Non-Destructive Testing (NDT) Practice Test

What happens to magnetic fields when AC is applied to a ferromagnetic material?

1. The magnetic field strengthens uniformly

2. The magnetic field penetrates deeper

3. Eddy currents are created, limiting depth of penetration

4. The material loses its magnetization

The correct answer is: Eddy currents are created, limiting depth of penetration

When alternating current (AC) is applied to a ferromagnetic material, eddy currents are induced due to the changing magnetic fields. These currents flow in closed loops within the material and generate their own magnetic fields, which counteract the applied field. This phenomenon limits the depth to which the magnetic field can penetrate into the material. The presence of eddy currents creates a barrier that effectively restricts the influence of the external magnetic field, leading to reduced magnetic field strength at greater depths in the material. This understanding elucidates why the development of eddy currents is a critical consideration in applications of non-destructive testing and magnetic inspection methods. The effect of eddy currents becomes especially significant in thicker ferromagnetic materials or at higher frequencies of AC. This relationship highlights the importance of understanding electromagnetic principles in evaluating the effectiveness and limitations of magnetic testing techniques.