Electromagnetic Testing-ASNT Level III Study Guide ET

Electromagnetic Testing

Study Guide Electromagnetic Testing Part 1 My ASNT Level III Pre-Exam Preparatory Self Study Notes 17th April 2015

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E&P Applications

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E&P 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 17th April 2015

http://meilishouxihu.blog.163.com/

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乱七八糟 – 随看随记

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乱七八糟 – 随看随记

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http://greekhouseoffonts.com/

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http://www.naturalreaders.com

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http://www.naturalreaders.cn/

IVONA TTS Capable.

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Chapter 1 Principles of Eddy Current Testing

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EDDY CURRENT an Overview Description of Eddy Current Detectors Coil configurations Appropriate coil selection is the most important part of solving an eddy current application, no instrument can achieve much if it doesn’t get the right signals from the probe. Coil designs can be split into three main groups: 1. 2. 3.

Surface probes used mostly with the probe axis normal to the surface, in addition to the basic ‘pancake’ coil this includes pencil probes and special-purpose surface probes such as those used inside a fastener hole. Encircling coils are normally used for in-line inspection of round products, The product to be tested is inserted though a circular coil. ID probes are normally used for in-service inspection of heat exchangers. The probe is inserted into the tube. Normally ID probes are wound with the coil axis along the centre of the tube.

Absolute probes These categories are not exhaustive and there are obviously overlaps, for example between non-circumferential wound ID probes and internal surface probes. To this point we have only discussed eddy current probes consisting of a single coil. These are commonly used in many applications and are commonly known as absolute probes because they give an ‘absolute’ value of the condition at the test point. Absolute probes are very good for metal sorting and detection of cracks in many situations, however they are sensitive also to material variations, temperature changes etc.

Differential’ probe Another commonly used probe type is the ‘differential’ probe this has two sensing elements looking at different areas of the material being tested. The instrument responds to the difference between the eddy current conditions at the two points. Differential probes are particularly good for detection of small defects, and are relatively unaffected by lift-off (although the sensitivity is reduced in just the same way), temperature changes and external interference. (assuming the instrument circuitry operates in a "balanced“ configuration)

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

Note the characteristic "figure of eight for differential probe" response as first one probe element, then the other, move over the defect. In general the closer the element spacing the wider the "loop" in the signal. Lift-off should be cancelled out assuming that the probe is perfectly balanced, but there will still be a "wobble" response as the probe is moved and tilted slightly.

Reflection or driver pick-up probes have a primary winding driven from the oscillator and one or more sensor windings connected to the measurement circuit. Depending on the configuration of the sensor windings reflection probes may give response equivalent to either an absolute or differential probe. The two coils (differential or absolute plus balancing coil) form the ‘legs’ of a bridge. When the bridge is balanced the measured voltage will be zero. Any change in the condition of either coil will result in an unbalanced bridge, the degree of imbalance corresponds to the change in coil impedance.

The diagram shows a typical response from a differential probe.

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Driver pick-up: As can be seen the essential elements are the same for a driver pick-up configuration as for a bridge, the necessary changes can be achieved by simple switching or probe connection changes

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

Tangential Probe

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

Orthogonal Probe

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

Electromagnetic Testing Advantages The following characteristics of the method can be used to advantage :  it can be used without making physical contact with the product ;  it does not need a coupling medium such as water ;  it is capable of being used at high throughput speeds.

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EN 12084 : 2001

Factors Affecting Eddy Current Responses The basic parameters which influence the measured quantity are all of the following properties of the product to be tested, alone or in combination :    

the conductivity of the material ; the magnetic permeability of the material ; (magnetic factor) the size and geometry of the product to be tested ; (magnetic factor) the geometry between the eddy current probe and the product to be tested. (magnetic factor)

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EN 12084 : 2001

Factors Affecting Eddy Current Response Material conductivity The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a material the greater the flow of eddy currents on the surface. Conductivity is often measured by an eddy current technique, and inferences can then be drawn about the different factors affecting conductivity, such as material composition, heat treatment, work hardening etc.

Permeability This may be described as the ease with which a material can be magnetised. For non-ferrous metals such as copper, brass, aluminum etc., and for austenitic stainless steels the permeability is the same as that of ‘free space’, i.e. the relative permeability (μr) is one. For ferrous metals however the value of μr may be several hundred, and this has a very significant influence on the eddy current response, in addition it is not uncommon for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc.

Frequency As we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is one property we can control.

Geometry In a real part, for example one which is not flat or of infinite size, geometrical features such as curvature, edges, grooves etc. will exist and will effect the eddy current response. Test techniques must recognise this, for example in testing an edge for cracks the probe will normally be moved along parallel to the edge so that small changes may be easily seen. Where the material thickness is less than the effective depth of penetration (see below) this will also effect the eddy current response

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

Proximity / Lift-off The closer a probe coil is to the surface the greater will be the effect on that coil. This has two main effects: The "lift-off" signal as the probe is moved on and off the surface. A reduction in sensitivity as the coil to product spacing increases.

Depth of penetration The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the metal being tested and declines with depth. It is mathematically convenient to define the "standard depth of penetration" where the eddy current is 1/e (37%) of its surface value. The standard depth of penetration in mm is given by the formula:

Where: δ is standard depth in mm ρ is resistivity in μΩ.cm f is frequency in Hz μr is relative permeability

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

from this it can be seen that depth of penetration: 1. Decreases with an increase in frequency 2. Decreases with an increase in conductivity 3. Decreases with an increase in permeability: this can be very significant penetration into ferrous materials at practical frequencies is very small.

δ

δ

The graph above shows the effect of frequency on standard depth of penetration. It is also common to talk about the "effective depth of penetration" usually defined as three times the standard depth, where eddy current density has fallen to around 3% (5%?) of its surface value. This is the depth at which there is considered to be no influence on the eddy current field.

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

The Impedance Plane Eddy current responses of a single coil may be conveniently described by reference to the "impedance plane". This is a graphical representation of the complex probe impedance where the abscissa (X value) represents the resistance and the ordinate (Y value) represents the inductive reactance.

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

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The Impedance Plane

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

Note that, while the general form of the impedance plane remains the same, the details are unique for a particular probe and frequency. The display of a typical CRT eddy current instrument represents a ‘window’ into the impedance plane, which can be rotated and "zoomed" to suit the needs of the application. For example in the above impedance plane diagram a rotated detail of the "probe on aluminum" area would appear as below:

This shows the display when moving over a series of simulated cracks of varying depths. Note that in the example shown both the amplitude and the phase of response from the different sized cracks varies.

Reliability Eddy currents are often generated in transformers and lead to power losses. To combat this, thin, laminated strips of metal are used in the construction of power transformers, rather than making the transformer out of one solid piece of metal. Insulating glue, which confines the eddy currents to the strips, separates the thin strips. This reduces the eddy currents, thus reducing the power loss. Beside that, Eddy-Current Detectors are very reliable as far as their industrial usage. They are so reliable that nuclear plants are using robots to the tests, instead of risking real human beings.

