NDT Ultrasonic Testing 5 — Questions and Answers
Question 1: What phenomenon is exploited when using Lamb waves (plate waves) in ultrasonic testing?
- Specular reflection from the top surface of thin plates
- Guided wave propagation through the entire thickness of thin plate structures (Correct answer)
- Diffraction from through-thickness cracks only
- Resonance of the plate at its natural frequency
Correct answer: Guided wave propagation through the entire thickness of thin plate structures
Lamb waves are guided waves that propagate through the full thickness of thin plates or shells, making them effective for long-range screening of large plate areas from a single probe position.
Question 2: In ultrasonic contact testing, what defect orientation is most difficult to detect with a standard normal-beam (0°) transducer?
- Planar flaws perpendicular to the beam (laminations)
- Planar flaws oriented at 45° or parallel to the beam path (Correct answer)
- Spherical porosity located at mid-thickness
- Shrinkage cavities in cast materials
Correct answer: Planar flaws oriented at 45° or parallel to the beam path
Planar flaws whose faces are parallel to the beam path (vertical cracks) reflect little energy back to a normal-beam transducer, requiring angled beams for reliable detection.
Question 3: What does the term 'signal-to-noise ratio' (SNR) mean in the context of ultrasonic testing?
- The ratio of transducer output voltage to instrument gain setting
- The ratio of flaw signal amplitude to the background material noise level (Correct answer)
- The ratio of transmitted to received pulse frequency
- The ratio of surface reflection to back-wall echo amplitude
Correct answer: The ratio of flaw signal amplitude to the background material noise level
SNR compares the flaw echo amplitude to the background 'grass' or noise level; a high SNR means the flaw signal clearly stands out from background noise, enabling reliable detection.
Question 4: When performing ultrasonic thickness measurement on a corroded pipe, which technique is best suited to measure minimum remaining wall thickness through internal corrosion pitting?
- Single-element contact testing with a fixed delay line
- Multiple-echo technique (echo-to-echo measurement) to ignore coating thickness (Correct answer)
- Through-transmission immersion testing
- Resonance frequency thickness gauging
Correct answer: Multiple-echo technique (echo-to-echo measurement) to ignore coating thickness
The multiple-echo (echo-to-echo) technique measures between successive back-wall echoes rather than from the front surface, eliminating the influence of coatings or surface irregularities.
Question 5: In full matrix capture (FMC) phased array UT, what data is collected compared to conventional PAUT?
- Only the highest-amplitude signal from each focal law firing
- The complete A-scan from every transmitter-receiver element combination (Correct answer)
- A single averaged waveform from all elements firing simultaneously
- Only backwall signals for thickness mapping purposes
Correct answer: The complete A-scan from every transmitter-receiver element combination
FMC collects a complete A-scan for every possible transmitter-receiver element pair combination, enabling post-acquisition reconstruction (TFM) and far superior imaging flexibility.
Question 6: What is the purpose of the 'reject' (suppression) control on a conventional UT flaw detector?
- To remove high-amplitude flaw signals exceeding the screen height
- To eliminate low-level noise and 'grass' from the A-scan display (Correct answer)
- To filter out signals from unwanted wave modes
- To suppress back-wall echoes during near-surface inspection
Correct answer: To eliminate low-level noise and 'grass' from the A-scan display
The reject (suppression) control clips low-amplitude signals below a set threshold from the display to reduce noise; however, it can also suppress small flaw signals if set too high.
Question 7: According to SNT-TC-1A, which level of UT certification is authorized to establish written procedures for ultrasonic examination?
- Level I
- Level II
- Level III (Correct answer)
- Level I with 3 years of experience
Correct answer: Level III
SNT-TC-1A Level III personnel are qualified to establish, review, and approve NDT procedures, methods, and techniques for their certified methods.
NDT Ultrasonic Testing 5
What phenomenon is exploited when using Lamb waves (plate waves) in ultrasonic testing?