Focussing of bending waves in functionally graded composite structures
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This work presents a new approach using composite materials to design GRadient-INdex lenses (GRIN lenses). Vibrational energy focusing is of great interest in fields such as energy harvesting. GRIN lenses are systems in which a profile of mechanical properties allows waves to propagate along curved paths to focus on a specific area. Various design strategies and manufacturing processes of varying complexity have been explored to achieve the gradient properties required for focusing [1,2,3]. The strategy implemented here is based on the control of the fiber mass ratio of a unidirectional (UD) composite with a constant thickness. Various manufacturing processes for UD composites with low fiber mass ratio were implemented and compared to select an efficient process suitable for lens manufacturing and with acceptable uncertainties. Numerical computations demonstrate the efficiency of energy focusing in a frequency range from 2kHz to 8kHz. Within this design framework, the comparison between the numerical and experimental results on the manufactured lens structure has shown that the energy density in a defined focusing zone can be increased by a factor of 5 for an incident plane wave at 8kHz. Furthermore, designs based on other profiles of fiber mass ratio along the axis transverse to the direction of propagation could be provided to control wave propagation in order to satisfy different specifications.
