Eddy current NDT

Question 1

What are the principle of eddy current testing?

Answer 1

• The presence of any defect in the volume inspected disturbs the circulation of these eddy currents and leads to a change in the apparent impedance
• NDT tests are performed by detecting on a screen the changes in the impedance
• the information collected is compared to a reference sample containing known defects

Question 2

What does the depth of penetration depend on?

Answer 2

• conductivity of the material
• magnetic permeability
• frequency of the current probe
• spatial relationship between the probe and object

Question 3

What is the standard depth of penetration?

Answer 3

The depth at which the current density is 37% of the surface depth

Question 4

What types of defects can eddy currents detect?

Answer 4

Defects and alterations that affect the magnetic properties of the material, its resistivity and the circulation of induced current into it.
Some examples are:
- cracks
- hidden corrosion damage
- measurement of thickness of material

Question 5

What are the advantages of eddy current NDT?

Answer 5

• Good sensitivity to surface defects
• Can do dimentional measurements (e.g defect depth)
• It is possible to have an automatic or continuous control of long products since the probe is not in contact with the part
• Safe

Question 6

What are the limitations of eddy current NDT?

Answer 6

• Only electrically conductive materials can be tested
• There can be alterations of the magnetization caused by previous NDT or plastic deformation
• The right selection of frequency and voltage, and the interpetation of signals needs to be done by a specialist