Impact and fatigue behaviour of hemp fibre composites

A series of experiments has been carried out to characterize the residual tensile and fatigue properties following impact of non-woven hemp fibre mat reinforced polyester. Additionally, the degradation of tensile modulus during fatigue cycling has been studied and related to the damage accumulation. For comparison purposes, ±45° glass fibre reinforced polyester samples have also been subjected to similar tests. It was found necessary to apply a relatively high pressure to the hemp composite during the curing stage in order to ensure a high enough fibre fraction to provide a significant reinforcing effect. With similar fibre weight fractions, the hemp and glass reinforced materials exhibited similar static tensile properties and fatigue lifetimes. Although the slightly steeper S–N curve of the hemp based material indicated a higher rate of reduction in fatigue strength with increasing cycles, it remained above the S–N curve for the glass based material showing that it was able to withstand slightly higher cyclic stress levels for equivalent numbers of cycles. The major difference in mechanical performance was the poorer resistance of the hemp based composite to impact. Also, the hemp based material failed in a much more brittle manner, without any visible signs of damage, such as the matrix cracking that was seen in the glass fibre based composite. It was found that, if the fatigue lifetime data of impact damaged samples were normalized against the post-impact residual tensile strength, then all data points lay close to a common S–N curve. This implies that residual fatigue lifetimes of damaged samples could be predicted from knowledge of their residual strength and the S–N curve for undamaged material.