What is a typical observation when a metallic specimen has large grains?

• A. All of the above
• B. Decrease in back surface reflection
• C. Hash/noise indications
• D. Decrease in penetration

Answer: (A)

When grains in a metal are large compared to the ultrasonic wavelength, scattering at grain boundaries becomes strong and energy is not transmitted cleanly. This causes several observable effects in UT.

First, energy that would travel to the back wall is partially scattered away, so the back surface reflection is weaker. Second, the wave attenuates more as it propagates, reducing penetration and making deeper features harder to detect. Third, the scattering from many grain boundaries creates random, fluctuating signals—grain noise or hash—which degrades the clarity of the indication.

Because these three behaviors—weaker back-wall echoes, reduced penetration, and grain noise—often occur together when grain size is large, the most accurate conclusion is that all of these effects can be observed.