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Abstract: Recently, considerable attention has been paid on the utilization of natural materials in structures. Utilizing natural materials over traditional, synthetic structures results in a reduction of carbon emissions from material synthesis; such a source of materials could also be renewable and recyclable. Currently, few reports exist on sound and vibrational properties in sandwich composites with the use of natural materials. Sandwich composites are commonly used in structures for their superior strength and stiffness-to-weight ratios, but from these same properties, they radiate noise efficiently. Here, in this study, the sound and vibration damping properties of natural material based sandwich composites were explored and characterized. It was experimentally observed that utilizing a balsa wood core with a natural fiber based face sheet has a 100% improvement in coincidence frequency, a metric of acoustic performance, and the combination of a natural fiber based face sheet with a Rohacell 51 WF synthetic core exhibits a 233% increase over a fully synthetic sandwich composite. As these improvements in acoustic performance are achieved with only small sacrifices in bending stiffness, these results suggest that, if optimized, natural material based sandwich composites could be an environmentally friendly solution to the sandwich structure-noise radiation problem. [Copyright &y& Elsevier]

Abstract: A sandwich composite is typically designed to possess high bending stiffness and low density and consists of two thin and stiff skin sheets and a lightweight core. Due to the high stiffness-to-weight and strength-to-weight ratios, sandwich composite materials are widely used in various structural applications including aircraft, spacecraft, automotive, wind-turbine blades and so on. However, sandwich composite structures used in such applications often suffer from poor acoustic performance. Ironically, these superior mechanical properties make the sandwich composites “excellent” noise radiators. There is a growing interest in optimizing and developing a new sandwich composite which will meet the high stiffness-to-weight ratio and offer improved acoustic performance. The focus of this study is to investigate the structural–vibrational performance of carbon-fiber face sheet sandwich composite beams with varying core materials and properties. Core materials utilized in this study included Nomex and Kevlar Honeycomb cores, and Rohacell foam cores with different densities and shear moduli. The structural–vibrational performance including acoustic and vibrational damping properties was experimentally characterized by analyzing the wave number response, and structural damping loss factor (η) from the frequency response functions, respectively. It was observed that the relationship between the slopes of the wave number data for frequencies above 1000Hz is inversely proportional to the core material’s specific modulus (G/ρ). The analysis also showed the importance of using a honeycomb core’s effective properties for equal comparison to foam-cored sandwich structures. Utilizing analytical modeling, the loss factors of the core materials (β) was determined based upon the measured structural loss factors (η) for a frequency range up to 4000Hz. It was determined that low shear modulus cores have similar material damping values to structural damping values. However as the core’s shear modulus increases, the percent difference between these values is found to increase linearly. It was also observed that high structural damping values correlated to low wave number amplitudes, which correspond to reductions in the level of noise radiation from the structure. [Copyright &y& Elsevier]