Electromagnetic Testing-ASNT Level III Study Guide ECT

Electromagnetic Testing

Study Guide Eddy Current Testing Revisited My ASNT Level III Pre-Exam Preparatory Self Study Notes 26th April 2015

Charlie Chong/ Fion Zhang

Aerospace Applications

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Aerospace Applications

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Aerospace Applications

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Aerospace & Defence Applications

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Power & Nuclear Applications

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Oil & Gas Applications

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Heavy Industry & Mining Applications

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http://independent.academia.edu/CharlieChong1 http://www.yumpu.com/zh/browse/user/charliechong http://issuu.com/charlieccchong

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Fion Zhang at Shanghai 26th April 2015

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CHAPTER 1 PRINCIPLES OF EDDY CURRENT TESTING

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HISTORICAL BACKGROUND Belore discussing the principles of eddy current testing, it seems appropriate to discuss brielly facets of magnetism and electromagnetism that serve as the foundation of our study of eddy current testing. In the period from 1775 to 1900, scientific experimenters Coulomb, A Ampere, Faraday, Oersted, Arago, Maxwell, and Kelvin investigated and cataloged most of what is known about magnetism and electromagnetism Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the magnet (Figure 1.1). He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field to the disk causing eddy currents to allow in the disk producing a magnetic field by the disk that attracted the magnet. Arago's simple model is a basis lor many automobile speedometers used today.

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Figure 1.1- Arago‘s Magnetic Experimentation, 1821.

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https://www.nde-ed.org/GeneralResources/Formula/ECFormula/ECFormula.htm

Oersted discovered the presence of a magnetic field around a currentcarrying conductor, and he observed a magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite currents Ilowing in adjacent conductors cancelled this magnetic effect. Ampere's observation is used in differential coil applications and to manufacture noninductive, precision resistors. Faraday's first experiments investigated induced currents by the relative motion of magnet and a coil (Fig. 1.2)

Figure 1.2一Induced Current with Coil and Magnet Charlie Chong/ Fion Zhang

Faraday's major contribution was the discovery of electromagnetic induction. His work can be summarized by the example shown· in Figure 1.3. Coil A is connected to a battery through a switch S. A second coil a connected to a galvanometer G is nearby. When switch S is closed producing a current in coil A in the direction shown, a momentary current is induced in coil a in a direction(- a) opposite to that in A. If S is now opened, a momeritary current will appear in coil a having the direction of (- b). In each case, current flows in coil a only while the current in coil A is changing.

Figure 1.3-lnduced Current, Electromagnetic Technique

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FARADAY LAW The electromotive force (voltage) induced in coil a of Figure 1.3 can be expressed as follows: E = K ∙ N ∙ ∆Ф/∆t E = Average induced voltage N = Number of turns of wire in coil B ∆Ф/∆t = Rate of change of magnetic lines of force affecting coil B K = 10-8 constant

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Maxwell produced a two-volume work "A Treatise on Electricity and Magnetism" first published in 1873, Maxwell not only chronicled most of the work done in electricity and magnetism at that time, but he also developed and published a group of relations known as Maxwell's equations for the elec tromagnetic field. These equations form the base that mathematically describes most of what is known about electromagnetism today. In 1849 Lord Kelvin applied Bessel.'s equation to solve the elements of an electromagnetic field. The principles of eddy current testing depend on the process of electromagnetic induction. This process includes a test coil through which a varying or alternating current is passed. A varying current flowing in a test coil produces a varying electromagnetic field about the coil. This field is known as the primary field.

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Faraday Law Increasing current in a coil of wire will generate a counter emf which opposes the current. Applying the voltage law allows us to see the effect of this emf on the circuit equation. The fact that the emf always opposes the change in current is an example of Lenz's law. The relation of this counter emf to the current is the origin of the concept of inductance. The inductance of a coil follows from Faraday's law.

E ∝ ∆ Ф/ ∆t (Faraday Law)

Since the magnetic field of a solenoid is:

E = - N ∆Ф/ ∆t

B = μNI ∙ (l -1)

Ф = BA

Thus:

B = flux density A = Area under the influence of B

E = - NA ∙∆B/ ∆t, becomes;

For a fixed area and changing current, Faraday's law becomes:

E = - N A ∙∆ [μNI (l -1)] / ∆t E = - NAμN ∙(l -1) ∙ ∆I/∆t

E = - N ∆Ф/ ∆t = -N ∆BA/ ∆t

for L = N2Aμ (l -1)

for Ф = BA

E = -L ∆I/∆t #

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http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indcur.html

Faraday Law

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GENERATION OF EDDY CURRENTS When an electrically conducting test object is placed in the primary field, an electrical current will be induced in the test object. This c urrent is known as the eddy current. Figure 1.4 is a simple model that illust rates the relations hips of primary and induced (eddy) curre nts. Conductor A represents a portion of a test coil. Conductor B represents a portion of a test object.

Figure 1.4-1 Induced Current Relationships Charlie Chong/ Fion Zhang

Following Lenz's law and indicating the instantaneous direction of primary current Фp, a primary field Фp is developed about Conductor A. When Conductor B is brought into the influence of Фp, an eddy current lE is induced in Conductor B. This electrical current lE produces an electromagnetic field ФE that opposes the primary electromagnetic field Фp. The magnitude of ФE is directly proportional to the magnitude of lE. Characteristic changes in Conductor B such as conductivity, permeability, or geometry will cause lE to change. When lE varies, ФE also varies. Variations of ФE are reflected to Conductor A by changes in Фp. These changes are detected and displayed on some type of readout mechanism that relates these variations to the characteristic that is of interest. Ip = Primary current IE = Eddy current Фp = Primary magnetic flux ФE = Secondary eddy current flux

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FIELD INTENSITY Ф Figure 1.5 presents a schematic view of an excited test coil. The electromagnetic field produced about the unloaded test coil in Figure 1.5 can be described as decreasing in intensity with distance from the coil and also varying across the coil's cross section. The electromagnetic field is most intense near the coil's surface. Ip Фp

Figure 1.5-Eiectromagnetic Field Produced by Alternating Current Charlie Chong/ Fion Zhang

The field produced about this coil is directly proportional to the magnitude of applied current, rate of change of current or frequency, and the coil parameters. Coil parameters include inductance, diameter, length, thickness, number of turns of wire, and core material. To better understand the principles under discussion, we must again look at the instantaneous relationships of current and magnetic flux. The exciting current is supplied to the coil by an alternating current generator or oscillator. With a primary current lr flowing through the coil, a primary electromagnetic field Фp is produced about the coil. When this excited test coil is placed on a conducting test object, eddy currents lE will be generated in that test object. Figure 1.6 illustrates this concept.

Figure 1.6-Generation of Eddy Current in a Test Object Charlie Chong/ Fion Zhang

Note the direction of lp, Фp, and the resultant eddy current lE. Although Figure 1.6 shows lE by directional arrows on the surface of the test object, lE extends into the test object some distance. Another important observation is that lE is generated in the same plane in which the coil is wound. Figure 1.7 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Figure 1.7- Induced Current Flow in a Cylindrical l Part

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A more precise method of describing the relationships of magnetic flux, voltage, and current is the phase vector diagram or phasor diagrams (4).

Figure 1.8-a. Phasor Diagram of Coil Voltage without Test Object b. Phasor Diagram of Coil Voltage with Test Object Charlie Chong/ Fion Zhang

Figure 1.8-a. Phasor Diagram of Coil Voltage without Test Object

E = Coil Voltage Ep = Primary Voltage Es = Secondary Voltage = 0 I = Excitation Current Фp = Primary Magnetic Flux Фs = Secondary Magnetic Flux = 0

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Figure 1.8 shows the effects of a non-ferromagnetic test object on a test coil. Figure 1.8a shows an encircling coil and the resultant phasor diagram for the unloaded coil . The components of phasor diagram 1.8a are as follows:

E = Coil Voltage Ep = Primary Voltage Es = Secondary Voltage = 0 I = Excitation Current Фp = Primary Magnetic Flux Фs = Secondary Magnetic Flux = 0

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object

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The current (I) and primary magnetic flux (ФP) are plotted in phase, and the primary voltage (EP) is shown separated by 90 electrical degrees. Secondary magnetic flux Фs is plotted at zero because without a test object no secondary flux exists. Figure 1.8b represents the action of placing a nonerromagnetic test object into the test coil. The components of phasor diagram 1.8b for a loaded coil are as follows:

E = Coil Voltage Ep = Primary Voltage Es = Secondary Voltage ET = Total Voltage I = Excitation Current Фp = Primary Magnetic Flux Фs = Secondary Magnetic Flux ФT = Total Magnetic Flux I = Excitation Current

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object Ep Es

nonferromagnet ic test object

Emeasured = ET ET∠ ≠90º

ФT Excitation current I

Фp

ФS ФT∠ ≠90º

Primary magnetic flux

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Ep + Es = ET

Secondary  magnetic flux

Observing Figure 1.8b we can see by vectorial addition of Ep and Es we arrive at a new coil voltage (ET) for the loaded condition. The primary magnetic flux cflp and secondary magnetic flux ells are also combined by vectorial addition to arrive at a new magnetic flux (ФT) for the loaded coil. Notice that for the condition of the test object In the test coil, ФT is not in phase with the excitation current I. Also observe that the included angle between the excitation current and the new coil voltage Ep is no longer 90 electrical degrees. These interactions will be discussed in detail later in this study guide.