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

Robotic

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Robotic

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Measurement Techniques (EN!) a) Absolute measurement. The measurement of the deviation from a fixed reference point. The reference point is defined by a calibration procedure and can be generated by a reference voltage or coi l. This technique can be used for sorting the product into classes based on physical properties such as hardness, dimensions or chemical composition. It can also be used for the identification of continuous or gradually changing discontinuities.

b) Comparative measurement. The subtraction of two measurements, one of which is taken as a reference. This technique is normally used to sort the product into classes.

c) Differential measurement. The subtraction of two measurements made at a constant distance between the measurement locations and on the same scanning path. This measurement technique reduces the background noise due to slow variations in the product to be tested. (?)

d) Double differential measurement. The subtraction of two differential measurements. This measurement technique provides high-pass filtering of a differential measurement independent of the relative speed between the probe and the product to be tested.

e) Pseudo differential measurements The subtraction of two measurements made at a constant distance between the measurement locations.

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EN 12084 : 2001

Historical Background Before discussing the principles of eddy current testing, it seems appropriate to briefly discuss the concept of magnetism and electromagnetism that serve as the foundation for this study. In the period from 1775 to 1900, scientific experimenters Andre Marie Ampere, Françios Arago, Charles Augustin coulomb, Michael Faraday, Lord William Thomson Kelvin, James Clerk Maxwell and Hans Christian Oersted had 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 conductor disk was placed near the magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today. This experiment can be modeled as shown in Figure 1.1.

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http://pegna.vialattea.net/2Arago_Disk.htm

Figure 1.1 Arago’s Experiment

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Arago’s Disk Experiment Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today.

■

https://www.youtube.com/embed/sChcqdkcLGE

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https://www.youtube.com/watch?v=sChcqdkcLGE

Oersted discovered the presence of a magnetic field around a current carrying conductor and observed magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite currents flowing in adjacent conductors cancelled this magnetic effect. Ampere's observation is used in differential coil applications and to manufacture non inductive precision resistor. Faraday's first experiments investigated induced currents by the relative motion of magnet and a coil (Figure 1.2). Faraday's major contribution was the discovery of electromagnetic induction. His work can be summarized by the example shown in Figure 1.3. A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil A is changing.

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Figure 1.2: Induced current with coil and magnet

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Figure 1.3: Induced current electromagnetic technique A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil A is changing.

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Electromagnetic induction is the production of an electromotive force across a conductor when it is exposed to a varying magnetic field. It is described mathematically by Faraday's law of induction, named after Michael Faraday who is generally credited with the discovery of induction in 1831. Electromagnetic induction was first discovered by Michael Faraday, who made his discovery public in 1831. It was discovered independently by Joseph Henry in 1832. In Faraday's first experimental demonstration (August 29, 1831), he wrapped two wires around opposite sides of an iron ring or "torus" (an arrangement similar to a modern toroidal transformer). Based on his assessment of recently discovered properties of electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other wire to a battery. Indeed, he saw a transient current (which he called a "wave of electricity") when he connected the wire to the battery, and another when he disconnected it. This induction was due to the change in magnetic flux that occurred when the battery was connected and disconnected. Within two months, Faraday found several other manifestations of electromagnetic induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near the bar magnet with a sliding electrical lead ("Faraday's disk"). Faraday explained electromagnetic induction using a concept he called lines of force. However, scientists at the time widely rejected his theoretical ideas, mainly because they were not formulated mathematically. An exception was Maxwell, who used Faraday's ideas as the basis of his quantitative electromagnetic theory. In Maxwell's model, the time varying aspect of electromagnetic induction is expressed as a differential equation which Oliver Heaviside referred to as Faraday's law even though it is slightly different from Faraday's original formulation and does not describe motional EMF. Heaviside's version (see Maxwell– Faraday equation below) is the form recognized today in the group of equations known as Maxwell's equations. Heinrich Lenz formulated the law named after him in 1834, to describe the "flux through the circuit". Lenz's law gives the direction of the induced EMF and current resulting from electromagnetic induction (elaborated upon in the examples below). Following the understanding brought by these laws, many kinds of device employing magnetic induction have been invented.

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http://en.wikipedia.org/wiki/Electromagnetic_induction

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http://en.wikipedia.org/wiki/Electromagnetic_induction

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http://en.wikipedia.org/wiki/Homopolar_generator

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http://en.wikipedia.org/wiki/Homopolar_generator

Faraday's Law - Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.

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http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

The law of physics describing the process of electromagnetic induction is known as Faraday's law of induction and the most widespread version of this law states that the induced electromotive force in any closed circuit is equal to the rate of change of the magnetic flux enclosed by the circuit. Or mathematically,

ε = dфB/ dt where ε (epsilon) is the electromotive force (EMF) and ΦB (Φ= BA) is the magnetic flux. The direction of the electromotive force is given by Lenz's law. This version of Faraday's law strictly holds only when the closed circuit is a loop of infinitely thin wire, and is invalid in some other circumstances. A different version, the Maxwell–Faraday equation (discussed below), is valid in all circumstances. For a tightly wound coil of wire, composed of N identical turns, each with the same magnetic flux going through them, the resulting EMF is given by

ε = -N dфB/ dt Faraday's law of induction makes use of the magnetic flux ΦB through a hypothetical surface Σ whose boundary is a wire loop. Since the wire loop may be moving, we write Σ(t) for the surface. The magnetic flux is defined by a surface integral:

фB = ∫Σ(t) B(r,t)∙dA where dA is an element of surface area of the moving surface Σ(t), B is the magnetic field, and B·dA is a vector dot product (the infinitesimal amount of magnetic flux). In more visual terms, the magnetic flux through the wire loop is proportional to the number of magnetic flux lines that pass through the loop.

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http://en.wikipedia.org/wiki/Electromagnetic_induction

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http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

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http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

Lenz's Law When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

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http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

Magnetic Force The magnetic field B is defined from the Lorentz Force Law, and specifically from the magnetic force on a moving charge:

The implications of this expression include: 1. The force is perpendicular to both the velocity v of the charge q and the magnetic field B. 2. The magnitude of the force is F = q∙v∙B sin θ where θ is the angle < 180 degrees between the velocity and the magnetic field. This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero. 3. The direction of the force is given by the right hand rule. The force relationship above is in the form of a vector product.

When the magnetic force relationship is applied to a current-carrying wire, the right-hand rule may be used to determine the direction of force on the wire. From the force relationship above it can be deduced that the units of magnetic field are Newton seconds /(Coulomb meter) or Newtons per Ampere meter. This unit is named the Tesla. It is a large unit, and the smaller unit Gauss is used for small fields like the Earth's magnetic field. A Tesla is 10,000 Gauss. The Earth's magnetic field at the surface is on the order of half a Gauss Charlie Chong/ Fion Zhang

Lorentz force In physics, particularly electromagnetism, the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force

F = - q∙ [ E + (v x B) ] (in SI units). Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes called Laplace force), the electromotive force in a wire loop moving through a magnetic field (an aspect of Faraday's law of induction), and the force on a charged particle which might be traveling near the speed of light (relativistic form of the Lorentz force). The first derivation of the Lorentz force is commonly attributed to Oliver Heaviside in 1889, although other historians suggest an earlier origin in an 1865 paper by James Clerk Maxwell. Hendrik Lorentz derived it a few years after Heaviside.

