An NDT technician is performing an ultrasonic inspection on a 50 mm thick carbon steel plate with a butt weld. During the scan, they detect an indication that appears as a strong, high-amplitude reflection at a specific depth, which significantly differs from the expected back wall echo. The technician suspects a planar defect oriented perpendicular to the sound beam. What type of discontinuity is most likely indicated by this ultrasonic signal?

Question: An NDT technician is performing an ultrasonic inspection on a 50 mm thick carbon steel plate with a butt weld. During the scan, they detect an indication that appears as a strong, high-amplitude reflection at a specific depth, which significantly differs from the expected back wall echo. The technician suspects a planar defect oriented perpendicular to the sound beam. What type of discontinuity is most likely indicated by this ultrasonic signal?

Answer options:

• Volumetric porosity
• Slag stringer ✅ Lack of fusion or crack
• Weld reinforcement

Correct answer: Lack of fusion or crack

Explanation: A strong, high-amplitude reflection differing from the back wall echo in UT often indicates a sharp, planar defect like a lack of fusion or a crack, especially if the reflector is oriented favourably to the sound beam. Volumetric porosity and slag stringers tend to produce broader, lower-amplitude signals compared to sharp planar defects.