Impact and fatigue properties of natural fibre composite

Low velocity impact and fatigue properties of hemp fibre reinforced polyester composites have been studied. The specific tensile properties and impact damage tolerance of hemp fibre composites were found to be very low compared to CSM glass fibre composites. However hybridisation of hemp with glass fibres even at low concentrations improved their tensile and impact properties considerably. Despite having poorer absolute fatigue strength, hemp fibre composites exhibited better fatigue sensitivity than CSM glass fibre composites in tension-tension fatigue. This correlated well with the lower stiffness degradation observed during fatigue of hemp fibre composites than glass fibre composites at the same normalised stress level. Also, images taken during fatigue loading showed that hemp fibre composites were better at resisting crack formation and growth than glass fibre composites. Alkalisation pre-treatment of hemp fibres at low concentrations of 1% and 5% NaOH solution resulted in improvements in tensile and fatigue properties of composites made from these fibres but no such improvements were observed for 10% alkalised fibre composites. The improvements were attributed to improvement in fibre/matrix bonding following this treatment which was also confirmed by SEM images. No improvement in impact damage tolerance was observed for any of these three alkalised fibre composites. Immersion of undamaged and impact damaged hemp fibre composites in water produced a degradation in tensile properties, particularly stiffness, and made them more ductile and susceptible to failure by shear. Most degradation occurred within the first 100 hours of immersion and longer immersion in water for up to 400 hours did not result in any further degradation, although there was some evidence of further reduction in strength over much longer periods. However, there was no deterioration in fatigue strength and fatigue sensitivity of hemp fibre composites following immersion in water, reinforcing the good fatigue properties of these composites.