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http://en.wikipedia.org/wiki/Lorentz_force

Generation of Eddy Currents When a conductor is place in the area influence by the primary field, eddy current is induced in the conductor, see Fig. 1.4. Following Lenz’s law, the induced eddy current IE will produce a secondary field фE that oppose the фP. The magnitude of фE is proportional to IE. The test objet, conductor B’s characteristic like, material conductivity, permeability and geometry will affect the IE, this in turn cause variation in фE. The variation in фE is reflected in conductor CA by фE influences on фp. The variations are recorded in media like meter, CRT, digital read out or chart. The

Ip = Primary Current Фp =Primary magnetic flux ФE = Secondary Eddy current magnetic flux IE = Secondary Eddy current Figure 1.4: Induced current relationships

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Generation of Eddy Currents

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

Factors Affecting Inductance There are four basic factors of inductor construction determining the amount of inductance created. These factors all dictate inductance by affecting how much magnetic field flux will develop for a given amount of magnetic field force (current through the inductor's wire coil): NUMBER OF WIRE WRAPS, OR "TURNS" IN THE COIL: All other factors being equal, a greater number of turns of wire in the coil results in greater inductance; fewer turns of wire in the coil results in less inductance. Explanation: More turns of wire means that the coil will generate a greater amount of magnetic field force (measured in amp-turns!), for a given amount of coil current. L ∝ N2

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

COIL AREA: All other factors being equal, greater coil area (as measured looking lengthwise through the coil, at the cross-section of the core) results in greater inductance; less coil area results in less inductance. Explanation: Greater coil area presents less opposition to the formation of magnetic field flux, for a given amount of field force (amp-turns). L ∝ A

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

COIL LENGTH: All other factors being equal, the longer the coil's length, the less inductance; the shorter the coil's length, the greater the inductance. Explanation: A longer path for the magnetic field flux to take results in more opposition to the formation of that flux for any given amount of field force (amp-turns). L ∝ (l)-1

COIL LENGTH

L ∝ (l)-1

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COIL LENGTH

L ∝ (l)-1

http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

CORE MATERIAL: All other factors being equal, the greater the magnetic permeability of the core which the coil is wrapped around, the greater the inductance; the less the permeability of the core, the less the inductance. Explanation: A core material with greater magnetic permeability results in greater magnetic field flux for any given amount of field force (amp-turns). L∝μ

μ0 = 4π x 10-7 H.m-1

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μr = 600, μiron = 600 x μ0 μ0 = 4π x 10-7 H.m-1

http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

Coil Inductance L An approximation of inductance L, for any coil of wire can be found with this formula: The electromagnetic field produced about an unloaded test coil can be described as decreasing in intensity with distance from the coil and also varying across the coil's cross section. The field is most intense near the coil's surface. The field produced about this coil is directly proportiona1 to the magnitude of applied current, rate of change of current or frequency and the coil parameters. Coil parameters inc1ude inductance, diameter, length， thickness, number of turns of wire and core material.

L = μr• (N2 x A /l) • 1.26 x 10-6 Henry μ0 = 4π x 10-7 H.m-1 or 1.26 x 10-6 H.m-1 EMF = L di/dt Volt Where: L= inductance in Henry H N = Numbers of turn in coil wire (straight wire N=1) μr = relative permeability l = average length of coil in m A = area of coil (not wire area?) in m2 μo = relative permeability in air 4π x 10-7 H.m-1 or 1.26 x 10-6 H.m-1

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

Coil Inductance L

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

Note the direction of the primary current (Ip) and the resultant eddy current (IE). IE extends some distance into the test object. Another important observation is that IE is generated in the same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Important observation is that IE is generated in the same plane in which the coil is wound.

Figure 1.6 Induction current flow in a cylindrical part. Charlie Chong/ Fion Zhang

Note the direction of the primary current (Ip) and the resultant eddy current (IE). IE extends some distance into the test object. Another important observation is that IE is generated in the same plane in which the coil is wound. Figure 1.6 emphasizes this point with a loop coil surrounding a cylindrical test object (4).

Important observation is that IE is generated in the same plane in which the coil is wound & in opposite direction of Ip

Figure 1.6 Induction current flow in a cylindrical part. Charlie Chong/ Fion Zhang

Generation of Eddy Current With a primary current 1p flowing through the coil, a primarr electromagnetic field фp is produced about the coil. When this excited test coil is placed on an electrically conductive test object, eddy currents IE will be generated in that test object Figure 1.5 illustrates this concept.

Figure 1.5 Generation of eddy current IE in a test object

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It must be understood that this formula yields approximate figures only. One reason for this is the fact that permeability changes as the field intensity varies (remember the nonlinear "B/H" curves for different materials). Obviously, if permeability (µ) in the equation is unstable, then the inductance (L) will also be unstable to some degree as the current through the coil changes in magnitude. If the hysteresis of the core material is significant, this will also have strange effects on the inductance of the coil. Inductor designers try to minimize these effects by designing the core in such a way that its flux density never approaches saturation levels, and so the inductor operates in a more linear portion of the B/H curve. If an inductor is designed so that any one of these factors may be varied at will, its inductance will correspondingly vary. Variable inductors are usually made by providing a way to vary the number of wire turns in use at any given time, or by varying the core material (a sliding core that can be moved in and out of the coil). An example of the former design is shown in this photograph:

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

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Permeability changes as the field intensity varies (remember the nonlinear "B/H" curves for different materials).

Figure 1: This unit uses sliding copper contacts to tap into the coil at different points along its length. The unit shown happens to be an air-core inductor used in early radio work. Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire: Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its wire coil is wound around a white ceramic tube for greater rigidity: Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores. Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design needs.

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

A Dual1: Variable inductors Figure This unit uses sliding copper contacts to tap into the coil at different points along its length. The unit shown happens to be an air-core inductor used in early radio work.

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Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire:

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Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its wire coil is wound around a white ceramic tube for greater rigidity:

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Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores.

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). These inductors are called "toroidal" because their wire coils are wound around donut-shaped ("torus") cores.

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design needs.

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http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

Grundig radio satellit 750

■ https://www.youtube.com/embed/yD7WAcSwz8o

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http://www.universal-radio.com/catalog/portable/0750.html

Phasor Vector Diagram of Coil Voltage A more precise method of describing the relationships of magnetic flux, voltage and current is the phase vector diagram or phasor diagrams (4). Figure 1.7 compares the electromagnetic events associated with an unloaded test coil and what happens when that same coil is placed on a nonferromagnetic test object. The components of phasor diagrams are as follows:

Fig.17(b) Ep = Primary coil voltage I = Exciting current (Primary coil current) Фp = Primary flux Фs = Secondary flux

Fig.17(b) Ep = Primary coil voltage I = Exciting current (Primary coil current) Фp = Primary flux Фs = Secondary flux Es = Secondary voltage ET= Total voltage ФT = Total flux

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Figure 1.7: Phasor Diagram of Coil Voltage (?)

