Eddy current fact card-01

Electromagnetic Testing- Eddy Current Method 2014-NovemberMy ASNT Level III Pre-Exam Preparatory Self Study NotesFact Cards学习卡 -01

Charlie Chong/ Fion Zhang





Fion Zhang at Shanghai2014/November

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

Charlie Chong/ Fion Zhang Shanghai 上海


Shanghai 上海


Shanghai 上海


Greek letter



Aeronautical NDT Inspection


Aeronautical NDT Inspection


Pipeline Remote Field Testing


Overview:Eddy current testing is an inspection method that can be used for a variety of purposes including the detection of cracks and corrosion, material and coating thickness measurement, material identification and, in certain materials, heat treatment condition. The process relies upon a material characteristic known as electro-magnetic induction. When an alternating current is passed through a conductor an alternating magnetic field is developed around the coil, the field expanding and contracting as the alternating current rises and falls. If the coil is then brought close to another electrical conductor the fluctuating magnetic field surrounding the coil permeates the material and induces a circulating or eddy current to flow in the conductor. This eddy current, in its turn, develops its own magnetic field. This ‘secondary’ magnetic field opposes the ‘primary’ magnetic field and thus affects the current and voltage flowing in the coil. Any changes in the conductivity of the material being examined such as near surface defects or differences in thickness will affect the magnitude of the eddy current and this change can be detected using either the primary coil or a second detector coil. This forms the basis of the eddy current inspection technique.


Fact Card: What is Ampere I?

Ampere, which represents a quantity of oneCoulomb of charge flowing per second. Quantitatively it is equal to the flow of 6.25 × 1018 electrons per second through a given point in a circuit.

Positive charges move in the direction of the field in a conductor


Fact Card: What is Voltage V?

The basic unit of potential in an electrostatic field is a Volt which is equal to the potential difference between two points for which one Coulomb of charge will do one Joule of work in going from one point to the other, i.e.

1 Volt = 1 Joule/Coulomb

EMF = VA -VB or VB - VA

VA VB


Fact Card: What is Resistance R (Ω)?

Resistance in any conducting material is the measure of the opposition to the motion of free electrons due to their continuous collisions against the atoms of the lattice. Resistance depends on the nature, dimension and physical state of the conductor. The unit of electrical resistance is Ohm (Ω). The Ohm is defined as the resistance of a conductor through which a current of one Ampere is flowing when the potential difference across it is one Volt i.e.

1 Ohm = 1 Volt /1Ampere


VA VB


Fact Card: What is resistivity p (Ω.m)?

The amount of resistance in a material is a factor that limits the amount of current that flows through the material for a given applied electromotive force (EMF). Since the resistance in a circuit results in the expenditure of energy, the result is the dissipation of that energy in the form of heat. For a pure conductor resistance is directly proportional to its length L and inversely proportional to its cross section area A i.e.

R ∝ L / A or R = p L / A ,


VA VB


Fact Card: What is conductivity (mho/m)?

Conductivity is defined as the ability of a material to conduct electric current. It is denoted by s. The unit of conductivity is Siemens per meter or mho per meter. The conductivity of a conductor decreases with the increase in the temperature. Each element has a unique value of conductivity. Copper, silver and gold have high conductivities where as, carbon has a verylow conductivity.


Fact Card: What is temperature coefficient of resistivity?

Experimentally the change in resistivity of a metallic conductor with temperature is found to be nearly linear over a wide range of temperatures below and above 0°C. Over such a range, the fractional change in resistivity per Kelvin is known as the temperature coefficient of resistivity.


Fact Card: What is %IACS?

In eddy current testing, conductivity is frequently given as a percentage of the international annealed copper standard (%IACS). In this system conductivity of pure annealed copper at 20 °C is set to 100% and conductivity of other materials is given as a percentage of copper. Conductivity of a material can be calculated from its resistivity.

%IACS = 172.41/p

Where: IACS = international annealed copper standard p = resistivity


Fact Card: The resistivity and conductivity values of various materials (1/2)


Fact Card: The resistivity and conductivity values of various materials (2/2)


Fact Card: What is Hall’s effect?

Hall effect is the accumulation of charge at one side of the charge carrier (conductor) when it pass thru an external magnetic field. The effect of accumulation of charges is reflected as potential difference.



If an electric current flows through a conductor in a magnetic field, the magnetic field exerts a transverse force on the moving charge carriers which tends to push them to one side of the conductor. This is most evident in a thin flat conductor as illustrated. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall effect after E. H. Hall who discovered it in 1879. Note that the direction of the current I in the diagram is that of conventional current, so that the motion of electrons is in the opposite direction. That further confuses all the "right-hand rule" manipulations you have to go through to get the direction of the forces.




Fact Card: What is right hand rule


Fact Card: What is Faraday’s Law (1/3)

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.

Simply:Changing magnetic field induces EMF in a conductor

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



Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct 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.

ΔΦ=ΔBA




Fact Card: What is Lenz’s Law (1/2)

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.


