topic 3 -ndt ec

7/24/2019 Topic 3 -NDT EC

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Use the principal of electromagnetism asthe basis for conducting examinations.

Several other methods such as Remote Field

Testing (RFT), Flux Leakage and BarkhausenNoise also use this principle.

Eddy currents are created through a processcalled electromagnetic induction


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When alternating current is applied to theconductor, such as copper wire, a magnetic fielddevelops in and around the conductor.

This magnetic field expands as the alternatingcurrent rises to maximum and collapses as thecurrent is reduced to zero.

If another electrical conductor is brought into theclose proximity to this changing magnetic field,current will be induced in this second conductor.

Eddy currents are induced electrical currents thatflow in a circular path. They get their name from

eddies that are formed when a liquid or gasflows in a circular path around obstacles whenconditions are right.


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Figure 1: Primary field of test coil enters the test part generateseddy currents that generate second field. Strength of the eddycurrents decreases with depth of penetration


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Larger eddy currents are produced near thetest surface. As the penetration of theinduced field increases, the eddy currentsbecome weaker.

The induced eddy currents produce an

opposing (secondary) magnetic field. Thisopposing magnetic field, coming from thematerial, has a weakening effect on theprimary magnetic field and the test coil cansense this change.

In effect, the impedance of the test coil isreduced proportionally as eddy currents areincreased in the test piece.


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A crack in the test material obstructs the eddy

current flow, lengthens the eddy current path,reduces the secondary magnetic field, andincreases the coil impedance.

If a test coil is moved over a crack or defect in

the metal, at a constant clearance and constantrate of speed, a momentary change will occur inthe coil reactance and coil current

This change can be detected, amplified, anddisplayed by an eddy current flaw detector.

Changes in magnetic flux density may also bedetected by Hall effect devices, amplified, anddisplayed on PCs and laptop computers.


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Figure 2: Schematic diagram of basic eddycurrent instrument.


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As shown in the figure 2, an AC generator isused to drive the test coil. As the test coilpasses over various defects, the coilimpedance and AC voltage changes.

The AC voltage is converted to DC voltage bya diode rectifier and compared to a stable DCvoltage of opposite polarity produced by abattery.

With the meter properly zeroed at the start,changes in coil voltage can be measured.


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Impedance (Z) in an eddy current coil isthe total opposition to current flow. In acoil, Z is made up of resistance (R) andinductive reactance (XL).

Definitions:

Resistance - The opposition of currentflow, resulting in a change of electricalenergy into heat or another form ofenergy.

Inductive Reactance (XL) - Resistance toAC current flow resulting fromelectromagnetic induction in the coil.

Impedance (Z) - The combinedopposition to current flow resulting frominductive reactance and resistance.

In an AC coil, induction fromthe magnetic field of one loopof the coil causes a secondarycurrent in all other loops. Thesecondary current opposes theprimary current.


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Figure shows an eddy current testcoil located at distance A above aconductive material.

The coil is considered to be an"ideal" coil with noresistive losses.

The impedance of the coil in thecomplex plane shown is a functionof the conductivity of the material

at distance "A". If the material in figure was an

insulator, its conductivity (thereciprocal of resistivity) would beinfinite. The coil's reactance wouldremain unchanged at point "PI".

However, if the material is a

conductor, eddy current losses willoccur. The coil will signal thischange by increases in resistivelosses with a simultaneousdecrease in reactance, and theoperating point of the system willshift to "P2".


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When the conductivity of thematerial approaches infinity (asuperconductor), the resistivelosses will again approach zero.With very highly conductivematerials, eddy current flow willbe very high and the strong

secondary field will reduce thereactance of the coil to point"P3".

Since the complex planeapproaches a semicircle asconductivity varies from zero toinfinity, it can be concluded thatthe conductivity of a material has

the greatest effect on coil impedance. Coil impedance is dependent on

the vector sum of the coil'sinductive reactance and the testpart's resistance to the eddycurrent field.


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If the conductivity of the material isheld constant and "A" is changed, thestraight line from point "PI" to "A0" isgenerated.

When attempting to measurechanges in conductivity, changes inspacing or lift-off are highlyundesirable. In order to minimizevariations in lift-off, eddy currentcoils may be recessed a shortdistance into the eddy current probehead, and the probe head may bespring loaded to maintain surfacecontact.

However, since the lift-off effect islinear over a limited probe clearancerange, eddy current probes can bedesigned to measure nonconductivecoating thickness over uniformlyconductive materials. Coil impedancecan be calculated for any knowncombination of conductivity andprobe clearance.


