Nondestructive measurement of surface cracks using ultrasonic Rayleigh waves
Surface breaking cracks are one of the most feared defects in engineering structures. Many methods are available today to locate them. There are procedures for gaging the depths of deep surface cracks and some other techniques for measuring the density of microcracking close to the surface. Intermediate cracks, with depths from about 1 mm to 6 mm, however, have yet to be gaged accurately. Rayleigh waves, which represent a special case of surface waves, are very energetic close to the surface of a medium. Most of their energy is, in fact, concentrated in the first two wavelengths of penetration of the wave into the medium. Broadband Rayleigh waves contain a wide range of wavelengths at the surface. Since the different frequencies have different depths of penetration, the deeper portion of such a wave will be devoid of the higher frequency components. The research presented in this dissertation is concerned mostly with the use of the aforementioned characteristic of the Rayleigh wave to gage surface breaking fatigue cracks. If a discontinuity exists in the field of propagation of a Rayleigh wave, it will act as a filtering device for the frequencies or wavelengths that do not penetrate deeper than the flaw itself;All of the research reported here was conducted on steel specimens. To be able to work in the range of crack lengths of 1 mm to 6 mm, it was necessary to use frequencies between 0.5 - 5.5 MHz. The method proposed in this dissertation is not, though, reserved to metals. Applicability could be found in the fields of geophysics, to study faults and their extension into the earth crust, and to obtain crack depth measurements in any material capable of transmitting acoustic waves.