The depth of eddy current penetration is primarily dependent on:

The depth of eddy current penetration is primarily influenced by test frequency because higher frequencies result in shallower penetration of the eddy currents, while lower frequencies allow for deeper penetration. This relationship arises from the skin effect, which is the tendency of alternating current (AC) to distribute itself within a conductor such that the current density is highest near the surface and decreases exponentially with depth.

While factors like coil shape and magnetic permeability do influence the testing results and the distribution of eddy currents, they do not primarily determine the penetration depth. Coil shape affects the field distribution and sensitivity, and magnetic permeability impacts how the currents flow in different materials, but the frequency is the most direct and critical parameter governing how deeply eddy currents can penetrate the material being tested. Thus, emphasizing test frequency as the key driver aligns properly with the principles of eddy current testing in this context.