Modeling of Fatigue Crack Growth Closure Considering the Integrative Effect of Cyclic Stress Ratio, Specimen Thickness and Poisson's Ratio

Key components of large structures in aeronautics industry are required to be made light and have long enough fatigue lives. It is of vital importance to estimate the fatigue life of these structures accurately. Since the FCG process is affected by various factors, no universal model exists due to the complexity of the mechanisms. Most of the existing models are obtained by fitting the experimental data and could hardly describe the integrative effect of most existing factors simultaneously. In order to account for the integrative effect of specimen parameters, material property and loading conditions on FCG process, a new model named integrative influence factor model (IIF) is proposed based on the plasticity-induced crack closure theory. Accordingly to the predictions of crack opening ratio (gamma) and effective stress intensity factor range ratio (U) with different material under various loading conditions, predictions of gamma and U by the IIF model are completely identical to the theoretical results from the plane stress state to the plane strain state when Poisson's ratio equals 1/3. When Poisson's ratio equals 0.3, predictions of gamma and U by the IIF model are larger than the predictions by the existing model, and more close to the theoretical results. In addition, it describes the influence of R ratios on gamma and U effectively in the whole region from -1.0 to 1.0. Moreover, several sets of test data of FCG rates in 5 kinds of aluminum alloys with various specimen thicknesses under different loading conditions are used to validate the IIF model, most of the test data are situated on the predicted curves or between the two curves that represent the specimen with different thicknesses under the same stress ratio. Some of the test data slightly departure from the predictions by the IIF model due to the surface roughness and errors in measurement. Besides, based on the analysis of the physical rule of crack opening ratios, a relative thickness of specimen is defined to describe the influence of material property, specimen thickness and so forth on FCG characteristics conveniently. In conclusion, the relative thickness of specimen simplifies the expression of FCG characteristic and provides a general parameter to analyze the fatigue characteristics of different materials with various thicknesses under different loading conditions. The IIF model describes the integrative effect of existing influence factors explicitly and quantitatively, and provides a helpful tool for fatigue property estimation of practical component and experiment design.