Abstract
This paper describes a new ultrasonic method for sensitive evaluation of surface connected small closed cracks of size ranging from 0.3 to 3.5 mm in steel plates. Ultrasonic echo obtained by shear wave (SW) beam of 50° incidence in material from the back‐wall crack is used. First, the ultrasonic response of open crack is determined as a function of crack size. The crack‐corner echo of closed crack is analyzed in the perspective of crack closing stress, and a relation between the crack closing stress and the echo amplitude is determined. Based on the relation, both the crack size and crack closing stress of unknown closed cracks are determined by an inverse analysis of the response of the crack. The quantitative characteristic of the method is demonstrated here by evaluating crack size as well as crack closing stress of small closed surface cracks.