Studies on Vibration and Acoustic Response Characteristics of Sandwich Aerospace Structures

Abstract

Numerical investigation carried out on vibration and acoustic response characteristics of structures used in aerospace application is presented. Sandwich panels are used as structural members in aircraft due to their high stiffness to weight ratio. Vibro acoustic characteristics of sandwich panels with honeycomb, truss and foam filled truss core are analysed in this work. Equivalent 2D finite element model is used to obtain the free and forced vibration response of sandwich panels using commercial finite element solver ANSYS. Further, vibration response of the sandwich panel is given as an input to the Rayleigh integral code built-in-house using MATLAB to obtain the acoustic response characteristics. Initially, influence of important geometrical parameters on vibration and acoustic response characteristics of sandwich panels which are typically used as aerospace structures are investigated. Different types of sandwich panels analysed are (a) Honeycomb core (b) Truss and Z core and (c) Foam core. It is found that for a honeycomb core sandwich panel in due consideration to space constraint, the better acoustic comfort can be achieved by reducing the core height and increasing the face sheet thickness. It is also observed that, triangular core gives better acoustic comfort for the truss core sandwich panel compared to other types of core. Further, a sandwich panel with fibre reinforced plastic (FRP) facing and aluminium honeycomb core is investigated to analyse the effect of inherent material damping associated with FRP facing on vibro-acoustic response characteristics. The result reveals that FRP panel has better vibro-acoustic and transmission loss characteristics due to high stiffness and inherent material damping associated with them. It is observed that resonant amplitude of the vibro-acoustic response is significantly controlled by modal damping factors which is calculated based on modal strain energy. It is also demonstrated that FRP facing can ivbe used to replace the aluminium panel without losing acoustic comfort with nearly 40 % weight reduction. Effect of foam filling in empty space of the truss core sandwich panel on sound radiation and transmission loss (STL) characteristic is also studied. Results revealed that polyurethane foam (PUF) filling in empty space of the truss core, significantly reduces resonant amplitudes of both vibration and acoustic responses. It is also observed that foam filling reduces the overall sound power level by about 12 dB. Similarly, sound transmission loss studies revealed that, at resonance frequencies nearly 20 dB is reduced. In order to validate the accuracy of results, free and forced vibration response of a honeycomb core sandwich panel made of aluminium is obtained experimentally. The experimental results are compared with the proposed numerical results. From the results, it is observed that numerical and experimental results are in good agreement.