Generally, the best ultrasonic testing method for detecting discontinuities oriented along the fusion zone in a welded plate is:

• A. An angle beam method using surface waves
• B. A contact test using a straight longitudinal wave
• C. An immersion test using surface waves
• D. An angle beam method using shear waves

Answer: (D)

To detect a discontinuity that runs along the fusion zone, you want energy that interacts most effectively with a flaw that lies in that weld plane. An angle-beam approach that uses shear waves does exactly that. When you send energy at an angle so it excites shear-wave components in the material, the shear motion aligns well with cracks or incomplete fusion that run along the weld. The energy is more readily reflected or mode-converted at the crack tip, producing a clearer, stronger echo from a flaw aligned with the weld. This combination—angle positioning to reach the weld region and shear waves that respond strongly to planar flaws along that plane—offers the best sensitivity for weld-oriented discontinuities.

Surface waves are largely surface-bound and don’t penetrate deeply enough to reliably detect flaws inside the fusion zone. Longitudinal waves in a straight, contact test aren’t as sensitive to cracks running parallel to the weld, so reflections from such flaws are weaker. Immersion with surface waves shares similar limitations and isn’t the typical choice for weld flaws. The angle-beam shear-wave method thus provides the most effective indication for defects aligned with the fusion line.