Analysis of compression failure in multidirectional laminates

Abstract

Even though the attractive properties of fibre-reinforced polymer composites have led to them becoming essential materials for a wide variety of high performance applications, they pose several drawbacks such as low compressive strength, low delamination resistance and high sensitivity to defects. These drawbacks have led designers to adopt a damage tolerance approach, whereby damage growth is deemed as failure. However, this approach has resulted in heavy composite structures with conservative configurations, which has somewhat negated the significant weight saving potential that composites offer over traditional materials. A step change in the approach by which damage growth is tolerated could provide designers with the freedom to develop novel composite structures. Despite an improvement in the understanding of composites failure, particularly in unidirectional laminates, compressive failure of multidirectional composites is still not fully understood. Therefore, the initial objective of this research project was to investigate the compressive failure processes multidirectional composites, leading to development of material-based approaches (i.e. introduction of a secondary material to the parent composite) which could offer compressive crack arrest/redirection. Such an approach would facilitate the adoption of a damage growth approach for composites design. An extensive experimental, fractographic, theoretical and numerical study on the compressive failure of multidirectional composites was conducted, resulting in the main failure mechanisms being identified and the sequence of events that lead to global fracture being deduced. The influence of the layup, specimen geometry (such as compact, plain and sandwich panel compression) and the proportion of shear loading on the compressive performance of multidirectional laminates were characterised. These observations were then used to validate numerical models, thus yielding more physically based predictions. In the process of formulating novel crack arrest/diversion solutions in composite structures, various concepts ranging from hybridisation to carbon nanotubes and piezoelectric actuators, were investigated. However, after consideration of the relative maturity of these technologies and the time constraints, the latter two approaches were not pursued. Given the absence of an explanation of the hybrid effects observed in composites in the literature, an extensive study was carried out to investigate the effect of hybridisation on the compressive performance of multidirectional composite laminates. For this study, two systems of unidirectional pre-preg tapes with the same epoxy resin but different carbon fibre types and tow sizes were employed. It was identified in this study that hybridisation of selective ply interfaces influenced the location and severity of the fracture mechanisms. Finally, in a complementary study on delamination fracture toughness of hybrid composites, a significant improvement was observed in the delamination resistance (doubling in Mixed Mode I/II toughness) compared to the monolithic composites, indicating that the behaviour of the hybrid interfaces was critical for the compressive performance of the hybrid laminates.