MODELLING THE POSTBUCKLING BEHAVIOUR OF IMPACTED COMPOSITE AEROSTRUCTURES Paola Apruzzese†, Brian G. Falzon‡*, Robin Olssonm †Department of Aeronautics, Imperial College London, London, UK; ‡Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Australia; mSwerea SICOMP AB, Sweden * brian.falzon@eng.monash.edu.au SUMMARY Two approaches to modelling the effects of embedded defects and impact damage in composite aerostructures are presented. These differ in the manner in which the damage is represented; one as an equivalent delamination and the other as a soft inclusion with non-linear homogenized material properties. These techniques are applied to study the effects of defects and impact damage on the performance of composite panels. Keywords: impact damage, postbuckling, stiffened panel, delamination, soft inclusion. INTRODUCTION A major challenge in developing a reliable virtual testing capability for composite structures is predicting damage growth and its effect on structural performance. This work aims at developing such a capability which will allow the design of advanced composite aerostructures where damage growth is arrested prior to significant structural degradation. In this paper, two different approaches to model the effects of defects and impact damage are discussed. These are modelled as equivalent delaminations or as soft inclusions. The aim is not to accurately simulate all the events leading to catastrophic failure but rather to provide an estimate of the progressive stiffness reduction of the structure caused by defects and impact damage and subsequently determine the maximum applied strain that a structure can carry prior to significant damage propagation leading to final failure. IMPACT DAMAGE MODELLED AS AN EQUIVALENT DELAMINATION Among several forms of damage, induced by low-velocity impact, delamination is undoubtedly one of the most critical. Impact damage is usually characterized by multiple delaminations through the thickness. Delaminations location and dimensions depend mainly on the span-to-thickness ratio of the impacted plate and on the impact energy. The mechanical performance of composite materials can be drastically reduced in the presence of a delamination, in particular, the compressive strength may be reduced by up to 60%.