In Figure 1.7(a) the current (I) and primary magnetic flux (фp) are plotted in phase. The primary voltage (Ep) is shown separated by 90 electrical degrees. The secondary magnetic flux (фs) is plotted at zero because without a test object no secondary flux exists.

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Figure 1.7(b) represents the action of placing the coil on a nonferromagnetic test object Observing the figure, one can see by vectorial addition of Ep and Es that a new coil voltage (ET) is arrived at for the loaded condition. The primary magnetic flux фp and secondary magnetic flux фs are also combined by vectorial addition to arrive at a new magnetic flux (фT) for the loaded coil.

In Figure 1.7(a) the current (I) and primary magnetic flux (фp) are plotted in phase. The primary voltage (Ep) is shown separated by 90 electrical degrees. The secondary magnetic flux (фs) is plotted at zero because without a test object no secondary flux exists. Figure 1.7(b) represents the action of placing the coil on a nonferromagnetic test object Observing the figure, one can see by vectorial addition of Ep and Es that a new coil voltage (ET) is arrived at for the loaded condition. The primary magnetic flux фp and secondary magnetic flux фs 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 no longer in phase with the excitation current I. Also observe that the included angle between the excitation current and the new coil voltage ET is no longer at 90 electrical degrees. These interactions will be discussed in detaillater in this study guide.

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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 exponential decay curve (1/ e) where ε (epsilon) is 2.718. 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% value on the ordinate axis. This 37% point is chosen because changes in X axis values produce significant changes in Yaxis values from 100% to 37% and below 37% changes in X axis values þroduce less signlficant changes in Y axis values (?).

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Applying this logic to eddy current testing, a term is developed to describe the relationship of current distribution in the test object. The eddy current generated at the surface of the test object nearest the test coil is 100%. The point in the test object thickness where this current is diminished to 37% of its previous strength is known as the standard depth of penetration. The term δ (delta) is used to represent this point in the material. Figure 1.8 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 1.8: Relative eddy current density

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The current density at any depth can be calculated as:

Jx =J0 e - x√(πfμσ) Where: Jx = Electrical density at depth x in A∙m-2 J0 = Electrical density at the surface x=0 x = distance fro surface in meter m f = Frequency of the AC primary current Hz μ = Permeability of the test object in H∙m-1 σ = Conductivity of the test object in Siemen∙m-1 e = Natural logarithm

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Relative Magnetic Permeability

Permeability of free space μ0 = 4π x 10-7 HM-1 Permeability of material can be expressed as relative to μ0 μmaterial = μr∙μ0

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The Standard Depth of Penetration δ The Standard Depth of Penetration can be expressed as:

δ = (πfμσ) -½ Where:

δ = One standard depth of penetration; 1/e of the surface current density (37%) in meter, m f = Frequency of the AC primary current in Hz μ = Permeability of the test object in Henry per meter, H∙m-1 σ = Conductivity of the test object in Siemens per meter, S∙m-1

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It 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. The graph in Figure 1.8 is used 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 kHz, the standard depth of penetration can be determined and current densities observed at other depths. Stainless steel (300 Series) is nonferromagnetic. Magnetic permeability (μ) is 4πX 10-7 H∙m-1, the conductivity σ is 0.14 X 107 siemens (mhos) per meter for 300 Series stainless steel. δ = (πfμσ) -½ δ = (π x 100 x 103 x 4 x π x 0.14) -½ δ = 0.00135m or 1.35mm#

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δ = (π x 100 x 103 x 4 x π x 0.14) -½ as 1000*(pi*x*4*pi*0.14*10^3)^(-.5)

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http://graph-plotter.cours-de-math.eu/

δ = (π x 100 x 103 x 4 x π x 0.14) -½ as 1000*(pi*x*4*pi*0.14*10^3)^(-.5)

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http://fooplot.com/

δ = (π x 100 x 103 x 4 x π x 0.14) -½ as 1000*(pi*x*4*pi*0.14*10^3)^(-.5)

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http://rechneronline.de/function-graphs/

Using 1.35 mm as depth x from surface, a ratio of depth/depth of penetration would be 1 Referring to Figure 1.8, a depth/ depth of penetration of 1 indicates a relative eddy current density of 0.37 or 37%. What is the relative eddy current density at 3 mm? The relative standard depth Drelative of x = 3mm is: Drelative = 3/δ = 3/1.53 mm = 2.22δ This ratio indicates a relative eddy current densityof about 0.1 or 10% [ (1/e)2.22 = 10.9% ]. With only 10% 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 delectability at a depth of 3 mm depth is to lower the test frequency. Frequency selection will be covered in detaillater in this text.

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Relative current density

f(x) = (1/e)x where x = depth/δ

Relative Standard depth x = depth/δ

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http://rechneronline.de/function-graphs/

Standard Depth for Different Conductive Materials

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https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet7/applet7.htm

Phase/Amplitude and Current Time Relationships Figure 1.9 reveals another facet of eddy current. Eddy currents are not generated at the same instant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and time are analogous meaning - phase is an electrical term used to describe timing relationships of electrical waveforms. Phase Lag = x/δ radian Where: x =depth below surface δ = Standard depth

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Current (?) Lagging Voltage lagging or current lagging?

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Current (?) Lagging Voltage lagging or current lagging?

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σº∙πμ■δ∝∞ωΩθ√ρβααδπ

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Phase is u,sually expressed in either degrees or radians. There'are 2πradians per 360 degrees. Each radian therefore is about 57 degrees (360/2π). Using the surface eddy current near the test coil as a reference, the deeper the eddy current the greater the phase lag. The amount of phase lag is determined by:

β = x/δ = x∙√(πfμσ) β or Φ = Phase lag angle in radian. Others as defined earlier

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Figure 1.9 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, using a standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to Figure 1.9 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 or 57 degrees.

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Projecting this examination, 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 to 5. This produces phase lag of 5 radians or about 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 0.017mm. That would be considered excellent resolution. There is an obvious Iimitation. Refer to Figure 1.8 and observe the resultant relative current density with an x/δ ratio of 5. The relative current density is near 0. It 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 Chapter 5 of this Study Guide.

