Development of a durability model applied to unidirectional composites beams reinforced with glass fibers

The interactions between hygrothermal ageing and the flexural fatigue behavior of unidirectional glass/epoxy composites are investigated within the frame of a Stress Corrosion Cracking model. The study is focused on the initial damage development stages, i.e. when the degradation mainly consists in the non interactive accumulation of delayed fibers failures at the microscopic scale. These processes are described on the basis of a knowledge of the initial fibers surface defects distribution and from the determination of the subcritical crack growth rates of these surface flaws under the combined action of moisture and stress. The stress corrosion behavior of the glass reinforcement is established experimentally using successively unimpregnated fibers bundles and microscopic observations of a composite elementary volume under tensile stresses. An empirical relationship between the broken fibers density and the flexural stiffness allows to predict the macroscopic stiffness loss from the calculation of the fibers survival probability. The model accounts for the activation of the stiffness loss kinetics as a function of the fatigue (strain level, frequency, strain ratio) and environmental (temperature, humidity) parameters. Theoretical predictions are compared to experiments carried out under monotonic, relaxation and cyclic fatigue loading.