Modeling the tensile strength of hemp fibers and short-hemp-fiber reinforced composites

Abstract

Ecological and economical considerations foster replacement of man-made fibers by natural renewable fibers in various industrial applications. Proper description of the scale effect of fiber strength is important for theoretical estimation of the load-carrying capacity of their products. Bast fibers of such plants as, e.g., flax, hemp and jute, are particularly attractive as reinforcement in polymer-matrix composites due to their high specific stiffness and strength. The elementary bast fibers exhibit pronounced strength scatter. This necessitates a probabilistic description of their strength via a distribution function that reflects damage morphology and severity in fibers. Fiber fracture is shown to originate from mechanical defects of the bast cell wall, the most prominent of them being kink bands. While the number of kink bands in a fiber is easily determined by optical microscopy, direct experimental measurement of their strength is complicated. Therefore, alternative approaches are sought, enabling extraction of strength characteristics of the kink bands from fiber tests via appropriate probabilistic models. An analytical distribution function of bast fiber strength has been derived, allowing for the effect of mechanical damage in the form of kink bands. The theoretical distribution is verified against experimental tensile strength data of elementary hemp fibers at several gauge lengths and found to provide acceptable accuracy in predicting the scale effect of strength. The fiber characteristics measured have been used to evaluate the kink band density and strength distributions. The fiber strength distribution is applied for predicting the tensile strength of short-fiber-reinforced composites.