Damage due to soft body impact on composite sandwich aircraft panels

Abstract

Due to high stiffness and strength to weight ratios, composite sandwiches are being increasingly used, particularly in aerospace applications. The main drawback of sandwich components is their relatively low resistance to impact damage. The assessment of damage due to impacts in composite sandwich panels and the component life reductions associated with such damage is important to the aerospace industry. This is because commercial profitability is reduced with aircraft delays. This study analyses minimum gauge skin thickness, non-metallic honeycomb wing panels subject to impact damage. In all instances the damage is caused by "soft body" impactors travelling at elevated velocities to simulate bird strike and other soft debris. The damage formed during these impacts is shown to be large in plan area but shallow in depth and primarily consists of a thin layer of crushed core. The degradation of honeycomb as it crushes forms a major part of this research study. This permits the formulation of a continuum damage model. The model is specifically developed to describe the compressive behaviour of honeycombs made from materials which are prone to elastic buckling. The material behaviour in compression is described by a combination of three distinct constitutive models namely elastic, inelastic and an elastic continuum damage model. This is interfaced with a commercial finite element package, LUSAS® and is used to model soft body impacts onto a minimum gauge Nomex™ honeycomb sandwich. The outcome of this work is the ability to evaluate several impact damage scenarios for various honeycomb sandwiches using a reliable numerical procedure. Experimental analysis of soft body impact damaged panels consists of loading post impact panel specimens under four point bending with the damaged zones in compression. These results provide estimates of the residual load carrying capacity and cyclic life of damaged components. Testing under quasi-static and cyclic conditions develops multiple compression wrinkles through the damage area. Propagation of one or more of these wrinkles leads to panel collapse. Close examination of the soft body impact damage zones identifies low amplitude wrinkles wavelength for wrinkling, are shown to precipitate the compression wrinkling mechanism. Collectively, the components of this research are applied to an actual on wing in service test case in the study of large scale BVID formed by tyre impact.