ultrasonic testing

Ultrasonic testing sends a pulse of very high-pitched sound — far above hearing, millions of vibrations per second — into a part and listens for the echoes, exactly like a ship's sonar or a medical ultrasound of a baby. Sound races cleanly through sound metal but bounces off any interface it meets: the far wall, a crack, a void. By timing the echoes, you find not just THAT there is a flaw but HOW DEEP it lies.

A transducer pressed to the surface (through a drop of gel or oil so the sound couples in) fires a short ultrasonic pulse and then listens. Sound travels at a known speed in the material — about 5900 metres per second in steel — so an echo returning after a time t came from a depth of speed times t divided by 2 (halved because the pulse goes there and back). On the screen you see a 'back-wall echo' from the far side; an extra echo arriving earlier marks a flaw, and its position pinpoints the depth. Its height hints at the flaw size.

Ultrasonics is the go-to method for finding internal cracks and measuring thickness deep inside thick sections — welds, forgings, rails, pipe walls thinning from corrosion — where surface methods cannot reach. It is safe (no radiation) and gives depth information radiography does not. Honest limits: it needs a couplant and good surface contact, coarse-grained or very attenuating materials scatter the sound, a flaw lying parallel to the beam can be nearly invisible, and interpreting the echoes takes a trained operator.