ET ASNT Level II Practice Exam – Prep, Study Guide & Practice Test

In eddy current testing, increasing the frequency leads to decreased penetration depth into the material being tested. This phenomenon is due to the skin effect, where higher frequencies cause the induced eddy currents to concentrate closer to the surface of the conductive material. As the frequency rises, the alternating magnetic field creates eddy currents that have a shallower penetration, which is characterized by a reduced depth of material affected by these currents.

The skin depth, which refers to the distance into the conductor at which the current density falls to approximately 37% of its value at the surface, decreases with increasing frequency. This relationship is critical in applications where assessing material properties at various depths is necessary, as it impacts the ability to inspect sub-surface flaws or imperfections.

This principle is a core concept in electromagnetic testing methods, where understanding the relationship between frequency, penetration, and the consequent ability to detect defects is essential for accurate material evaluation.