Which material would most likely exhibit the lowest penetration depth during eddy current testing?

In eddy current testing, penetration depth is influenced significantly by the material's electrical conductivity and magnetic permeability. Ferromagnetic materials tend to have high magnetic permeability, which leads to the generation of a magnetic field that concentrates the eddy currents near the surface of the material. This behavior results in a reduced penetration depth, as most of the eddy currents do not penetrate deeply into the material.

In contrast, non-ferrous metals and aluminum typically have lower magnetic permeability and thus a greater penetration depth compared to ferromagnetic materials. Soft iron, while also a ferromagnetic material, can exhibit similar characteristics. However, due to the high permeability of ferromagnetic materials in general, they are more likely to show the lowest penetration depths in eddy current testing scenarios.

Therefore, the answer that indicates ferromagnetic materials will experience the lowest penetration depth aligns with the principles of how eddy current testing operates and the properties of materials involved.