is a portable unit designed for testing aluminum alloys, copper, brass, and other relatively soft materials. Approximate range of the tester is 25 to 100 Brinell. The unit can be used in any PRESTON UNIVERSITY, LAHORE CAMPUS

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position and in any space that will allow for the operator’s hand. The hardness is indicated on a dial conveniently divided in 100 graduations.

Fig. 1-30-Cutaway of Barcol tester. Figure 1-30 is a cutaway drawing of the tester, showing the internal parts and their general arrangement within the case. The lower plunger guide and point are accurately ground so that attention need be given only to the proper position of the lower plunger guide within the frame to obtain accurate operation when a point is replaced. The frame, into which the lower plunger guide and spring-tensioned plunger are screwed, holds the point in the proper position. Adjustment of the plunger upper guide nut, which regulates the spring tension, is made when the instrument is calibrated at the factory.

CAUTION The position of this nut should not be changed. Any adjustment made to the plunger upper guide nut will void the calibrated settings made at the factory. The leg is set for testing surfaces that permit the lower plunger guide and the leg plate to be on the same plane. For testing rivets or other raised objects, a block may be placed under the leg plate to raise it to the same plane. For permanent testing of this type, the leg maybe removed and washers inserted, as shown in the drawing. The point should always be perpendicular to the surface being tested. The design of the Barcol tester is such that operating experience is not necessary. It is only necessary to exert a light pressure against the instrument to drive the spring-loaded indenter into the material to be tested.

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Table 1-5.-Typical Barcol Readings for Aluminum Alloys Alloy and temper Barcol number 35 1100-0 3003-0

42

3003 -1/2H

56

2024-0

60

5052-0

62

5052-1/2H

75

6061-T

78

2024-T

85

The hardness reading is instantly indicated on the dial. Several typical reading for aluminum alloys are listed in table 1-5. The harder the material, the higher the Barcol number. To prevent damage to the point, avoid sliding or scraping when it is in contact with the material being tested. If the point should become damaged, it must be replaced with a new one. No attempt should be made to grind the point. Each tester is supplied with a test disc for checking the condition of the point. To check the condition of the point, press the instrument down on the test disc. When the downward pressure brings the end of the lower plunger guide against the surface of the disc, the indicator reading should be within the range shown on the test disc. To replace the point, remove the two screws that hold the halves of the case together. Lift out the frame, remove the spring sleeve, loosen the locknut, and unscrew the lower plunger guide, holding the point upward so that the spring and plunger will not fall out of place. Insert the new point and replace the lower plunger guide, screwing it back into the frame, Adjust the lower plunger guide with the wrench that is furnished until the indicator reading and the test disc average number are identical. After the lower plunger guide is properly set, tighten the locknut to keep the lower plunger guide in place, This adjustment should be made only after installing anew point; any readjustment on a worn or damaged point give erroneous readings.

Fig. 1-31.-Ernst portable hardness tester. PRESTON UNIVERSITY, LAHORE CAMPUS

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THE ERNST PORTABLE HARDNESS TESTER HAS A DIAMOND-TIPPED PENETRATOR AND READS IN ROCKWELL OR BRINELL SCALES. NOTE : MATERIAL MUST BE SOLIDLY SUPPORTED FROM BEHIND. PRESS DOWN WITH A STEADY, EVEN FORCE.

5.6 ERNST TESTER The Ernst tester is a small versatile tool that requires access to only one side of the material being tested. There are two models of the tester-one for testing hardened steels and hard alloys and one for testing unhardened steels and most nonferrous metals. It has a diamond point penetrator, and it is read directly from the Rockwell A or B or the Brinell scales, depending on the model used. Figure 1-31 shows the Ernst portable hardness tester and its proper use. The correct procedures for using the Ernst tester are as follows: 1. Solidly support the metal being tested by placing a bucking bar behind the metal. This will minimize flexing of the metal and yield a more accurate reading of hardness. 2. The handgrip must be pressed down with a steady, even force to ensure accurate readings. 3. Press down until the fluid column has stopped moving. The hardness value is given at the point where the fluid column has stopped moving on the scale. As with other portable testers of similar type, the material must be smooth and backed up so there will be no tendency to sag under the load applied on the tester. The test block supplied with each tester should be used frequently to check its performance.

5.7 UNIVERSAL HARDNESS TESTER For customers who require the flexibility of three hardness testing machines in one then Indentec's range of Universal Hardness Testers are an ideal choice. The units are capable of Rockwell, Vickers and low load Brinell hardness tests, with the option of a hand held portable microscope or built in optical system for Vickers and Brinell indentation measurement. If you need to eliminate uncertainty from Vickers and Brinell testing, you should check out our CAMS system. This computeraided innovation replaces operator judgement with a CCD camera for distinguishing impressions. Through mouse-driven software, the indentation is electronically projected onto a PC monitor and measured automatically eliminating operator influence and reducing gang R+ R values. Available on all our Universal machines, CAMS makes Vickers and Brinell testing very easy and trouble free. The multi-test facility makes the machine ideal for educational purposes in helping to demonstrate the three classical hardness tests. The unit is supplied with UKAS accredited test blocks and indentors for each of the three test methods.

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5.8 MICRO VICKERS HARDNESS TESTER Our advanced line of QV-1000 Series Vickers hardness testers are low-cost and precise testing systems suitable for hardness analysis of metallic specimens in metallography laboratories or production environments. Features • • • • • • • •

Motorized turret High quality microscope with digital reading (QV-1000DAT Model) Fully automatic load control Easy operating system Two optical paths Built-in high speed thermal printer XY stage with minimum reading of 0.01mm QV-MONITOR or QV-CCD system (optional)

Micro Vickers Hardness Tester - Analogue with Auto Turret (motorized)

Standard configuration includes: • • • • • • • • •

Main unit Diamond indenter Vickers Objectives 10x, 40x Eyepiece 15x XY-stage with micrometers 3 adjustable feet 3 clamping devices Extension tube for CCD-camera Digital eyepiece incl. protection cover

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• • • • • •

Spirit level Micro-Vickers test plates 2x Spare light bulb 12V-30W Spare fuses 2x Installation & user manual Portable tester

•

If you can't bring the specimen to the tester, then you can take the test to the remote or large specimen with one of our handheld portable hardness testers. Producing on-the-spot readings and print-outs of any popular hardness scale, these pocketsized instruments can be applied to any surface, from any direction, including upside down. Top models store up to 200 test results and display hardness, scale, time, material tested, number of tests, running average hardness, test direction and ultimate tensile strength. The units are totally portable, requiring no external power source cables and connect to a printer via an infra-red port. Instruments are supplied with an Indentec calibration certificate and a UKAS traceable test block.

• • • •

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