Simulation Of Mixed Mode Fatigue Crack Propagation Behavior And Fatigue Life Prediction Based On Extended Finite Element Method

The renewal and progress of science and technology have led to the rapid development of aerospace,marine,transportation,petrochemical,and other industrial fields.Many mechanical structures are widely used in people’s production and life.Due to manufacturing process and other reasons,mechanical structures often have structural defects.Under reciprocating cyclic load,defects will evolve into fatigue cracks and eventually lead to structural failure and fracture.Therefore,the research on fatigue cracks has great economic value and practical significance.The extended finite element method（XFEM）does not need to redivide the mesh when simulating crack propagation,and it is more concise and efficient to deal with the problems of discontinuous and complex working conditions.In this paper,the fatigue crack propagation under static and dynamic conditions is simulated by the extended finite element method,and this method is extended to the analysis of pressure pipeline with defects.The specific research contents are as follows:（1）This paper summarizes the basis of fracture mechanics and introduces the theory of extended finite element method.The static analysis of three-dimensional rectangular specimens with central penetrating oblique crack is carried out.The results show that the stress intensity factor K_I presents a"convex"distribution and K_II presents a"concave"distribution along the thickness direction.The crack shows the trend of I-II mixed mode propagation and the stress intensity is related to the size of the initial crack(6₀,Angleαbetween the initial crack and central axis,the length L of the sample.Applying stop-crack holes at the crack tip can reduce the stress concentration and stress intensity factor at the crack tip.（2）A fatigue life prediction model based on energy release rate is established,and the accuracy of the model is verified by comparing with the experimental data.On this basis,under tensile load and bending moment load,the effects of the stop-crack holes and initial crack angleθ,the distance d between the center of the stop-crack holes and the initial crack tip,the radius r of the stop-crack holes on the fatigue life of the specimen are analyzed.The results show that the crack propagates in mode I under tensile load,the crack propagates in I-II mixed mode under bending moment load.And The best values are obtained for the fatigue life of the specimens prolonged by stop-crack holes under differentθ,d and r conditions.（3）The effects of the ratio a/c and a/t of different initial semi-elliptical cracks on the distribution of stress intensity factor and crack profile in penstock is simulated by using the above methods.The results show that the trend distribution of stress intensity factor K_I under static state is wave.Fixed a/c value under dynamic crack growth,crack profile tends to be semi-circular when the initial value of a/t is large,crack profile changes slightly when the initial value of a/t is small.The fatigue life value of the penstock is obtained and basically agrees with the theoretical value of the numerical calculation.