Fact Card: What is Lenz’s Law (2/2)

Simply:The polarity of the induced EMF is such that it produces a current whose magnetic field opposes the change which produces it.


Fact Card: What is Maxwell's equations?

Maxwell's equations represent one of the most elegant and concise ways to state the fundamentals of electricity and magnetism. From them one can develop most of the working relationships in the field. Because of their concise statement, they embody a high level of mathematical sophistication and are therefore not generally introduced in an introductory treatment of the subject, except perhaps as summary relationships.These basic equations of electricity and magnetism can be used as a starting point for advanced courses, but are usually first encountered as unifying equations after the study of electrical and magnetic phenomena.

Read more: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html


Fact Card: AC Current- Amplitude and Phase

At any time t the instantaneous voltage

Vt = V0 sin ωt = V0 sin(2πft)

Where:Vt = instantaneous voltage, V0 = initial voltage, ω = 2πf, angular frequencyt = time, f = frequency

2ππ


Fact Card: What is Inductive reactance?

Inductance, like resistance, imposes a limit on the current which a given ac voltage will cause to flow in a circuit. The magnitude of opposition to current flow by an inductance is called inductive reactance (XL), which, unlike resistance, varies with frequency. If the frequency is zero (dc), there is no inductance and a coil will act as an ordinary conductor, but as frequency increases (AC), the rate of change of the coil magnetic field increases and the coil will become more and more inductive, thereby increasing the opposition to current flow. Therefore the higher the frequency, the higher the inductive reactance.

XL = 2πf L

Where XL = inductive reactance, f = frequency, L = inductance


Fact Card: What is Capacitive reactance?

The magnitude of opposition to current flow by a capacitanceis called capacitive reactance (XC) and is measured in Ohms. Capacitive reactance also varies with frequency but unlike inductive reactance, the higher the frequency, the lower the capacitive reactance.

XC = 2πf C

Where XC = capacitive reactance, f = frequency, C = Capacitance


Fact Card: In circuit having pure inductance, what is the phase change between current & voltage

Current is lagging voltage by 90° (π /2)


Fact Card: In circuit having pure capacitance, what is the phase change between current & voltage

Current is leading voltage by 90°


Fact Card: What is impedance Z?

Total opposition to flow of current in a circuit is called impedance (Z). If the resistance, inductive reactance and capacitive reactance are drawn vectorially(vector addition) with magnitude and direction and with a horizontal line representing zero phase angle, then the resistance will be drawn as a horizontal line whose length is proportional to its magnitude. As an inductance causes a phase lag of 90°, the inductive reactance can be drawn as a vertical line upwards whose length is proportional to its magnitude. Similarly, as a capacitance causes a phase lead of 90°, capacitance reactance can be drawn as a vertical line downwards


Fact Card: Impedance Z- a vector addition of reactance & resistance

R

XC

XL


Fact Card: What is Magnetism (simplest definition) (1/4)

Magnetism is a property possessed by certain material by which this material can exert a mechanical force of attraction and repulsion on other like materials. The most well known example of the effects of magnetism is the attraction that the magnet has for an iron nail.


Fact Card: What is Magnetism (2/4)

Magnetism arise from three atomic effects: Nuclear Spin, Electron Spin and Electron Orbital Motion. These effects lie in the realm of quantum mechanics. And, well, I don't really understand quantum mechanics all that well. However, what I can say is that magnetic dipole moment, m → is a measure of the contribution of these three effects to a material's magnetic properties. This moment is typically defined in terms of electric current, I , and current loop area, A → , as shown in the equation below (also see figure 13). We can also represent magnetic moment in a more arbitrary volumetric case where current distribution is more complex (i.e. current density J ⃗ ).

http://weblog.sirajs.com/article/who-devil-maxwell



http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/09._The_Hydrogen_Atom/Atomic_Theory/Electrons_in_Atoms/Electron_Spin



http://wps.prenhall.com/wps/media/objects/3311/3390683/blb0607.html


Fact Card: What is magnetic field?

The space around a magnet where the influence of the magnet is felt by another magnet or a magnetic substance (paramagnetic?, ferromagnetic?) is called magnetic field. The magnetic field is represented by magnetic field lines (or magnetic lines of force). The path along which an isolated north pole of a magnet moves in the magnetic field is called the field line. The field lines are directed from N-pole of the magnet towards the S-pole. The field lines do not intersect one another.The magnetic field in a test sample can be created either by passing currents directly into the sample or by indirect way, whereby the field is created into the ferromagnetic sample by induction. The sample gets magnetized in this way by placing it axially in the pre-wound coil, solenoid or by placing it around the conductor carrying current. The lines of force as a result of the applied field would be circular or longitudinal.


