18370060 Eddy Current Testing

7/28/2019 18370060 Eddy Current Testing

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EDDY CURRENT TESTING

By: Jabran Younas

A NEW BEGINNING


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Applications

Metal

thickness

Alloy sorting

Heat treat

condition

Heat damage

Plating

thickness

CladdingThickness

Insulation

Thickness

Nonmetallic

coatingthickness

vibration

Diameter of

tubing or bar

stock

Cracks

Seams

Porosity

Corrosion

ErosionSegregation

Inclusion


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R2 XL2 Z2

Resistance Inductive Reactance Impedance


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Ohm's Law

R=L/A

Resistivity (Ohm-cm)Conductivity (mho or % IACS)


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Faraday Law

Ohm's Law


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Faraday Law

Ohm's Law

emf=Nt

emf=Induced emfN= Number of Turns

= Change in Flux


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Faraday Law

Ohm's Law

Lenzs Law The direction of induced current is such asto oppose the cause producing it.


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Faraday Law

Ohm's Law

Lenzs Law

Biot SavartsLaw

B=o/2 x I/r


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Coils Magnetic Field



Coils

CoilsAC

Coils

CoilsCoils

Conducting MaterialConducting Material

EddyCurrents

Eddy Currents

Magnetic Field


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Magnetic Field

From Test Coil

Magnetic FieldFrom

Eddy Currents

Eddy Currents

Crack

Crack Detection


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Depth of Penetration & Skin Effect

Eddy current density is greatest

at surface

Reduces exponentially with depth

At standard D of P = 1/e (37%) of

surface value

= 50

.

rf


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Effect Of Frequency


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Effect of conductivity


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Effect of Permeability


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De

pth

Eddy Current Density

De

pth

Eddy Current Density

Low Frequency

Low ConductivityLow Permeability

High Frequency

High ConductivityHigh Permeability

Standard Depth

of

Penetration(Skin Depth)

1/e or 37 %

of surface density


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Edges produce signals

just like cracks and

mislead inspector

Different techniques

can be used to avoid

edge effect

Edge effect


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Inductive Coupling


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Lift Off and Fill Factor

Usually, 70-90% "fill-factor" is targeted for reliable inspection


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Phase Lag


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Eddy Current Variables Comparison


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A Typical ECT Instrument


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Types of Probes

Surface

Probe

External

Probe

Internal

Probe


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Mode of 0peration

Absolute Differential

Sensitive to both sudden and

gradual changes in properties.

Not Sensitive gradual changes

in properties

Easy to interpret Difficult to interpret

Show total length of long flaws Detect only ends of long flaws

Sensitive to drift due to

temperature changes

Less Sensitive to drift due to

temperature changes

Sensitive to probe wobble Less Sensitive to probe wobble

Single Coil Pair of coils

Absolute value of impedance

and induced voltage is

measured

Changes in impedance or

induced voltage is mesured


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Eddy Current Probes


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Eddy Current Test Equipments

Digital Meter Analogue Meter

Multi-Freq Equipments


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Calibration Standards


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Testing Procedure

Balance on sound portion Set Sensitivity (Drive Level & Gain)

Set Frequencies

Draw Calibration Curve

Take signal with constant speed

Note Phase lag and amplitude for each

indication


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Effect of frequency on

signal


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Effect of Frequency

44khz 22khz

5khz


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Wobble Dent

High SpeedCorrect Speed


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Signal Filtering

Signal+ Noise Correct Filter is

used

Incorrect Filter is

used


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Stripper


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Thickness Monitoring

Methodology


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Stripper Tube Signal


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Selection of Frequency


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Selection of Probe


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10% OD

shown 36%

OD

Defect Sizing

Defect Sizing


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Defect Sizing

60% OD

80% OD

Support

60% changed

to 78% near

support

80% OD

60% OD


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Defect Sizing


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Wrong Indications


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RFT Theory


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RFT Zones


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RFT Signals


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Effect of Support plate


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Tube Inspection


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Tube Inspection


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Depth Curve


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Shi ldi Eff


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Magnet

Card Board

Non

Magnetic

Strip

Magnetic

Strip

Shielding Effect

18370060 Eddy Current Testing

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