THE ESTIMATION OF LIFETIME OF THE THICK-WALL TUBES AND THE TUBULAR WELDED JINTS AT THE STAGE OF FETIGUE SURFACE SEMIELLIPTICAL CRACK PROPAGATION

Abstract

The work is devoted to the problems of assessing the lifetime of tubular structural elements at the stage of surface cracks propagation under cyclic loading effect with account of residual stresses. The mathematical model is based on a partial differential equation to determine the kinetics of fatigue cracks in a three-dimensionаl elastoplatic body under high-cycle or low-cycle fatigue. In this equation, the basic parameter, responsible for crack growth rate, is the effective crack tip opening displacement range Ddef. To investigate the fatigue crack growth kinetics in structural elements, depending on stress ration using the proposed differential equation, it is sufficient to use the diagram of fatigue fracture alone in coordinating the rate – displacement range (V ~Ddef), thus allowing us to save the materials and time of experimental investigations, related with construction of fatigue curves. The analytical method for approximate determination of Ddef with account of the plastic stretches at the fatigue crack edges has been developed. An analytical approach to approximate determination of the d – opening for a surface semi-elliptical crack in tubular structural elements under non-uniform distribution of stresses over the element thickness has been proposed. The method for determination of fatigue surface semielliptical crack kinetics and residual lifetime of the tubular welding joint is proposed based on nonlinear fracture mechanics approaches. The basic parameter, responsible for crack growth rate, is the effective crack tip opening displacement range. Computional results of fatigue surface semielliptical crack kinetics the residual lifetime of tubular structural elements were compared with the known experimental data.The analytical relationships for estimation of material constants, that are used in differential equations of fatigue cracks kinetics have been established. The residual lifetime and growth of fatigue surface semi-elliptical cracks for such structural elements such as thick-wall tubes, and tubular welded joints of marine stationary platforms have been assessed. Numerical results of the residual lifetime of thick-wall tubes and T-shaped tubular welded joints were compared with the known experimental data.