Micromechanical modelling of damage and failure in fibre reinforced composites under loading in the transverse plane

Fibre reinforced composites exhibit a gradual damage accumulation to failure, with a multitude of hierarchical dissipative mechanisms responsible for the deterioration of mechanical properties. In order to predict failure and gain a novel insight into the failure process, a micromechanics damage model was developed which predicts the onset and evolution of local microscopic damage mechanisms by representing the fibre and matrix phases discretely in the form of a Representative Volume Element (RVE). The micromechanical model was used to examine the influence of intra-ply properties on the mechanical behaviour of a high strength carbon fibre/epoxy composite under a range of loading scenarios in the transverse plane. The Nearest Neighbour Algorithm (NNA) was initially developed in order to create statistically equivalent fibre distributions for high fibre volume fraction composites. This technique uses nearest neighbour distribution functions, measured from the microstructure of a high strength carbon fibre composite, to define inter-fibre distances in the RVE. The statistical distributions, characterising the resulting fibre arrangements, were found to be equivalent to those in the actual microstructure, allowing for an accurate representation of the microscopic stress state. Damage was implemented by using a Mohr-Coulomb material model to predict the onset and evolution of matrix plasticity, while a cohesive zone model was used to examine the effects of fibre-matrix debonding. It was found that, under both transverse tensile and transverse shear loading, the fibre-matrix interface strength had a significant effect on the macroscopic strength, while the interface fracture energy had a marked effect on the strain to failure of the material. Interestingly, it was found that tailoring the properties of the fibre-matrix interface to a suitable level could promote the occurrence of certain sub-critical damage mechanisms, resulting in more favourable responses through a more effective dissipation of damage over the entire material microstructure. The presence of thermal residual stress had a significant influence on the interfacial stress state and, consequently, the onset of damage in the microstructure. However, its effect on the macroscopic strength was less pronounced. Under cyclic loading, the micromechanical model provided novel insight into the microscopic damage accumulation that forms prior to ultimate failure, and clearly highlighted the different roles that fibre-matrix debonding and matrix yielding play in forming the macroscopic response of the composite. Finally, the Composite Micromechanics (COMM) Toolbox was developed which provides efficient pre- and post-processing capabilities for micromechanical analyses of composite materials. It is thought that the COMM Toolbox will advance multiscale/multilevel modelling strategies towards more practical industry based implementation, as it could prove useful in determining application specific properties for composite material systems by numerically carrying out various parameter studies on variables such as cure temperature and/or interface strength, for example.