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object Ep Es

nonferromagnet ic test object

Emeasured = ET Ep + Es = ET ФT

Excitation current I

Фp Primary magnetic flux

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ФS

Secondary  magnetic flux

CURRENT DENSITY The distribution of eddy currents in a test object varies exponentially. The current density in the test object is most dense near the test coil. This exponential current density follows the mathematical rules for a natural expo.nential decay curve (1/e) Usually a natural exponential curve is illustrated by a graph with the ordinate (Y axis) representing magnitude and the abscissa (X axis) representing time or distance. A common point described on such a graph is the "knee" of the curve. The knee occurs at the 37 percent value on the ordinate axis. This 37 percent point, or knee, is chosen because changes in X axis values produce significant changes in Y axis values from 100 percent to 37 percent, and below 37 percent changes in X axis values produce less significant changes in Y axis values. Applying this logic to eddy current testing, a term is developed to describe the relationship of current density in the test object. Consider the eddy current generated at the surface of the test object nearest the test coil to be 100 percent of the available current, the point in the test object thickness where this current is diminished to 37 percent is known as the standard depth of penetration (4). Figure 1.9 is a relative eddy current density curve for a plane wave of infinite extent with magnetic field parallel to the conducting test object surface.

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Figure 1.9 - Relative Eddy Current Density

δ = 1.98√(ρ/fμr)

0.37

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Current Density at Depth “x” The current density at any depth can be calculated as follows:

Jx = Jo e –x√(πfμσ) Where: Jx = Current density at depth x , amperes per square meter Jo = Current density at surface, amperes per square meter π = 3.1416 f = Frequency in hertz μ = Magnetic permeability, henries per meter (H∙m-1) x = Depth from surface, meters σ = Electric conductivity, mhos per meter (Siemens∙m-1?) The siemens (SI unit symbol: S) is the unit of electric conductance, electric susceptance and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is equal to the reciprocal of one ohm, and is also referred to as the mho. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.In English, the same form siemens is used both for the singular and plural

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MAGNETIC PERMEABILITY

Magnetic permeability μ is a combination of terms. For nonmagnetic materials: μ= μo = 4π∙ 10-7 H/m For magnetic materials: μ = μr∙μo Where: μr = Relative permeability, henries per meter (H∙m-1) μo = Magnetic permeability of air or nonmagnetic material, (H∙m-1)

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THE STANDARD DEPTH OF PENETRATION δ The standard depth of penetration can be calculated as follows:

δ = (πfμσ) -½ where: δ = Standard depth of penetration, meters π = 3.1416 f = Frequency in. hertz μ = Magnetic permeability, H/m σ = Electric conductivity, mhos· per meter

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Exercise: lt should be observed at this point that as frequency, conductivity, or permeability is increased, the penetration of current into the test object will be decreased. We can use the graph in Figure 1.9 (p. 6) to demonstrate many eddy current characteristics. Using an example of a very thick block of stainless steel being interrogated with a surface or probe coil operating at a test frequency of 100 kilohertz (kHz), we can determine the standard depth of penetration and observe current densities at other depths. Stainless steel {300 Series) is non-ferromagnetic. Magnetic permeability μ is 4π∙ 10-7 H/m and the conductivity is 0.14∙107 mhos per meter for 300 Series stainless steel. δ = (πfμσ) -½ δ = (100 x 103 x π x 4 π x 10-7 x 0.14 x 107) -½ m δ = 1.35 x 10-3m = 1.35mm

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Exercise: Using 1.35 mm as depth “x” from surface a ratio of depth/depth of penetration would be 1. Referring to Figure 1.9, a depth/depth of penetration of 1 indicates a relative eddy current density of 0.37 or 37 percent. What is the relative eddy current density at 3 mm? Depth “x” equals 3 mm and depth of penetration is 1.35 mm, therefore: 3/1.35 = 2.22δ Current density = (1/e) 2.22 = 0.11 or 11% This ratio indicates a relative eddy current density of about 0.1 or 10 percent. With only 10 percent of the available current flowing at a depth of 3 mm, detectability of variables such as conductivity, permeability, and discontinuities would be very difficult to detect. The obvious solution for greater detectability at the 3 mm depth is to lower the test frequency. Frequency selection will be covered in detail later in this text.

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PHASE/AMPLITUDE AND CURRENT/TIME RELATIONSHIPS Figure 1.10 reveals another facet of the eddy current. Eddy currents are not generated at the same in stant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and time are analogous; i.e., phase is an electrical term used to describe timing relationships of electrical waveforms. Phase angle lagging

Figure 1.10 - Eddy Current Phase Angle Radians Lagging β = x/δ radian

Depth of penetration Charlie Chong/ Fion Zhang

Phase is usually expressed in either degrees or radians. There are 2π radians per 360 degrees. Each radian therefore is approximately 57 degrees. Using the surface current phase angle near the test coil as a reference, phase angle current deeper in the test object lags the surface current. The amount of phase lag is determined by: β = x/δ = x(πfμσ) -½ in radian where β equals the phase angle lag in radians. Figure 1.10 should be used as a relative indicator of phase lag. The exact phase relationship for a particular system may be different due to other variables, such as coil parameters and excitation methods. The amount of phase lag for a given part thickness is an important factor when considering resolution. Resolution is the ability to separate variables occurring in the test object; for example, distinguishing two discontinuities occurring at different depths in the same test object. As an example, let us establish a standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to Figure 1.10 and observe the phase lag of the current at one standard depth of penetration. Where depth of interest (X) is 1 mm and depth of penetration (δ) is 1 mm, the X/ δ ratio is 1 and the current at depth X lags the surface current by 1 radian.

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Projecting this examination, let us observe the phase lag for the entire part thickness. The standard depth of penetration is 1 mm, the part thickness is 5 mm; therefore, the ratio X/δ equals 5. This produces a phase lag of 5 radians or approximately 287 degrees for the part thickness. Having a measurement capability of 1 degree increments, the part thickness could be divided into 287 parts, each part representing 0.017 mm. That would .be considered excellent resolution. There is an obvious limitation. Refer to Figure 1.9 and observe the resultant relative current density with an X/δ ratio of 5. The relative current density is near 0. lt should become apparent that the frequency can be adjusted to achieve optimum results for a particular variable. These and other variables will be discussed in Section 5 of this study guide.

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CHAPTER 1 REVIEW QUESTIONS

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Answer to Questions

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0.1-1 Generation of eddy currents depends on the principle of: A. wave guide theory. B. electromagnetic induction. C. magneto-restrictive forces. D. all of the above. 0.1-2 A secondary field is generated by the test object and is: A. equal and opposite to the primary field. B. opposite to the primary field, but much smaller. C. in the same plane as the coil is wound. D. in phase with the primary field. 0.1-3 When a non-ferromagnetic part is placed in the test coil, the coil's voltage: A. increases. B. remains constant because this is essential. C. decreases. D. shifts 90 degrees in phase.

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0.1-4 Refer to Figure 1.8b (p. 5): If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper? A. ET would decrease and be at a different angle. B. ET would increase and be at a different angle. C. Because both materials are non-ferromagnetic, no change occurs. D. None of the above. 0.1-5 Eddy currents generated in a test object flow: A. in the same plane as magnetic flux. B. in the same plane as the coil is wound. C. 90 degrees to the coil winding plane. D. Eddy currents have no predictable direction. 0.1-6 The discovery of electromagnetic induction is credited to: A. Arago. B. Oersted. C. Maxwell. D. Faraday.

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Figure 1.8-a. b. Phasor Diagram of Coil Voltage with Test Object Ep Es

nonferromagnet ic test object

Emeasured = ET ET∠ ≠90º

ФT Excitation current I

Фp

ФS ФT∠ ≠90º

Primary magnetic flux

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Ep + Es = ET

Secondary  magnetic flux

Discussion Subject: Reason out on the following: 0.1-4 Refer to Figure 1.8b (p. 5): If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper? A. ET would decrease and be at a different angle. B. ET would increase and be at a different angle. C. Because both materials are non-ferromagnetic, no change occurs. D. None of the above.

