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.
Highlights
We can deduce that the temperature is an important factor for the determination of the ineffective length passing from the carbon to hemp fiber, because it generates a degradation of the mechanical characteristics
To illustrate the effect of the environmental conditions on the degradation of the ineffective area surrounded by the broken hemp fibers, in Figure 4, we present the evolution of the stress concentrations as function of the broken fibers for different variations of moisture concentrations and the humidity
We note that the longitudinal displacement for less than 5 broken fibers is less important while comparing by those of 40 broken fibers
Summary
The composites give an additional liberty for a designer to choose the material behaviour [1, 2] according to the type of fibers and resin, the rate of the reinforcement, the fiber orientations and the process of manufacturing. The two authors studied the damage of a structured fiber arrangement and where they determinated the effective length to estimate the tensile strength, while basing on the fiber shearing analysis oriented in unidirectional direction Their models do not take in account the effects of the stress concentrations in the adjacent fibers to those of broken fibers. An analysis of the process of longitudinal failure including the micromechanical interactions between natural fibers and the matrix is carried out by taking in account the effect of the environmental conditions on the degradation of the composite components This model is based on Gao and Reifsnider [12] which allow to predict the resistance and the durability of the unidirectional composite by using micromechanical techniques. The modified Gao and Reifsnider model was used to predict the tensile behaviour of hemp fiber/epoxy composite subjected to moisture and temperature aging
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