Effect of the environmental conditions on the ultimate load of damaged unidirectional of natural hemp fiber/epoxy composite

Abstract

Natural continuous natural fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibers. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fiber composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of natural fibers compared to glass fibers. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). The present paper proposes a strength reliability model for unidirectional composites with natural fibers in a hexagonal array. The model assumes that, a central core of broken fibers flanked by unbroken fibers which are subject to stress concentrations from the broken natural fibers. Thermal and hygroscopic residual stresses are neglected because they haven’t more effect when the composite was subjected to tensile than transverse loading. The approach of the model consists of using a modified shear lag model to calculate the ineffective lengths and stress concentrations around the broken fibers. In this paper, we attempt to incorporate in the proposed model the unidirectional composite property variation with temperature and moisture in order to predict even composite strength degradation. Strength degradation is often seen as a result of changes in ineffective lengths at natural fiber breaks and the corresponding stress concentrations in intact neighboring fibers.