EDDY CURRENT TESTING-BASIC THEORY

KEY PHRASE

“Changes In Impedance”

EDDY CURRENT TESTING

• Based on electromagnetic induction principles

• Used to identify or differentiate between variety of physical structural & metallurgical properties

• Can be used to measure conductivity, grain size, heat treatmentcondition, hardness etc.

• Used to detect cracks, voids & inclusions, sort dissmilar metals, measure depth of non-conductive coatings

• Can test entire production owing to non-contacting nature

BASIC EDDY CURRENT TESTING

When an AC current flows in a coil in close proximity to aconducting surface the magnetic field of the coil will inducecirculating (eddy) currents in that surface. The magnitudeand phase of the eddy currents will affect the loading on thecoil and thus its impedance.

BASIC EDDY CURRENT TESTING

• As an example, assume that there is a deep crack in thesurface immediately underneath the coil. This will interrupt orreduce the eddy current flow, thus decreasing the loading onthe coil and increasing its effective impedance.

• This is the basis of eddy current testing, by monitoring thevoltage across the coil in such an arrangement we candetect changes in the material of interest.

BASIC EDDY CURRENT TESTING

Note that cracks must interrupt the surface eddy current flowto be detected. Cracks lying parallel to the current path willnot cause any significant interruption and may not bedetected

ECT In Progress

ECT & RFT

ECT- Conventional Eddy Current Testing (applicable to non-ferromagnetic materials)

RFT-Remote Field Testing (applicable to ferromagnetic materials)

• Magnetic field in test piece due to eddy currents can be sensed through effect of field on primary coil or by an independent sensor.

• In non-ferromagnetic materials the field in test piece is merely due toeddy currents alone.

• In ferromagnetic materials additional effects due to magnetic permeability of material can overshadow EC field and due to NORMAL variations in permeability in a material test is inconclusive. Thus in order to get a response permeability variations are suppressed by magnetic saturation.

PRINCIPAL OPERATING VARIABLES

Coil Impedance

Electrical Conductivity

Magnetic permeability

Lift-off

Fill Factor

Edge & Skin Effect

Impedance: In a dc circuit, current flow is limited by resistance (R). In an ac circuit an additional phenomena (XL) occurs, inductive reactance. This is measure of reduction in current flow due to self induced currents. Impedance is additive result of pure resistance and inductive reactance.

Z= R+XL

XL= 2πfL

Conductivity curve is generated for material variables i.e thickness, chemical composition, type of flaws e.g specimens of different thickness varying in resistance would cause different points on the curve

Note: Increase in conductivity towards Copper

Impedance Graph

20

50

200

freq

1.0

Thickness

R/ L0Probe dia effectFrequency Effect

Increases separation between lift off & conductivity curves

Inner most curve represents thickest tube

• Coil has inductance L0 & negligible resistance “R”

• Part is a very thin tube closely coupled to the coil

Under steady state conditions for current flow, a portion of energy stored in system and the other dissipated as heat. Thus XL energy stored,

R heat dissipated.

We know in an ac circuit current and voltage have phase difference,

If only coil is present then impedance is purely reactive.

If only conductance is present impedance is purely resistive.

In an R-L circuit i.e for both inductive & resistive components impedance is expressed as a vector at a phase angle θ and expressed as follows:

Z= SQRT (R2+XL2), θ= Sin-1 X L / Z

The current though both elements must be the same, and at any instant the total voltage across the circuit is the sum of the values across the elements, however the maximum voltage across the resistance coincides with zero voltage across the inductor and vice versa.

Coil Impedance

Problem:

Calculate the inductive reactance of a coil with L=40 microhenry operating at 50 KHz. Also calculate the impedance of coil if its resistance is 3 ohm

XL= 2 fL= 12.568

Z=SQRT(R2+XL2)= SQRT(9+157.95)=12.92 ohm

Electrical Conductivity

Capacity for passing electric current is measured by conductivity or resistivity.

In ECT measure of conductivity is IACS (International Annealed Copper Standard)

• Arbitrarily conductivity of annealed pure copper is taken as 100%. Conductivity of other materials is expressed as a percentage of this standard.

EXAMPLE Conductivity of pure Aluminium is 61 % IACS.