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0.1-7 A standard depth of penetration is defined as the point in a test object where the relative eddy current density is reduced to: A. 25 percent. B. 37 percent. C. 50 percent. D. 100 percent. 0.1·8 Refer to Figure 1.9 (p. 6). If one standard depth of penetration was established at 1 mm in an object 3 mm thick, what is the relative current density on the far surface? A. 3 B. 1?

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Electromagnetic Testing with Bobbin Coil Expert at Works

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http://www.concosystems.com/sites/default/files/userfiles/files/techical-papers/energy-tech-magazine-ndt-testing-article-jk.pdf

For this example, the system parameters are as follows: (a) Unloaded coil voltage equals 10 volts, (b) Test object effective permeability (5) equals 0.3. (c) Test coil inside diameter equals 1 inch. (d) Test object outside diameter equals 0.9 inches. Fill Factor η = 0.81 An equation demonstrating coil loading is given by:

E = E0(1- η + ημeff) When the nonmagnetic test object is inserted into the test coil with μeff=0.3, the coil's voltage will decrease. E = 10 (1-0.81 + 0.81 • 0.3) E = 10 (0.19 + 0.243) E = 1 0 (0.433) E = 4.3 volts where: E0 = Coil voltage with coil affected by air E = Coil voltage with coil affected by test object η = Fill factor μeff = Effective permeability This allows 10 - 4.3 or 5.7 volts available to respond to test object changes caused by discontinuities or decreases in effective conductivity of the test object. it is suggested that the reader calculate the resultant loaded voltage developed by a 0.5 inch bar of the same material and observe the relativesensitivity difference.

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DISCONTINUITIES Any discontinuity that appreciably changes the normal eddy current flow can be detected. Discontinuities, such as cracks, pits, gouges, vibrational damage, and corrosion, generally cause the effective conductivity of the test object to be reduced. Discontinuities open to the surface are more easily detected than subsurface discontinuities. Discontinuities open to the surface can be detected with a wide range of frequencies; subsurface investigations require a more careful frequency selection. Discontinuity detection at depths greater than 0.5 inch in stainless steel is very difficult. This is in part due to the sparse distribution of magnetic flux lines at the low frequency required for such deep.penetrations. Figure 1.9 (p. 6) is again useful to illustrate discontinuity response due to current distribution. As an example, consider testing a non-ferromagnetic tube at a frequency that establishes a standard depth of penetration at the midpoint of the tube wall. This condition would allow a relative current density of approximately 20 percent on the far surface of the tube. With this condition, identical near and far surface discontinuities would have greatly different responses. Due to current magnitude alone, the near surface discontinuity response would be nearly 5 times that of the far surface discontinuity. Discontinuity orientation has a dramatic effect on response. As seen earlier, discontinuity response is maximum when eddy currents and discontinuities are at 90º, or perpendicular. Discontinuities parallel to the eddy current flow produce little or no response. The easiest method to insure detectability of discontinuitles is to use a reference standard or model that provides a consistent means of adjusting instrumentation.

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SIGNAL-TO-NOISE RATIO Signal-to-noise ratio Is the ratio of signals of interest to unwanted signals. Common noise sources are test object variations of surface roughness, geometry, and homogeneity. Other electrical noises can be due to such external sources as welding machines, electric motors and generators. Mechanical vibrations can increase test system noise by physical movement of test coil or test object. In other words, anything that interferes with a test system's ability to define a measurement is considered noise. Signal-to-noise ratios can be improved by several methods. If a part is dirty or scaly; signalo-noise ratio can be lmproverl tly cleaning the part. Electrical interference can be shielded or isolated. Phase discrimination and filtering can improve signalto-noise ratio. lt is common practice in nondestructive testing to require a minimum signal-to-noise ratio of 3: to 1. This means a signal of interest must have a response at least three times that of the noise at that point.

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Chapter 4 REVIEW QUESTIONS

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Answers:

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Q.4·1 Materials that hold their electrons loosely are classified as: A. resistors. B. conductors. C. semiconductors. D. insulators. Q.4·2 100% IACS is based on a specified copper bar having a resistance of: A. 0.01 ohms. B. 100 ohms. C. 0.017241 ohms. D. 172.41 ohms. Q.4·3 A resistivity of 13 micro ohm-cm is equivalent to a conductivity in % lACS of A. 11.032 B. 0.0625 C. 16.52 D. 13.26

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Q.4·4 A prime factor affecting conductivity is: A. temperature. B. hardness. C. heat treatment. D. all of the above. Q.4·5 Materials that tend to concentrate magnetic flux lines are ____ _ A. conductive B. permeable C. resistive D. inductive Q.4·6 Diamagnetic materials have ____ _ A. a permeability greater than air B. a permeability less than air C. a permeability greater than ferromagnetic materials D. no permeability

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0.4·7 When an increase in field intensity produces little or no additional flux in a magnetic test object, the object is considered: A. stabilized. B. balanced. C. saturated. D. at magnetic threshold. 0.4·8 Edge effect can be reduced by: A. shielding. B. selecting a lower frequency. C. using a smaller coil. D. both A and C. 0.4·9 Lift-off signals produced by a 0-10 mil spacing change are approximately _____ times greater than a 80-90 mil spacing change. A. 10 B. 2 C. 5 D. 100 Charlie Chong/ Fion Zhang

0.4-10 Calculate the effect of fill factor when a conducting bar 0.5 inches in diameter with an effective permeability of 0.4 is placed into a 1-inch diameter coil with an unloaded voltage of 10 volts. The loaded voltage is ____ _ A. 2 volts E = E0(1- η + ημeff) B. 4.6 volts η = 0.25 E = 10(1-.25+.25x0.4) = 8.5V C. 8.5 volts D. 3.2 volts 0.4·11 Laminations are easily detected with the eddy current (probe coil) method. A. True B. False 0.4-12 Temperature changes, vibration, and environmental effects are test coil inputs that generate: A. unwanted signals. B. magnetic fields. C. eddy currents. D. drift. Charlie Chong/ Fion Zhang

5. SELECTION OF TEST PARAMETERS As NDT engineers and technicians, it is our responsibility to industry to provide and perform nondestructive examinations that in some way assure the quality or usefulness of industry products. In order to apply a nondestructive test, it is essential that we understand the parameters affecting the test. Usually, industry establishes a product or component and then seeks a method to inspect it.This practice establishes test object geometry, conductivity, and permeability prior to the application of the eddy current examination. Instrumentation, test coil, and test frequency selection become the tools used to solve the problem of inspection. Test coils were discussed previously, and instrumentation will be discussed later in this text. Test frequencies and their selection will be examined in detail in this Section.

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Frequency Selection In Section 1, we observed that eddy currents are exponentially reduced as they penetrate the test object. We also observed a time or phase difference in these currents. The currents near the test coil happen first, or lead the current that is deeper in the object. A high current density allows good delectability, and a wide phase difference between near and far surfaces allows good resolution.

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http://www.eng.morgan.edu/~hubert/IEGR470/eddycurrent.html

Standard Depth δ

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http://www.suragus.com/en/company/eddy-current-testing-technology

Single Frequency System unfortunately, if a low frequency is selected to provide good penetration and detectability, the phase difference between near and far surface is reduced. Selection of frequency often becomes a compromise. lt is common practice in in-service inspection of thin wall, non-ferromagnetic tubing to establish a standard depth of penetration δ just past the mid-point of the tube wall. This permits about 25 percent of the available eddy current to flow at the outside surface of the tube wall. In addition, this establishes a phase difference of approximately 150 to 170 degrees between the inside and outside surface of the tube wall. The combination of 25 percent outside, or surface current, and 170 degrees included phase angle provides good detectability and resolution for thin wall tube inspection. The depth of penetration formula discussed in Section 1, although correct, has rather cumbersome units. Conductivity is usually expressed in percent of the "International Annealed Copper Standard“ (% IACS). Resistivity is usually expressed in terms of micro-ohm-centimeter (μΩcm). Depths of penetration are normally much less than 0.5 inch.

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A formula using these units may be more appropriate and easier to use. A depth of penetration formula using resistivity, frequency, and permeability can be expressed as follows: δ = √(2/ ωσμ) = √2 / √(ωσμ) = √2 / √(2πfσμ) = √1/(πfσμ) = (πfσμ) -½ For non-magnetic conductor μr ≈ 1 δ = K (ρ/f)½ (given that μ = μr x μ0 = 4π∙10-7Hm-1 and For magnetic conductor μr ≠ 1 δ = K (ρ/fμr)½ where: δ = Depth of penetration in inches K = Constant = 1.98 Q = Resistivity in μΩcm f = Frequency in hertz μrel or μr = 1 for non-ferromagnetic conductors

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ρ = 1/σ)

As technicians and engineers, our prime variable is frequency. By adjusting frequency we can be selectively responsive to test object variables. Solving the non-ferromagnetic depth of penetration formula for frequency requires a simple algebraic manipulation as follows: δ = K (ρ/f)½ δ2/K2 = (ρ/f), f = K2ρ / δ2

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for English system

f =1.982ρ / δ2 f in Hertz. for a given standard penetration δ ρ in micro-ohm-cm. δ in inches. As an example of how this may be used consider inspecting an aluminum plate 0.3 inch thick, fastened to a steel plate at the far surface. Effects of the steel part are undesirable and require discrimination or elimination. The aluminum plate has a resistivity of 5 micro-ohm-cm. By establishing a depth of penetration at 0.1 inch, the far surface current will be less than 10 percent of the available current, thus reducing response to the steel part. The frequency required for this can be calculated by applying: f = 1960Hz.

