MULTIAXIAL FATIGUE BEHAVIOR OF UNIDIRECTIONAL PLIES BASED ON UNIAXIALFATIGUE EXPERIMENTS .2. EXPERIMENTAL EVALUATION

To validate the generalized residual material property degradation mod el, developed in the first part of this paper, an experimental program was conducted using graphite/epoxy AS4/3501-6 material. The main adva ntage of the model is that it can simulate the fatigue behaviour of a unidirectional ply under multiaxial state of stress and arbitrary stre ss ratio by using the results of uniaxial fatigue experiments. The fat igue behaviour of the unidirectional ply under combined tensile matrix stress and in-plane shear stress (biaxial conditions) is considered. To prepare input for the model the material must be fully characterize d in the matrix direction and in-plane shear condition under static an d fatigue loading. For this purpose the static strength, residual stre ngth and fatigue life of the material in the transverse direction and under in-plane shear conditions are measured. The results obtained by these series of experiments are used as input data for the model and t he behaviour of a 30 degrees off-axis specimen under uniaxial fatigue load, which in fact is a unidirectional ply under biaxial stress condi tions, is simulated by the model. To evaluate the results calculated b y the model, a series of uniaxial tension-tension fatigue tests are pe rformed on unidirectional 30 degrees off-axis laminates. A comparison between S-N curves (curve fitted mathematically and simulated by the m odel) shows the one simulated by the model slightly overestimates the fatigue life of the 30 degrees off-axis specimen. However, the S-N cur ve simulated by the model is located reasonably inside the range of ex perimental scatter. It should be noted that the model proposed in this study is a deterministic model and further research is needed to coup le a probabilistic for a more realistic simulation. Copyright (C) 1997 Elsevier Science Limited.