The penetration of eddy currents in a conductive material is decreased when:

The correct answer indicates that the penetration of eddy currents in a conductive material is decreased when the test frequency, conductivity of the specimen, or permeability of the specimen is increased. This is based on the principles of electromagnetic induction and the skin effect.

As frequency increases, the eddy currents generated by alternating current tend to remain closer to the surface of the conductive material due to the skin effect. This phenomenon causes a decrease in the effective penetration depth of the eddy currents. Therefore, higher test frequencies lead to shallower penetration.

Similarly, an increase in the material's conductivity also contributes to reduced penetration. High conductivity facilitates the efficient conduction of eddy currents on the surface, which strengthens the skin effect, resulting in diminished depth of penetration into the material.

The permeability of the specimen relates to its ability to support magnetic field lines. Higher permeability materials can also lead to increased eddy current losses, which further confines the currents to the surface and hence reduces penetration.

Understanding these relationships helps in selecting the appropriate frequencies and materials for non-destructive testing applications using eddy current techniques, particularly when aiming to detect surface-breaking defects or to characterize thin coatings.