Hygrothermal aging behaviour of VARTMed three-dimensional braided carbon-epoxy composites under external stresses

The hygrothermal aging of three-dimensional braided carbon fibre-epoxy resin composites prepared by vacuum assisted resin transfer moulding (VARTM) was investigated. Emphasis was given to the effect of external stresses, both tensile and compressive modes at stress levels of 10 and 20% ultimate tensile strength (UTS). Unstressed, tensile-stressed and compressive-stressed specimens were immersed in distilled water at 37 °C. Specimens were characterized by examining weight changes and evaluating variations of flexural and shear properties due to moisture ingress. It was concluded that the application of external stresses did not change the mechanism of moisture sorption. However, external stresses exerted apparent effects on moisture uptake, equilibrium moisture content and mechanical degradation. Moisture sorption was promoted and resisted, respectively under the actions of tensile and compressive stresses. The degree of mechanical deterioration of the composites depended on the level of moisture content. A mechanism which attributed the external stresses effects to changes of matrix strain, propagation of matrix cracks, and moisture transport at interfaces was proposed. Moreover, a comparison between the 3D and unidirectional composites was made to assess the influence of fibre structure.