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Figure 1.8: Relative eddy current density

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Chapter 1 Review Questions

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Q.1.1 Generation of eddy currents depends on the principle of: A. wave guide theory. B. electromagnetic induction. C. Magnetostriction force D. All of the above Q.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. Q.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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Q.1.4 Refer to Figure 1.7(b). 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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Q.1.5 Eddy current generated 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. Q.1.6 The discovery of electromagnetic induction is credited to A. Arago B. Oersted. C. Maxwell. D. Faraday. Q.1.7 A standard depth of penetration is defined as the point in a test object where the relative current density is reduced to: A. 25%. B. 37% C. 50%. D. 100%

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Q.1.8 Refer to Figure 1.8. 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. 19%. b. Report all other indications that appear to be relevant. c. Identify the axial position of all indications with respect to a known structural member. v. Pre-service inspections a. Report all indications observed. Include the axial position of the indication with respect to a known structural member. b. Interpretation i. All data shall be reported on a digital Final Report form. ii. The conversion from signal phase angles (or amplitudes) to discontinuity depths shall be accomplished per calibration curves established on the appropriate channels using the calibration standards and techniques defined in the site specific data analysis specifications. iii. All data shall be reviewed in its entirety. iv. Any abnormal signals observed shall be reported.

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G. REFERENCES The following documents or files are required for the performance of eddy current inspection programs utilizing the methods described in this procedure. 1. Required Documentation a. Eddy current inspection specific calibration procedure documents applicable to the plant to be inspected. b. Inspection plans showing tube sheet maps marked to designate the extent of examination to be performed and extent of completion. c. Final Reports including all indications resolved by the Data Resolution Analyst.

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Chapter 9 Review Questions

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Answers

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Q.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. Q.9.2 A statement that comprises one or more terms with explanation is a: A. practice. B. classification. C. definition. D. proposal. Q.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

Q.9.4 Military Standards are designated by MIL-C-(number). A. True MIL-STD-XXXXX B. False Q.9.5 In the structure of American Society of Mechanical Engineers (ASME) the subcommittee reports to the subgroup. A. True B. False Q.9.6 In example QA 3, personnel interpreting results must be: A. Level I or higher. B. Level II or higher. C. Level IIA or higher. D. Level Ill.

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Q.9.7 The prime artificial discontinuity used to calibrate the system described in QA 3 is: A. 20% inside diameter. B. 50% outside diameter. C. 100%. D. 50% inside diameter. Q.9.8 In QA 3, equipment calibration must be verified at least: A every hour. B. each day. C. every 4h. D. every 8 h. Q.9.9 QA 3 specifies a maximum probe traverse rate of: A. 305 mm/ s (12 in./ s). B. 355.6mm/s (14in./s). C. 152.4 mm/s (6 in./s). D. not specified.

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Q.9.10 The system in QA3 is calibrated with an approved standard that is traceable to: A. NBS. B. American Society of Mechanical Engineers (ASME). C. a master standard. D. American Society for Testing and Materials (ASTM). Q.9.11 In accordance with QA 3, a tube whose data are incomplete must be: A. reinspected. B. reported. C. reevaluated. D. removed from service.

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■ωσμ∙Ωπ∆º≠δ≤>η

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More Reading http://www.allaboutcircuits.com/vol_1/index.html

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Further Reading

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Discussion Subject: discuss on the standard requirements on the differences in frequency Hz used on specific applications for thickness checks and weld examination.

BS EN 1711:2000 6.4.2 Surface probes 6.4.2.1 Probes for measuring thickness of coating and material evaluation relative to calibration block To be acceptable for this purpose, the probe shall be capable of providing a full screen deflection lift off signal on the instrument when moved from an uncoated spot on a calibration block to a spot covered with the maximum coating thickness expected on the structure to be tested. The probe shall operate in absolute mode at a selected frequency in the range from 1 kHz to 1 MHz. All the probes shall be clearly marked with their operating frequency range. (See Figure 1). 6.4.2.2 Probes for weld examination For examination of ferritic welds, probes specially designed for this purpose shall be used. The probe assembly shall be differential, orthogonal, tangential or equivalent which is characterized by having a minimal dependency on variations in conductivity, permeability and lift off in the welded and heat-affected zones. The diameter of the probe shall be selected relative to the geometry of the component under test. Such probes shall be able to operate when covered by a thin layer on non-metallic wear-resistant material over the active face. If the probe is used with a cover, then the cover shall always be in place during calibration. The probe shall operate at a selected frequency in the range from 100 kHz to 1 MHz. Key • 1,2,3,4 Deflections representing variations of thickness of simulated coatings on calibration block • 5 Deflection representing material of calibration block • 6,7 Deflection representing range of material to be examined using calibration block 0 Balance

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BS EN 1711:2000

Discussion Subject: discuss on the standard requirements on the frequency used on the specific applications for thickness testing and defect detections. BS EN 1711:2000 6.4.2 Surface probes 6.4.2.1 Probes for measuring thickness of coating and material evaluation relative to calibration block To be acceptable for this purpose, the probe shall be capable of providing a full screen deflection lift off signal on the instrument when moved from an uncoated spot on a calibration block to a spot covered with the maximum coating thickness expected on the structure to be tested. The probe shall operate in absolute mode at a selected frequency in the range from 1 kHz to 1 MHz. All the probes shall be clearly marked with their operating frequency range. (See Figure 1). 6.5.2 Procedure for examination of welds in ferritic materials 6.5.2.1 Frequency The frequency shall be optimized with respect to the sensitivity, the lift off and other unwanted signals. Underusual conditions a frequency of about 100 kHz is recommended.

Key • 1,2,3,4 Deflections representing variations of thickness of simulated coatings on calibration block • 5 Deflection representing material of calibration block • 6,7 Deflection representing range of material to be examined using calibration block 0 Balance

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BS EN 1711:2000

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Comparison of OD and ID Eddy Current Inspection of Tubing Scope The Eddy Current test (ECT) test is the primary nondestructive test (NDT) used in tube mill certification testing for condenser, feedwater heater, and balance of plant (BOP) power generation tubing. When tested at the tube mill, the procedure is performed using encircling differential outside diameter (OD) coils. Such OD testing techniques are well accepted by industry and consumers alike. This technique has, in fact, been the standard tubing NDT practice for several decades and is incorporated into the ASME Boiler & Pressure Vessel Code. Details of this type of test and its advantages and limitations are defined in the HEI Tech Sheet #129. Although ultrasonic testing (UT), remote field testing (RFT) and flux leakage testing may be acceptable alternatives, they are only used when specified by the customer, or by a small number of product specifications such as ASTM B338. As a result, this document will only discuss the EC test. Once the tubing is manufactured, the owner or end user may specify an additional EC test using an ID probe. The purpose of this document is to describe the major differences between the two tests and what each is intended to accomplish. Charlie Chong/ Fion Zhang

http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

Inspection from the Tube OD Most ASTM and ASME tubular product specifications require a nondestructive electric test NDE. The NDE tests may include eddy current testing, ultrasonic testing, or flux leakage testing. The product specifications do not necessarily designate which of these three must be used, and unless agreed upon in the purchase order, the test choice is at the option of the tube producer. The test that is the quickest, with the highest reliability and provides good sensitivity for finding sharp, abrupt defects is the OD eddy current test. It is the overwhelming choice of both tube manufacturers and end users.

