NDT Ultrasonic Testing 4 — Questions and Answers
Question 1: What is the critical angle in ultrasonic testing?
- The probe angle that produces maximum flaw signal amplitude
- The incident angle beyond which total internal reflection occurs and no transmitted longitudinal wave enters the second medium (Correct answer)
- The angle of the weld bevel relative to the transducer axis
- The minimum scanning angle required for full weld coverage
Correct answer: The incident angle beyond which total internal reflection occurs and no transmitted longitudinal wave enters the second medium
The first critical angle is the incident angle at which the refracted longitudinal wave travels at 90° (along the interface), beyond which no refracted longitudinal wave enters the second medium.
Question 2: What is 'gate' in an ultrasonic testing instrument?
- A physical barrier placed between the transducer and the test piece
- An electronic time window set to monitor a specific depth range for signals (Correct answer)
- The connector port on the instrument for the transducer cable
- The frequency filter applied to suppress low-frequency noise
Correct answer: An electronic time window set to monitor a specific depth range for signals
A gate is an electronic time window on the A-scan display that triggers alarms or records signal amplitude when signals appear within the selected time/depth range.
Question 3: Which pulse duration characteristic has the greatest impact on ultrasonic axial (depth) resolution?
- Peak frequency of the pulse
- Pulse duration (wavelength count in the pulse) (Correct answer)
- Total acoustic power transmitted
- Pulse repetition frequency (PRF)
Correct answer: Pulse duration (wavelength count in the pulse)
Shorter pulse duration (fewer wavelengths per burst) gives better axial resolution because two closely spaced reflectors produce distinct, non-overlapping echoes.
Question 4: In automated ultrasonic testing of pipelines, what scanning pattern is typically used to achieve 100% volumetric coverage?
- Random spot-check pattern
- Helical (spiral) scanning pattern (Correct answer)
- Single-pass axial scan along the weld centerline
- Zigzag manual pattern by the operator
Correct answer: Helical (spiral) scanning pattern
Helical scanning wraps the transducer path in a spiral around the pipe, ensuring complete 100% volumetric coverage of the weld zone and base material.
Question 5: Why is a higher frequency ultrasound generally preferred for detecting small flaws?
- Higher frequency signals travel faster, reducing inspection time
- Shorter wavelengths improve sensitivity to small reflectors and enhance resolution (Correct answer)
- Higher frequencies penetrate deeper into coarse-grained materials
- Higher frequency reduces mode conversion at interfaces
Correct answer: Shorter wavelengths improve sensitivity to small reflectors and enhance resolution
Higher frequencies have shorter wavelengths, improving both sensitivity (ability to detect small flaws) and axial resolution, because a flaw must be at least half a wavelength to reflect detectable energy.
Question 6: What is the main limitation of the pulse-echo technique when evaluating a laminar flaw (planar flaw parallel to the test surface)?
- Laminar flaws reflect excessive energy that saturates the receiver
- Normal-beam pulse-echo may not detect laminar flaws if they are below the near-field zone (Correct answer)
- The technique cannot penetrate far enough to reach the flaw
- Laminar flaws cause so much beam scatter that no signal is returned
Correct answer: Normal-beam pulse-echo may not detect laminar flaws if they are below the near-field zone
Laminar (plate-parallel) flaws in the near-field zone may be difficult to resolve from the entry signal, requiring careful technique or special probes to reliably detect them.
Question 7: According to ASME codes, when recording UT indications for welds, an indication is typically recordable if its amplitude exceeds what threshold?
- 20% of the DAC curve
- 50% of the DAC curve (Correct answer)
- 100% of the reference reflector amplitude (DAC)
- 200% of the DAC curve
Correct answer: 50% of the DAC curve
ASME Section V typically requires recording all indications that exceed 50% of DAC, with further evaluation required for those exceeding 100% DAC.
NDT Ultrasonic Testing 4
What is the critical angle in ultrasonic testing?