Experimental and numerical analyses of damage in laminate composites under low velocity impact loading

Prediction of damage in laminate composites due to low velocity impact is an important step in evaluating the service life of composite components. This is a combined experimental and numerical study aimed at improving the understanding of damage initiation and growth in composite laminates. The focus is on the real-time characterization of damage and explicit accounting of damage under conditions of low velocity impact. In the experimental part, a novel experimental technique for real-time monitoring of damage in the form of delamination is developed. A three-point bend configuration is used. The impact loading is achieved using a modified Split Hopkinson bar apparatus, with a full set of diagnostics for load, deformation, and input energy measurements. The experiment uses a dual laser interferometer system to detect delamination through time-resolved velocity and displacement measurements. In the numerical part, a framework for the simulation of impact deformation and explicit resolution of damage in the forms of inter-ply delamination and in-ply cracking is presented. This framework of analysis is based on the cohesive finite element method (CFEM). The model is used to study the effect of loading mode, interlaminar bonding strength, and material lay-up on the initiation and growth of damage.; This work shows that mode-II dominated loading induces higher rates of damage growth as compared to mode-I loading. In the case of mode-II loading, a complicated interaction is shown to exist between the formation of multiple matrix-cracks and delamination. The simulations show that the location at which the first matrix-crack forms is influenced by loading rate, specimen span and the interfacial bonding strength. Increasing inter-ply bonding strength decreases the damage rate and increases the energy release rate. It is observed that the damage mechanism changes at higher inter-ply bonding strengths, causing the energy release rate to decrease. Comparisons of high-speed digital photographs with results from simulations show that the model accurately predicts the deformation progression of damage. Finally, results from the new experimental approach and CFEM results agree well, indicating that the CFEM is a promising and powerful tool for predicting progressive failure in composite materials.