Life prediction and stiffness degradation modeling of glass/epoxy composites subjected to flexural fatigue loading

Textile composite are extensively used as structural materials for automotive, aerospace, energy, transportation and construction applications. During their service life these structures are subjected to different types of static and cyclic loading. For structural health monitoring of these structures, it is important to know the fatigue life and damage occurred at any stage of the life of the structure. Fatigue life is generally estimated using suitable life prediction model, while fatigue damage can be predicted by monitoring measurable damage parameters such as stiffness and strength. Two mathematical models namely fatigue life prediction model and stiffness degradation model are proposed for plain weave glass/epoxy composite subjected to flexural fatigue loading. Three different functions namely linear, exponential and sigmoid are evaluated to represent S-N diagram for plain weave glass/epoxy composite. Using predicted fatigue life along with initial modulus as inputs, the stiffness degradation model can predict residual stiffness at any stage of the fatigue loading life cycle. Logarithmic function used to represent stiffness degradation in the model is derived by inverting Boltzmann sigmoid function. The results of both, fatigue life model and stiffness degradation model were found to be in good agreement with those of the experimental results.