Hygrothermal ageing effects on failure behaviour of fibre-reinforced polymer composite materials under in-situ SEM testing

This study aims to analyse the effects of hygrothermal ageing on mechanical properties, weight gain phenomena, and failure mechanisms of non-crimp fabric (NCF) based fibre-reinforced polymer (FRP) epoxy composites following immersion in two different aqueous media – deionised water and seawater at 35 °C until complete saturation. A comparative analysis has been performed between the effect of hygrothermal ageing on moisture absorption characteristics, flexural properties, and corresponding failure mechanisms of NCF basalt FRP and NCF E-glass FRP composites with similar fibre-volume fraction of approx. 54 %. Specimens were tested in-situ using a JEOL JSM-5600 scanning electron microscope (SEM) fitted with a 3-point bend test fixture. Overall, NCF basalt FRP composites demonstrated relatively lower moisture absorption characteristics than NCF E-glass FRP composites in deionised water and seawater. For both NCF basalt and NCF E-glass, moisture ageing significantly affected the flexural strength of composites. NCF basalt FRP demonstrated a reduction of approx. 30–35 % in flexural strength while for NCF E-glass, the reduction in flexural strength was relatively higher ranging to approx. 35–40 %. Flexural modulus of NCF basalt FRP was not significantly affected while for NCF E-glass, a reduction in flexural modulus by approx. 15 % was observed. However, during in-situ SEM testing, the failure mechanisms of moisture-aged NCF basalt and NCF E-glass FRP composites were similar, where failure initiated in the form of fibre/matrix debonding in the 90° sub-ply at the bottom ply on the tension side, but final failure took place due to fibre kinking in the 0° sub-ply of the top ply on the compression side.