Fact Card: What is permanent magnetic? (1/3)

magnetic dipole, generally a tiny magnet of microscopic to subatomic dimensions, equivalent to a flow of electric charge around a loop. Electrons circulating around atomic nuclei, electrons spinning on their axes, and rotating positively charged atomic nuclei all are magnetic dipoles. The sum of these effects may cancel so that a given type of atom may not be a magnetic dipole. If they do not fully cancel, the atom is a permanent magnetic dipole, as are iron atoms. Many millions of iron atoms spontaneously locked into the same alignment to form a ferromagnetic domain also constitute a magnetic dipole. Magnetic compass needles ... (100 of 460 words)

http://www.cyclopaedia.info/wiki/Magnetic-Dipole



A permanent magnet, such as a bar magnet, owes its magnetism to the intrinsic magnetic dipole moment of the electron. The two ends of a bar magnet are referred to as poles (not to be confused with monopoles), and may labeled "north" and "south". However, more properly they are labeled "north-seeking" and "south-seeking" poles,The only known mechanisms for the creation of magnetic dipoles are by current loops or quantum-mechanical spin since the existence of magnetic monopoles has never been experimentally demonstrated.

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



Example of permanent magnet:Rare earth (lanthanoid) elements have a partially occupied f electron shell (which can accommodate up to 14 electrons). The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore, these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.

http://www.cyclopaedia.info/wiki/Magnetic-Dipole


Fact Card: What is permanent magnetic?

http://solarscience.msfc.nasa.gov/magmore.shtml


Fact Card: What is Magnetic Flux

The total number of lines of induction threading through a surface is called themagnetic flux through the surface and is denoted by f. In a special case where B is uniform and normal to a finite area A,

Φ = B x A

Where:Φ = magnetic flux at surface, Weber.B = flux density, Weber/m2.A = area, m2.

Since B is in Wb/m2 and A is in m2. The SI unit of magnetic flux is the weber(Wb) (in derived units: volt-seconds). Since the induction B at a point equals the flux per unit area, it is often referred to as the flux density.


Fact Card: What is Gauss?

The largest values of magnetic induction that can be produced in the laboratory are of the order 10Wb/m2 or 105 Gauss (1 Weber/m2 = 104 Gauss), while in the magnetic field of the earth the induction is only few hundredth thousandths of Weber per square meter or a few tenths of a Gauss.

Simply:Gauss = flux density1 Weber/m2 = 10000 Gauss.


Fact Card: What is Permeability?

In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter μ. The term was coined in September 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity.In SI units, permeability is measured in henries per meter (H·m−1), or newtons per ampere squared (N·A−2). The permeability constant (μ0), also known as the magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum. The magnetic constant has the exact (defined) value µ0 = 4π×10−7 H·m−1≈ 1.2566370614…×10−6 H·m−1 or N·A−2).


Fact Card: Magnetic Permeability

Magnetic permeability is an intrinsic property of a material. It is the ability of a material to concentrate magnetic lines. It is denoted by the Greek letter m. Any material that is easily magnetized, such as soft iron, concentrate the magnetic flux. This is the main feature separating magnetic materials from nonmagnetic materials. The magnetic permeability is equal to the induced magnetic flux density B divided by external magnetic field intensity (magnetizing force) H. i.e.

μ = B / H

Where:μ(mu) = magnetic permeability (Weber/ampere-meter).B = flux density (tesla = Weber/m2).H = magnetizing force (amperes/meter).


Fact Card: Relative Permeability

For air, vacuum, and non-magnetic materials the m is constant. For air and vacuum the value of m is given as, μo = 4 π x 10-7 Weber/ampere-meter. The numerical values of μ for different materials are assigned in comparison with air or vacuum. This is called the relative permeability and is defined as:

μr= μ/μ0

Where:μr = relative permeability, μ = permeability, μ0 = permeability in vacuum.

The μr is a dimensionless quantity because it is a ratio comparing two flux densities. Since air, vacuum and any other non-magnetic material cannot affect a magnetic field by induction, they all have the μr equal to 1. For magnetic materials μr can be very large. Typical values for iron are 100 to 5,000.


Fact Card: What is Reluctance?

Reluctance in the magnetic circuit is comparable to resistance in the electrical circuit. It is defined as the opposition to the establishment of magnetic flux in the material under the influence of the magnetizing field. The material with high permeability has low reluctance and vice versa. Reluctance of the material determines the magnitude of the flux produced by the MMF as given by the magnetic Ohm’s law.


Fact Card: What is magnetic Ohm’s law MMF?


Fact Card: What is Diamagnetism?

Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment in the direction opposing the external field. Diamagnets are materials with a magnetic permeability less than μ0 (a relative permeability less than 1).Consequently, diamagnetism is a form of magnetism that a substance exhibits only in the presence of an externally applied magnetic field. It is generally a quite weak effect in most materials, although superconductorsexhibit a strong effect.

http://en.wikipedia.org/wiki/Permeability_(electromagnetism)


Fact Card: What is Paramagnetism?

Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a relative magnetic permeability greater than one(or, equivalently, a positive magnetic susceptibility). The magnetic moment induced by the applied field is linear in the field strength and rather weak. It typically requires a sensitive analytical balance to detect the effect. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field, because thermal motion causes the spins to become randomly oriented without it. Thus the total magnetization will drop to zero when the applied field is removed. Even in the presence of the field there is only a small induced magnetization because only a small fraction of the spins will be oriented by the field. This fraction is proportional to the field strength and this explains the linear dependency. The attraction experienced by ferromagnets is non-linear and much stronger, so that it is easily observed.


