Basics (1)

Question 1

What current does Eddy Current testing use?

Answer 1

AC

Alternating Current

Question 2

Why are Eddy Currents named what they are?

Answer 2

they resemble the circling currents formed in flowing streams of water.

Question 3

Relationship between probe coil current and eddy current

Answer 3

Eddy currents flow in an alternate direction but cycle at the same rate (frequency) as the current in the probe coil.

Question 4

ET is a ____ test

Answer 4

conductive test

- between known and unknown material factors that affect the flow of eddy currents.

Question 5

What currents are flowing during a EC inspection

Answer 5

Primary field
- magnetic field generated by probe coil

Secondary field
- magnetic field generated by the eddy currents

Question 6

What is “Magnetic Coupling”

Answer 6

• between the primary and secondary magnetic field

- feeds information about the material back to the ET instrument via probe coil (electrical signal)

Question 7

Eddy currents cycle in the _____ direction to the primary current flowing in the probe coil

Answer 7

opposite direction

Question 8

Where are eddy currents the strongest?

Answer 8

at the surface of test material directly below the primary magnetic field that created them.

Question 9

Eddy current depth of penetration:

Answer 9

usually less than 10mm from the surface

Question 10

the stronger the test materials conductivity the _____ the eddy current concentration at the material surface

Answer 10

stronger the eddy current concentration at the material surface

Question 11

Secondary field direction is _____ to the direction of the primary field

Answer 11

opposite to the direction of the primary field

Question 12

What happens to an eddy current when the test material has an increased magnetic permeability?

Answer 12

an increase in material magnetic permeability will decrease eddy current penetration and create a third residual magnetic field that interferes with the primary and secondary field making instrument interpretation difficult

Question 13

What can eddy currents be used for?

Answer 13

• tiny surface and slight subsurface cracks and corrosion
• material sorting
• heat treatment and overheat checks
• hardness
• measure conductive and non-conductive coating thickness
• measuring thin material thickness
• test aircraft, nuclear and petrochemical components
• test heat-exchanger tubing
• “remote field testing” used in steel pipe inspections
• military use, detection of explosives and weapons
• treasure hunting, pay phones, vending and slot machines, activating traffic lights