The flaws like internal cracks, etc. in the metal blocks are detected by using ultrasound, it is based on the fact that internal crack or hole in the block does not allow ultrasonic waves to pass through it. To detect a flaw in the metallic block, a source of ultrasonic sound is placed in front of one face of the block and a detector on the opposite face. The ultrasonic waves are produced and made to pass through one face of the block and detected on the opposite face.
(a) If the ultrasonic waves pass uninterrupted through all parts of the block, then there is no defect in the block.
(b) If the ultrasonic waves do not reach the opposite face, then the block has a defect.