Fact Card: What are the type of magnetic materials.

Ferromagnetic and ferrimagnetic materials are the ones normally thought of as magnetic; they are attracted to a magnet strongly enough that the attraction can be felt. These materials are the only ones that can retain magnetization and become magnets; a common example is a traditional refrigerator magnet. Ferrimagnetic materials, which include ferrites and the oldest magnetic materials magnetite and lodestone, are similar to but weaker than ferromagnetics. The difference between ferro- and ferrimagneticmaterials is related to their microscopic structure, as explained in Magnetism.



If an object is placed in a magnetic field and become magnetized. Some metals are attracted to a magnet, these are paramagnetic metals of which ferromagnetic materials are a sub group. Others are repelled by magnets; these are diamagnetic metals. An illustration of these relationships is shown in FIG below.

Subgroup of paramagnetic

Subgroup of diamagnetic?



Paramagnetic substances, such as platinum, aluminum, and oxygen, are weakly attracted to either pole of a magnet. This attraction is hundreds of thousands of times weaker than that of ferromagnetic materials, so it can only be detected by using sensitive instruments or using extremely strong magnets.



Diamagnetic means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances, such as carbon, copper, water, and plastic, are even more weakly repelled by a magnet. The permeability of diamagnetic materials is less than the permeability of a vacuum. All substances not possessing one of the other types of magnetism are diamagnetic; this includes most substances. Although force on a diamagnetic object from an ordinary magnet is far too weak to be felt, using extremely strong superconducting magnets, diamagnetic objects such as pieces of lead and even mice can be levitated, so they float in mid-air. Superconductors repel magnetic fields from their interior and are strongly diamagnetic.

Question:Nonmagnetic = Diamagnetic?


Fact Card: Magnetic flux density of a infinitely long conductor

The magnetic field surrounds the current carrying conductor. For a long straight conductor carrying a unidirectional current, the lines of magnetic flux are closed circular paths concentric with the axis of the conductor. Biot and Savart deduced, from the experimental study of the field around a longstraight conductor, that the magnetic flux density B associated with the infinitely long current carrying conductor at a point P which is at a radial distance r, as illustrated in FIG. below, is

B

http://electrical4u.com/magnetic-flux-density-definition-calculation-formula/


Fact Card: What is Fleming Right Hand Rule?

The lines of magnetic induction are circles concentric with the wire and lying in planes perpendicular to it. The direction of this concentric closed loop of magnetic lines is given by right hand rule, which states, ‘If the conductor is grasped in the right hand with the thumb pointing in the direction of the current, the curled fingers of the hand will point in the direction of the magnetic field’ as shown in FIG. 2.15.below.

http://electrical4u.com/magnetic-flux-density-definition-calculation-formula/


Fact Card: ASTM Standards (1/2)

Recommended practice for examination and evaluation of pitting corrosion ASTM G 46 Definition of terms relating to electromagnetic testing ASTM E 1316

Electromagnetic (eddy current) examination of type F continuously welded (CW) ferromagnetic pipe and tubing above the Curie temperature ASTM E 1033

Electromagnetic (eddy current) measurements of electrical conductivity ASTM E 1004Electromagnetic (eddy current) sorting of nonferrous metals ASTM E 703

In-situ electromagnetic (eddy current) examination of nonmagnetic heat-exchanger tubes ASTM E 690

A Electromagnetic (eddy current) examination of nickel and nickel alloy tubular products ASTM E 571

Electromagnetic (eddy current) sorting of ferrous metals ASTM E 566


Fact Card: ASTM Standards (2/2)

Electromagnetic (eddy current) testing of seamless and welded tubular products austenitic stainless steel and similar alloys ASTM E 426

Measuring coating thickness by magnetic field or eddy current (electromagnetic) test methods ASTM E 376

Eddy current examination of steel tubular products using magnetic saturation ASTM E 309

Electromagnetic (eddy current) testing of seamless copper and copper alloy tubes ASTM E 243

Standardizing equipment for electromagnetic testing of seamless aluminum alloy tube ASTM E 215

Recommended practice for measurement of thickness of metallic coatings on nonmetallic substrates ASTM B 659

Method for measurement of thickness of anodic coatings of aluminum and other nonconductive coatings on nonmagnetic base materials with eddy current instruments

ASTM B 244

General requirements for carbon, ferritic alloys and austenitic alloy steel tubes ASTM A 450


Fact Card: BS Standards

1984 Eddy current methods for measurement of coating thickness of nonconductive coatings on nonmagnetic base material. Withdrawn: now known as BS EN 2360 (1995).