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

The ASTM has developed recommended practices on how those tests may be performed. These include as follows: 1. ASTM E309 / SE309 –Standard Practice for Eddy-Current Examination of Steel Tubular Products Using Magnetic Saturation 2. ASTM E426 / SE426 –Standard Practice for Electromagnetic (EddyCurrent) Examination of Seamless and Welded Tubular Products, Austenitic Stainless Steel and Similar Alloys 3. ASTM E571 / SE571 –Standard Practice for Electromagnetic (EddyCurrent) Examination of Nickel and Nickel Alloy Tubular Products 4. ASTM E2096 - Standard Practice for In Situ Examination of Ferromagnetic Heat-Exchanger Tubes Using Remote Field Testing 5. ASTM E-690 / ASTM E690 - Standard Practice for In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

Approximately 25 years ago, the ASTM A01.09/A01.10 NDE task group recognized that the “E” practices identified above did not have sufficient detail to ensure that tube mills were incorporating all of the necessary ASTM requirements. In addition, there was no validation that the procedures used at one manufacturing plant would provide similar test results as those at another mill. As a result, the ASTM developed a number of additional requirements which were then added into the general tubular product specifications. These additional requirements specified items such as calibration size & location (artificial defect size & type, i.e., drilled hole or notch), and calibration procedures to ensure consistent & repeatable results. These requirements also included training and certification of operators, signal to noise ratio recommendations, and required equipment calibration standards. The general specifications that include the additional requirements are as follows: ■ ASTM A450 / A450M – Standard Specification for General Requirements for Carbon and Low Alloy Steel Tubes

■ ASTM A1016/A1016M - Standard Specification for General Requirements for Ferritic Alloy Steel, Austenitic Alloy Steel, and Stainless Steel Tubes

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

The ASTM/ASME OD EC testing of tubes is usually accomplished using only one frequency, typically within a range of 25 KHz to 100 KHz. As the magnetic field penetrates the metal, the ability to receive a signal lessens or attenuates. This phenomenon is called standard depth of penetration or alternatively, the “skin effect”. The depth of penetration decreases with increasing frequency, conductivity and magnetic permeability. As a result, the signal returning from an imperfection near the OD will be stronger than an identically sized imperfection away from the OD surface. The specifications do not address the imperfection’s location. Rejection is typically decided on a go/no-go signal amplitude criteria from an artificial defect described in the general specification or in the supplementary requirements of the product specifications. Magnetic properties or anomalies can be created in non-magnetic materials through minor parent metal alloy excursions, manufacturing, welding, strain-induced cold work and other processes and may not be detected using conventional OD saturation. These signals are defined as anomalies or discontinuities and are not considered a manufacturing defect. This magnetic coupling is achieved by using encircling coils to create a saturating magnetic field. This magnetic saturation does not necessarily improve the testing sensitivity or repeatability but does allow penetration of eddy currents in magnetic materials.

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

One additional advantage of OD ECT inspection is that the tubing can be fully magnetically saturated during testing to ensure the maximum level of sensitivity and repeatability, vastly reducing the occurrence of false indications on those materials which have ferromagnetic domains. As noted earlier, carbon and alloy steels, stainless steels and some nickel alloys may contain small magnetic domains that must be magnetically coupled during testing to minimize “noise” providing for “quiet” or higher signal to noise inspection with eddy currents. Even austenitic stainless steels which are considered to be non-magnetic, may have small magnetic regions from residual delta ferrite formed during the welding process or strain induced martensite from cold working. Special tube configurations such as integral ID and/or OD fins will require unique technologies that are not covered in this document but should be reviewed with the manufacturer prior to the onset of testing.

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

Inspection from the tube ID ID eddy current testing employing internal probe coils was developed as an in-service or baseline inspection tool to identify tube damage, discontinuities or operational wear. This damage may include pitting, cracking, wear from vibration or abrasion and other environmentally induced mechanisms. The indications are identified using ID probes that are passed down the length of the tube on a tethered cable that is connected to specially designed equipment containing an alternating current power source and electronics for recording and analyzing the output. The probes can be designed with differential encircling coils highly sensitive in identifying sharp, abrupt or axial damage (similar to OD testing), or can use pancake coils to identify longitudinally oriented damage. Internal probe coils can be operated in both the differential and absolute modes simultaneously for identifying both abrupt and gradually occurring discontinuities. The ID test is not only sensitive to tube damage and wear but may also identify other discontinuities including scratches and dents caused by transport and handling, installation, and OD and ID debris that can come from a variety of sources. Therefore, if a one-time test is performed on an existing heat exchanger, it may be difficult to determine which indications are the results of service vs. those that are a result of the manufacturing and installation process.

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

The baseline eddy current test The baseline eddy current test was developed to separate service related damage vs. manufacturing/installation process defects. The baseline test is most effective when performed immediately after the installation of the tubing and is typically done in the fabricator’s shop. It should be noted however that even with specialized electronics, the use of a bobbin coil will have considerable difficulty determining the precise discontinuity shape. Depth can be determined with a single frequency but multiple frequencies will improve the analysis. Signal length and a comparison of the absolute and differential signals from the same discontinuity can also help. The ability & knowledge of the signal analyst to correctly & accurately interpret potential failure mechanisms for the tubes serviced and the signals’ location in the heat exchanger becomes of paramount importance. A baseline test can be performed for the following reasons: ■ To determine if the tube was damaged during installation in the heat exchanger ■ To develop a database of discontinuities and anomalies, including their locations in the heat exchanger for comparison with future examinations

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http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

An initial ID baseline map facilitates in-service evaluations by comparing the initial readings to a second test after a given service exposure. Tracking indication changes becomes a useful tool to understand the effect of pits, cracks and other wall loss damage in the tubing over the service timeline. This information can then facilitate predictive maintenance programs. During ID testing, indications are normally identified as a percentage of wall loss which is determined by a combination of phase angle shift and different responses to multiple frequencies. Because natural damage may not provide identical size data to the artificial defects used to calibrate the equipment, accurate sizing of the damage needs to be verified by removing samples with indications and comparing them to the calculation made during the analysis. However, removal of actual samples may not always be practical; in that case, the analysis must rely on past experience.

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There is no standard accept/reject criteria for ID electric testing of feedwater heater, condenser or balance of plant heat exchangers. Steam generator tubing has several criteria for rejection and many are associated to the artificial defects machined into the reference standard per ASME, Section V, Article 8, Appendix II. This reference standard does not simulate the feedwater heater, condenser and BOP heat exchanger indications and may result in excessive reject rates from non-injurious indications. ID testing to develop a baseline condition map is a mature technology and can be a very useful tool to help track tube damage and wear and future heat exchanger tube problems. Testing should be performed after the tubing is installed, rolled, seal welded and other surrounding manufacturing processes are completed. Detailed test information such as frequencies, probe speeds, phase angles and other parameters need to be carefully documented as well as probe descriptions and model numbers. When the test is duplicated, it can be compared easily to the baseline map to identify any changes to the tubes.

