Non-Destructive testing - Eddy Current testing

Question 1

What is the definition of Eddy currents?

Answer 1

Electrical currents which are generated in conductive materials by inducing an alternating magnetic field

Question 2

How do changes in the material effect the Eddy current?

Answer 2

Interrupt the flow of the Eddy current

Question 3

What type of material can Eddy Current testing be carried out on?

Answer 3

On all electrically conducting materials with a reasonably smooth surface

Question 4

What 3 components make up the equipment for Eddy current testing?

Answer 4

the test equipment consists of a generator (AC supply), a test coil and recording equipment

Question 5

What is detected using Eddy current testing?

Answer 5

• Crack detection
• Material thickness
• Corrosion detection
• Sorting materials
• Coating thickness measurements
• Metal detection

Question 6

Where are Eddy currents concentrated at in a material?

Answer 6

Near the surface adjacent to the excitation coil

Question 7

Eddy current density decrease exponentially with depth. What is the effect called?

Answer 7

Skin effect

Question 8

The STANDARD DEPTH OF PENETRATION is the depth when the Eddy current density has decreased to what percentage of the surface density

Answer 8

37%

Question 9

Name the 3 major probe types

Answer 9

• Surface probe
• Internal bobbin probe
• Encircling probe

Question 10

Name 4 advantages of Eddy Current testing (there are 8)

Answer 10

• Sensitive to small cracks and other defects
• Detects surface and near surface defects
• Inspection surface and near surface defects
• Inspection gives immediate results
• Equipment is portable
• Method can be used for more than flaw detection
• Minimum part preparation is required
• Test probes does not need to contact the part
• Inspects complex shapes and sizes of conductive materials

Question 11

Name 4 limitations of Eddy current testing (there are 7)

Answer 11

• Only conductive materials can be inspected
• Surface must be accessible to the probe
• Skill and training required is more extensive than other techniques
• Surface finish and roughness may interfere
• Reference standards need setup
• Depth of penetration is limited
• Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable

Question 12

Describe the principle of Eddy current testing

Answer 12

• When a AC passes through a test coil, a primary magnetic field is set up around the coil.
• The AC primary field includes Eddy current in the test object held below the test coil
• A secondary magnetic field arises due the the Eddy current
• The strength of the second field depends on the electrical and magnetic properties and structural integrity of the test object
• If cracks or other defects are present, the Eddy current and second field are both effected
• The changes in the secondary field will be ‘fed back’ to the primary coil and affect the primary current
• The variations of the primary current can be easily detected by a simple circuit which is zeroed properly beforehand