Estimation of longitudinal and transverse stiffness of hemp fibers and their composites using experimental material parameters and invariants

Abstract The use of hemp is becoming increasingly favored due to its low carbon footprint and biodegradability advantages combined with its low density, high specific stiffness, and strength. This paper aims to simplify, normalize, and present published experimental data on the stiffness of long unidirectional hemp composites to avoid dispersion and to provide new effective stiffness parameters for comparison with other composites. Trace normalized values of longitudinal, transverse, and shear moduli of untreated and treated long hemp composites were calculated based on over 100 tensile experimental values normalized to a volume fraction of 0.4% for comparison. The trace normalized longitudinal stiffness values of untreated and chemically modified hemp composites are 0.7 and 0.85, with a coefficient of variation of 4.8% and 6.8%, respectively. These are comparable with non‐natural composites such as carbon (0.88), Kevlar (0.88) and flax (0.78). Predicting moduli of composites and micromechanics has been used to obtain back‐calculations, leading to an average value for hemp fiber of 56.3 GPa longitudinal modulus, 6.8 GPa for transverse moduli, and 7.4 GPa for shear. The relationship between the longitudinal modulus and the U2 parameter was found to be linear, with a ratio of 0.42 and 0.48 for hemp and chemically modified composites, respectively. Due to the limited experimental data available, only preliminary compression results are presented. Highlights Simple estimation of transverse and shear modulus of hemp composites and fibers. Influence of treatment on the stiffness of hemp composites. Experimental material properties of hemp composites. Stiffness comparison between composites at a volume fraction of 0.4%.