Which factor primarily influences the penetration depth of eddy currents in a test material?

The penetration depth of eddy currents in a test material is primarily influenced by the test frequency applied. As the frequency of the electromagnetic field increases, the penetration depth decreases. This relationship is described by the skin effect, where higher frequencies cause eddy currents to flow closer to the surface of the material, thereby reducing the penetration depth.

While the conductivity of the material is indeed an important factor, it primarily affects the magnitude of the eddy currents rather than their penetration depth. Materials with high conductivity will have stronger eddy currents for a given frequency, but this does not change how deep those currents can penetrate. The geometric shape and temperature of the material can affect other aspects of testing and the resulting data but are not the primary factors in determining penetration depth. Therefore, considering the relationship between frequency and penetration depth, the test frequency is the most influential factor.