MECHANICAL PROPERTIES OF HEMP FIBRE COMPOSITES

Hemp fibre reinforced polyester composite plaques were made using two layers of non-woven randomly oriented hemp fibre mat hand laid up in a conventional unsaturated polyester matrix and subjected to relatively high moulding pressure, giving fibre weight fractions of up to 56%. End-tabbed parallel-sided samples (125mm by 20mm by 2.5mm thick) were cut from the plaques. Tensile, impact and fatigue testing was carried out by using standard test methods. Incorporation of hemp fibres into the polyester resin resulted in negligible improvement in tensile strength of the neat polyester resin but almost doubled the tensile modulus to 7.2 GPa. Replacement of about 11% of the hemp fibres by glass fibres resulted in almost 50% improvement in tensile strength and 10% improvement in tensile modulus. The hemp fibre composites exhibited poor damage tolerance in low velocity impact. They lost almost 70% of their intrinsic strength and stiffness following and impact of 4J energy. However, hemp-glass hybrid composites showed considerable improvement in impact damage tolerance. 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 compared with glass fibre composites at the same normalised stress level