Production of quasi-unidirectional woven fabrics from water-retted hemp fibers and mechanical characterization of their composites

Natural fiber reinforced polymer composites have attracted great attention in the past few decades due to increasing environmental awareness and various advantages of these materials in terms of mechanical properties, sustainability, renewability and biodegradability. In this study, hemp rovings with various amounts of twist were prepared using water-retted continuous hemp fiber bundles. Then these rovings were used to produce quasi-unidirectional (quasi-UD) woven hemp fabrics. These fabrics were utilized to manufacture epoxy composites. Tensile and flexural tests were carried out to determine the mechanical properties of the developed composites and the effect of yarn twist. The fracture pattern of the composites was investigated using scanning electron microscopy (SEM) analysis. The tensile strength of the composite with 45 turns/m twisted rovings was approximately 20% lower than that of the composite sample with zero-twist hemp rovings. Composites with a twist level of 45 turns/m also showed approximately 37% and 48% lower flexural strength and modulus values respectively when compared to composites with zero-twist yarns. This was attributed to the fact that the yarn twist causes the fibers to align at an angle to the loading direction resulting in reduced properties in the axial direction of the composites as well as fiber damage induced by twisting. SEM observations showed that the composite failure took place predominantly as total fiber fracture, fiber pull-out from the yarns and matrix cracking. There were also limited cases of fiber pull-out from the matrix.