Numerical Simulations Of Three-dimensional Fatigue Fracture Strength Of Complex Structures

Many widely used mechanical equipments in engineering such as air crafts or aerospace crafts, motorcars, lathes and so on usually consist of complex components, which were designed to bear static and cyclic loading during service. The most important failure form of engineering structures is fatigue fracture. Fatigue fracture mechanics theory has been widely recognized as the basis of structure strength analyses. However, the current applications of fatigue fracture methodology are mainly based on traditional two-dimensional (2D) fracture theory. Furthermore, the strong three-dimensional (3D) effects often occur in the complex structures. Therefore, the research on the three-dimensional effects becomes important and necessary in the engineering structures failure and destruction. In the thesis, we focused on the stress concentration 3D crack propagating simulations. The mainly findings are as following:1.The influence of Poisson's ratio on the thickness-dependent stress concentration factor along the root of elliptic holes in elastic plates subjected to tension is systematically investigated by use of three-dimensional finite element method. It is found that the influence of Poisson's ratio on the thickness-dependent stress concentration factor is very significant. Based on the three parameters of elliptic hole shape factor, plate's dimensionless thickness and Poisson's ratio, a team of empirical formulae provide the relationships among the maximum value of stress concentration factor, the value of the surface stress concentration factor and the corresponding planar solution by fitting the numerical results with satisfied accuracy, which will be take an important advantage on the measurement of stress concentration in the engineering practice.2.The influence of Poisson's ratio-dependent 3D stress concentration to the fatigue intensity of structures is investigated based on the local stress principle. It is found that the fatigue intensity of structures based on the 2D stress concentration or the surface concentration is less than that based on the maximum 3D stress concentration. Therefore, it will be very dangerous that the effect of 3D concentration to the fatigue intensity is hardly considered.3.With the development of computer technique, the three-dimensional crack analysis has become feasible. However, how to model the three-dimensional cracks in complex structures is still a great challenge to the wide application of three-dimensional damage tolerance technique, The simulation of fatigue propagation of corner crack at the root of elliptic holes in the elastic plates has been studied. The simulation method on the crack self propagation has been developed. Based on this method, arbitrary complex cracks will be simulated easily, which will put an impulse on the applications of the three-dimensional damage tolerance technique.