Why are relatively low frequencies used when inspecting ferromagnetic materials?

When inspecting ferromagnetic materials, relatively low frequencies are utilized primarily due to the lower penetrability of these materials. Ferromagnetic materials, such as iron and steel, exhibit high magnetic permeability, which significantly affects how eddy currents behave within them. At higher frequencies, the skin depth—the distance into the material where the eddy currents penetrate—becomes shallower, which can limit the effectiveness of the inspection by restricting the probe's ability to sense defects located deeper within the material.

Using lower frequencies increases the skin depth, allowing eddy currents to penetrate more deeply and providing better access to features or flaws that may lie beneath the surface. As a result, the inspection becomes more effective for identifying subsurface discontinuities and defects that would otherwise be undetectable at higher frequencies.

The other options do not accurately capture the relationship between frequency and ferromagnetic materials in the context of eddy current testing. Higher resolution is often associated with higher frequencies, which would not be advantageous when dealing with ferromagnetic materials due to the reasons outlined regarding skin depth and penetrability.