Interaction of Rayleigh Waves Induced by Interdigital Transducer with Fatigue Crack

Compared to bulk waves, Rayleigh waves can propagate much longer distances with lower signal attenuation levels, which are nominally proportional to r / 1 for a given frequency (r=distance). In addition to the small attenuation, Rayleigh waves are also non-dispersive and sensitive to surface-breaking defects. The surface penetration depth of Rayleigh waves is also approximately one wavelength, which can be beneficial for many NDT applications. However, one major drawback of using Rayleigh waves for nondestructive testing has been the difficulty of generating Rayleigh waves, where the most commonly used method involves the combination of a plastic wedge and a conventional longitudinal mode transducer. Unfortunately, a number of wedges with different angles are needed to generate Rayleigh waves in different materials or at different operating frequencies to satisfy the critical Rayleigh launch angle requirement. In this study, instead of using wedge transducers, we have designed, fabricated, and tested narrow-band Interdigital Transducers (IDTs), where precision electrode patterns were etched on piezoelectric ceramic substrate materials using micro laser machining techniques. These IDTs can be directly applied to the surface of test specimens to generate Rayleigh waves in MHz range. A laser interferometry system was used to characterize the IDT sensors, where a well-defined and directional Gaussian beam profile was observed. Investigations were also made for Rayleigh waves interacting with fatigue cracks in aircraft qualified metallic alloys. It was observed that some of the Rayleigh wave energy can propagate through a tight crack at both normal and oblique incidence.