Answer 1

Ultrasound waves are made to pass through one face of the metal block and ultrasound detectors are placed on the opposite face of the block to detect the transmitted ultrasound. In doing so, we have two possibilities:
(i) If the ultrasound waves pass uninterrupted through all parts of the metal block, then the block is flawless.
(ii) If the ultrasound waves are not able to pass through a part of the metal block and get reflected back, then there is a flaw in the metal block.
Hence, the flaw can be detected without damaging the metal block.