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So far, we have described howeddy current resistance(heating) losses, conductivity,probe spacing, and defectsaffect coil impedance; nomention has been made of theeffect of frequency on coilimpedance.

We know that conductivereactance and impedance ofthe coil are affected by testcoil frequency in accordancewith Eq. (1):

......(1)

where XL = the inductive reactance of the coil in ohms , = 3.1416, f =

frequency in Hertz (Hz) and L = inductance in Henrys (H)

Equation (1) shows that both inductance and frequency directly affect coilimpedance. Thus, conductivity and frequency have exactly the same effect oncoil impedance.

Figure above shows the effect of holding frequency constant and varyingconductivity and vice versa. Assuming that material conductivity is reasonablyconstant, we can use the frequency relationship to our advantage.

XL=2fL


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For a particular materialconductivity, a test coilfrequency may be selectedthat will create a favorableoperating point for detectingflaws while differentiatingagainst non-relevant

indications.

The frequency "fg" is the limiting frequency or the point where further

increases in frequency will not increase the ohmic losses in the test

material.

When material conductivity is known, optimum test coil operatingfrequency can be calculated or determined experimentally.


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In theory, the maximum eddy current testing speed is

determined by test coil frequency. In turn, the test frequency selected determines the initial

impedance of the eddy current test coil; as operatingfrequency increases, empty coil impedance increases.

If test frequency strength is held constant, the surface eddycurrent density increases. Small discontinuities are classifiedas high-frequency variables because they are isolating at high

frequencies. The relationship between coil impedance andfrequency is given by Eq. (13):

XL=2fL

where XL = inductive reactance of the coil in ohms

= 3.1416f = test frequency in Hertz (Hz)

L = inductance in Henrys (H)


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As shown in Eq. (13), test frequency affects the inductance of the coil.Lowering the test frequency increases the depth of eddy current

penetration. Lower test frequencies are typically used withferromagnetic materials because of their low permeability.

XL=2fL

Frequency, temperature, material hardness, and permeability affect the formation

of the skin effect that limits the depth of eddy current penetration. At a fixedfrequency, eddy current penetration will be the greatest in a metal with the lowest-

percentage International Annealed Copper Standard (% I ACS) conductivity.

For any given set of test conditions, there is a range of suitable frequencies

centered on the optimum test frequency. In modulation analysis, conductivity, part

dimensions, and defects modify frequency.

Chemical composition, alloy, and heat treatment changes produce low-frequency

modulation.

The oscillator section of the eddy current instrument controls the test frequency.

Proper selection of frequency, centering, and adjustment of phase obtain the

optimum sensitivity to a known defect.


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The ratio of test frequency (f) to limit frequency (fg) provides a useful number for

evaluating the effects of various variables based on their impedance diagram.

The limit frequency, limit frequency equations, and impedance diagrams are

different for solid rods and thin-walled tubing. A change in f/fg ratio will cause a

change in both the phase and magnitude of voltage developed across the test coil.

The limit frequency is the frequency at which additional frequency which increases

eddy current losses. Limit frequency is defined when the mathematical function

describing the electromagnetic field within a part is set equal to one.

The limit frequency is also known as the "characteristic" frequency of the material.If the characteristic frequency is 100Hz, the test frequency that is required for an

f/fg ratio of 10 is 1.0 kHz.

The characteristic frequency for a solid magnetic rod is calculated by

fg = 5060/d2

where = conductivity

= permeability

d = diameter of the rod


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Aluminum tube having 30mm outer diameterand 26mm inner diameter was inspectedusing eddy current encircling coil of 0.9

filling ratio at frequency test ratio of f/fg =

10.1-What is the actual frequency of test coil?2- What is the eddy current depth of

penetration

Resistivity for Aluminum = 2.9x10-8ohm-m, susceptibility (Xm)=2.07x10-5 anduo= 4x10-7 H/m


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In many cases, we do notwant to measure the effect ofprobe clearance orconductivity on coilimpedance. Instead we wantto locate and measure the

effect of discontinuities oncoil impedance and probeoutput.

Figure shows the effect thatcracks and defects have oncoil impedance. When the coil

passes over a crack, theimpedance of the coil variesby the value shown by thevector point "PI".


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A significant change in vectordirection occurs and the vectorpoints toward "P0" when probeclearance changes. This change invector direction is used toadvantage by modern instrumentsas will be described later.

