Constitutive modelling of composite materials under impact loading

In this thesis a constitutive model is developed for the numerical prediction of UD composite material behaviour under impact loading. Impact induced loading usually results in three dimensional stress states which significantly influences the failure behaviour. The heterogeneous nature of composite materials, in particular, results in a complex failure behaviour which manifests itself in various failure modes. Predicting the onset and evolution of these failure modes requires the use of physically based three dimensional theories for the prediction of the onset of damage and subsequent damage evolution. Furthermore, the use of polymeric matrices in continuous fibre reinforced composites results in a distinct directional strain rate dependent material behaviour which needs to be incorporated in constitutive models for the numerical simulation of impact events. The developed constitutive model relies on the prediction of the onset of damage evolution by the use of physically based three dimensional stress based failure criteria. A special feature of the proposed model is the identification of potential fracture planes. Numerically efficient algorithms for finding such planes are developed thus enabling the implementation into an explicit FE environment which was prohibitive so far. Damage evolution is simulated by degrading the tractions which are acting on the failure mode dependent fracture planes. The damage evolution and consequent energy dissipation is thereby driven by physically based dissipation potentials which consider only stresses which contribute to damage growth. The well known mesh dependent energy dissipation in Continuum Damage Mechanics is reduced by the introduction of an element size dependent parameter into the constitutive equations. An experimental program is conducted to investigate the compressive behaviour of composites. The focus of the study is on the rate dependent failure behaviour. The experiments are designed such that the failure mechanisms can be studied at varying strain rates with identical boundary conditions. This allows for direct conclusions about the strain rate dependent material behaviour. Novel optical measurement techniques are applied across all investigated strain rates thus ensuring an improved observation of the failure modes. The proposed constitutive model is finally verified by modelling of three point beam bending experiments which were performed quasi-statically and at impact velocities. The experimental technique for beam bending at impact loading was therefore improved thus yielding significantly more accurate experimental data.N.B. Some figures have been removed from this thesis for copyright reasons prior to it being added to ORA