In which of the following materials is eddy current penetration greatest at a fixed test frequency?

Eddy current penetration is influenced by the electrical conductivity and magnetic permeability of materials. At a fixed test frequency, penetration depth increases as material conductivity decreases. The skin effect, which is particularly pronounced in conductive materials, causes eddy currents to concentrate near the surface, thereby limiting penetration depth.

Lead, with a conductivity of 7 percent IACS, has the lowest conductivity among the options presented. This lower conductivity means that eddy currents will not concentrate as much near the surface, allowing for greater penetration compared to the other materials listed. In contrast, aluminum, brass, and copper have higher conductivities, which leads to a stronger skin effect and, as a result, reduced penetration depth.

Thus, in the context of eddy current testing, lead would have the greatest penetration at a given frequency because its relatively low conductivity allows eddy currents to penetrate deeper into the material.