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If detection of the presence of the steel part was required, the depth of penetration could be reestablished at 0.3 inch in the aluminum plate, and a new frequency could be calculated: f = 218Hz Area of interest aluminum plate

δ= 0.3 in.

steel part

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Another approach to frequency selection uses argument "A" of the Bessel function where argument "A" is equal to unity or 1. A = fσμrd2/ 5066 f = Frequency in hertz σ = Conductivity meter/ohm-mm2 d = Diameter of test object, cm μr = Relative permeability A frequency can always be selected to establish factor "A" equal to 1. This frequency is known as the limit frequency and is noted by the term fg By substituting 1 for factor "A" and fg for f, the equation becomes:

fg = 5066/σμrd2 Limit frequency (fg) is then established In terms of conductivity, permeability dimension, and a constant “5066”. · Since limit frequency is based on these parameters, a method of frequency determination using a test frequency to limit frequency ratio f/fg can be accomplished. High f/fg ratios are used for near surface tests, and lower f/fg ratios are used for subsurface tests. Charlie Chong/ Fion Zhang

Often results of such tests are represented graphically by diagrams. These diagrams are called impedance diagrams. Impedance illustrated by vector diagrams in Section 3 shows inductive reactance represented on the ordinate axis and resistance on the axis of abscissa. The vector sum of the reactive and resistive components is impedance. This impedance is a quantity with magnitude and direction that is directly proportional to frequency. In order to construct a universal Impedance diagram valid for all frequencies, the jmpedance must be normalized. Figure 5.1 illustrates a normalization process.

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Figure 5.1-Effect of Frequency Change: (a) Primary Impedance Without Secondary Circuit; (b) Primary lmpedance with Secondary Circuit

R1

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Figure 5.1 a shows the effect on primary impedance Zp with changes in frequency (ω = 2πf). Figure 5.1 a represents primary impedance without a secondary circuit or test object. Figure 5.1b Illustrates the effect of frequency on primary impedance with a secondary circuit or test object present. The primary resistance R1 in Figure 5.1 a has been subtracted in Figure 5.1 b since resistance is not affected by frequency. The term ωLsG in Figure 5.1 b represents a reference quantity for the secondary impedance. The units are secondary conductance G and ωLs secondary reactance. Further normalization is accomplished by dividing the reactive and resistive components by the term ωLo or the primary inductive reactance without a secondary circuit present. Figure 5.2 shows a typical normalized impedance diagram. The terms ωL/ωLo and R/ωLo represent the relative impedance of the test coil as affected by the test object.

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Fig. 16 Normalized impedance diagram for a long coil encircling a solid cylindrical nonferromagnetic bar showing also the locus for a thin-wall tube. k, electromagnetic wave propagation constant for a conducting material, or √(ωμσ) ; r, radius of conducting cylinder, meters; ω , 2πf;f, frequency; √(ωLoG) , equivalent of √(ωμσ) for simplified electric circuits; μ, magnetic permeability of bar, or = 4π × 10-7 H/m if bar is nonmagnetic; σ, electrical conductivity of bar, mho/m; 1.0, coil fill factor.

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Signals generated by changes in ωL or R caused by test object conditions such as surface and subsurface discontinuities may be noted by ∆ωL or ∆R. The ∆ωLo and ∆R notation indicates a change in the impedance. Figure 5.3 shows the impedance variation in a non-ferromagnetic cylinder caused by surface and subsurface discontinuities. Figure 5.3 also illustrates a sensitivity ratio for surface and subsurface discontinuities. Notice with an f/fg ratio of 50, a relatively high frequency, the response to subsurface discontinuities is not very pronounced.

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Figure 5.3-lmpedance Variations caused by surface and subsurface cracks in non-ferromagnetic cylinders, at a frequency ratio f/f 9 = 50.

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Figure 5.4 shows responses to the same discontinuities with an f/fg ratio of 15. This lower frequency allows better detection of subsurface discontinuities as shown in Figure 5.4.

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Figure 5.4-lmpedance Variations caused by surface and subsurface cracks in non-ferromagnetic cylinders, at a frequency ratio f/fg = 15

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Multifrequency Systems lt becomes obvious that the technician must have a good working knowledge of current density and phase relationships in order .to make intelligent frequency choices. The frequency choice discussed to date deals with coil systems driven by only one frequency. Test systems driven by more than one frequency are called multifrequency or multiparameter systems. lt is common for a test coil to be driven with three or more frequencies. Although several frequencies may be applied simultaneously or sequentially to a test coil, each of the individual frequencies follows rules established by single frequency methods. Signals generated at the various frequencies are often combined or mixed in electronic circuits that algebraically add or subtract signals to obtain a desired result. One multifrequency approach is to apply a broadband signal, with many frequency components, to the test coil. The information transmitted by this signal is proportional to its bandwidth, and the logarithm of 1 plus the signalto-noise power ratio. This relationship is stated by the equation:

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C = W Log2 (1 + S/N) C = Rate of information transmitted in bits per second W = Bandwidth of the signal S/N = Signal-to-noise power ratio This is known as the Shannon-Hartley Jaw. Another approach to multi parameter methods is to use a multiplexing process. The multiplexing process places one frequency at a time on the test coil. This results in zero cross-talk between frequencies and eliminates the need for band pass filters. The major advantages of a multiplex system are (1) lower cost, (2) greater flexibility in frequency selection, and (3) no cross-talk between frequency channels. If the multiplex switching rate is sufficiently high, both broadband and multiplex systems have essentially the same results. The characterization of eddy current signals by their phase angle and amplitude is a common practice and provides a basis for signal mixing to suppress unwanted signals from test data. Two frequencies are required to remove each unwanted variable. Charlie Chong/ Fion Zhang

Keywords:  Multiple frequency testing- Multifrequency systems  Multiple frequency testing- Multiparameter systems  Broad band technique for multiple frequency testing.  Multiplexing technique for multiple frequency testing.  Phase angle and amplitude for characterization of eddy current signals.  Two frequencies are required to remove each unwanted variable (prime & subtractor frequencies).

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Practical multiparameter frequency selection can be demonstrated by the following example: Problem: Eddy current inspection of installed thin-wall non-ferromagnetic heat exchanger tubing. Tubing is structurally supported by ferromagnetic tube supports at several locations. lt is desired to remove the tube support response signal from tube wall data. Solution: Apply a multiparameter technique to suppress tube support signal response. First, a frequency is selected to give optimum phase and amplitude information about the tube wall. We shall call this the prime frequency. At the prime frequency, the response to the tube support and a calibrating through- all hole are equal in amplitude response. A second frequency called the subtractor frequency is selected on the basis of tube support response. Since the tube support surrounds the OD of the tube, a low frequency is selected. At the subtractor frequency the tube support signal response is approximately 10 times greater than the calibrating through-wall hole. If the mixing unit amplitude adjustments are set so that both prime and subtractor tube support signal amplitudes are equal and phased in a manner to cause signal subtraction, the tube support signals cancel, leaving only slightly attenuated prime data information. For suppressions of inside or near surface signals, a higher subtractor frequency would be chosen. A combination of prime, low, and high subtractor frequencies is often used to suppress both near and far surface signals, leaving only data pertaining to the part thickness and its condition. Optimization of frequency then depends on the desired measurement or parameter of interest Charlie Chong/ Fion Zhang

Typical Heat Exchanger Since the tube support surrounds the OD of the tube, a low frequency is selected. At the subtractor frequency the tube support signal response is approximately 10 times greater than the calibrating through-wall hole.

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Typical Heat Exchanger Since the tube support surrounds the OD of the tube, a low frequency is selected. At the subtractor frequency the tube support signal response is approximately 10 times greater than the calibrating through-wall hole.

