What is the most useful range of incident longitudinal wave angles for angle beam ultrasonic testing?

• A. Zero to First Critical Angle
• B. First Critical Angle to Second Critical Angle
• C. Second Critical Angle to Third Critical Angle
• D. Above the Third Critical Angle

Answer: (B)

In angle beam UT, how useful an incident angle is depends on how the energy inside the material is transmitted and what kinds of wave modes appear. The material has different wave speeds for longitudinal (P) and shear (S) waves, and as you raise the incidence angle, the transmitted energy can change modes at certain boundaries called critical angles. The first critical angle marks the onset of a change in the interior wave mode, and the second critical angle marks another such transition. Above the second critical angle, energy tends to move along the surface as a surface wave, which reduces penetration and interior defect interrogation.

The range between the first and second critical angles provides a good balance: the wave inside the plate is a mode (typically a shear-type path) that interacts well with common flaw geometries and offers clear, well-controlled reflections for sizing. Below the first critical angle, the response is less sensitive to many flaws; above the second critical angle, surface waves dominate and interior detectability drops. That’s why this middle range is considered the most useful for angle beam inspections.