Fatigue response of thick section fiberglass/epoxy composites

One of the significant mechanical property variations under long term fatigue is the change in material or component stiffness. So stiffness reduction can be used as a damage parameter to assess damage in composite laminates under fatigue loading. It has been observed that the stiffness decreases monotonically under cyclic loading. One of the main advantages of using stiffness as a damage parameter is, it can be measured nondestructively in service. In this investigation the S-N diagram of two thick section fiberglass/epoxy composites has been determined and their stiffness degradation during fatigue loading has been monitored. A constitutive model has been used to simulate the stiffness degradation of composite laminates under fatigue loading. The sensitivity of model parameters has been tested and the values of the parameters can be determined from the experimental data for different loading conditions. Static tensile tests have also been carried out in order to determine the ultimate strength of this composite. During subsequent fatigue tests loads ranging from 45% to 70% of the ultimate load were applied to each specimen.