NDT Ultrasonic Testing 3 — Questions and Answers
Question 1: What is the significance of the 'acoustic impedance mismatch' at a material interface?
- It determines the electrical resistance of the transducer
- It controls the proportion of sound energy reflected versus transmitted at the interface (Correct answer)
- It sets the maximum testable thickness of the material
- It defines the frequency range usable for inspection
Correct answer: It controls the proportion of sound energy reflected versus transmitted at the interface
The greater the acoustic impedance mismatch between two materials, the higher the percentage of sound energy reflected at the interface rather than transmitted through it.
Question 2: In phased array ultrasonic testing (PAUT), what enables steering of the beam without physically moving the probe?
- Mechanical gimbals inside the probe housing
- Electronic time delays applied to individual array elements (Correct answer)
- Variable frequency modulation of the pulser
- Magnetic field adjustments around the transducer
Correct answer: Electronic time delays applied to individual array elements
PAUT steers and focuses the beam by applying precisely calculated electronic time delays to fire individual array elements in a controlled sequence.
Question 3: Which type of wave travels along the surface of a material and is useful for detecting shallow surface-breaking flaws?
- Longitudinal wave
- Shear wave
- Surface (Rayleigh) wave (Correct answer)
- Lamb wave
Correct answer: Surface (Rayleigh) wave
Rayleigh (surface) waves travel along the material surface with particle motion that is elliptical, making them sensitive to surface and near-surface discontinuities.
Question 4: What does 'DAC' stand for in ultrasonic testing, and what is its purpose?
- Distance Amplitude Correction — compensates for signal loss with increasing sound path distance (Correct answer)
- Defect Area Calibration — sizes flaws by their surface area
- Digital Analog Conversion — converts signals to a readable waveform
- Diffraction Angle Correction — adjusts for beam bending at interfaces
Correct answer: Distance Amplitude Correction — compensates for signal loss with increasing sound path distance
Distance Amplitude Correction (DAC) is a reference curve that accounts for natural signal amplitude decrease with increasing depth, enabling consistent flaw evaluation at all depths.
Question 5: When calibrating an angle beam transducer using an IIW block, which feature is used to set the beam index point (BIP)?
- The 50 mm radius curved surface (Correct answer)
- The rectangular notch at the top
- The 1.5 mm diameter side-drilled holes
- The graduated ruler on the side face
Correct answer: The 50 mm radius curved surface
The 100 mm (or 50 mm) radius curved surface of the IIW block is used to locate the beam exit point (BIP) by maximizing the reflection from the curved surface.
Question 6: In time-of-flight diffraction (TOFD) ultrasonic testing, what signals are used to detect and size flaws?
- Specular reflections from flaw faces
- Diffracted signals from the tips of a flaw (Correct answer)
- Backwall echoes that are blocked by the flaw
- Surface wave signals that circumnavigate the flaw
Correct answer: Diffracted signals from the tips of a flaw
TOFD detects and sizes flaws using diffracted signals from flaw tips; the time difference between upper and lower tip diffractions is used to calculate flaw height.
Question 7: Which factor causes 'mode conversion' when an ultrasonic beam strikes an interface at an angle?
- The difference in material density on either side of the interface
- The change in wave velocity between the two media causing the beam to split into different wave modes (Correct answer)
- The roughness of the interface scattering the beam into multiple directions
- The magnetic permeability difference between the two materials
Correct answer: The change in wave velocity between the two media causing the beam to split into different wave modes
Mode conversion occurs when a wave strikes an interface at an angle, causing some energy to convert from longitudinal to shear wave (or vice versa) due to velocity differences.
NDT Ultrasonic Testing 3
What is the significance of the 'acoustic impedance mismatch' at a material interface?