A new coupled approach of residual stiffness and strength for fatigue of fibre-reinforced composites

During the last decades, fibre-reinforced composites have been established as competitive materials for naval, automotive and aerospace industry. However, the fatigue behaviour of fibre-reinforced composites is so diverse and complex that present knowledge is far from complete. Two commonly used approaches to model fatigue damage are the residual stiffness and the residual strength approach. In this paper, the change in the modulus of elasticity due to fatigue damage is studied for uni-axial bending. The proposed modelling approach is new in two ways: (i) the damage growth rate—a measure for stiffness loss—is expressed by two separate terms representing the initiation and propagation phase of damage respectively, (ii) a static failure criterion is modified to represent the decreasing reserve to ultimate static strength. In that way this coupled approach of residual stiffness and strength is capable of simulating the three stages of stiffness degradation: initial decline, gradual reduction and final failure, as well as the stress redistribution due to the loss of stiffness in the damaged zones. The model has been applied to displacement-controlled bending fatigue experiments of plain woven glass/epoxy specimens.