Material Conditions Affecting Eddy Current Flow (4)

Question 1

Two categories that affect Eddy Current flow:

Answer 1

• Material Conditions

- Probe Conditions

Question 2

Material Conditions

Answer 2

Electrical conductivity/resistivity
- skin effect
- atomic composition
- thickness
- heat treatment
- hardness
- part temperature
- magnetic permeability
Discontinuities
Shape
Surface Condition

Question 3

Electrical conductivity/resistivity - an increase in conductivity will:

Answer 3

increase in conductivity =

• increase eddy current density at the surface (due to skin effect)
• decrease in depth of penetration

Question 4

Skin Effect

Answer 4

the smallest surface impediment will have the greatest disruptive effect on the flow of eddy currents.
- increasing surface inspection sensitivity, at the loss of subsurface penetration and subsurface sensitivity.

Question 5

Atomic Composition

Answer 5

conductivity/resistivity will vary due to the number of openings in the valence energy level (permitting the flow of free electrons/electricity from one atom to another)
- less valence shell electrons = higher %IACS

Question 6

Thickness

Answer 6

thicker parts provide more area for eddy currents = less resistance.

• play a role in calculating base frequency needed
• millimeter units

Question 7

Heat Treatment

Answer 7

can increase material hardness, reducing EC strength or reduce internal stress, increasing flow and strength

• dependant on which type of heat treatment and quenching was done
• grain structure, orientation, and boundaries

Question 8

Hardness

Answer 8

increase in material hardness,
increases atomic bonding in the material,
increasing opposition to current flow,
reducing EC concentration and strength,
decreasing inspection sensitivity

Question 9

Part Temperature

Answer 9

increase in part temperature, increases erratic valence electron activity.

• increase in electrical resistance
• decrease in EC strength

Question 10

Magnetic Permeability

Answer 10

increase in magnetic permeability will increase signal noise and make inspection difficult to interpret.
Increase in permeability will reduce depth of EC penetration and sensitivity.
- DC saturation coils are used
- material can be heated above curie temperature
- use of remote field testing equipment

Question 11

Discontinuities

Answer 11

the larger the discontinuity, the greater the distortion of EC.

the greater EC density the smaller the discontinuity can be detected.

Question 12

Describe the ideal discontinuity Orientation compared to EC flow

Answer 12

We want indications to be 90° to EC flow.

Question 13

Shape

Answer 13

more complex shapes cause more disruption in the flow of EC, reducing strength and sensitivity.

Question 14

Surface Condition

Answer 14

smooth surfaces produce uniform EC concentration, increasing sensitivity and reducing signal noise.