For detecting discontinuities oriented along weld fusion lines, which beam configuration and wave type is typically most effective?

• A. Angle beam with surface waves
• B. Straight longitudinal waves
• C. Immersion with surface waves
• D. Angle beam with shear waves

Answer: (D)

Discontinuities that run along the weld fusion line form a planar flaw whose plane is parallel to the weld. To reveal such a flaw, you want energy that interacts strongly with that crack plane. A shear-wave in an angle-beam setup delivers particle motion perpendicular to the crack faces and, when launched at an angle, directs energy along the flaw such that the crack produces a strong reflected signal (often enhanced by mode conversion at the crack faces). This combination makes the echo from a crack along the fusion line most detectable.

Longitudinal waves tend to produce weaker reflections from cracks whose faces lie parallel to the weld line, and surface waves (including immersion surface waves) are more sensitive to near-surface features than to through-thickness, orientation-specific flaws. Therefore, angle-beam with shear waves offers the best sensitivity for defects aligned with the fusion line.