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Conclusion Investigative efforts in producing this HEI Tech Sheet have not identified any known research or studies that have been performed comparing the results of OD EC testing vs. ID EC testing of new commercial grade tubes. As such, this Tech Sheet is compelled to identify concerns relative to this issue as follows: 1. 2. 3.

4. 5.

The impact of attenuation needs to be better understood and addressed. Testing performed from the OD will accentuate OD imperfections while ID testing will accentuate those on the ID. The use of different frequencies will also have a significant impact on the signal vs. depth of the discontinuity. With any eddy current testing, fill factor, or the distance between the coil and the tube, is critical for determining discontinuity sizing. A high fill factor and precise coil centering improves sensitivity while a low fill factor results in a less precise response. When OD testing is performed the tubing is rigidly held and centering within the coil is ensured through the use of stationary rolls in both in-line and offline testing. Depending on the calibration process, OD-tested tubes can either be held stationary or rotated during testing. ID probes rarely have effective centering devices and no requirement or specification currently exists to prove centering. In the case of ID probe coils, a high fill factor results in better centering. Poor centering results in less sensitivity in the hemisphere of the tube that has a larger gap between the probe coil and tube wall. In a baseline test, a good fill factor is usually achievable because the tubes are clean. Testing tubes that have been in service may result in lower fill factors because of ID fouling. Most ID eddy current probes do not have a method for saturation to ensure that small magnetic domains do not produce false indications. Those probes with saturation only have sufficient energy to saturate thin walls and the testing is significantly slower. If the ID testing is performed before installation in the bundle, imperfections developed during the installation process are typically ignored.

The OD ECT is the current industry norm for NDE certification of new tubing. Considering all of the issues above and in the absence of detailed comparative studies, the use of ID testing as an acceptance criterion for new tubing is not only controversial but highly subjective. In light of these concerns, it is therefore recommended that users discuss these issues in detail with the proposed tube manufacturers before specifying an ID test. Charlie Chong/ Fion Zhang

Pulsed Eddy Currents Systems

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Reading 1： Pulsed Eddy Currents - PEC Technology Conventional eddy current testing uses a single frequency sinusoidal to excite a coil and, among many applications, measure flaw responses as voltage and phase changes on an impedance plane. Since different frequencies present different sensitivity behaviors, multi-frequency testing is sometimes performed. In that case, multi-frequency eddy current measurements are either performed by simultaneous injection or multiplexing of multiple frequency components. In pulsed eddy current (PEC) testing, multi-frequency inspections are performed by driving a coil with a broadband pulse instead of a monochromatic excitation. This results in broader frequency contents than standard eddy current signals, as well as offering a better penetration into the depth of a material. The measured response of a PEC inspection is a waveform, similar to an ultrasonic A-Scan, from which features can be extracted to characterize flaws or for example perform thickness measurements. A temporal analysis of the transient response of the coil that results from this excitation can provide useful information about the depth of a defect. Pulsed eddy current is an ongoing research field as novel probes as well as new ways of interpreting and quantifying results are still required to fully exploit their potential.

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http://www.pecscan.ca/BasicsPECtech.html

Although still an active research field, PEC technology already has its place among the NDT techniques. The first application that benefits from the use of PEC is the detection of corrosion under insulation (CUI), where PEC has been used for many years to measure the remaining wall thickness of material buried below up to 6” of insulation material. PEC has also been used to detect deeply embedded corrosion or cracks in the multi-layered aluminum structures used in the aerospace industry.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

PEC Definitions Transient response: A transient event or response is a short-lived oscillation caused by a sudden change of voltage, current, or load. In pulsed eddy current, it expresses the time-dependant behavior of the coil-inspected material response to the input pulse. Balanced signal: Result of the subtraction of a voltage response by a reference signal, generally taken on an unflawed area of a test sample. The balanced signal is null for unflawed regions and displays amplitude variations when a defect or thickness change is encountered. It is similar in nature to performing a null with a standard eddy current apparatus.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

LOI (Lift-Off point of Intersection): The LOI is the location of the crossing point between a transient voltage response acquired on a sample and a response taken with a certain probe lift-off: the LOI is a position where the signal does not vary with probe lift-off. Monitoring the voltage response in the vicinity of the LOI point location therefore provides a mean of performing Pulsed Eddy Current inspections that are free of lift-off.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Monitoring the actual displacement of the LOI point also provides useful information as the position of the LOI is dependant of the sample properties (material, thickness, etc.). An important application of shift is the possibility to perform thickness measurements from the variations of the LOI point coordinates.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Gap Point : Similar to the LOI, the gap point defines a coordinates of the voltage response that is independent of gap variations between two layers of a multi-layered sample. This point is located further in time than the LOI. Spectral analysis: Spectral analysis consists of performing a conventional eddy current analysis of the frequencies contained in a PEC signal. The spectral analysis approach is a variation of the multi-frequency eddy current field but benefits a complete spectrum instead of finite frequencies.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Probes Probes play an important part in Pulsed Eddy Current inspections. Their selection basically depends on the dimensions and shape of the flaws that need to be detected in relation with the properties of the part (material, number of layers, etc.). The probe that is most commonly used for Pulsed eddy current inspections is a reflection type, which means that the device used to induce the pulsed eddy currents is different than the device that receives their effects. Different combinations of driving/receiving sensors can be used to perform pulsed eddy current measurements. Driving Coil: Induction of Pulsed eddy currents The generation or induction of the pulsed eddy currents is typically done using a coil. The purpose of the coil is to convert an electrical pulse (driving pulse) into a magnetic field which induces eddy currents into the tested material following Faraday's laws of induction. The physical and electromagnetic characteristics of the driving coil partly define the bandwidth and footprint of the induced pulsed eddy currents.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Driving Coil

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Coil receiver: Conventional eddy current probes Conventional eddy current probes use both a coil as the driving and receiving sensor. The reception of the eddy currents is again based on Faraday's induction laws. When a voltage I applied to the driving coil, it creates a magnetic field that induces eddy currents in the tested material. In return, these eddy currents generate an additional magnetic field that interacts with the initial one. A receiving coil picks up the variations of that resulting magnetic field and converts it into a measurable electrical signal.

Driving Coil Receiving Coil

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Hall effect sensors In opposition to coils which measure variations of a magnetic field, Hall effect sensors allow for the direct measurement of a magnetic field. This difference allows for a better measurement of magnetic fields that do not vary rapidly. GMR sensors Giant Magneto Resistance sensors (GMR sensors) make use of a phenomenon discovered in 1988 and observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers, where the electrical resistance of the GMR varies in the presence of a magnetic field. While it does not rely on the same principles, this sensor is equivalent to a Hall sensor in the sense that it also provides a voltage output that is proportional to the magnetic field.

