Mechanical properties of asymmetric bouligand structured carbon fibre reinforced plastics

Abstract

This thesis adopted two novel bioinspired microstructural approaches for the purpose of investigating the effect of inter-ply pitch angles on the tensile and viscoelastic behaviours of composite structures. Two bioinspired concepts were investigated: asymmetric helicoidal structured composite and asymmetric discontinuous helicoidal structured composite.

The concept of the asymmetric helicoidal structured composite was drawn from naturally occurring armour such as impact resistant periodic region of the mantis shrimp's dactyl club and crab carapaces. Seven (7) pitch angles such as 0 (UD), 5 (ID5), 10 (ID10), 15 (ID15), 20 (ID20), 25 (ID25) and 30 (ID30) were employed to investigate their effects on the tensile and viscoelastic behaviour of asymmetric helicoidal structured composite. The composite structures were designed, manufactured, hygrothermally aged, tested and analysed.

The experimental and numerical studies show that asymmetric helicoidal structured composite achieved enhanced energy storage capacity and tensile strength at pitch angles 15 (ID15), 20 (ID20), and 25 (ID25) due to their fibre orientations within the composite microstructures. Also, they have fewer ageing effects than unidirectional composites in terms of percentage of water intake and deterioration of tensile strength, especially at higher ageing temperatures. Compared to unidirectional composites, bioinspired asymmetric helicoidal structured composite achieved approximately an 85% increase in energy storage capacity, 86% increase in total dissipated energy and 76% delay in tensile failure.

In the asymmetric discontinuous helicoidal structured composite, the concept was adopted from arapaima fish as a model to realise the toughening design of composite materials. The basic principle was to use discontinuous (cut) UD prepreg plies laid up in helicoidal architectures with combinations of minor and major inter-ply pitch angles (90o and 120o) to fabricate the composite laminates aiming to achieve higher modulus and tensile strength than continuous and aligned counterparts. Eight (8) pitch angles such as 5:90, 10:90, 15:90, 25:90, 5:120, 10:120, 15:120 and 25:120 were used to investigate their effects on the tensile and viscoelastic behaviours of asymmetric discontinuous helicoidal structured composite. The composite structures were designed, manufactured, hygrothermally aged, tested and analysed.

The results show that asymmetric discontinuous helicoidal structured composite achieved high-energy dissipation capability, better energy storage and improved tensile strength at pitch angle 5:120. Due to the increased tortuosity in the polymer phase and interfaces created by fibre orientations, asymmetric discontinuous helicoidal structured composite absorbs less water than unidirectional composite. The stiffness and strength of UD composite samples were adversely affected more than the asymmetric discontinuous helicoidal composite samples, as reduction of stress transfer capacity from the fibres to the matrix result from weakened interface composites, which are more likely to fail at lower stress levels. Increased tortuosity of the structure, therefore, decreases the interfaces that water can interact with, and we suggest that the discontinuous Bouligand structure limits the opportunity for fibre-matrix interface deterioration by decreasing the amount of water that can interact with the interfaces as the travel path for the water increased in a discontinuous Bouligand. We compared Hashin model predictions to fracture paths observed from experimental samples, which confirmed that some plies oriented close to 90o had pronounced failure compared to those oriented to 0o. The results established that asymmetric discontinuous helicoidal structured composite could be successfully tailored to enhance tensile strength and achieve higher energy dissipation and storage capability.