A multi-scale case study on the variation of flax fiber properties: From single fiber and yarn to woven fabric and bio-based HDPE composites

Bio-based polymer composites and natural fiber reinforcements continue to gain interest as sustainable alternatives to synthetic, non-renewable composites while targeting similar or even superior mechanical performance. However, the high variability in the mechanical properties of natural fibers remains a primary concern for designers. This study compares the mechanical properties and underlying multi-scale variability in flax fiber material, from dry single fiber and yarn, to dry woven fabric and finally consolidated composite levels. For the composite level, samples were made of 95 % bio-based high-density polyethylene (G-HDPE) reinforced with 2 × 2 twill Flax Fiber (FF) fabrics and fabricated using compression molding. The coefficient of variation in the Young’s Modulus of G-HDPE/30 %FF (13 %) was significantly lower than that of flax single fibers (59 %), yarns (24 %), and fabric reinforcements (21 %), indicating a favorable reduction in property variability at higher levels of the application/design scale. Using 30 wt% of FF in G-HDPE enhanced the composite’s tensile strength by 225 % and Young's modulus by 250 % compared to neat G-HDPE, despite only a 10 % increase in density. Additionally, thermogravimetric analysis showed a low decomposition temperature for the composite (∼300 °C) due to the presence of flax fibers, as compared to the virgin matrix (440 °C), while differential scanning calorimetry revealed no impact on the melting and crystallization temperatures.