A Numerical Procedure for the Residual Compressive Strength Prediction of Impacted CFRP Laminates.

Today the use of composite materials in the aircraft industry is very widespread also for the primary components (wings, fuselage, etc.). However, low velocity impacts are still recognized as a debilitating threat since composites are not able to absorb energy by plastic deformation and damage, which may be undetectable by visual inspection and can grow and propagate during service leading to premature failure of the structure. This kind of behaviour has been confirmed by a number of works in the literature where impacted laminated materials presented a rapid and severe degradation of their tensile and compressive properties. For these reasons, the prediction of damage induced by an impact load and the prediction of the residual strength are of primary importance in the design of composite aircraft structures. The aerospace industry and the research community has traditionally used laboratory drop tests on coupon size specimens to explore the nature of impact damage and to seek ways to minimizing it. Compression after impact (CAI) tests are considered to be a crucial evaluation in the design process of composite primary structures. Indeed, design allowable strain limits of Carbon Fiber Reinforced Plastics (CFRP) are mainly defined by CAI tests which leads to very conservative design since the structural withstanding capability of the composite components is not fully exploited. Hence, a tool able to predict the residual strength of impacted laminates is needed in order to reduce the number of experimental tests which are very expensive and in order to reduce the conservatism of the design process. In this work, a numerical procedure aimed at predicting the compressive residual strength of impacted laminates and thus the knock down factor, is proposed. Such procedure consists of two main steps: prediction of the damage induced by the impact (fibre failure, matrix cracking and delamination) and prediction of damage evolution under the service loads. The first step belongs to the topic of damage resistance while the second step belongs to the subject of damage tolerance. These two problems are usually faced by using two different kinds of commercial FE codes. Indeed, impact analysis are usually performed by using explicit FE codes which are most suitable for the simulation of dynamic events, while the damage evolution is mostly studied by using implicit FE codes which are more reliable for the simulation of quasi-static event. The proposed procedure tries to create a link between the two disciplines with the aim of predicting the residual strength of an impacted composite laminate once known the impact conditions. The proposed tool first execute an explicit FE solver (LS-DYNA) to simulate impact and to generate a mapping of the damage. Using these results, the residual compressive strength is predicted by means of a nonlinear implicit progressive failure analysis performed by using the B2000++ FE code. A routine written in Ansys Parametric Design Language (APDL) was developed to exchange data between the explicit and implicit FE solvers. The proposed methodology was validated against experimental data (CAI tests). The residual compressive strength and the knock down factor were predicted with a very good accuracy.