Non-Destructive Testing (NDT) Practice Test

What characteristics of a test piece lead to eddy currents with little depth of penetration?

1. Low conductivity and low permeability

2. High permeability and high conductivity

3. High density and low conductivity

4. Medium conductivity and medium permeability

The correct answer is: High permeability and high conductivity

The choice indicating high permeability and high conductivity is correct because both of these characteristics influence the behavior of eddy currents in a conductive material. When a material has high conductivity, it allows electric currents to flow easily, which facilitates the generation of eddy currents. High permeability, on the other hand, relates to the material's ability to support the establishment of magnetic fields. In materials that are both highly conductive and highly permeable, eddy currents tend to develop but primarily remain near the surface, leading to a reduced depth of penetration. This is fundamental to the phenomenon of electromagnetic induction. When alternating current is applied to conductive materials, such as during eddy current testing, the resulting magnetic fields interact with the material's conductive properties and the material's electromagnetic characteristics. High conductivity results in stronger eddy currents, while high permeability tends to trap these currents closer to the surface, thus limiting their penetration depth. Therefore, the combination of these two properties directly results in eddy currents that have little depth of penetration in the material being tested.