NDT Ultrasonic Testing Principles 3 — Questions and Answers
Question 1: In pulse-echo ultrasonic testing, what does the time-of-flight measurement directly indicate?
- The amplitude of the reflected signal
- The distance from the transducer to the reflector (Correct answer)
- The size of the detected discontinuity
- The attenuation coefficient of the material
Correct answer: The distance from the transducer to the reflector
Time-of-flight is the time for the pulse to travel to the reflector and return; knowing wave velocity allows calculation of distance to the reflector.
Question 2: What is the critical angle in angle-beam ultrasonic testing?
- The angle at which maximum signal amplitude is achieved
- The incident angle at which the refracted wave travels along the interface at 90° (Correct answer)
- The angle that produces maximum beam divergence in the far field
- The angle at which the transducer must be held to avoid surface wave generation
Correct answer: The incident angle at which the refracted wave travels along the interface at 90°
The first critical angle is the incident angle at which the refracted longitudinal wave travels exactly along the interface (90° refraction), leaving only a shear wave entering the second medium.
Question 3: Which scanning method uses a fixed transducer while the part moves, and is commonly used for immersion testing of plates?
- Contact scanning
- Raster scanning (Correct answer)
- Sector scanning
- Phased array linear scanning
Correct answer: Raster scanning
Raster scanning moves the part (or transducer) in a systematic grid pattern to ensure complete volumetric coverage, commonly used in immersion tank testing.
Question 4: In immersion ultrasonic testing, what is the recommended water path distance to position the interface echo?
- As short as possible to maximize signal strength
- Equal to the thickness of the part being tested
- Sufficient to place the interface echo before any back-wall echo
- Greater than the near field length of the transducer (Correct answer)
Correct answer: Greater than the near field length of the transducer
The water path should exceed the near field (Fresnel zone) length so the transducer is operating in the more uniform far field when the beam enters the part.
Question 5: What does a Distance-Amplitude Correction (DAC) curve compensate for in ultrasonic testing?
- Variations in couplant thickness across the test surface
- The decrease in signal amplitude from identical reflectors at increasing depths due to beam spread and attenuation (Correct answer)
- Differences in material grain structure between test specimens
- The frequency shift caused by the Doppler effect during scanning
Correct answer: The decrease in signal amplitude from identical reflectors at increasing depths due to beam spread and attenuation
DAC curves account for the natural decrease in echo amplitude from same-size reflectors as depth increases, establishing an amplitude reference that varies with distance.
Question 6: Rayleigh (surface) waves are most useful for detecting:
- Sub-surface volumetric flaws deep within heavy sections
- Laminar discontinuities parallel to the test surface
- Discontinuities open to or just beneath the surface of a component (Correct answer)
- Flaws located at the mid-thickness of a weld
Correct answer: Discontinuities open to or just beneath the surface of a component
Rayleigh waves travel along the surface and penetrate to a depth of approximately one wavelength, making them ideal for detecting surface-breaking or near-surface discontinuities.
Question 7: The beam spread half-angle of an ultrasonic transducer in the far field is primarily controlled by:
- The pulse repetition frequency (PRF) of the instrument
- The ratio of wavelength to transducer active element diameter (Correct answer)
- The impedance of the couplant used
- The gain setting on the UT instrument
Correct answer: The ratio of wavelength to transducer active element diameter
Far-field beam divergence is governed by the ratio λ/D (wavelength divided by transducer diameter); larger transducers or higher frequencies produce narrower, more focused beams.
NDT Ultrasonic Testing Principles 3
In pulse-echo ultrasonic testing, what does the time-of-flight measurement directly indicate?