Interaction of Rayleigh surface waves with a tightly closed fatigue crack

A miniature interdigital transducer (IDT) tuned to resonate at 3.1 MHz of frequency is designed, fabricated, and used to generate narrow band Rayleigh waves on aircraft grade metallic alloys. Electrodes of the IDT are precisely machined by a laser machining technique and its acoustical wave properties are characterized by using a scanning laser interferometry system with a spatial resolution of less than 10 microns. Unlike the conventional contact surface wave generation methods using wedges or combs, the biggest advantage of an IDT are its miniature physical size and its high efficiency in converting electrical signals to mechanical vibrations. Narrow band surface waves with finite amplitudes generated by an IDT are used to investigate the wave interaction with a tightly closed fatigue crack on a metallic test specimen. High-resolution images for the time-resolved ultrasonic field and time-averaged amplitude displacement are generated to understand the interaction of the wave with a surface-breaking fatigue crack. From the amplitude displacement data, three-dimensional surface contour plots of the wave energy are generated to find out how the elastic energy interacts with the tight fatigue crack interfaces. Results show that a tightly closed fatigue crack can transmit Rayleigh waves through the crack due to interfacial contact within the crack, where transmission of elastic energy was found to be more dominant towards the crack tip as expected. The sum of transmitted and reflected energy at the crack interface suggests that there is an additional energy loss mechanism which is proved experimentally by visualizing part of the incident Rayleigh wave energy propagating along the interfacial surfaces of the crack and continues its propagation along the surface of the opposite side of the test specimen.