BS 5411 (part 3)

ASME, Section V, Article 8, Appendix 1 and 2), Electromagnetic (eddy current) testing of heat exchanger tubes

BS 3889

1965 (1989) Eddy current flaw detection glossary BS 3683 (part 5)

British Standards (BS)

DescriptionStandard

1966 (1987) Eddy current testing of nonferrous tubes part 213

1986 (1991) Automatic eddy current testing of wrought steel tubes part 2A

Methods for non-destructive testing of pipes and tubes. Methods of automatic ultrasonic testing for the detection of imperfections in wrought steel tubes


Fact Card: What are the Recent Trends in Eddy Current Testing? (1/3)

1. Pulsed EC testing for sub-surface defect detection2. Remote field EC testing for ferromagnetic tubes3. Eddy current imaging to produce images or pictures of defects and to

automate inspection4. Signal and image processing methods to extract more useful information of

defects for enhanced detection and characterization of defects5. Low-frequency eddy current testing6. Numerical modelling (finite element, boundary element / volume integral,

hybrid etc.) for • Simulation of inspection technique / situation• Prediction of ECT signals for inversion• Optimization of probes / test parameters

7. Design of Phased-array and special focused probes8. Realization of expert systems and data-base systems



Eddy Current Image of a Stainless Steel Weld

Grey Level Eddy Current Image of a Stainless Steel Disc consisting of a Fatigue Crack



This is an eddy current image of a small defect (hole) in a stainless steel plate. As compared to the time-domain and impedance plane signals shown earlier, it is possible to have a visual feel of the defect and can have an idea about the spatial extent/shape/size of the defect.


Fact Card: How Eddy Current Works The alternating current flowing through the coil at a chosen frequency

generates a magnetic field around the coil. When the coil is placed close to an electrically conductive material, eddy

current is included in the material. If a flaw in the conductive material disturbs the eddy current circulation, the

magnetic coupling with the probe is changed and a defect signal can be read by measuring the coil impedance variation.

http://www.victor-aviation.com/Eddy-Current-Inspection.php


Fact Card: Sensitivity versus current density

As with any inspection method there are both advantages and disadvantages to eddy current testing. The method can be used only on conductive materials and, although all metals can be inspected, the depth of penetration of the eddy currents varies. Eddy current density is higher and defect sensitivity greatest at the surface and decreases with depth, the rate of the decrease depending on the “conductivity” and “permeability” of the metal. The conductivity of the material affects the depth of penetration with a greater flow of eddy current at the surface in high conductivity metals and a subsequent decrease in penetration in metals such as copper and aluminium.

Simply: ■ Current density high – testing sensitivity high.■ Conductivity high + permeability high – skin effect greatest.■ Conductivity high + permeability high – Lower penetration.■ Al & Cu are high conductivity thus eddy current penetration low.

http://www.twi-global.com/technical-knowledge/job-knowledge/eddy-current-testing-123/


Fact Card: What is Magnetic Permeability and how it affect eddy current testing?

Permeability is the ease with which a material can be magnetized. The greater the permeability the smaller will be the depth of penetration. ’Non-magnetic’ metals such as austenitic stainless steels, aluminium and copper have very low permeability whereas the ferritic steels have a magnetic permeability several hundred times greater.

Simply: ■ Magnetic material – High permeability.■ Non-Magnetic material – Low permeability.■ High Magnetic permeability – Low penetration, high skin effect.

.


Fact Card: How to increase the depth of penetration of Eddy Current?

The depth of penetration may be varied by changing the frequency of the alternation current – the lower the frequency the greater is the depth of penetration. Unfortunately, as the frequency is decreased to give this greater penetration the defect detection sensitivity is also reduced. There is therefore, for each test, an optimum frequency to give the required depth of penetration and sensitivity.

Simply: ■ Lower frequency – High penetration.■ Lower frequency – Less Sensitivity.■ Lower frequency – Sub-surface detection.■ Choice of frequency- Optimize between penetration and sensitivity.■ Choice of frequency- Types of testing.


Fact Card: What is the useful depth of detection of eddy current

A parameter known as the “standard depth of penetration”, taken as the depth at which the eddy current value has reduced to 37% of that at the surface, can be calculated from the magnetic permeability, the metal’s conductivity and the frequency of the alternating current in the probe. The standard depth of penetration is generally regarded as the criterion by which the efficiency of detection can be judged, although changes in the eddy current can be detected at depths of up to three times this figure. A simple calculation may be used to select the optimum probe frequency.

Simply: ■ 1 standard depth is that the eddy current density is reduce by 1/e or 37%

of surface current value.■ Eddy current can be detected at a depth up to 3 standards depth.


Fact Card: Depth of one standard depth (The Math)

Where:

δ = Standard Depth of Penetration (mm)π = 3.14f = Test Frequency (Hz)μ = Magnetic Permeability (H/mm)σ = Electrical Conductivity (% IACS)


Fact Card: What is Multi-Frequency eddy current testing

Some inspections involve sweeping through multiple frequencies to optimize results, or inspection with multiple coils to obtain the best resolution and penetration required to detect all possible flaws. It is always important to select the right probe for each application in order to optimize test performance.


Fact Card: What are the factors affecting eddy current?

