Estimation of the height of surface-breaking cracks using ultrasonic timing methods

This study was designed to obtain information on the accuracy and reliability of an ultrasonic timing method based on the transit time of the waves following various paths around the cracks. The study was confined to simulated cracks (machined slits). The bulk wave timing method includes a technique that uses the diffraction of longitudinal waves at the crack tip and a technique which uses the reflection of shear waves at the crack tip. The steel test blocks used for the study contained 0.5 mm-wide vertical slits of various heights ranging from 1 mm to 30 mm. Another set of blocks contained inclined slits (10°, 15° inclination) of various heights ranging from 5 to 20 mm. Both the vertical and inclined slits were open to the top surface. An ultrasonic flaw detector capable of measuring the beam path to an accuracy of 0.01 mm was used along with angled longitudinal beam probes and shear wave probes. Using the tip-diffraction technique, the smallest vertical slit of 4.82 mm was measured with an accuracy of 0.5 mm and the largest slit height of 30 mm was measured with an accuracy of 0.35 mm. However, the smallest inclined slit which could be estimated by this technique was 7.35 mm with an accuracy of 0.3 mm. Using a tip-corner timing method it was possible to estimate the vertical slit height of 1.82 mm with an accuracy of 0.22 mm and the inclined slit of 2.56 mm with an accuracy of 0.25 mm, whereas the largest slit height of 19.98 mm was measured with an accuracy of 0.52 mm.