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What factors affect eddy currents?

TWI Frequently asked questions

by Malcolm Spicer

The size of the current is affected by the following:

Electrical conductivity

The ease of electron flow
Inverse of resistivity
Symbol is σ
Units are:

I.A.C.S.
Siemens/m
m/ Ω·mm ²

Permeability

Has a dominant effect on eddy currents.

The 'noise' created by permeability changes in ferrous materials makes eddy current inspection of welds, etc, difficult.

Magnetic saturation can negate the effect of permeability.

Measurement of permeability is the basis of sorting bridges.

Frequency

Affects depth of penetration (skin effect).

The standard depth of penetration (SDP), δ, is the depth at which the eddy current intensity has fallen to 37% of its surface intensity: _{Ω mm ²}

Edge effect

µ - relative permeability
500 - a constant to define the units in use

_{This refers to the effect that the component's edge or
sharp changes in geometry have on the eddy currents.}

_{Can be negated by placing a balancing probe near to the
edge and scanning at that distance.}

_{Lift-off/Stand-off distance}

_{The term used for the distance between a surface coil and the
test surface.}

_{Small lift off gives pronounced effects.}

_{Most high frequency sets employ lift-off
compensation.}

_{Lift-off can be used to measure non-conductive coating
thickness.}

_{Fill factor}

_{Equivalent to lift-off when using encircling coils.}

Fill factor

(internal coil) Or

(external coil) η must be less than 1.0 η is usually about 0.7

Specimen dimensions

Material thickness.

Component geometry.

Flaws

Planar discontinuities which cut eddy currents will be detected.

Planar discontinuities which are parallel to eddy currents will not be detected.

The depth of a crack cannot be measured accurately by eddy current testing.