The relationship shown at point

"P1" applies to a specific value of conductivity. If the conductivityvalue decreases to point "P2",vector direction differences are lesssignificant and it is harder todifferentiate between theimpedance caused by the crack andthe impedance change that caused

by probe clearance. The planar diagram shows that it ismore difficult to distinguishbetween defect indications and lift-off indications with low conductivitymaterials.


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Encircling coils are used morefrequently than surface-mounted coils.

With encircling coils, thedegree of filling has a similar

effect to clearance withsurface-mounted coils.

The degree of filling is theratio of the test materialcross-sectional area to the

coil cross-sectional area. Figure shows the effect of

degree of filling on theimpedance plane of theencircling coil.


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For tubes, the limiting frequency (point where ohmic losses of

the material are the greatest) can be calculated precisely fromEq. (2):

For most applications, two coils are employed where the primary (field) coil

generates the eddy currents and the secondary (pickup) coil detects thechange in coil impedance caused by the changes in conductivity andpermeability.

As previously discussed, the magnitude of the eddy current depends onfrequency of the field current, conductivity and permeability of the testmaterial, and geometry of the test part. Because of the skin effect (eddycurrent heating), the depth of penetration of eddy currents is relatively smalland can be calculated from Eq. 3:


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Depth of Penetration of eddy

currentsEddy currents are strongest at the surface of the material and decrease in strengthbelow the surface. The depth that the eddy currents are only 37% as strong as theyare on the surface is known as the standard depth of penetration or skin depth. Thisdepth changes with probe frequency, material conductivity and permeability.


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The Hall voltage produced is given in Eq. (4):

Equation (4) holds true only when the semiconductor has an infinite length to-

width ratio. For practical purposes, Eq. (4) reduces to eq. (5):

where k is a constant that combines the Hall coefficient, temperature, and

semiconductor geometry

esl


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Some hypothetical metal is known to have anelectrical resistivity of 4 X 10-8 (ohm-m).Through a specimen of this metal that is 25mm thick is passed a current of 30 A; when a

magnetic field of 0.75 tesla is simultaneouslyimposed in a direction perpendicular to thatof the current, a Hall voltage of -1.26 X 10-7

V is measured. Compute the electron mobilityfor this metal.


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For reliable flaw detection with eddy currents, various

forms of interference, such as coil clearance, must be

reduced and suppressed. The signal-to-noise ratio of

the eddy current system can be favorably enhancedthrough the use of:

Probe design

Vector analysis equipment

Filtering techniques

Elimination of permeability variations in ferromagneticmaterials


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The externally applied magnetic field, sometimes called the magnetic field strength,is designated by H. If the magnetic field is generated by means of a cylindrical coil

(or solenoid) consisting of N closely spaced turns, having a length l, and carrying a

current of magnitude I, then

H=NI/l.......(6)

The magnetic induction, or magnetic flux density, denoted by B, represents the

magnitude of the internal field strength within a substance that is subjected to an

H field. The units for B are teslas [or webers per square meter (Wb/m2)]. Both B

and H are field vectors, being characterized not only by magnitude, but also by

direction in space.


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The magnetic field strength and flux density are related

according toB=uH........(7)

The parameter u, is called the permeability, which is a property of the specific

medium through which the H field passes and in which B is measured. The

permeability has dimensions of webers per ampere-meter (Wb/A-m) or henries

per meter (H/m).

In a vacuum,

Bo=uoH.......(8)

where uo is the permeability of a vacuum, a universal

constant, which has a value of 1.257 X 10-6 H/m.

The parameter Bo represents the flux density within a

vacuum.


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The magnitude of M is proportional to the applied field as follows:

M=XmH ...... (11)

and Xm is called the magnetic susceptibility, which is unitless.The magnetic

susceptibility and the relative permeability are related as follows:

Xm=ur-1 ........(12)


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A coil of wire 0.25 m long and having 400turns carries a current of 15 A.

(a) What is the magnitude of the magneticfield strength H?

(b) Compute the flux density B if the coil is ina vacuum.(c) Compute the flux density inside a barof chromium that is positioned within the

coil.

The susceptibility for chromium is 3.13 X 10-4. Take the permeability 1.257x10-6H/m


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Conductivematerial

CoilCoil'smagnetic field

Eddycurrents

Eddy current'smagnetic field


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Eddy current testing is particularly well suited for detecting surface

cracks but can also be used to make electrical conductivity andcoating thickness measurements. Here a small surface probe is

scanned over the part surface in an attempt to detect a crack.

topic 3 -ndt ec

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