The factors which affect eddy currents are:

1. Conductivity σ (Sigma)2. Permeability μ (mu)3. Frequency f4. Proximity (Lift off/fill factor)5. Geometry6. Probe Handling7. Discontinuities (Defects)

Due to the large number of variables in eddy current inspection, in order to correctly interpret the cause on an indication, all of the above seven (7) factors must be considered.


Fact Card: Eddy current considerations: Conductivity (1/3)

While the inherent conductivity of a material is always the same, there are internal factors that can cause what appears to be a change in the inherent conductivity:

Alloys.Alloys are combinations of other metals and/or chemical elements with a base metal. Each metal or chemical element has an individual effect on the conductivity of the base metal. The conductivity of the base metal is changed to a value related to the composition of the alloy. Thus it is possible to identify basic metals and their alloys by measuring their conductivities.



Hardness.When a metal or alloy is subjected to heat treatment (or to excessive heat during normal operation) the metal will become harder or softer depending on the material. This change in hardness is brought about by an internal change in the material which also affects the conductivity of the material. This change in conductivity can also be detected by eddy current test methods. An improper heat treatment can be detected in this manner.

Temperature and Residual Stresses.The ambient temperature and internal residual stresses of a material under test also have an effect on the conductivity of the material. These changes can also be detected by eddy current testing. An increase in the temperature of the material normally results in a decrease in the conductivity of the material. Residual stresses cause an unpredictable, but detectable, change in conductivity.



Conductive Coatings.The presence of a conductive coating on a conductive material changes the inherent conductivity of the base metal just as an alloy would. However, if the thickness of the cladding varies, the conductivity will vary. This change in thickness can be detected by eddy current testing methods.


Fact Card: Eddy current considerations: Ferromagnetic Materials

When an energized test coil is placed upon non-magnetized ferromagnetic material, the field is greatly intensified by the magnetic properties of the material so that a large change in the impedance of the test coil occurs. If the magnetic field strength at various locations varies even slightly, these small variations have a large effect on the impedance of the coil. These changes in the impedance of the coil are often so large (in comparison to the changes caused by changes in conductivity or dimension) that they mask all other changes. When specimen geometry permits, this effect may be overcome by magnetizing the material to saturation using a separate coil powered by direct current. Magnetic saturation effectively eliminates any variations in the residual magnetic field due to magnetic variables, and thus allows other variations to be measured. After testing is completed, the article must be demagnetized.


Fact Card: Eddy current considerations: Test Frequency

Frequency is one of the few operator controlled variables in eddy current testing. The main use of frequency is controlling depth of penetration, density and phase of induced eddy currents. In general terms, higher frequencies are used to detect surface breaking discontinuities, and lower frequencies for sub surface testing with less sensitivity.


Fact Card: Eddy current considerations: Proximity

Lift off and fill factor are the terms used to describe any space that occurs between the article under test and the inspection coil. Each has an identical effect on the eddy currents. Lift off and fill factor are essentially the same thing; one is applied to surface coils and the other to encircling and internal coils.

Lift Off- When a surface coil is energized and held in air above a conductor the impedance of the coil has a certain value. As the coil is moved closer to the conductor the initial value will change when the field of the coil begins to intercept the conductor. Because the field of the coil is strongest close to the coil, the impedance value will continue to change until the coil is directly on the conductor. Conversely, once the coil is on the conductor any small variation in the separation of coil and conductor will change the impedance of the coil. The lift off effect is so pronounced that small variations in spacing can mask many indications.


Fact Card: Eddy current considerations: Proximity (2/2)

Fill Factor - In an encircling coil, or an internal coil, fill factor is a measure of how well the conductor (test specimen) fits the coil. It is necessary to maintain a constant relationship between the diameter of the coil and the diameter of the conductor. Again, small changes in the diameter of the conductor can cause changes in the impedance of the coil. This can be useful in detecting changes in the diameter of the conductor but it can also mask other indications.


Fact Card: Eddy current considerations: Geometry (1/2)

The two main factors in component geometry affecting eddy currents are thickness and edge/end effect.

Thickness - Changes in material thickness may be caused by manufactured geometry or in service corrosion/erosion. If the material thickness is less than the effective depth of penetration, any change in the material thickness will affect the eddy currents, this can be used to good effect to measure material thickness.


Fact Card: Eddy current considerations: Geometry (2/2)

Edge/end effect - Eddy currents are distorted when the end, or an edge, of a part is approached with the test coil since the currents have no place to flow. The distortion results in a false indication that is known as ‘edge effect’. However, when scanning into tight radii the opposite effect occurs. Edge effect is also apparent at the junction of different materials. Since, to the test coil, the edge of the part looks like a very large crack or hole, there is a very strong reaction that will mask any changes due to other factors. The limit as to how close to the edge a coil can be placed is determined by the size of the coil and any shielding applied.


