A mechanism-based modeling methodology has developed for prediction of long-term durability of composites for emerging facilities in different climatic zones. The objective of the research was to develop a predictive tool using the Arrhenius principles adapted to the TTS (Time Temperature Superposition) to measure degradation of carbon-fiber/epoxy composite under hygrothermal exposure and applied tensile stress. The hygrothermal conditions capture the synergistic effects of field exposure and extreme temperatures, viz., hot/dry, hot/wet, cold/dry, and cold/wet. Short term tests were performed to determine the flexural strength of environmentally aged composite specimens in accordance with ASTM D2344-84 and ASTM D7264 respectively. Carbon/epoxy specimens of [02/902]2s configuration were manufactured for flexure tests using Vacuum Assisted Resin Transfer Molding (VARTM). A unique strain fixture was designed to apply constant strain on the specimens during ageing and applied a simple methodology to eliminate excessive creep in the specimens. A two-dimensional cohesive layer constitutive model with a cubic traction-separation law has being developed in order to predict the life of the composite under hygrothermal conditions. The model simulated the test conditions and predicted the progressive failure mechanism of the specimen as observed in the tests, under various loading conditions. The model also incorporated synergistic interactions between temperature, moisture and stress effects and predicted degradation in strength and stiffness as a function of different ageing conditions and ageing times. Model predictions have been benchmarked using test data.