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Chapter 5 REVIEW QUESTIONS

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Answers:

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Table of information

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0.5·1 What frequency is required to establish one standard depth of penetration of 0.1 inch in Zirconium? A. 19.6 kHz δ = (πfμσ) -½ B. 196 Hz f = 1.982ρ/δ2 = 1.982 x 50 / (0.1)2 C. 3.4 kHz D. 340Hz 0.5-2 In order to reduce effects of far surface indications, the test frequency ____ _ A. must be mixed B. must be raised C. must be lowered D. has no effect 0.5-3 The frequency required to establish the Bessel function Argument "A" equal to 1 is called A. optimum frequency B. resonant frequency C. limit frequency D. penetration frequency Charlie Chong/ Fion Zhang

0.5·4 Calculate the limit frequency for a copper bar (σ = 50.6 meter/ohm-mm2) 1 cm in diameter. The correct limit frequency is ____ _ A. 50kHz 2 fg = 5066/σμrd B. 50.6 Hz fg = 5066 /(50.6 x 1 x12) = 100Hz C. 100Hz D. 100kHz 0.5-5 Using the example in Question 5.4, what is the f/fg ratio if the test frequency is 60 kHz? A. 1.2 B. 120 C. 60 D. 600 0.5-6 In Figure 5.1b the value ωLsG equaling 1.4 would be indicative of ____ A. a high resistivity material B. a high conductivity material C. a low conductivity material D. a nonconductor Charlie Chong/ Fion Zhang

Figure 5.1- Effect of Frequency Change: (a) Primary Impedance Without Secondary Circuit; (b) Primary lmpedance with Secondary Circuit

R1

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0.5·7 Primary resistance is subtracted from Figure 5.1 b because ____ _ A. resistance is always constant B. resistance is not frequency dependent C. resistance does not add to the impedance D. none of the above . 0.5-8 The reference quantity is different for solid cylinder and thin-wall tube in Figure 5.2 because A, the frequency is different B. the conductivity is different C. the skin effect is no longer negligible D. the thin-wall tube has not been normalized 0.5-9 A 25 percent deep crack open to the near surface gives a response ___ times greater than the same crack 3.3 percent of diameter under the surface (ref. Figure 5.4). A. 10 B. 2.4 C. 1.25 D. 5 Charlie Chong/ Fion Zhang

ratio = 5

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ration = 3

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0.5-10 When using multifrequency systems, low subtractor frequencies are used to suppress A. conductivity changes B. far surface signals C. near surface signals D. permeability changes

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6. INSTRUMENT SYSTEMS Most eddy current instrumentation is categorized by its final output or display mode. There are basic requirements common to all types of eddy current instrumentation. Five different elements are usually required to produce a viable eddy current instrument. These functions are: ■ ■ ■ ■ ■

excitation, modulation, signal preparation, Demodulation, signal analysis, and signal display.

An optional sixth component would be test object handling equipment. Figure 6.1 illustrates how these components interrelate.

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Figure 6.1- internal Functions of the Electromagnetic Nondestructive Test

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1. The generator provides excitation signals to the test coil. 2. The signal modulation occurs in the electromagnetic field of the test coil assembly. 3. Next, the signal preparation section, usually a balancing network, prepares the signal for demodulation and analysis. In the signal preparation stage, balance networks are used to "null" out steady-value alternating current signals. Amplifiers and filters are also part of this section to improve signal-to-noise ratio and raise signal levels for the subsequent demodulation and analysis stage. 4. The demodulation and analysis section is made up of detectors, analyzers, discriminators, filters, and sampling circuits. Detectors can be a simple amplitude type or a more sophisticated phase/ amplitude or coherent type. 5. The signal display section is the key link between the test equipment and its intended purpose. The signal can be displayed many different ways. Common displays include cathode ray tube (CRT) oscilloscopes, meters, recorders, visual or audible alarms, computer terminals, and automatic signaling or reject equipment.

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series of simple eddy current instruments is shown in Figure 6.2 a, b, c, and d (19).

Figure 6.2-Four Types of Simple Eddy Current Instruments In Figure 6.2a, the voltage across the inspection coil is monitored by an ac voltmeter. This type of instrument could be used to measure large lift-off variations where accuracy was not critical. Figure 6.2b shows an impedance bridge circuit. This instrument consists of an ac exciting source, dropping resistors, and a balancing impedance. Figure 6.2c is similar to Figure 6.2b. In Figure 6.2c a balance coil similar to the inspection coil is used to provide a balanced bridge. Figure 6.2d illustrates a balancing coil affected by a reference sample. This is commonly used in external reference differential coil tests. In all cases, since only the voltage change or magnitude is monitored, these systems can all be grouped as impedance magnitude types (5). Charlie Chong/ Fion Zhang

Eddy current testing can be divided into three broad groups. The groups are: 1. Impedance (magnitude) testing, 2. Phase analysis testing, and 3. Modulation analysis testing.  Impedance testing is based on gross changes in coil impedance when the coil is placed near the test object.  Phase analysis testing is based on phase changes occurring in the test coil and the test object's effect on those phase changes.  Modulation analysis testing depends on the test object passing through the test coil's magnetic field at a constant rate. The amount of frequency modulation observed as a discontinuity passes through the test coil's field and is a function of the transit time of the discontinuity through the coil's field. The faster the transit time, the greater the modulation.

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1. IMPEDANCE TESTING With impedance magnitude instrumentation it is often difficult to separate desired responses, such as changes in conductivity or permeability, from dimensional changes. A variation of the impedance magnitude technique is the reactance magnitude instrument. In reactance magnitude tests, the test coil is part of the fundamental frequency oscillator circuit. This operates like a tuned circuit where the oscillator frequency is determined by the test coil's inductive reactance. As the test coil is affected by the test object, its inductive reactance changes, which in turn changes the oscillator frequency. The relative frequency variation ∆f/f is, therefore, an indication of test object condition. Reactance magnitude systems have many of the same limitations as impedance magnitude systems.

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2. PHASE ANALYSIS TESTING Phase analysis techniques are divided into many subgroups depending on the type of data display. Some of the various types are (1) vector point, (2) impedance plane, (3) ellipse, and (4) linear time base. The vector point circuit and display are illustrated in Figure 6.3. 2.1 Vector Point The vector point display is a point of light on a CRT. The point is the vector sum of theY and X axis voltages present in the test coil (2). By proper selection of frequency and phase adjustment, voltage V1 could represent dimensional changes and voltage V2 could represent changes in conductivity.

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Figure 6.3-Vector Point Method (2, p. 3-15) Reprinted with permission.

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Figure 6.3-Vector Point Method (2, p. 3-15) Reprinted with permission. (continued)

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2.2 Ellipse The ellipse method is shown in Figure 6.4. As with the vector point method, the test object and reference standard are used to provide a balanced output. A normal balanced output is a straight horizontal line. Figure 6.5 shows typical ellipse responses. With the ellipse method the vertical deflection plates of a CRT are energized by an amplified voltage from the secondary test coils. The horizontal deflection plates are energized by a voltage that corresponds to the primary magnetizing current. With this arrangement, an ellipse opening occurs when a discontinuity signal is perpendicular to a dimensional variation in the impedance plane. The ellipse method can be used to examine many test object variables such as conductivity, permeability, hardness, dimensions, and discontinuities. When testing ferromagnetic parts with the ellipse and vector point methods, the relative permeability of the part will vary due to the nonlinear magnetization of the magnetizing field. This nonlinear magnetization creates odd harmonic frequencies to appear in the output data.

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Figure 6.4-EIIipse Method (2, p. 3-16) Reprinted with permission.

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Figure 6.5-CRT Displays for Dimension and Conductivity (2, p. 3-17) Reprinted with permission.

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2.3 Linear Time Base A test instrument system that is well suited to determine harmonic distortions present in the fundamental frequency uses the linear time base method of analysis. The linear time base unit applies a sawtooth shaped voltage to the horizontal deflection plates of a CRT. This provides a linear trace of the CRT beam from left to right across the CRT screen. The linear trace is timed so that it is equal to one cycle of the magnetizing current. This allows one cycle of the magnetizing sine wave voltage to appear on the CRT. Figure 6.7 illustrates a linear time base display.

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Figure 6.6-Linear Time Base Instrument Diagram (5, p. 40-29)

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Figure 6.7- Screen Image of a Linear Time Base Instrument with Sinusoidal Signals (5, p. 40·31)

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A slit or narrow vertical scale is provided to measure the amplitude of signals present in the slit. The base voltage is normally adjusted to cross the slit at "0" volts, the 180°point on the sine wave. The slit value "M" is used to analyze results. The slit value "M" is described by the equation: M = A sine ϴ where: M = Slit value A = Amplitude of the measurement in the slit ϴ = Angle between base signal and measurement effect In Figure 6.7, the angle difference A to B is approximately 90 degrees.

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MODULATION ANALYSIS TESTING Test instruments may also be classified by mode of operation. The mode of operation is determined by two functional areas within the instrument type. The first consideration is the method of test coil excitation. The second area is the degree of compensation, or nulling, and the type of detector used. The types of excitation include single frequency or multifrequency sinusoidal, single or repetitive pulse, and swept frequency. Compensation and detection can be accomplished by three modes. The three main input-detector modes are: 1. null balance with amplitude detector, 2. null balance with amplitude-phase detectors, and 3. selected off-null balance with amplitude detector.

