Fibre Volume and Strength
Fibre volume plays an important role in composite strength and ductility. Critical fibre volume is necessary to induce pseudo strain hardening behaviour and multiple cracking. The following equation can be used to estimate the critical fibre volume in a composite containing parallel-aligned fibres:
Where ofu and omu represent the ultimate strength of fibre and matrix. Ef and Em refer to the Young’s moduli of the fibre and matrix [67, 74]. The critical volume content of a composite that contains fibres organised in a random (isotropic) discontinuous manner can be expressed as follows:
Where Jc is the composite crack tip toughness, g is the snubbing factor, т is interface frictional bond strength, Lf and df are the fibre length and diameter and 80 is the crack opening when the fibre bridging stress reaches maximum. Compared with random oriented fibres, aligned fibres requires lower critical fibre content and low bonding strength requires with high fibre content at fibre-matrix interface [67, 71].
The interfacial strength (transfer of load between the fibre and matrix) can be determined by single fibre pull-out test or using a uniaxial tensile test where the tensile stress vs. the opening relationship is continuously monitored [72, 75-77]. Various methods have been developed to improve interfacial strength, e.g. coatings to the fibres and introducing functional groups on the fibre surface [78-80].