In a contact ultrasonic test, the hash on the CRT display could be caused by which microstructural feature?

• A. Fine grains in the structure
• B. Dirt in the water couplant
• C. Coarse grains in the structure
• D. A thick but tapered back surface

Answer: (C)

Grain size directly affects how an ultrasonic wave scatters inside the material. Coarse grains produce more scattering at grain boundaries, creating many small, random echoes that interfere with each other. This interference shows up as a hash or speckled background on the CRT display in a contact test, especially at higher frequencies where scattering is stronger. Fine grains scatter less, so the trace remains crisper. Dirt in the water couplant can degrade coupling or attenuate signals but isn’t a microstructural feature causing the hash. A back surface that is thick and tapered changes reflections but doesn’t generate the grain-boundary scattering pattern responsible for the hash.