Fact Card: Eddy current considerations: Probe Handling

Under ideal conditions the probe coil should be presented to the test surface at a constant angle to the surface with constant lift off and pressure. Changes in probe angle, contact pressure, or the way the probe is held (hand capacitance) will cause changes to the signal from the probe. Innondestructive testing where the majority of inspections utilize the hand held surface contact coil method, the influence of a bad probe handling technique cannot be over emphasized. The effects of probe handling can be reduced with the use of special spring loaded probes which maintain the probe at a constant angle and pressure to the surface. These are usually used where scanning is to be carried out on flat surfaces, or where conductivity or paint thickness measurements are being taken. When scanning close to changes of section (geometry effect) the use of simple probe guides will assist in good probe handling resulting in a more effective inspection.


Fact Card: Eddy current considerations: Discontinuities

Cracks cause a distortion of the eddy current field due to the fact that the eddy currents have to flow around them. This results in an increased resistance path and a corresponding reduction in eddy current strength. Similarly corrosion causes an increased resistance within the material with a corresponding reduction in eddy current strength. In each case a meter orspot display change will result. To ensure that inspections are carried out to a repetitive standard, reference blocks with artificial defects are used. These blocks should be of similar material specification (alloying, heat treatment, conductivity) to the component under test. By setting up the flaw detector (standardization) to give a known response from the artificial defect, an inspection can be carried out repeatedly to the same standard.


Fact Card: What are the type of read out or display commonly available to the operator

The results are displayed either as a digital read-out for the more simple examinations such as thickness measurements or displayed on an oscilloscope screen as an X-Y display of resistance versus the inductive reactance. This gives a characteristic curve, the shape and size of which can be used to detect and size a defect as illustrated in Fig. below to determine heat treatment condition or, as a quick sorting test, to establish the type of alloy.


Fact Card: What are the parameters that could be decided by the operator during eddy current testing?

The eddy current operator is faced with a material whose;

■ conductivity and ■ permeability are ■ physical properties

Are outside of the operator’s control.

The parameters that can be selected are (1) probe size, (2) probe type and (3) frequency of the alternating current.

The selection depending upon the test requirements i.e. crack detection, corrosion depth, coating thickness, heat treatment condition etc.


Fact Card: How the probe sizes affect eddy current testing?

A large diameter coil will inspect a larger volume of metal and therefore reduce the inspection time – a small diameter probe, however, is more sensitive and better suited to detecting small flaws. The large diameter probes are often used for the detection of large sub-surface flaws in castings and forgings and for the detection of corrosion (good for pitting check?) ; the small diameter pencil type probes for detecting cracks.

Weld examination requires special probes to reduce noise from the permeability change across a weld. (more information?)


Fact Card: What are the similarity between MT & eddy current testing

A major advantage is that the process may be used underwater and can be used to scan welds through paint and other coatings.

With respect to detection of linear defects such as cracks and lacks of fusion the defect should break the lines of the eddy currents ideally at right angles –as with magnetic particle inspection (magnetic lines of force) defects parallel to the eddy currents (current) are likely to remain undetected. It is important therefore that the weld is scanned in the correct direction. Cracks as small as 0.5mm deep and 5mm in length are capable of being detected.


Fact Card: How Defect orientations affect eddy current flaw detectability?

the defect should break the lines of the eddy currents ideally at right angles –defects parallel to the eddy currents (current) are likely to remain undetected.


Fact Card: What is the normal sensitivity of eddy current in weld crack detection?

Cracks as small as 0.5mm deep and 5mm in length are capable of being detected by commercial grade equipment. (0.5mm deep crack- scratch marks misinterprets as crack?)

0.5mm deep

mm


Fact Card: The principle of coating thickness measurement by eddy current

Measuring the proximity of a component to the probe can also be used to determine coating thickness provided the coating is non-conductive. The “lift-off”, the distance of the probe tip from the conductive surface, causes a change in eddy current flow which is measurable.


Fact Card: What are the essential characteristic of reference standards?

All of the systems must be calibrated using appropriate reference standards –as for any NDT method, this is an essential part of any eddy current examination procedure. The calibration blocks must be of the;

■ Same material, ■ Heat treatment condition, ■ Shape and size of the item to be tested.

For defect detection the calibration block contains artificial defects simulating defects; for corrosion detection a calibration block of different thicknesses is used.

The eddy current method requires more skill on the part of the operator than, say, MPI and penetrant inspection – it goes without saying that operator training is essential.


Fact Card: Is eddy current testing requires more skillful operator?

Yes! The eddy current method requires more skill on the part of the operator than, say, MPI and penetrant inspection – it goes without saying that operator training is essential.


Fact Card: What is the advantages of Eddy Current Testing?

Pros: Some of the advantages of eddy current technique include:

1. Nearly all metallic materials can be tested. (must be conductive)2. Detects surface and near surface defects. 3. Inspections can be proceeded quickly, results will be provided immediately. 4. Equipment could be very portable. 5. Method can be used for much more than flaw detection. 6. Minimum part preparation is required. 7. Testing machine can be calibrated to accept parts with a certain range of

signal interpretation by utilizing a series of known good samples. 8. Test probe does not need to contact the part. 9. Inspects complex shapes and sizes of conductive materials.


Fact Card: Could eddy current uses to Inspects complex shapes and sizes of conductive materials?

