Low velocity impact is a serious hazard for laminated composite structures. It can result in considerable loss of mechanical performance and must be taken into account during the design process. Extensive knowledge of the composite damage processes and advanced numerical simulation tools can help find optimal designs and reducing development costs.
In addition to normal service loads such as bending moments, shear forces, torques, pressure loads, etc. airframe structures also have to withstand impact loads resulting from hailstones, runway debris or tool drops during maintenance work. These impacts are likely to happen while the airframe is stressed under normal service loads. The superposition of service loads and impact loads is likely to alter the impact response of a structure compared to an unloaded structure. In this work, the influence of in-plane compressive loads on the low velocity impact response of carbon fibre epoxy composites is studied. Low velocity impact experiments on T800s/M21 UD carbon fibre epoxy laminates, under various compressive pre-strains, have been carried out with impact energies of up to 45J. The compressive pre-load applied to the structure was observed to significantly increase the impact damage and reduce the post-impact strength. To predict the damage resulting from impacts with and without pre-loads, a 2D damage model has been developed and implemented into the commercial finite element code ABAQUS/Explicit. The model is based on a combination of continuous damage mechanics and fracture mechanics with interactions between damage modes considered for both, damage initiation and damage propagation. Thereby damage degradation is following non-linear propagation laws. The model's material degradation is governed by the material's fracture toughnesses which are important material input parameters for the damage model. A detailed series of laboratory tests have been conducted to develop test set-ups for the measurement of translaminar fracture toughness values, which are used as input units for the damage model.