Durability of hybrid flax fibre-reinforced epoxy composites with graphene in hygrothermal environment

Natural fibre composites are highly sensitive to the hygrothermal environment (humidity and elevated in-service temperature). Enhancing the long-term behaviour of such composites can be achieved through additive manufacturing using nanomaterials as a constituent. Thus, this study investigated the mechanical properties of hybrid flax fibre-reinforced epoxy composites with 0%, 0.5%, 1% and 1.5% of graphene nanoparticles after exposure to a relative humidity of 98% for 1000, 2000, and 3000 hours and at temperature of 20 degrees C, 40 degrees C, and 60 degrees C. The degradation behaviour of hybrid natural fibre composites was then evaluated by flexural and interlaminar shear tests. Hygrothermal conditioning simulation of these hybrid composites was then performed using Arrhenius model based on accelerated aging data. The results of this study showed that graphene nanoparticles played a significant role in the reduction of moisture absorption and in the improvement of mechanical properties after hygrothermal conditioning. Compared to the specimens without graphene nanoparticles, the flexural and interlaminar shear strength in hybrid composites with 0.5%, 1.0%, and 1.5% graphene increased by 77.7%, 72.0%, and 77.1%, respectively, and up to 75.5%, 70.6%, and 73.5%, respectively after exposure for 3000 hours at 40 degrees C. However, the degradation of the hybrid composites increased with the increase of conditioning temperature and exposure duration due to the moisture diffusion into the flax fibres and resin plasticisation. Nevertheless, the 0.5% graphene nanoparticles were found optimal in retaining the mechanical properties of aged hybrid composites due to their better distribution within the matrix. Accelerated test results showed that the hybrid composites can retain at least 57% and 49% of its flexural and interlaminar shear strength, respectively, after 100 years in service in hygrothermal environment at a temperature of 30 degrees C representing the average annual temperature in Australia.