Yes and No, the answer is yes, if specific reference standard is available.


Fact Card: What is the disadvantages of Eddy Current Testing?

Cons: Some of the limitations of eddy current technique include: 1. Only conductive materials can be inspected. 2. Surface must be accessible to the probe. 3. Skill and training required is more extensive than other techniques. 4. Testing equipment is relatively expensive and complex in nature.5. Surface finish and roughness may interfere. 6. Reference standards needed for setup. (specific?)7. Flaws such as delaminations that lie parallel to the probe coil winding and

probe scan direction are undetectable. (parallel to the direction of eddy current)

8. Depth of penetration is limited.


Fact Card: The Applications of Eddy Current Testing

Eddy current is best suited for: 1. Quality assurance during manufacturing and in-service inspections. 2. Sorting of materials with different heat treatment. 3. Detection of flaws in metallic plates, tubes, rods and bars. 4. Detection and characterization of intergranular corrosion in stainless steel. 5. Measurement of conductive and non-conductive coating thickness. 6. Measurement of electrical conductivity and magnetic permeability.

More information:Detection and characterization of intergranular corrosion in stainless steel.


Fact Card: What are the Modes of eddy current testing

Modes: (1) Absolute, (2) differential and (3) reflection

Reflection Probe


Fact Card: What is Electromagnetic Coupling (Lift-off / Fill-factor)

Coupling of magnetic field to the material surface is important in ECT. For surface probes, it is called "lift-off" which is the distance between the probe coil and the material surface. In general, uniform and very small lift-off is preferred for achieving better detection sensitivity to defects. Similarly, the electromagnetic coupling in the case of tubes/bars/rods is referred to as "fill-factor". It is the ratio of square of coil diameter to square of tube diameter, in the case of encircling coils and is expressed as percentage (dimensionless). Usually, 70-90% "fill-factor" is targeted for reliable inspection.


Fact Card: Magnetic Flux Lines

Magnetic flux line contours of an eddy current probe in air, in an Inconel tube and in the tube surrounded by a carbon steel support plate.


Fact Card: What are the principles used in measuring the non-conductive coating thickness by eddy current (1/2)

(1) Magnetic Induction Method

The excitation current of the probe generates a low frequency magnetic field with a strength that is dependent on the coating thickness and is amplified by the magnetic base material. The signal of the measurement coil that captures this amplification is converted to the coating thickness reading by means of the probe characteristic stored in the instrument.

Magnetic induction method according to DIN EN ISO 2178

http://www.gardco.com/pages/filmthickness/df/fischer_FMP10_40.cfm


Fact Card: What are the principles used in measuring the non-conductive coating thickness by eddy current (2/2)

(2) Eddy Current Method

The excitation current of the probe generates a high frequency primary magnetic field that induces eddy currents in the base material. Its secondary magnetic field weakens the primary field. This weakening effect corresponds to the distance (= coating thickness) between the probe and the base material and is converted to the coating thickness reading by means of the probe characteristic stored in the instrument.

ddy current method according to DIN EN ISO 2360

http://www.gardco.com/pages/filmthickness/df/fischer_FMP10_40.cfm


Fact Card: What is Magnetic Flux Leakage?

With MFL technology, permanent magnets are used to temporarily magnetize the steel pipe and the magnetic field changes are recorded and analyzed. The magnetic flux is uniform if there are no flaws in the wall of the pipe. If internal or external flaws are present, such as pitting, corrosion or other forms of damage, the magnetic flux is distorted beyond the wall of the pipe, and this distortion or ‘leakage’ is measured by Hall Effect sensors.

http://www.puretechltd.com/services/electromagnetics/magnetic_flux_leakage.shtml


Fact Card: What are the advantages of MFL?

1. Provides high resolution data of entire circumference of the pipe wall allowing for comprehensive structural evaluations of the pipe

2. MFL tool is capable of collecting high resolution data through interior mortar linings

3. Recognized as an industry standard for metallic pipe wall assessment4. May be deployed manned or free-swimming

http://www.puretechltd.com/services/electromagnetics/magnetic_flux_leakage.shtml


If the eddy current circuit is balanced in air and then placed on a piece of aluminum, the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil, which shows up as resistance R∝I) and the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance).

Fact Card: Eddy current impedance plane respond (1/3)

If a crack is present in the material, less (fewer) eddy currents will be able to form and the resistance will go back down and the inductive reactance will go back up. Changes in conductivity will cause the eddy current signal to change in a different way.



When the probe is brought near a conductive but nonmagnetic material, the coil's inductive reactance goes down since the magnetic field from the eddy currents opposes the magnetic field of the coil. The resistance in the coil increases since it takes some of the coil's energy to generate the eddy currents and this appears as additional resistance in the circuit. As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater. Therefore, the signals will be come more vertical as the conductivity increases, as shown in the image above.



As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater (+ or -?) . (see the brown dotted lines)

Impedance shift of more conductive material

Lift-offLift-off


Good Luck!

Engineering

Eddy current fact card-01