Investigation of sound and vibrational performance of sandwich composite structures through wave number analysis
Date
2012
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Publisher
University of Delaware
Abstract
Sandwich structures are utilized in many applications for their superior mechanical performance including strength and stiffness-to-weight ratios compared to metallic structures. Unfortunately as a result of these mechanical properties, sandwich composites are also excellent radiators of noise beginning at low vibrational frequencies. However, in applications which require the materials with the highest mechanical performance at a low-weight, such as aircraft, rotorcraft, wind turbine blades, or automobiles, acoustic performance remains a secondary design requirement. Current state-of-art methods involve additional vibration and sound absorbing layers, which are costly and increase manufacturing time. Therefore, solutions are sought to achieve improved acoustic and vibrational performance without the sacrifice of mechanical performance or weight. This study employs a wave number approach to characterize the acoustic performance from the vibrational response of sandwich composite beams, along with their structural loss factor (η). It was first determined that the effect of changing the beam’s core thickness on the acoustic performance is non-linear, having a hyperbolic profile. Secondly, the core material’s specific shear modulus is inversely proportional to acoustic performance. Moreover, outstanding damping properties can help mitigate noise radiation over a broad range of frequency ranges, especially for frequencies under 1000 Hz. With such a characterization, the wave number and damping properties of natural material based sandwich structures was explored. The purpose of investigating such materials is that they provide environmentally friendly alternatives to traditional, synthetic sandwich composite structures. Utilizing a cotton or bamboo fiber face sheet with a Rohacell foam core achieved 100% improvement in coincidence frequency, while using the same face sheets coupled with a balsa-wood core showed a 233% improvement in coincidence frequency. All beams showed substantial reductions in wave number amplitudes, which correlate to the level of noise radiation. Finally, the use of a natural cork agglomerate as a core material showed unprecedented improvements in coincidence frequency; the coincidence frequency was not observed for vibrational frequencies up to 10 kHz. Along with such improvements, reductions in wave number amplitudes and increases in damping values were observed over all frequency ranges. Thus, it is concluded that a cork-agglomerate core sandwich composite could be a “noise free” sandwich structure with almost no compromise in flexural bending stiffness. Moreover, cork is another natural material which is not only a renewable resource, but only requires minimal carbon emissions during its fabrication and processing into an agglomerate.