EDDY CURRENT TESTING

By: Jabran Younas

A NEW BEGINNING

Applications

Metal thickness

Alloy sorting

Heat treat condition

Heat damage

Plating thicknessCladding Thickness

Insulation Thickness

Nonmetallic coating thicknessvibrationDiameter of tubing or bar stock

CracksSeamsPorosityCorrosionErosionSegregationInclusion

R2 XL2 Z2

Resistance

Inductive Reactance

Impedance

Ohm's Ohm's LawLaw

R=ρL/A

Resistivity (Ohm-cm)

Conductivity (mho or % IACS)

Faraday Faraday LawLaw

Ohm's Ohm's LawLaw

Faraday Faraday LawLaw

Ohm's Ohm's LawLaw

Єemf=NΔφΔt

Єemf=Induced emfN= Number of TurnsΔφ= Change in Flux

Faraday Faraday LawLaw

Ohm's Ohm's LawLaw

Lenz’s Lenz’s LawLaw

The direction of induced current is such as to oppose the cause producing it.

Faraday Faraday LawLaw

Ohm's Ohm's LawLaw

Lenz’s Lenz’s LawLawBiot Biot

Savart’s Savart’s LawLaw

B=μo/2π x I/r

Coils Magnetic Field

Coils Magnetic Field

Coils Magnetic Field

Coils

CoilsAC Coils

CoilsCoils

Conducting MaterialConducting Material

Eddy Currents

Eddy Currents Magnetic Field

Magnetic FieldFrom Test Coil

Magnetic Field From

Eddy Currents

Eddy Currents

Crack

Crack DetectionCrack Detection

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

• δ = 50 ……ρ…. μr f

Effect Of Frequency

Effect of conductivity

Effect of Permeability

Dep

th

Eddy Current Density

Dep

th

Eddy Current DensityLow FrequencyLow ConductivityLow Permeability

High FrequencyHigh ConductivityHigh Permeability

Standard Depthof

Penetration(Skin Depth)

1/e or 37 % of surface density

Crack parallel to eddy

Currents - not detected

Crack interrupts eddy

Currents - detected

Discontinuities

• Edges produce signals just like cracks and mislead inspector

• Different techniques can be used to avoid edge effect

Edge effect

Inductive Coupling

Lift Off and Fill Factor

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

Phase Lag

Eddy Current Variables Comparison

A Typical ECT Instrument

Types of Probes

Surface

Probe

External Probe

Internal Probe

Mode of 0perationAbsolute 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

Eddy Current Probes

Eddy Current Test Equipments

Digital Meter Analogue Meter

Multi-Freq Equipments

Calibration Standards

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

Effect of frequency on signal

Effect of Frequency

44khz 22khz

5khz

Wobble

Dent

High Speed

Correct Speed

Signal Filtering

Signal+ Noise Correct Filter is used

Incorrect Filter is used

Stripper

Thickness Monitoring Methodology

Stripper Tube Signal

Selection of Frequency

Selection of Probe

10% OD shown

36% OD

Defect Sizing

Defect Sizing

60% OD

80% OD

Support

60% changed to 78% near

support

80% OD

60% OD

Defect Sizing

Wrong Indications

RFT Theory

RFT Zones

RFT Signals

Effect of Support plate

Tube Inspection

Depth Curve

Magnet

Card Board

Non Magnetic Strip

Magnetic Strip

Shielding Effect