Damage-tolerant, delamination-free high-performance fibre-reinforced polymer composites

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The multi-scale, anisotropic structure of fibre-reinforced polymer composites (FRP) leads to complex failure behaviour. Thus, different types of failure occur and the matrix failure at the micro level influences the failure process at all higher levels of observation. Delamination refers to the layer separation (delamination) of differently oriented layers in an FRP and is a critical failure mode as it leads to a significant reduction in the load-bearing capacity of the laminates, especially under compressive loading or loads outside the laminate plane. Thin-ply refers to composites with a layer thickness less than 60 m. Quasi-static UD tensile tests on unnotched, multidirectional thin-ply laminates have shown that the initiation of matrix cracking is delayed until just before complete fracture, significantly increasing strength. However, suppression of matrix damage results in a significant reduction in the strength of notched laminates, as no degradation of stress concentrations due to delamination occurs. The suppression of matrix damage in thin-ply laminates also affects the resulting damage from low velocity impact loading. However, thin-ply laminates also allow completely new approaches to influence the failure behaviour due to the extended degrees of freedom in the design of the layer structure. Thus, a new approach to influencing the damage behaviour of FRP is the use of novel, nature-inspired layer structures. Promising are helicoidal layer structures found in impact-loaded body parts of crustaceans, so-called Bouligand structures.As shown in the applicant's preliminary work, the use of bio-inspired helicoidal ply structures can control the failure behaviour of FRP such that delamination is completely suppressed and only subcritical matrix damage occurs before ultimate failure:- Delamination-free ply constructions can be realised by thin-ply and small angles between the plies.- Layer build-ups with almost no notch effect can be achieved by thin-ply and small layer angles.- The residual strength after impact damage was improved by 15% compared to conventional laminate structures.However, questions also arise from the results:- How thin must the single ply be and how small must the angle between the plies be to take full advantage of the effects? It must be taken into account that these geometric factors are essentially determined by the stiffness of the fibres and the interlaminar energy release rate GIIc.- What are the critical angle and the boundary layer thickness at which the damage mode changes from subcritical matrix cracking to interlaminar delamination?This research project aims to fully understand the mechanical behaviour of helicoidal laminate structures and to model the behaviour in order to reduce the high experimental effort and thus make helicoidal laminates technically usable.