Residual Stress Influence on Fatigue Life

Abstract To investigate the influence of cure cycle-induced residual stresses on the fatigue life in CFRP-steel laminates, experimental fatigue tests with a constant amplitude and tension-tension loading are performed. The notched FML specimens are monitored with digital image correlation, thermography, and electrical resistance measurements throughout the dynamic tests. The evaluation of the test data shows that cracks are initiated at the stress concentrations around the edge of the notch and propagate through the metallic facesheets. Thermography reveals that failure of the inner metal sheets happens considerably after the failure of the facesheets. The influence of different residual stress states in otherwise identical specimens is seen in the evaluation of the dynamic stiffness and, in particular, when evaluating the number of cycles until crack initiation and facesheet failure. With reduced residual stresses, crack initiation is delayed, and crack growth is slowed down. The importance of the residual stresses for the fatigue life characterization becomes evident when the prediction bounds for the fit of the experimental crack initiation and crack propagation data are calculated. These prediction bounds become considerably smaller when the correct residual stress states for each laminate are considered in the calculation. Fatigue in the investigated CFRP-steel laminates is governed by the metal, and the propagation of a fatigue crack takes up most of the time of the fatigue life. This is particularly advantageous for an SHM system in FMLs.