In inspecting a 4 inch diameter threaded steel cylinder for radial cracks extending from the root of the threads, which wave transmission is preferable?

• A. Longitudinal waves from the end of the cylinder and perpendicular to the direction of the thread roots
• B. Shear waves at an angle to the threads
• C. Surface waves perpendicular to the thread roots
• D. Shear waves around the circumference of the cylinder

Answer: (A)

The most reliable way to detect radial cracks from thread roots is to use longitudinal waves traveling from the end of the cylinder along the axis, with the beam oriented so the propagation is perpendicular to the direction of the thread roots. This setup makes the crack plane, which runs radially, present a boundary that the incident P-wave encounters more or less head-on. The result is a strong impedance mismatch and a clear back reflection from the crack tip, making the flaw easier to detect.

Other wave types or orientations are less effective here. Shear waves arriving at the crack at an angle can suffer mode conversion and typically produce weaker reflections for this crack orientation. Surface waves travel along the exterior and walls near the surface, so they’re not as sensitive to subsurface radial cracks in the threaded region. Waves traveling around the circumference are less likely to intersect the radial crack plane at a useful angle for reflection.