Design of toughened composite sandwich structures

Abstract

Composite sandwich structures are used across a wide range of industries due to their exceptional stiffness and strength-to-weight ratios. These structures are brittle in nature and their failure is often more complex than the materials they replace. This research project considers methods aimed at increasing the damage resistance of these structures. Specifically, the work presented in this thesis shows that the impact performance of composite sandwich structures can be improved in a systematic manner. Initial impact tests were carried out to evaluate the resulting damage types. Following this, each aspect of the sandwich structure, the core, the skin, and the skin-core bond, was considered in an isolated manner. For each component, alterations aimed at improving the performance of the sandwich structure were devised. These toughening alterations were then tested under static conditions. The most successful alterations were a 3 wt% core shell rubber toughened skin matrix, a 2.5 parts per hundred resin 0.75 mm aramid fibre-reinforced foam core, and 1.5 mm deep 3 mm wide 13 mm spacing grid of grooves in the foam core. These alterations were then combined in sandwich structures that were subject to further impact testing. The result was a 26.5% improvement in back face deflection from 13.2 mm to 9.7 mm. Furthermore, an aramid fibre-reinforced foam core sandwich structure completely resisted perforation in a test where the control panel only absorbed 74.2% of the projectile energy. The key findings are that the toughness of all constituents and interfaces are important when considering the damage tolerance of a sandwich structure. If one element is toughened more than another, damage will likely be experienced in another element of the structure. An optimised structure will distribute damage in such a way as to maximise post-impact strength. It is considered that the techniques developed are very appropriate in optimising composite sandwich structures for transport, wind energy, and other applications. Throughout this thesis, further studies were carried out to gain a more complete understanding of the materials used. Epoxy foams were investigated over a range of densities, as a result, a transition in fracture performance has been identified. Architected polymers were studied with systematically varying bulk polymer properties. The outcome of this work has informed the design of toughened epoxy foams. Tensile testing of composite laminates was carried out to investigate the effect of adding tougheners on the tensile properties. Novel strain measurement techniques allowed micro cracks and small damage areas to be investigated. Tougheners were found to reduce the extent of these cracks and damage areas.