Influence of loading rate, alkali fibre treatment and crystallinity on fracture toughness of random short hemp fibre reinforced polylactide bio-composites

Plane-strain fracture toughness (KIc) of random short hemp fibre reinforced polylactide (PLA) bio-composites was investigated along with the effect of loading rate, fibre treatment and PLA crystallinity. Fracture toughness testing was carried out at loading rates varying from 0.5 to 20 mm/min using single-edge-notched bending specimens with 0 to 30 wt.% fibre. KQ (trial KIc) of composites decreased as loading rate increased, until stabilising to give KIc values at a loading rate of 10 mm/min and higher. The reduction of crazing and stress whitening, as well as a more direct crack path observed in PLA samples combined with reduced plastic deformation observed in composites provided explanation for this reduction. KIc of composites was found to decrease with increased fibre content and fibre treatment with sodium hydroxide. Studies controlling the degree of PLA crystallinity by heat treatment or “annealing” showed that reduction of KIc can be attributed to increased crystallinity.