Investigation into Mode II interlaminar fracture toughness characteristics of flax/basalt reinforced vinyl ester hybrid composites

In this work, the influence of water absorption of flax and flax/basalt hybrid laminates is presented with the aim to investigating the Mode II interlaminar fracture toughness. Four types of composite laminates namely, neat vinyl ester (neat VE), flax fibre reinforced vinyl ester (FVE), flax fibre hybridised basalt unstitched (FBVEu) and flax hybridised basalt stitched (FBVEs), were fabricated by vacuum assisted resin infusion technique. Three-point-end-notched flexure (3ENF) tests were performed to evaluate the critical Mode II strain energy release rate (GIIC) and the crack length (R-curve) at dry and wet conditions, by using two data reduction methods. The morphology of delamination and the fracture shear failure of composite laminates were evaluated using scanning electron microscopy (SEM) and X-ray micro computed tomography (μCT). The results obtained that the fracture energy of FBVEu composites, GIIC init. and GIIC prop. were increased by 58% and 21%, respectively compared to that of FVE dry specimens. Moisture absorption phenomenon caused increasing in the ductility of matrix that improved the resistance to crack initiation. However, this was inverted to a reduction in the fibre/matrix interfacial strength of FBVEu wet composites and a deterioration in the delamination resistance to crack propagation. The critical strain energy release rate of neat VE increased from 157.84 J/m2 to 239.85 J/m2 with reinforcement of flax fibre composites. The experimental results confirmed that basalt fibre hybridisation enhanced the durability and water repellence behaviour of natural fibre reinforced composites.