ET ASNT Level I Practice Exam 2025 - Free ASNT Level I Practice Questions and Study Guide

In eddy current testing, increasing the test frequency typically leads to a decrease in penetration depth. This phenomenon is based on the principles of electromagnetic induction and skin effect.

As the frequency of the alternating current applied to the coil increases, the eddy currents that are induced in the conductive material also become more concentrated near the surface of the material. This is due to the skin effect, which causes the alternating current to flow primarily within a thin layer of the material, known as the skin depth. The skin depth is inversely related to frequency, meaning that higher frequencies will result in a thinner skin depth.

Consequently, with an increased test frequency, the currents penetrate less deeply into the material, leading to a reduced effective detection range. Thus, for applications requiring greater penetration, lower frequencies are often favored, while higher frequencies are used for surface level inspections and detecting superficial flaws.