Multiaxial fatigue behaviour of unidirectional plies based on uniaxial fatigue experiments—II. Experimental evaluation

Abstract

To validate the generalized residual material property degradation model, developed in the first part of this paper, an experimental program was conducted using graphite/epoxy AS4/3501-6 material. The main advantage of the model is that it can simulate the fatigue behaviour of a unidirectional ply under multiaxial state of stress and arbitrary stress ratio by using the results of uniaxial fatigue experiments. The fatigue 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 characterized in the matrix direction and in-plane shear condition under static and fatigue loading. For this purpose the static strength, residual strength 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 the behaviour of a 30° off-axis specimen under uniaxial fatigue load, which in fact is a unidirectional ply under biaxial stress conditions, is simulated by the model. To evaluate the results calculated by the model, a series of uniaxial tension-tension fatigue tests are performed on unidirectional 30° off-axis laminates. A comparison between S- N curves (curve fitted mathematically and simulated by the model) shows the one simulated by the model slightly overestimates the fatigue life of the 30° off-axis specimen. However, the S- N curve simulated by the model is located reasonably inside the range of experimental scatter. It should be noted that the model proposed in this study is a deterministic model and further research is needed to couple a probabilistic for a more realistic simulation.