Many factors affect the magnitude of eddy current flow in the material under inspection. Some of these can create problems during the inspection but many can be exploited to determine specific material characteristics. The most significant factors are described in the following:
Electrical conductivity is often described as the ease of electron flow within a material. Conductivity is the inverse of resistivity and the commonly used symbol is σ. The SI unit for conductivity is siemens per metre. An earlier, metric unit which may be encountered is metres per Ω·mm2 however, the unit most commonly employed by NDT practitioners is the International Annealed Copper Standard (IACS), which compares the conductivity of the metal as a percentage of the conductivity of pure copper (e.g. aluminium is about 37% IACS).
The magnetic permeability of the material under inspection has a dominant effect on the magnitude of eddy current flow. The 'noise' created by permeability changes in ferrous materials makes eddy current inspection of carbon steel welds difficult and the strong signal from the steel supports of cupronickel heat exchanger tubes will mask defect indications.
The effect of permeability can be negated by magnetic saturation, multi-frequency inspection or differential coil arrangements. The measurement of permeability is the basis of material sorting bridges.
The frequency of the alternating current passing through the eddy current test coil affects the depth of penetration of the eddy current field in the test material. This is also known as the skin effect. The intensity of the eddy current flow will decrease exponentially with increasing depth into the material.
The standard depth of penetration (SDP), δ, is defined as:
1/e x surface intensity of eddy currents
where e = 2.71828
This gives the depth at which the eddy current intensity has fallen to c. 37% of its surface intensity.
The SDP can be calculated using the formula:
δ = SDP (mm)
f = frequency (Hz)
σ = conductivity (m/Ωmm2)
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.
When inspecting for cracks the edge effect can be negated by placing and balancing the probe near to the edge and scanning at that distance.
The term used for the proximity between the coil and the test surface. A small amount of lift off will give a pronounced effect on the signal amplitude. When analysing the eddy current signal using the impedance plane display the lift off signal will be at a different phase angle from a crack signal or a change in conductivity.
The lift-off effect is used to measure non-conductive coating thickness.
The fill factor is the equivalent to lift-off when using encircling coils. It is used to determine the correct allowance between the inspection coil and the tubular sample to ensure freedom of movement during scanning while maintaining the proximity of the coil to the sample to generate sufficient eddy currents to perform the inspection.
The fill factor (ƞ) is given by:
η must be less than 1.0
η is usually about 0.7
When inspecting plate material, if the plate thickness approaches the SDP a specific signal will be generated. If this is unexpected this could give rise to a false defect call. However, the effect can be used to estimate material loss, for example from blind side corrosion.
When scanning a sample with a complex geometry, false signals may be generated from the geometric changes. This needs to be taken into account when interpreting the signals.
Planar discontinuities (e.g. cracks or lack of weld fusion) which are perpendicular to the flow of eddy currents will be detected.
Planar discontinuities (e.g. laminations) which are parallel to the flow of eddy currents will not be detected.
The depth of a crack cannot be measured accurately by eddy current testing.
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