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 pro portional to r / 1 for a given frequency (r=distance). In addition to the small attenuation, Ra yleigh waves are also non-dispersive and sensitive to surface-breaking defects. The surface penetra tion 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 trans ducer. Unfortunately, a number of wedges with different angles are needed to generate Rayl eigh 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, fab ricated, and tested narrow-band Interdigital Transducers (IDTs), where precision elect rode patterns were etched on piezoelectric ceramic substrate materials using micro laser machining t echniques. These IDTs can be directly applied to the surface of test specimens to generate Rayle igh waves in MHz range. A laser interferometry system was used to characterize the I DT sensors, where a well-defined and directional Gaussian beam profile was observed. Inves tigations were also made for Rayleigh waves interacting with fatigue cracks in aircraft qualifi ed metallic alloys. It was observed that some of the Rayleigh wave energy can propagate through a tight crack at both normal and oblique incidence.
Optoelectronic techniques for constructive characterization of SAW microdevices
The demand for high-frequency low-loss surface acoustic wave - SAW - filters for telecommunications and remote sensing has led to extensive research focusing on new SAW microdevices. SAW devices have been the subject of increasing interest since 1960 till present and the applicability of optical measurement techniques to detection of SAWs were published in the late 1960s. As a consequence, many types of both constructive and operating characterization methods have been developed. The paper provides a short introduction regarding the developed level of SAW microdevices. Some optoelectronic techniques used for constructive characterization of SAW micro-devices are reviewed. Three optoelectronic measurement techniques are known for detection of surface acoustic waves: diffraction grating technique, the knife-edge technique, and the detection of ultrasonic vibrations using optical interferometry. We describe the results of literature investigation research of optoelectronic techniques for constructive characterization of SAW microdevices. The research work given in this paper concentrates on describing of optoelectronic techniques used for constructive characterization of SAW microdevices. Samples of SAW quartz filters were measured. The input and output inter digital transducers (IDTs) contain 22 paired electrodes which were photolithographed on the quartz substrates (20mm x 18mm x 0.5mm). The space between electrodes of measured channel was 11.25 μm. Finally, we try to draw some conclusions where: optoelectronics tchniques are predicted to be the fundamental measurement methods for measurement of future SAW microdevices.
Observed properties of an acoustic-surface-wave multistrip coupler
The properties of an acoustic-surface-wave multistrip coupler constructed on YZ LiNbO3 are reported. The device was made by standard photolithographic techniques and consisted of 100 separate aluminium strips at a repeat distance of 16 μm. Over a bandwidth of at least 30% centred at 86 MHz, the coupling factor was less than 1 dB with a directivity of approximately 30 dB. Some applications of the coupler are discussed; in particular, it is shown that the device may be used as a filter to discriminate between bulk and surface acoustic waves.