Multiscale characterisation of damage in laminated composite materials under bending loads

In-situ micro-mechanical testing of small composite laminates has been carried out under SEM where video recordings allowed examination of the evolution of damage processes at the microscale. Samples are tested under three- and four-point bending to encourage failure under various combinations of normal and shear stresses. SEM micrographs of the distinct phases of failure are presented and their transitions described, from initial debonding at fibre-matrix interfaces, through to catastrophic failure. For [90/0]s laminates, the transverse crack density is varied by altering the thickness of the exterior 90◦ ply block. This influences the distance between evenly spaced transverse cracks, which allows the relationship between transverse cracking and the resulting micro-delamination behaviour to be investigated. It was found that the process of micro-delamination coalescence is impeded for laminates with lower transverse crack densities, hence delaying catastrophic failure. A virtual experimental sub-modelling method is presented to numerically predict the cracking behaviour of real laminate microstructures, which can be compared directly with experimental results to validate the numerical modelling approach. The onset and evolution of matrix plasticity is modelled by implementing a Mohr-Coulomb material model, while the fibre-matrix interface region is represented by a cohesive zone model. Numerical models closely corresponded to experimental results, validating the modelling approach which provides a method for non-destructive prediction of cracking behaviour for real composite structures in the future. The Nearest Neighbour Algorithm (NNA) is used to generate statistically equivalent random fibre distributions for large microstructure cells, embedded into homogenised laminate models. These models are used for a parametric study of certain key cohesive properties. Optimised values for the cohesive interface strength and fracture energy were determined through comparison with experimentally observed transverse cracking behaviour. The resin rich ply boundary line often seen between adjacent plies is characterised and the NNA is modified to include boundary lines, increasing the realism of the microstructure representations for micromechanical models of multiple plies. This allowed the influence of resin rich lines on transverse cracking to be established. It was found that while the inclusion of resin rich lines of average thickness at ply boundaries influences crack growth in the early micro-cracking stages of the microstructure loading history, they do not significantly alter the overall macroscopic response of the microstructures to thermal or mechanical loading.