FACTORS AFFECTING CONDUCTIVITY

Chemical Composition

Heat Treatment

Grain Size

Hardness

Resistivity

L=Length

A=Area

ρ=Resistivity

R=Resistance

G=Conductance

R= ρl/A

ρ= RA/l µΩ-cm

G=1/ρ %IACS

Also ρ=172/%IACS

Thus ECT can be used to monitor chemicals composition and metallurgical characterisitcs provided their influence on conductivity is sufficient to display a change. (point on impedance plane diagram)

Magnetic PermeabilityEase of magnetization

Ferromagnetic metals and alloys are strongly attracted to a magnet e.g Fe, Ni and Co.

Magnetic Permeability = Strength Of Magnetic Field

Amount Of Magnetic Flux within materialAt saturation there is no change in induced flux for a change in field strength, thus permeability is constant for small change in field strength.

•Eddy currents are influenced by fluctuating magnetic field, thuspermeability affects ECT response, hence the difference between RFT and conventional ECT. HOWEVER the factors that influence conductivity ALSO influence permeability that is chemical composition, hardness etc. Thus ECT is applicable to magnetic aswell as non-magnetic substances.

• Although magnetic materials also experience conductivity changes with material conditions but effect of permeability change is more pronounced.

Lift -offChanges in eddy current response with variations in spacing between coil and conductor are called lift off effects

When a coil is energized in air some response is indicated (0% IACS) even when no conductive material is close by.

As coil is moved closer to conductor, change in indication will increase till coil is on material

Advantages & Disadvantages Of Lift Off

• Lift off effect can be so pronounced that small variations in spacing can mask conditions of tube

•Due to lift off complex geometry is harder to inspect.

• Utilising lift off ECT instruments can measure thickness of non conductive coating e.g paint and anodized coating.

Fill FactorIn an encircling coil, fill factor is an effect comparable to lift off.

It is a measure of how well a test piece fills a coil

Changes in fill factor resulting from OD to be changed, since it can give large indications

Edge EffectWhen a coil is brought near the edge of a part eddy currents are distorted, since they are unable to flow beyond the edge. This causes a distortion near the edge and thus inspection is limited 1/8” from the edge.

Skin EffectEddy currents are densest at the surface immediately beneath thecoil and become less dense with increasing distance below surface. This is skin effect. At some distance below surface of a thick part there will be no current

Standard Depth Of PenetrationDepth at which density of eddy current is 37% is called one standard depth of penetration.

PROBLEM: Calculate the fill factor of a coil with a dia of 0.6 inch in 3/4” tube that is 0.050 inch thick.

Fill factor= d2/D2 d= coil diameter

D= tube ID

Fill Factor= 0.62/(0.75-2x0.050)2

= 0.85

Probe Size= Tube ID - 0.040 ! 0.010

If depth of penetration is greater than thickness of material then eddy current response will be related to material thickness

If thickness is not to be considered then select a test frequency which will limit the eddy current within thickness.

This depth of penetration DECREASES with

1. Increase in frequency

2. Increase in conductivity

3. Increase in permeability

Eddy currents donot cease at one standard depth of

penetration but continue for some distance

Thus part should have thickness of 2-3 standard depths

Phase Lag & Depth Of Penetration Phase B

570

Calculation1.0

Bδ =1.98 SQRT(ρ/Fµ)X/δ

For Cu-Ni (70-30) ρ=37, f=100khZ, =1.0

δ = 1.98 SQRT (37/1X100X1000) = 0.038 in = 38 mils

PHASE AT 60 MILS

(60/38)57= 900

•Use Of higher frequency increases phase separation between ID & OD flaws

• Higher frequency results in lower depth of penetration

Probe Types

Three Basic Types

1. Surface Probes (pan cake coils, pencil coils)

• The probes have axis parallel to surface of test piece.

2. Encircling coils

• Intended for in-line inspection of round products

3. ID probes

• Used for inspection of heat exchanger tubes.

Differential & Absolute CoilDifferential Coil• Responds to differences in impedance between coils 1& 2

•Used for flaw detection

Absolute Coil• Responds to differences between coils 1 & 3

•Used for thickness measurement

•Used for axial cracks