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Mode 1 responds to any signal irrespective of phase angle. Mode 2, using amplitude-phase detectors, can discriminate against signals having a particular phase angle. With this system, the total demodulated signal can be displayed on an X-Y oscilloscope to show amplitude and phase relationships. Figure 6.8a shows a commercial null balance instrument with amplitude phase detectors.

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Figure 6.8a-Null Balance Instrument with Amplitude-Phase Detectors (Zetec, Inc.)

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Figure 6.8b-Typical Response to a Thin Wall Non-ferromagnetic Tube Calibration Standard (Zetec, Inc.)

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Mode 3 is a phase-sensitive system although it has only amplitude detectors. lt achieves phase sensitivity by operating at a selected off-balance condition. This off-null signal is very large compared with test object variations. Under this condition, the amplitude detector output varies in accordance with the test object signal variation on the large off-null signal. Two off-null systems are required to present both components of the test coil output signal. Figure 6.9 shows a block diagram of a stepped single frequency phaseamplitude instrument.

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Figure 6.9-lnstrument Providing Any One of Four Operating Frequencies

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The circuit in Figure 6.9 is capable of operating at any of the four frequencies. If the four frequencies are over a wide range, several different test coils may be required to use the instrument over the entire range. Most modern single frequency instruments use this principle; however, the four individual generators are usually replaced by one variable frequency generator with a wide operating range. A typical frequency range for such an instrument is 100 Hz to several megahertz. Figure 6.10 shows a block diagram for a multifrequency instrument operating at three frequencies simultaneously.

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Figure 6.10-Multifrequency Instrument Operating at Three Frequencies Simultaneously

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In Figure 6.10, excitation currents at each frequency are impressed on the coil at the same time. Multiple circuits are used throughout the instrument. The test coil output carrier frequencies are separated by filters. Multiple dual phase amplitude detectors are used and their outputs summed to provide separation of several test object parameters. A system similar to this is described in "In-Service Inspection of Steam Generator Tubing UsingMultiple Frequency Eddy Current Techniques“. another approach to the multifrequency technique uses a sequential coil drive called multiplexing. The frequencies are changed by a step-by-step sequence with such rapidity that the test parameters remain unchanged. The multiplex technique has the advantages of lower cost, continuously variable frequencies, and little or no cross-talk between channels.

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Figure 6.11 illustrates a commercial multifrequency instrument capable of operating at four different frequencies sequentially. Each of the frequency modules may be adjusted over a wide range of frequencies. In addition, two mixing modules are used to combine output signals of the various channels for suppression of unwanted variables. Results of such suppression are described in "Multi-Frequency Eddy Current Method and the Separation of Test Specimen Variables" .

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Figure 6.11-Commercial Multifrequency Instrument (Zetec, Inc.)

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Instruments are being developed that are programmable, computer or microprocessor based. With microprocessor controlled instruments, test setups can be stored in a programmable memory system. This allows complicated, preprogrammed test setups to be recalled and used by semiskilled personnel. Systems are designed with preprograms having supervisory code interlocks that prevent reprogramming by other than authorized personnel. Microprocessor-based instruments can interface with larger computer systems for control and signal analysis purposes. Figure 6.12 shows a single frequency portable microprocessor-based instrument. The CRT display applies the phase analysis technique for signal interpretation.

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Figure 6.12- Commercial Microprocessor-Based Instrument (Nortec Corporation)

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Other instruments being developed will be microprocessor based with the ability to excite several coils .at several frequencies. This would allow automatic supp-ression of unwanted variables and a direct link to larger computers for computer enhancement of test signal information. A test system using pulsed·excitation is shown in Figure 6.13. A pulse is applied to the test coil, compensating networks, and analyzers simultaneously. Systems having analyzers with one or two sampling points perform similar to a single frequency tester using sinusoidal excitation. Pulsed eddy current systems having multiple sampling points perform more like the multifrequency tester shown in Figure 6.10.

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Figure 6.13-Pulsed Waveform Excitation

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TEST OBJECT HANDLING EQUIPMENT Test object handling equipment is often a necessary component of a test system. Bars and tubes can be fed through encircling coils by means of roller fed assemblies. The stock being fed through the coil is usually transported at a constant speed. The transport speed is selected with instrument response and reject system response being of prime importance to the test. Pen marking and automatic sorting devices are common in automated systems. Spinning probes are used where the probe is rotated and the tube or bar is translated. Probe rotational speeds must be compatible with instrument response and translation speeds in order to obtain the desired inspection coverage and results. Small parts are often gravity fed through coil assemblies.

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A major problem with small parts is loading, inspecting, and unloading. A speed effect occurs when a conducting object is passed through a coil. As the object moves through the coil's magnetic field, an additional induced voltage within the object occurs. This additional induced voltage has the same frequency as the exciting current, and it causes a current flow and associated magnetic fields that produce signals proportional to the speed of the object through the coil. For larger or stationary structures, test probes are scanned over the part surface by manual or remotely operated systems. Scanning considerations are the same as tor tube and bar stock instrument response, marking or reject system response, and desired coverage. In the case of large heat exchangers, a probe positioning device is used to position the test probe over each tube opening to be inspected.

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Keywords: Speed Effect A major problem with small parts is loading, inspecting, and unloading. A speed effect occurs when a conducting object is passed through a coil. As the object moves through the coil's magnetic field, an additional induced voltage within the object occurs. This additional induced voltage has the same frequency as the exciting current, and it causes a current flow and associated magnetic fields that produce signals proportional to the speed of the object through the coil.

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Tubes to be inspected are identified by manual templates, or their coordinates are pro· grammed into computer memory. Positive feedback is supplied to computer positioning systems by encoder devices. In manual template systems the tube end is viewed by a video camera. Tube identification and control feedback are supplied to the operator via a video display system. In each system, as the probe guide is positioned correctly, the probe is inserted and withdrawn from the heat exchanger tube bore, and res ults of the scan are recorded on chart paper and magnetic tape.

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Chapter 6 REVIEW QUESTIONS

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Answers:

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0.6·1 Signal preparation is usually accomplished by: A. detectors. B. samplers. C. balance networks. D. discriminators. 0.6·2 Most eddy current instruments have _____ coil excitation. A. square wave B. triangular wave C. sine wave D. sawtooth wave 0.6·3 When only coil voltage is monitored, the system is considered a(an) _____ type system. A. impedance magnitude B. phase analysis C. reactance magnitude D. resistance magnitude

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0.6·4 lt is easy to distinguish dimensional variations from discontinuities in a reactance magnitude system. A. True B. False 0.6·5 Eddy current systems can be grouped by: A. output characteristics. B. excitation mode. C. phase analysis extent. D. both A and B. 0.6·6 In modulation testing the test object must be ____ _ A. stationary B. moving C. polarized D. saturated

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0.6·7 Using the vector point method, undesired responses appear _____ on the CRT. A. vertical B. horizontal C. at 45º to horizontal D. random and cannot be set 0.6·8 When ellipse testing a rod, the f/fg ratio is lowered from 50 to 5 percent. The response from a 5 percent surface flaw: A. will appear more elliptical. B. will appear less elliptical. C. is unchanged. D. rotates 90 D clockwise. 0.6·9 Using the linear time base, harmonics appear: A. as phase shifts of the fundamental waveform. B. as distortions of the fundamental waveform. C. to have no effect on the fundamental waveform. D. as modified sawtooth signals. Charlie Chong/ Fion Zhang

0.6·10 Calculate the slit value "M" for a signal phase shift of 45 degrees at 10 divisions vertical amplitude. A. 14 B. 7 C. 0.7 D. 1.4 0.6·11 A multifrequency instrument that excites the test coil with several frequencies simultaneously uses the concept. A. multiplex B. time share C. broadband D. synthesized 0.6·12 A multifrequency instrument that excites the test coil with several frequencies sequentially usesthe concept. A. multiplex B. time base C. broadband D. Cartesian Charlie Chong/ Fion Zhang

0.6·13 In a pulsed eddy current system using a short duration and a long duration pulse, the short duration pulse is used to reduce ____ A. edge effect B. skin effect C. lift-off effect D. conductivity variations 0.6·14 When selecting feed rates for automatic inspection of tube and bar stock, consideration is given to: A. instrument response. B. automatic sorting response. C. speed effect. D. all of the above.

