Effect of moisture absorption and temperature on quasi-static and fatigue behavior of nonwoven flax epoxy composite

Abstract

This study investigated the effect of moisture and temperature on the evolution of the mechanical properties in the monotonic and cyclic loading of nonwoven flax epoxy composite.The effect of water uptake was investigated based on the monitoring of the tensile modulus during the first 100 h of the composite immersion, where a drastic decrease has preceded a modulus stabilization until the full saturation. A significant effect of the temperature was also observed, where the tensile modulus and stress were reduced at 75 °C by 57% and 53%, respectively.The coupled effect of moisture absorption and temperature on the fatigue behavior was investigated. A high increase in the first 10% of the sample life was followed by a linear trend until the final failure of the composite, where the increase of the stress level increases the slope of the linear stage. However, the strain trend was seen to be inversely correlated to the fatigue modulus and hysteresis energy. Again, a sharp decrease of both of them was observed in the early part of the fatigue test, which reflect that the damage activity is generally high during the first stage of cycling. The fatigue behavior of aged composite has shown a significant effect of the water uptake in the residual strain, which exhibit a short acceleration phase before the final failure of the composite. The observed trend was attributed to an induced irreversible plastic effect by water uptake, where the hysteresis energy was highly increased before the composite failure. The increase of the temperature to 50 °C seems to keep the global fatigue behavior of the unaged composite, even if the residual strain was doubled. However, a further increase of the temperature to 75 °C showed a more pronounced effect in both strain and modulus. Beyond all expectations, the increase of the temperature hasn't affected the global behavior of the aged composites, where only a slight increase of the hysteresis was developed and a more pronounced decrease of the dynamic modulus was observed at 75 °C.