Rayl

Abstract: An impulse technique has been used to investigate the effect of up to 40% by weight moisture content on the impedance of grassland, barren earth, and sand. Layering of the soil after the addition of water causes resonances in the impedance values at frequencies around 5 kHz which are found to be position dependent. Comparison with layer theory is made although the predicted resonances are much broader than the experimental peaks. The variation of the peak height of the reflected impulse has been used to determine the change of effective flow resistivity of the top surface with moisture content. This may change from 200 cgs rayl at 10% moisture to 1500 cgs rayl at 35% moisture content. Results taken over grassland and sand follow the same trend if normalized to their saturation value.

Abstract: 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.

Abstract: The present disclosure provides an acoustic composite. The acoustic composite includes a first porous layer having a flow resistance in a range of from about 100 Rayl to about 150,000 Rayl. The acoustic composite further includes a second porous layer having a flow resistance in a range of from about 100 Rayl to about 150,000 Rayl. The acoustic composite further includes a perforated membrane adjacent to at least one of the first porous layer and the second porous layer. The perforated membrane includes a first surface and a second surface opposed to the first surface. The perforated membrane further includes a patterned arrangement of a plurality of through-holes each independently extending from a first open end, the first surface including the first open end, to a second open end, the second surface including the second open end.