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7. READOUT MECHANISMS Eddy current test data may be displayed or indicated in a variety of ways. The type of display or readout depends on the test requirements. Test records may require archive storage on large inservice components so that corrosion or discontinuity rates of change can bemonitored and projected. In some production tests, a simple GO/NO-GO indicator circuit is all that is required. Some common readout mechanisms are indicator tights, audio alarms, meters, digital displays, cathode ray tubes, recorders, and computer printout or displays. INDICATOR LIGHTS A simple use of the indicator light is to monitor the eddy current signal amplitude with an amplitude gate circuit. When the signal reaches a preset amplitude limit, the amplitude gate switches a relay that applies power to an indicator light or automatic sorting device. With the amplitude gate circuit, high-low limits could be preset to give GO/NO-GO indications.

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AUDIO ALARMS Audio alarms can be used in much the same manner. Usually the audio alarm indicates only the abnormal condition. Alarm lights and audio alarms are commonly incorporated in eddy current test equipment. The indicator light and audio alarm give only qualitative information about the item, whether a condition is present or not. The degree of condition cannot normally be determined with these devices. Indicator lights and audio alarms are relatively inexpensive and can be interpreted by semi skilled personnel. METERS Meters can present quantitative information about a test object. Meters operate on the D'Arsonval galvanometer principle. The principle is based on the action between two magnetic fields. A common meter uses a strong permanent magnet to produce one magnetic field; the other magnetic field is produced by a movable coil wound on a core. The coil and core are suspended on jewelled bearings and attached to a pointer or "needle." The instrument output current is passed through the coil and produces a magnetic field about the coil that reacts to the permanent magnetic field surrounding the assembly. The measuring coil is deflected, moving the meter pointer. The degree of pointer movement can be related to test object variables as presented by the tester output signals.

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DIGITAL DISPLAYS Numerical digital displays or indicators provide the same type of information as analog meter systems. Many eddy current instruments have analog output circuits. Data handling of analog information in digital form requires analog information to be processed through analog-to-digital (A-D) converters. The A-D converter transforms analog voltages to numerical values tor display.

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CRTs Cathode ray tubes (CATs) play an important role in the display of eddy current information. Most CRTs are the "electrostatic" type. Three main elements comprise a cathode ray tube: (1) electron gun, (2) deflection plates, and (3) a fluorescent screen. The electron gun generates, focuses, and directs the electron beam toward the face or screen of the CAT. The deflection plates are situated between the electron gun and the screen. They are arranged in two pairs, usually called horizontal and vertical, or X and Y. The plane of one pair is perpendicular to the other pair and therefore considered X and Y. The screen is the imaging portion of the CAT. The screen consists of a coating or coatings that produce photochemical reactions when struck by the electron beam. The photochemical action appears in two stages. Fluorescence occurs as the electron beam strikes the screen. Phosphorescence enables the screen to continue to give off light after the electron- beam has been removed or has passed over a section of the screen. All screen materials possess both fluorescence and phosphorescence. Screen materials are referred to as phosphors. The color of fluorescence and phosphorescence may differ as the case for zinc sulfide: the fluorescence is bluegreen, and the phosphorescence is yellow-green.

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Fluorescence may appear blue, white, red, yellow, green, orange, or a combination of colors, depending on the chemical makeup of the screen. The amount of light output from the fluorescent screen depends on the electron beam accelerating potential, screen chemical composition, thickness of screen material, and writing speed of the electron beam. The duration of the photochemical effect is called persistence. Persistence is grouped as to low, medium, or high persistence. To display repetitive signals, a low or medium persistence CAT may be used. To display non recurrent or single events, a high persistence CAT should be used. Many modern CRTs have the capability of both low or medium and highpersistence. Storage or memory CRTs have the ability to display non recurrent signals. The image from a single event may remain visible on the CRT for many hours, if desired.

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Figure 7.1 illustrates a typical eddy current signal response on a storage CAT.

The amplitude of the signal in Figure 7.1 is an indicator of the volume of the discontinuity. The phase angle with respect to the X axis represents discontinuity depth and origin, origin indicating whether the discontinuity originated on the inside or outside surface of the tube.

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RECORDERS Recorders are also used to display data and to provide a convenient method of data storage. Recording is accomplished on paper strip charts, facsimile paper, facsimile photosensitive, magnetic tape (AM, FM, or video), or digital memory disks. Strip chart recordings are common in testing tubing or nuclear fuel rods where the discontinuity's location down the length of rod or tube is critical. The strip chart length is indexed to time or distance and pen response indicates normal or abnormal conditions. Fascimile recording is a technique of displaying data signals as a raster of lines which have varying levels of blackness which correspond to data-signal voltage changes. Facsimile recording is commonly referred to as C-scan recording. If no data is transmitted to the facsimile recorder, a uniform light or dark (depending on preference) line or series of lines (raster) would be recorded. In the case of light rasters, the incoming data signal would produce areas of different darkness. The darkness would be dependent on the incoming data signal. Facsimile recorders are used in conjunction with scanning mechanisms and scan rates, and locations are synchronized with the facsimile recorder to present an image of the object variances. Figure 7.3 illustrates a typical facsimile recording.

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Figure 7.2-Commercial Strip Chart Recorder (Gould Instruments)

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Figure 7.3-Facsimile Recording of Saw-cut Specimen (Copyright, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA. 19103. Reprinted, with permission.)

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Another common type recorder is the X·Y recorder. X-Yrecorders are usually used to present scanning type data. In X-Y systems, only data signals are printed; no raster is produced in a conventional X-Y recorder system. Magnetic tape recorders, usually frequency-modulated multichannel types, are used to provide a permanent record of test results. In the case of eddy current equipment with X·Y outputs, quadrature information is recorded and played back into analyzers for post inspection analysis.

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COMPUTERS Computers may be used to control data acquisition and analysis processes. Data handling techniques take a wide variety of approaches. Dodd and Deeds describe a computer-controlled multifrequency system. Figure 7.4 shows a computer-controlled eddy current system. Figure 7.4- Computer-controlled Eddy Current System (Oak Rid.ge Nationa l Laboratory, No. 1747-49)

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Chapter 7 REVIEW QUESTIONS

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0.7-1 Display requirements are based on: A. test applications. B. records requirement. C. need for automatic control. D. all of the above. Q.7-2 Amplitude gates provide a method of controlling: A. reject or acceptance limits. B. instrument response. C. display amplitude. D. all of the above. Q.7-3 Alarms and lights offer only: A. qualitative information. B. quantitative information. C. reject information. D. accept information.

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Q.7-4 The galvanometer principle is the basis for ____ A. corrosion rates B. metallographic deterioration C. a voltmeter D. light source illumination Q.7-5 In order for analog information to be presented to a digital computer, it must be processed through _______ _ A. an A-D converter B. a microprocessor C. a phase detector D. an amplitude detector 0.7-6 In a cathode ray tube, the electron gun: A. directs the beam. B. focuses the beam. C. generates the beam. D. all of the above.

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0.7·7 Photochemical reactions produced by electrons striking a CAT screen cause: A. photosynthesis. B. phosphorescence. C. fluorescence. D. both B and C. 0.7·8 High persistance CRT screens are normally used for repetitive signal display. A. True B. False 0.7-9 Length of a strip chart can indicate: A. flaw severity. B. distance or time. C. orthogonality. D. all of the above.

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0.7-10 A series of lines produced in facsimile recording is/are called: A. grid lines. B. raster. C. crosshatch. D. sweep display.

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8. APPLICATIONS Electromagnetic induction and the eddy current principle can be affected in many different ways. These effects may be grouped by discontinuity detection, measurement of material properties, dimensional measurements, and other special applications. With discontinuity, or the flaw detection group, we are concerned with locating cracks, corrosion, erosion, and mechanical damage. The material properties group includes measurements of conductivity, permeability, hardness, alloy sorting or chemical composition, and degree of heat treatment. Dimensional measurements commonly made are thickness, profilometry, spacing or location, and coating or cladding thickness. Special applications include measurements of temperature, flow metering of liquid metals, sonic vibrations, and anisotropic conditions.

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FLAW DETECTION The theoretical response to discontinuities has been discussed in previous Sections of this guide. In this Section, some actual practice examples are given to enhance the understanding of applied theory. A problem common to the chemical and electric power industries is the corrosion of heat exchanger tubing. This tubing is installed in large vessels in a high density array. It is not uncommon for a 4 foot diameter heat exchanger to contain 3000 tubes· This high density and limited access to the inspection areas often preclude the use of other NDE methods. Heat exchanger inspection systems and results are described. In most of these cases, the severity of the discontinuity is determined by analyzing the eddy current signal phase and amplitude. ■ ■

The signal amplitude is an indicator of the discontinuity volume. The phase angle determines the depth of the discontinuity and also the originating surface (ID or OD) of that discontinuity. (See Figure 6.8, above)

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Phase angle and amplitude relationships are usually established by using a reference standard with artificial discontinuities of known and documented values. The geometry of real discontinuities may differ from reference standard discontinuities. This difference produces interpretation errors as discussed by Sagar. Placement of real discontinuities near tube support members causing a complex coil impedance change is also a source of error. This, of course, is dependent upon the size of the discontinuity and its resultant eddy current signal in relation to the tube support signal. This follows the basic principle of signal-to-noise ratio. The signal-to-noise ratio can be improved at tube to tube support intersections by the use of multi-frequency techniques.

