We have developed physically-based failure models for different failure modes in composites, namely matrix cracking, fibre kinking and fibre tensile failure. These failure models were used in the participation of the group in the second World-wide Failure Exercise, a benchmark exercise where selected researchers were asked to provide blind predictions for several cases. The blind predictions from the models were ranked first both quantitatively and qualitatively.
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In 2002-2004, when developing the first regularised smeared-crack model for laminated composites, we needed ply-level translaminar fracture toughness values. At the time, these toughnesses did not exist even as a concept. Since then, we have developed and perfected experimental tests to measure translaminar toughness in tension and compression, for UD and woven plies, for plies with fibres at an angle, and we have demonstrated how the toughness depends on the thickness of the plies.
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By noticing the key role played by hierarchy in the fibre tensile failure mode in UD plies, the group developed the first analytical model in the literature that is capable of predicting the translaminar fracture toughness of UD plies. This model can be used not only for structural design, but also for material development, since the predictions are obtained from fibre / matrix / interface properties.
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When analysing the mechanical response of recycled composites composed of short fibres and bundles dispersed stochastically in a resin matrix, we noticed that the hierarchy of bundles of different sizes was key in determining the energy dissipated during fracture of these materials. These bundles, which were until then considered to be a manufacturing defect in recycled composites, became a material design parameter which can be used to achieve desired damage tolerance, in both recycled and virgin discontinuous carbon fibre systems.
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Nature does not design materials the way engineers and scientists do. There is much to benefit from learning why certain biological materials have evolved the way they did so that we can understand what failure mechanisms are responsables for their often outstanding properties. Equipped with this, we can then investigate whether we can create corresponding artificial materials that aim at exhibiting equivalent failure mechanisms.
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