Modelling damage due to low speed impact in laminated composites of toughened interfaces through 'ex situ' technique

Laminated composites are widely employed in the aerospace industry for their high performances. One of the major drawbacks in using these materials for aerospace applications has been their weaknesses in resisting lateral impacts which tend to induce damage in laminates. Delamination is one of the major forms of damage which compromise the integrity of the laminates as load carrying structures. To tackle this issue, the 'ex-situ' toughening technique, was developed by Yi et al [1,3] in Beijing Institute of Aeronautical Materials (BIAM), China and has since been applied successfully to processing laminated composites made from prepregs and later to those made using RTM. Experimental evidences have demonstrated the toughening effects of such technique. Effects of toughening are reflected mainly through critical energy release rates for mode I and II, G_Ic and G_IIc, which to a large extent dictate the impact damage resistance of the laminates. These properties were obtained through standard tests, namely, double cantilever beam (DCB) and end-notched flexure (ENF) delamination tests for mode I and II, respectively. Three types of laminates made through RTM were used, neat BMI (non-toughened) as the control and 'ex-situ' RTM-16.8% and 20.2% PAEK, respectively, in the experiments conducted by BIAM. This paper is to present the capability of theoretical modelling the delamination damage of 'ex-situ' toughened laminates due to lateral impact using finite elements with appropriate treatments. Models representing the DCB and ENF specimens were generated to reproduce the results of these tests, and the predicted critical energy release rates is shown to agree well with the input values. The numerical modelling for these cases facilitated the development of FE model simulating the standard mixed mode tests, the prediction for which are also given in the paper.