How can ultrasound be used to detect defects in a metal block?
Ultrasound is those waves which have frequency greater than 20 KHZ. Now, metal blocks are subjected to ultrasound at one end and detectors on the other side If the metal block does not contain any defect then ultrasound travels through and is detected by detectors. If the metal block has any defect, then from that region ultrasound is not detected clearly and rather it is reflected back indicating defects.
