Ultrasonic spectroscopic studies of the interactions of Rayleigh waves with surface-breaking defects

Ultrasonic Rayleigh waves provide a depth-dependent probe to investigate surface breaking defects. This dissertation explores the use of broadband surface waves to examine crack-like defects which open to the surface of a material. An ultrasonic spectroscopic system has been developed to analyze the reflected and transmitted surface waves. This system was used to investigate the frequency-dependence of Rayleigh wave interactions with surface-breaking cracks and notches. Several analytical approaches to treat Rayleigh wave interactions with surface-breaking cracks have been analyzed. Experimental spectra agree favorably with predictions of exact and ray theories. Frequency-domain techniques for determining defect depth and length are examined. It is shown that features in the frequency spectra of the backscattered and forward scattered fields may be utilized to nondestructively infer crack depth. Additionally, an imaging technique for determining the location of even tightly-closed fatigue cracks is described.