On delamination directionality and migration in laminated composites

Abstract

Delamination is recognised as the most critical damage process in laminated composites under compressive or bending loading conditions and managing this failure mode has been key to using composites in primary structures. However, predictive modelling of delamination growth has been limited, since even a simple, single plane defect can result in multiplane delamination growth which is often associated to other secondary processes such as intralaminar or translaminar damage. The objective of this thesis has been to experimentally investigate the fundamental phenomena associated with delamination growth which were isolated from the observations made in embedded defects: delamination preferentially grows along the direction of the fibres at a ply interface (directionality) and, if forced to grow obliquely to the fibres, a change of delamination plane typically ensues (migration). Directionality was demonstrated with a bespoke test to grow delamination in a preferential direction. The geometrical effect of the fibres on the direction of delamination growth was also studied using a macroscopic model of the delaminating interface. A numerical strategy was introduced to model directionality. Cohesive elements and the virtual crack closure technique were modified to include the effect of fibre orientation at the interface at which delamination propagated. Migration was isolated with a series of delamination tests in cross-ply laminates under variable mode mixities. Finally, the insight gained in delamination growth mechanisms was demonstrated on a set of sandwich configurations with embedded defects to understand growth in complex composite geometries. Fractographic studies were used to glean a detailed understanding of the migration process. This work has successfully characterised the detailed processes by which delaminations grow and provided knowledge to develop damage tolerant designs.