In a straight beam ultrasonic test, when a discontinuity indication is small in amplitude compared with the loss in amplitude of back reflection, the orientation of the discontinuity is most likely:

• A. Parallel to the test surface
• B. Perpendicular to the sound beam
• C. Parallel to the sound beam
• D. At an angle to the test surface

Answer: (D)

In a straight-beam ultrasonic test, how a flaw appears to the probe depends on the flaw’s orientation relative to the incident beam. The strongest back echo comes from a discontinuity whose plane is oriented so that the reflected energy returns directly to the transducer—that is, roughly perpendicular to the beam. If the discontinuity is oriented parallel to the surface, the energy tends to be scattered along the plane or away from the transducer, giving a weak back echo. When the flaw is at an angle to the surface, a large portion of the reflected energy is directed away from the transducer, producing a small indication. If the observed indication is small compared with the overall loss of back-reflected energy, that points to the flaw being inclined relative to the surface, not normal to the beam. Therefore, the discontinuity is at an angle to the test surface.