Enhancement of impact damage resistance and tolerance in CFRP laminates using a single embedded polyurethane film

The aim of this study is to develop composite structures of high impact resistance abilities. The PhD study involves the design, the manufacturing and the testing of unidirectional carbon fibre reinforced polymer (CFRP) composites with a single embedded polyurethane (PU) interleave to enhance the damage resistance to low velocity impact (LVI) and damage tolerance to quasi static indentation (QSI). The study involves three lay-up cases. The objectives of this study include the understanding of the dynamics and the overall behaviour of the manufactured laminates to low velocity impact damage. It is essential to understand how the damage develops and how the failure modes occurr in specific lay-up cases. Another objective is to draw a conclusion on how the use of a single PU interleave would affect the damage path, the failure modes and the overall behaviour of the laminates both under dynamic or static indentation. There is an increased interest in impact resistant laminates to LVI due to the increasing use of CFRP laminates in primary structures. Due to the nature of the study, it is crucial to deploy laminates with the least manufacturing void content, as voids can be a source of damage initiation that disguise the effect of impact-derived damage. For the manufacturing of the examined laminates, out-of-autoclave process was used. Compression moulding is a manufacturing process that leads to superior laminate quality compared to other manufacturing processes such as vacuum bagging. The study examines three laminate lay-up configurations: Case I, a cross-ply lay-up, Case 2, a quasi-isotropic lay-up, and Case 3 a bio-inspired lay-up. In each case, the inclusion of a single PU interleave caused significant improvements in damage resistance and damage tolerance. The compatibility of the PU interleave with the epoxy resin and the carbon fibres is crucial for the investigation of the interfacial damage. Thus, adhesion of the epoxy-PU bond was initially investigated. The depth position of the single PU interleave in the laminate on its impact response is the main focus of this PhD study. Each possible sub-case, featuring a different depth position of a single PU interleave, resulted in a different extent of damage area and different delamination mechanisms. Thus, both the laminate ply configuration and the position of the interleave resulted in particular damage patterns, which are discussed and analysed. The damage path is strongly dependant on the stacking sequence and the depth position of the PU interleave. Overall, the PU interleave is a positive inclusion in the laminate. It has enhanced the damage resistance of the structure under low velocity impact and the damage tolerance under static loading conditions. Depending on the application needs the energy absorbance can be altered by incorporating in specific locations a single PU interleave