NDT - Non-Destructive Testing Eddy Current Testing Theory Questions and Answers — Questions and Answers
Question 1: An inspector needs to detect subsurface defects in a thick aluminum plate. According to the principles of eddy current testing, which adjustment to the test frequency would be most effective?
- Increase the frequency to improve surface resolution.
- Decrease the frequency to increase the depth of penetration. (Correct answer)
- Use a multi-frequency setup to cancel out lift-off effects.
- Keep the frequency constant but increase the coil current.
Correct answer: Decrease the frequency to increase the depth of penetration.
In eddy current testing, there is an inverse relationship between the test frequency and the depth of penetration. Lower frequencies generate eddy currents that penetrate deeper into the material, which is necessary for detecting subsurface defects. Higher frequencies concentrate the eddy currents near the surface (the skin effect) and are better for surface flaw detection.
Question 2: The 'standard depth of penetration' in eddy current testing is defined as the depth at which the eddy current density decreases to approximately what percentage of its surface density?
- 50%
- 10%
- 37% (Correct answer)
- 75%
Correct answer: 37%
The standard depth of penetration (δ) is a key theoretical concept where the density of the induced eddy currents falls to 1/e (where 'e' is Euler's number, approx. 2.718) of its value at the surface. This is approximately 36.8%, commonly rounded to 37%.
Question 3: Which of the following material properties has the MOST significant influence on the magnitude and distribution of induced eddy currents?
- Tensile Strength
- Ductility
- Thermal Expansion Coefficient
- Electrical Conductivity (Correct answer)
Correct answer: Electrical Conductivity
Eddy current testing is fundamentally an electromagnetic method. The ease with which eddy currents can be induced and flow within a material is primarily determined by the material's ability to conduct electricity, i.e., its electrical conductivity. Variations in conductivity directly alter the impedance of the test coil.
Question 4: An inspector is using an eddy current surface probe on a flat plate and observes a signal change as the probe is moved slightly away from the surface. This phenomenon is known as:
- Fill Factor
- Lift-Off (Correct answer)
- Edge Effect
- Phase Lag
Correct answer: Lift-Off
Lift-off refers to the effect of the distance, or gap, between the test probe and the surface of the conductive material. Even small changes in this distance can cause significant changes in the coil's impedance, which must be accounted for during inspection. 'Fill Factor' is the analogous term used for encircling coils.
Question 5: On an impedance plane diagram, the phase angle (or phase lag) of a defect signal provides crucial information about the defect's:
- Length
- Orientation
- Depth (Correct answer)
- Width
Correct answer: Depth
The phase of the eddy current signal lags as it penetrates deeper into the material. Because of this, the phase angle of a signal from a disruption (defect) correlates to its depth below the surface. Surface defects will have a different phase angle compared to subsurface defects.
Question 6: When testing a ferromagnetic material, an inspector notes that the depth of penetration is very shallow compared to a non-ferromagnetic material with similar conductivity. This is primarily due to the ferromagnetic material's high:
- Electrical Resistivity
- Magnetic Permeability (Correct answer)
- Modulus of Elasticity
- Density
Correct answer: Magnetic Permeability
High magnetic permeability in ferromagnetic materials causes a much stronger magnetic field to be concentrated at the surface. This, in turn, generates stronger surface eddy currents that create a powerful opposing secondary field, rapidly reducing the overall field strength and limiting penetration.
NDT - Non-Destructive Testing Eddy Current Testing Theory Questions and Answers
An inspector needs to detect subsurface defects in a thick aluminum plate.
According to the principles of eddy current testing, which adjustment to the test frequency would be most effective?