CWI NDE Methods and Symbols 5 — Questions and Answers
Question 1: On an NDE symbol, what does the flag symbol (a small filled triangle at the arrow-reference line junction) indicate?
- Inspection is required in the field rather than in the shop (Correct answer)
- The weld requires all five NDE methods
- A full radiographic survey is mandatory
- The symbol applies to the entire joint length
Correct answer: Inspection is required in the field rather than in the shop
A filled triangular flag at the bend of the arrow indicates the NDE or welding operation is to be performed in the field at the erection site.
Question 2: What is 'acoustic impedance' in ultrasonic testing, and why is it important?
- It is the product of material density and acoustic velocity; mismatches cause reflections at interfaces (Correct answer)
- It is the frequency response of the transducer to mechanical vibration
- It measures the ability of couplant to transmit sound
- It is the maximum decibel level the transducer can generate
Correct answer: It is the product of material density and acoustic velocity; mismatches cause reflections at interfaces
Acoustic impedance (Z = ρv) determines how much sound is transmitted versus reflected at a boundary; large mismatches (like metal/air) cause near-total reflection.
Question 3: Which NDE method would be BEST suited to detect a lack-of-fusion defect in a thick-section weld when the defect may not be open to the surface?
- Ultrasonic testing (UT) (Correct answer)
- Liquid penetrant testing (PT)
- Magnetic particle testing (MT)
- Visual testing (VT)
Correct answer: Ultrasonic testing (UT)
UT can detect internal volumetric and planar defects such as lack of fusion because sound waves penetrate through the material and reflect from internal discontinuities.
Question 4: In radiographic testing, what is the effect of increasing the kilovoltage (kV) of an X-ray machine?
- It increases penetrating power but decreases contrast (Correct answer)
- It decreases penetrating power but increases contrast
- It has no effect on image quality, only on exposure time
- It increases both contrast and film speed simultaneously
Correct answer: It increases penetrating power but decreases contrast
Higher kV increases the energy and penetrating power of X-rays, allowing thicker sections to be radiographed, but it reduces subject contrast making subtle thickness differences harder to distinguish.
Question 5: When performing MT using the yoke method, how do you verify the yoke's lifting power meets code requirements?
- Lift a steel weight meeting the minimum specified mass (e.g., 10 lb for AC yokes per ASME) (Correct answer)
- Measure the magnetic flux density with a gaussmeter on the part surface
- Verify the yoke creates a visible spark when placed on the test surface
- Check that the yoke pole spacing equals the material thickness
Correct answer: Lift a steel weight meeting the minimum specified mass (e.g., 10 lb for AC yokes per ASME)
ASME Section V requires AC yokes to lift at least 10 lb and DC yokes at least 40 lb to confirm adequate magnetic strength before use.
Question 6: What is the purpose of the 'all-around' symbol (a circle at the arrow-reference line junction) on an NDE symbol?
- The NDE examination is required on all welds joining the members at that joint (Correct answer)
- The weld must be inspected from all sides simultaneously
- All NDE methods must be applied to that weld
- The symbol applies to all welds on the entire drawing
Correct answer: The NDE examination is required on all welds joining the members at that joint
The circle at the arrow junction indicates the examination applies to all welds around the joint connection, not just one specific weld.
Question 7: In Time-of-Flight Diffraction (TOFD) ultrasonic testing, what signals are used to detect and size defects?
- Diffracted waves from defect tips, compared to lateral wave and back wall timing (Correct answer)
- Reflected pulses from the defect face measured in amplitude
- Resonance frequency shifts caused by defect mass loading
- Phase reversal of the main beam at the defect boundary
Correct answer: Diffracted waves from defect tips, compared to lateral wave and back wall timing
TOFD uses the time difference between the lateral wave, tip-diffracted signals from top and bottom of a defect, and the back wall echo to accurately size defect height.
CWI NDE Methods and Symbols 5
On an NDE symbol, what does the flag symbol (a small filled triangle at the arrow-reference line junction) indicate?