GMR sensors

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

PEC Results PEC is considered a new technology rather than an improvement of conventional eddy current. By changing the pulse excitation to a square wave, we input and receive signals that are quite different form conventional eddy currents. For this reason, PEC requires particular signal processing techniques which differ from the usual amplitude and phase analysis techniques. There is no denying that considerable information is available in the temporal and spectral analysis of these pulses. Because of the considerable amount of information available and inherent to the technology, the physical phenomenon must be well understood to discriminate between flaws and other artifacts

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/BasicsPECtech.html

Crack Detection using Pulsed Eddy Currents - PEC Crack detection on multilayered aircraft structures is achieved with two different PEC analysis methods. The PEC analysis method is selected based on the layer thickness and the rivet head physical properties. The designated PEC system requires proper calibration to obtain the desired detection.

Introduction The detection of cracks is of great importance in aerospace structures as they can rapidly grow to cause catastrophic failures. Eddy currents, ultrasounds and radiography are the most common ways of inspecting this type of defect. While radiography has a limited use in tight spaces and because of security reasons, eddy current and ultrasonic inspections fail to detect cracks in all situations. Ultrasonic inspections require a mechanical bonding in order to propagate through multiple layers, which is not always the case for riveted structures. On the other hand, eddy currents can penetrate through unbounded layers, but at limited depths (typically 2 layers). Like eddy currents, Pulsed Eddy Currents have the particular advantage of being able to monitor multiple layers without the need for mechanical bonding. In the case of multilayered aerospace structures, a magnetic field that is strong enough to penetrate all layers of interest must be generated. When this is achieved, pulsed eddy currents are produced on both surfaces of each layer and, from the principles of mutual-inductance, generate an additional magnetic field that interact with the one coming from the driving coil. The presence of cracks affects the pulsed eddy currents and can be monitored in the resulting field. Multiple features can be used to detect cracks from either the transient waveform or its spectral representation.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

Experiments C-Scan inspections of multilayered aircraft structures can be done using the ARMANDA scanner (figure 1a), which is a portable scanner that can be fixed on the structure. A PEC inspection was performed using this scanner on a riveted eddy current standard (2 aluminum layers of 0.04” with the bottom layer containing EDM notches of lengths of 0.250”, 0.200”, 0.150” and 0.100” on the rivet holes edge and identified from {1} to {4} on figure 2a). For sample inspection, we selected a conventional reflection eddy current probe (700 Hz - 15 kHz). The PecScan™ driver/receiver unit (figure 1b) is used to drive the probe, generate and receive the PEC signals.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

Fig. 1 (a) ARMANDA - Automated scanner Pulsed Eddy Current generation and reception. utomated for PEC testing (b) PecScan™ Driver/Receiver unit for Pulsed Eddy Current generation and reception.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

A picture of the sample is presented in figure 2a. Figure 2b shows the result obtained by analyzing the PEC waveforms using a temporal method (feature: total energy in a time gate) in the form of a C-Scan image. On the other hand, figure 2c shows the C obtained through spectral analysis (feature: single frequency component of 10 kHz extracted from the PEC waveforms). This spectral analysis allows displaying the content of the selected frequency on an impedance plane the same way it is performed in conventional eddy current inspections. Based on the impedance response measured on a good rivet, a rotation is applied on the 10 kHz component to minimize the effects of the rivet edge, leading to the result presented in figure 2 (c).

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

Fig. 2. Images of the Eddy current standard samples {1}, 0.200” {2}, 0.150” {3} and 0.100” waveforms (energy within a time gate); all scales in analysis of the PEC waveforms: imaginary part of the 10 kHz component after rotation of the rivet edge signals. (d) Color palette used to display the C samples. (a) Picture showing the EDM notches of 0.250” {4}. (b) C-Scan obtained from the temporal analysis of the PEC mm. (c) CScan obtained from the spectral C-Scans.

Charlie Chong/ Fion Zhang

http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

Pulsed Eddy Currents Systems Pulsed Eddy Currents offer great potential for corrosion detection and location in thick structures. The wide band frequency spectrum of Pulsed Eddy Currents allows the determination of a large number of parameters, such as defect size and location. In fact, Pulsed Eddy Current techniques have the potential to become the primary method of corrosion detection in multi-layered structures. Our research concerning Pulsed Eddy Current technologies concentrates on the detection of corrosion and measurement of wall thickness of insulated pipelines. In order to optimize inspection productivity and costs, it is imperative to improve the quality of inspection and corrosion data interpretation. Our research efforts therefore revolve around the interpretation of corrosion data and the integration of Pulsed Eddy Current techniques to commercial inspection systems.

Charlie Chong/ Fion Zhang

http://www.tecscan.ca/solutions/advanced/pulsed-eddy-current/

Reading 2： 4.9.2.3 Pulsed Eddy Current Testing. Conventional multifrequency systems usually utilize two or three frequencies. Additional frequencies require very complex multiplex mixing systems to analyze the information from the test. A variety of experimental techniques have utilized the multifrequency characteristics of a short electrical pulse to achieve the same type of results as the multifrequency test technique. In principle, this technique is advantageous in that it requires simpler electronics to process the data. It can potentially generate higher frequencies than fixed frequency systems. This would allow testing of thinner materials, and materials with very low electrical conductivity (high resistivity). The eddy current pulse can also be a very short, high voltage pulse that can be used to momentarily produce magnetic saturation in a ferromagnetic part. This will allow detection of subsurface flaws in ferromagnetic materials.

4.9.2.4 Low Frequency Eddy Current Inspection. In the past most eddy current testing utilized test frequencies of 10 kHz to 1 MHz .Improved equipment and data processing techniques now allow the use of test frequencies as low as 55 Hz. Along with impedance plane equipment to measure signal phase, this has provided a means for testing multilayer materials and thick materials. Detection of deep subsurface cracks, cracking in intermediate layers of material, and corrosion on the backside of a material are possible.

4.9.2.5 Barkhausen Noise Testing Of Ferromagnetic Materials. Abnormal stresses induced by shot peening, other cold working processes, and grinding burns affect the structural properties of a material and can lead to flaw growth and part failure. In ferromagnetic materials, these processes affect the ease with which the magnetic domains in the surface of the material can be moved. In un-magnetized ferromagnetic material, the magnetic domains are randomly oriented. If the material is subjected to a magnetic field, the magnetic domains tend to align themselves in the direction of the magnetic field. When the domains move to align themselves, electrical pulses are generated during the domain movement. This is called Barkhausen noise. This electrical noise can be detected and measured by Hall effect sensors. If the material is free of abnormal stresses, the domains are relatively free to move and little Barkhausen noise is generated. Areas of tensile stress parallel to the applied magnetic field cause an increase in Barkhausen noise. Examples of applications of this test method are ferromagnetic engine components and landing gear. Barkhausen noise measurements are also used to detect the quality of drilling and reaming of holes in ferromagnetic material.

Charlie Chong/ Fion Zhang

http://chemical-biological.tpub.com/TM-1-1500-335-23/css/TM-1-1500-335-23_419.htm

Good Luck

Charlie Chong/ Fion Zhang

Good Luck

Charlie Chong/ Fion Zhang

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