At a fixed test frequency, which material will have the greatest eddy current penetration?

The material with the greatest eddy current penetration at a fixed test frequency is, in fact, lead. This is primarily due to its low electrical conductivity. In eddy current testing, the penetration depth of eddy currents into a conductive material is influenced by the material's conductivity and magnetic permeability.

As conductivity decreases, the skin depth—the depth at which the current density falls to about 37% of its value at the surface—increases, allowing eddy currents to penetrate deeper into the material. Lead, with its low electrical conductivity (7 percent IACS), allows for the greatest penetration compared to the other materials listed, which have higher conductivities.

To further elaborate, while aluminum, brass, and copper are more conductive and therefore will have a smaller skin depth at the same frequency, they allow eddy currents to dissipate more quickly to the surface. This results in reduced penetration compared to lead. Thus, when assessing materials for eddy current testing, lower conductivity typically correlates with greater depth of eddy current penetration, making lead the most effective choice in this context.