Combined and isolated effects of humidity and temperature on monotonic and fatigue response of flax fibre/epoxy composites

Abstract

FRP structures are subjected to a combination of environmental and mechanical loads that act in an interactive way, determining service life. This study investigates the isolated and combined effects of in-situ temperature and relative humidity on monotonic and tension-tension fatigue response of two flax/epoxy laminates ([0/90/0]_S and [+45/-45]_2S), benchmarked against equivalent GFRP laminates. Particular emphasis was given to stiffness evolution, strain accumulation, and hysteretic behaviour particularly energy dissipation. Increasing temperature consistently reduced stiffness, strength, and fatigue life for both flax FRP laminates, leading to downward shifts and tilts of the S–N curves. The effect of moisture alone was laminate-dependent: elevated moisture content reduced stiffness, strength and fatigue life in the shear-dominated [+45/-45]_2S laminate, whereas the [0/90/0]_S laminate showed increased fatigue life attributed to enhanced ductility and increased laminate strength. Combined elevated temperature and moisture content lead to reduced monotonic stiffness and strength whilst their effects on fatigue life were cumulative. The largest effect was observed for the [+45/-45]_2S laminate, where fatigue life decreased by approximately three orders of magnitude. Across all hygrothermal conditions, energy dissipation was found to be an indicator of fatigue life with higher hysteretic energy dissipation per cycle correlated with reduced fatigue life. When assessed relative to baseline S–N behaviour, flax FRPs exhibit a proportional sensitivity to combined temperature and humidity comparable to GFRPs, indicating that flax composites are not disproportionately penalised under hot–wet fatigue loading.