Ultrasonic shear wave technique for sensitive detection and sizing of small closed cracks

This paper describes an experimental investigation on the ultrasonic nondestructive evaluation of small cracks in the size range of 0.25 to 3mm. The cracks treated are 0.1 mm-wide EDM slits (as open cracks) and closed fatigue cracks situated vertically on the back surface of the stainless steel specimens. In order to develop a method for the enhancement of sensitivity of ultrasonic evaluation of small cracks, firstly, the investigating feature and capability of usual ultrasonic techniques, specially, the normal incidence, small-angle incidence and large-angle incidence techniques of longitudinal wave and the large-angle shear wave beam technique are examined based on the open cracks. Then, the effect of incident beam angle of the longitudinal as well as the shear waves on the crack corner echo response is investigated by using a single probe transducer. Finally, the principle of a new angle-beam approach is proposed, where the use of shear wave with incidence around 50degrees upon the specimen back wall is emphasized. Highly sensitive feature of the present angle-beam approach is realized, especially for small closed cracks, when compared with the above techniques.