ACS Non-Destructive Testing (NDT) Methods 2 — Questions and Answers
Question 1: What is the primary principle behind eddy current testing?
- High-frequency sound waves are transmitted through the material
- Alternating magnetic fields induce eddy currents in conductive materials, and changes in those currents indicate defects (Correct answer)
- Ionizing radiation passes through the material to expose a film
- Fluorescent dye penetrates surface cracks and is revealed under UV light
Correct answer: Alternating magnetic fields induce eddy currents in conductive materials, and changes in those currents indicate defects
Eddy current testing works by inducing eddy currents in electrically conductive materials via alternating magnetic fields; discontinuities or material variations alter the eddy current flow, changing the coil impedance that is measured.
Question 2: What is a couplant used for in contact ultrasonic testing?
- To magnetize the test piece
- To eliminate the air gap between the transducer and the test surface, enabling sound transmission (Correct answer)
- To develop the penetrant indication
- To enhance fluorescent visibility under UV light
Correct answer: To eliminate the air gap between the transducer and the test surface, enabling sound transmission
A couplant (such as water, oil, or gel) fills the air gap between the transducer and test surface because ultrasonic sound waves cannot effectively transmit through air.
Question 3: On an A-scan ultrasonic display, what do the peaks (echoes) represent?
- Magnetic field intensity at various depths
- Reflections of sound energy from surfaces, boundaries, or discontinuities within the material (Correct answer)
- Electrical impedance changes caused by eddy currents
- Radiation intensity measured by the detector
Correct answer: Reflections of sound energy from surfaces, boundaries, or discontinuities within the material
On an A-scan display, peaks represent reflected ultrasonic echoes from surfaces or internal discontinuities; the horizontal axis shows time (depth) and the vertical axis shows signal amplitude.
Question 4: Which eddy current probe orientation is most effective for detecting cracks perpendicular to the surface on a flat plate?
- Surface (pancake) probe (Correct answer)
- Encircling probe
- Bobbin probe
- Reflection probe
Correct answer: Surface (pancake) probe
A surface (pancake) probe placed flat against the test surface is most effective for detecting surface cracks in flat materials because its coil orientation maximizes sensitivity to surface discontinuities.
Question 5: What effect does increasing the frequency of an ultrasonic transducer have on inspection capability?
- Increases penetration depth but reduces sensitivity to small defects
- Increases sensitivity to small defects but reduces penetration depth (Correct answer)
- Has no effect on penetration or sensitivity
- Increases both penetration depth and sensitivity equally
Correct answer: Increases sensitivity to small defects but reduces penetration depth
Higher ultrasonic frequencies produce shorter wavelengths, improving resolution and sensitivity to small defects, but are attenuated more rapidly, reducing penetration depth in the material.
Question 6: What type of material condition can eddy current testing detect that magnetic particle inspection cannot?
- Subsurface cracks in ferromagnetic steel
- Surface cracks in non-ferromagnetic alloys such as aluminum and titanium (Correct answer)
- Porosity in welds
- Delaminations in composites
Correct answer: Surface cracks in non-ferromagnetic alloys such as aluminum and titanium
Eddy current testing works on all electrically conductive materials, including non-ferromagnetic alloys like aluminum and titanium, whereas magnetic particle inspection is limited to ferromagnetic materials.
Question 7: In ultrasonic testing, what is 'back-wall echo loss' an indication of?
- Insufficient couplant application
- A large internal discontinuity or severe attenuation blocking the return of the back-wall reflection (Correct answer)
- Calibration error in the time base
- Surface roughness causing signal scatter at the entry point only
Correct answer: A large internal discontinuity or severe attenuation blocking the return of the back-wall reflection
Loss of the back-wall echo indicates that a large internal flaw or severe attenuation is preventing the ultrasonic pulse from reaching and reflecting off the far wall of the test piece.
ACS Non-Destructive Testing (NDT) Methods 2
What is the primary principle behind eddy current testing?