The use of head-type acoustic waves for ultrasonic monitoring

Longitudinal subsurface (head) elastic waves may be excited and used with inclined probes for ultrasonic monitoring. The waves detect defects under rough surfaces, such as screw threads and cracks under welds. They also measure the velocity of longitudinal waves in materials. Individually matched prismatic probes with an angle of inclination equal to the first critical angle may be used to excite and receive head waves. At a depth 4-10 mm from the surface the optimal sensitivity in defect detection has been found.