Depth of Penetration
In principle, eddy current testing is to be classified as a surface testing method. Depending on the method, the induced eddy currents are concentrated on a more or less thin layer near the surface. The strongest eddy currents flow directly at the surface. Therefore, the maximum test sensitivity can be achieved there.
The decrease in eddy current intensity with increasing depth (distance from the surface) is caused by the shielding effect of flowing eddy currents (“skin effect”). In eddy current testing, the so-called standard penetration depth is usually used as a measure of the depth-dependent decrease in eddy current intensity.
The standard depth of penetration describes the depth (dependent on the electrical conductivity and magnetic permeability of the workpiece, as well as the excitation frequency used) at which the density of the eddy current has dropped to ca. 37%. Penetration performance depends on the nature of the eddy currents. Principally the eddy current field penetrates very deeply into the workpiece at low excitation frequencies, and at high frequencies it only forms near the periphery. Technically, tests at up to 3 to 5 times the standard penetration depth are possible. The following figure depicts standard penetration depths of various materials, based on the excitation frequency. Put simply, with the excitation frequency one can adjust the depth resolution of the eddy current test.
The standard penetration depth δ corresponds to the distance at which the eddy current intensity has fallen to approx. 37 % of the value at the test specimen surface (this corresponds to a decrease by a factor of 1 / e ~ 1 / 2.7). It is not a fixed value, but depends on the respective test conditions: the test frequency (f), the electrical conductivity (σ) and the relative permeability of the test object (µr) and can be calculated approximately with the following formula:
δ – Standard penetration depth in mm
σ – electrical conductivity in MS / m
µr – relative Permeability (unitless)
f – Test frequency in Hz
So it is valid:
The greater the electrical conductivity or relative permeability or the higher the test frequency, the more the eddy currents are concentrated on the surface of the test object and the smaller the standard penetration depth.
The relative eddy current strengths for selected integer multiples of the standard penetration depth are δ:
1δ: -> 36,8 %
2δ: -> 13,5 %
3δ: -> 5,1 %
5δ: -> 0,7 %.
The depth 3δ is also known as the “effective penetration depth”. Material changes or defects located at greater depths can generally no longer be reliably detected with sufficient sensitivity because the eddy current intensity has already dropped too much.
Test objects with a wall thickness greater than 5δ are considered “thick-walled”; a further increase in wall thickness would not cause any further change in the measured value at the eddy current coil.
On the basis of the standard penetration depth, the depth detection capability can thus be roughly estimated, taking into account the existing test conditions (material properties and test frequency).