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In multifrequency applications, an optimum frequency is chosen for response to the tube wall and a lower than optimum frequency is chosen for response to the tube support. The two signals are processed through comparator circuits called mixers where the tube support response is subtracted from the tube wall response signal, leaving only the response to the tube wall discontinuity. Another industry that uses eddy current testing extensively is the aircraft industry. Many eddy current examinations are conducted on gas turbine engines and airframe structures. A common problem with gas turbines is fatigue cracking of the compressor or exhaust turbine blade roots.

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Usually these inspections are performed with portable instruments with meter response capability. The meter response is compared to the response of known discontinuities in a reference specimen. A determination is then made of the part's acceptance. The reference specimen and its associated discontinuities are very critical to the success of the test. Often models are constructed with artificial discontinuities that are exact duplicates of the item being inspected. The low frequency eddy current inspection of aircraft structures is explained by D.J. Hagemaier. The low frequency (100 - 1000 Hz) technique is used to locate cracks in thick or multiple layer, bolted or riveted aircraft structures. Again, models are constructed with artificial cracks, and their responses are compared to responses in the actual test object. Pulsed eddy current systems also are used for crack detection in thick structures.

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DIMENSIONAL MEASUREMENTS Dimensional measurements, such as thickness, shape, and position, or proximity of one item to another, are important uses of the eddy current technique. Often materials are clad with other materials to present a resistance to chemical attack or to provide wear resistance. Cladding or plating thickness then becomes an important variable to the serviceability of the unit. For nonconductive coatings on conductive bases, the "probe-tospecimen spacing", or lift-off technique can be applied. The case of conductive plating or cladding on conductive bases requires more refinement. The thickness loci respond in a complex manner on the impedance plane. The loci for multilayered objects with each layer consisting of a material with a different conductivity follow a spiral pattern. In certain cases, two frequency or multifrequency systems are used to stabilize results or minimize lift-off variations on the thickness measurement

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The depth of case hardening can be determined by measuring the nitride case thickness in stainless steel. The nitride case thickness produces magnetic permeability variations. The thicker the nitride, the greater the permeability. The coil's inductive reactance increases with a permeability increase. This variable is carefully monitored and correlated to actual metallographic results. Eddy current profilometry is another common way to measure dimensions; for example, the measurement of inside diameters of tubes using a lift-off technique. For this measurement, several small probe coils are mounted radially in a coil form. The coil form is inserted into the tube and each coil's proximity to the tube wall is monitored. The resultant output of each coil can provide information about the concentricity of the tube. An obvious problem encountered with this method is cantering of the coil holder assembly. The center of the coil holder must be near the center of the tube. When inspecting for localized dimensional changes, a long coil holder is effective in maintaining proper centering. Another function of the long coil form is to keep the coils from becoming "cocked" or tilted in the tube.

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CONDUCTIVITY MEASUREMENTS Conductivity is an important measured variable. In the aircraft industry, aluminum is used extensively. Aluminum conductivity varies not only with alloy but also with hardness and tensile strength. Eddy current instruments scaled in % IACS are normally used to inspect for conductivity variations. Secondary conductivity standards are commonly used to check instrument calibration. Common secondary conductivity standards range from 8% IACS to approximately 100% IACS. The secondary standards are usually certified accurate within ± 0.35 percent or ± 1 percent of value, whichever is less. Temperature is an important variable when making conductivity measurements. Most instruments and standards are certified at 20°C. Primary conductivity standards are maintained at a constant temperature by oil bath systems. Primary standards are measured by precision Maxwell bridge type instruments. This circuit increases measurement accuracy and minimizes frequncy dependence of the measurement

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HARDNESS MEASUREMENTS Hardness of steel parts is often measured with low frequency comparator bridge instruments. The reference and test coil are balanced with sample parts of known hardness. As parts of unknown hardness affect the test coil, the instrument output varies. The amount of output variation depends upon the degree of imbalance created by the unknown test object hardness. Signal output is then correlated to test object hardness by comparing to known hardness samples of the same geometry. For example, if a cathode ray tube were used to display hardness information, the "balance" hardness could be adjusted to center screen, lower hardness values could appear below center, and higher hardness values could appear above center on the CRT.

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ALLOY SORTING Alloy sorting can be accomplished in the same comparator bridge manner as hardness. A major consideration in both cases is the selection of correct and accurate reference specimens. Since most eddy current instruments respond to a wide range of variables, the reference specimen parameters must be controlled carefully. Test object and reference specimens must be the same or very similar in the following characteristics: 1. geometry, 2. heat treatment, 3. surface finish, 4. residual stresses, and 5. metallurgical structure.

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In addition, it is advisable to have more than one reference specimen for backup in case of loss or damage. In the case of steel parts, they should be completely demagnetized to remove the effects of residual magnetism on instrument readings. As in most comparative tests, temperature of specimen and test object should be the same or compensated. Many other measurements can be made using eddy current techniques. The electromagnetic technique produces so much information about a material, its application is only limited by our ability to decipher this information.

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Chapter 8 REVIEW QUESTIONS

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0.8-1 Conductivity, hardness, and composition are part of the group. A. defect detection B. material properties C. dimensional D. special 0.8·2 Using an ID coil on tubing and applying the phase/amplitude method of inspection, a signal appearing at 90º on a CRT would be caused by: A. ID flaw. B. OD flaw. C. dent. D. bulge. 0.8·3 Discontlnuitles in heat exchangers at tube support locations are easier to detect because the support plate concentrates the electromagnetic field at that point. A. True B. False

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0.8·4 Using multifrequency techniques on installed heat exchanger tubing, a tube support plate signal can be suppressed by adding a ____frequency signal to the optimum frequency signal. A. low B. high C. A orB D. none of the above 0.8·5 In the aircraft industry, a common problem in gas turbine engines is: A. corrosion. B. fatigue cracking. C. vibration damage. D. erosion. 0.8-6 Thick or multilayered aircraft structures are normally inspected by: A. low frequency sinusoidal continuous wave instruments. B. high frequency sinusoidal continuous wave instruments. C. pulsed systems. D. A and C. Charlie Chong/ Fion Zhang

0.8·7 Response to multilayer varying conductivity structures follow _____ loci. A. orthogonal B. spiral c. linear D. stepped 0.8·8 Nitride case thickness can be monitored in stainless steel cylinders by measuring ____ _ A. conductivity B. dimensions C. permeability D. none of the above 0.8-9 Conductivity is not affected by temperature. A. True B. False 0.8-10 Residual stresses in the test part produce such a small effect that they are usually ignored when selecting reference specimens. A. True B. False Charlie Chong/ Fion Zhang

Chapter 9 REVIEW QUESTIONS

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0.9·1 A precise statement of a set of requirements to be satisfied by a material, product, system, or service is a----'--A. standard B. specification C. procedure D. practice 0.9·2 A statement that comprises one or more terms with explanation is a ____ _ A. practice B. classification C. definition D. proposal 0.9-3 A general statement of applicability and intent is usually presented in the _____ of a standard? A. summary B. scope C. significance D. procedure Charlie Chong/ Fion Zhang

0.9·4 Military Standards are designated by "MIL-C-(number}." A. True MIL-STD-1537A B. False 0.9·5 In the structure of ASME the subcommittee reports to the subgroup. A. True B. False 0.9·6 In example QA 3, personnel Interpreting results must be: A. Level I or higher. B. Level 11 or higher. C. Level IIA or higher. D. Level Ill.

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0.9-7 The prime artificial discontinuity used to calibrate the system described in QA 3 is: A. 20% ID B. 50% OD C. 100% D. 50% ID 0.9-8 In QA 3, equipment calibration must be verified at least ____ _ A. every hour B. each day C. every 4 hours D. every 8 hours 0.9·9 QA 3 specifies a maximum probe traverse rate of _______ _ A. 12"/sec B. 14"/sec C. 6"/sec D. not specified

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0.9·10 The system in QA 3 is calibrated with an approved standard that is traceable to ___ _ A. NBS B. ASME C. a master standard D. ASTM Q.9·11 In accordance with QA 3, tubes whose data are incomplete or uninterpretable must be A. reinspected B. reported C. reevaluated D. removed from service 0.9·12 Referring to QA 3, QA 4.1 is a ____ _ A. calibration form B. data interpretation table C. data report form D. certification form Charlie Chong/ Fion Zhang

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More Reading http://www.ndt.net/article/ecndt02/322/322.htm http://www.proprofs.com/quiz-school/story.php?title=eddy-current-practise

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Further Reading

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https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang