APPLICATION OF MODE-CONVERTED WAVES FOR THE IMAGING OF VERTICAL CRACKS FROM MULTIFREQUENCY DIGITAL ACOUSTIC HOLOGRAMS

A new technique for the high-resolution imaging of vertical cracks is described based on the digital coherent processing of data obtained by recording the field of shear waves generated as a result of mode conv ersion of an irradiating longitudinal wave at the face of the crack. T he data recording process is a modified delta method, which utilizes a moving longitudinal wave source and a scanning shear wave receiver. T he proposed method permits vertical cracks to be reliably identified i n samples having a highly reflecting bottom surface. The results of ex periments with models of cracks opening at the bottom, suspended crack s (which do not open at the testpiece surface), and fatigue cracks are given.