NDT Eddy Current Testing Theory 3 — Questions and Answers
Question 1: Which of the following materials has the HIGHEST electrical conductivity, making eddy current testing most effective at low frequencies?
- Carbon steel
- Titanium alloy
- Copper (Correct answer)
- Stainless steel 316
Correct answer: Copper
Copper has electrical conductivity of approximately 58 MS/m, far higher than titanium, stainless steel, or carbon steel, allowing eddy currents to flow very freely.
Question 2: What is 'phase analysis' used for in eddy current testing?
- Determining the exact shape of a defect
- Distinguishing between different types of discontinuities and their depth (Correct answer)
- Measuring the surface roughness of the test piece
- Calibrating the instrument's voltage output
Correct answer: Distinguishing between different types of discontinuities and their depth
Phase analysis uses the phase angle of the eddy current signal to discriminate between defect types (surface vs. subsurface) and estimate their depth.
Question 3: An eddy current technician notices the signal drifts continuously during scanning of a large aluminum plate. The MOST likely cause is:
- A growing crack propagating during the test
- Thermal drift changing probe impedance as temperature varies (Correct answer)
- The calibration block material is different from the test piece
- The frequency is set too low for aluminum testing
Correct answer: Thermal drift changing probe impedance as temperature varies
Thermal drift occurs when probe or material temperature changes during scanning, causing a gradual impedance shift unrelated to defects.
Question 4: In the context of eddy current testing, what does 'fill factor' describe for encircling coil probes?
- The percentage of the coil wound with conducting wire
- The ratio of the test piece cross-sectional area to the coil's inside area (Correct answer)
- The fraction of the coil length occupied by the test piece
- The ratio of eddy current density at the surface to the interior
Correct answer: The ratio of the test piece cross-sectional area to the coil's inside area
Fill factor (η) is the ratio of the test piece's cross-sectional area to the coil's inner cross-sectional area, and a higher fill factor improves coupling and sensitivity.
Question 5: When testing nonconductive coatings over a conductive substrate, the eddy current technique can measure coating thickness because:
- The coating itself generates eddy currents that are measured
- Coating thickness changes the effective lift-off distance from the conductor (Correct answer)
- The coating alters the substrate's magnetic permeability
- Higher coating thickness increases eddy current frequency
Correct answer: Coating thickness changes the effective lift-off distance from the conductor
A nonconductive coating acts as a spacer, increasing the effective lift-off between the probe and the conductive base material, which is directly related to coating thickness.
Question 6: What is the characteristic impedance plane trace for a surface-breaking crack as probe scan speed increases?
- The signal amplitude decreases proportionally to speed
- The signal shape remains the same but rotates clockwise
- The signal may become distorted due to filtering effects at high speeds (Correct answer)
- The signal disappears above a critical scan speed
Correct answer: The signal may become distorted due to filtering effects at high speeds
At high scan speeds, the instrument's filter bandwidth may not keep up with rapid signal changes, causing waveform distortion and potentially missing narrow cracks.
Question 7: Which eddy current testing application most commonly uses a 'horseshoe' or 'U-shaped' probe?
- Bolt hole inspection
- Weld inspection on flat plates
- Fastener site inspection through multiple layers (Correct answer)
- Tube internal diameter scanning
Correct answer: Fastener site inspection through multiple layers
U-shaped (transmission) probes are used for multi-layer fastener site inspection, where the driver coil is on one side and the pickup coil on the other side of the structure.
NDT Eddy Current Testing Theory 3
Which of the following materials has the HIGHEST electrical conductivity, making eddy current testing most effective at low frequencies?