Research On Fatigue Crack Growth With Overload Retardation Effect Based On XFEM

Fatigue cracks are one of the major causes of ship structural failures.In order to evaluate the safety of ship structures more accurately,the effect of actural load sequence on fatigue crack growth life should be considered.At present,the fatigue strength research based on fracture mechanics mainly focuses on the fatigue crack growth life assessment under constant amplitude loading.The research on fatigue strength under variable amplitude loading is limited to simple structures with type I crack propagation,and rarely involves the residual fatigue strength of complex structures under actural load sequence.For the simulation of fatigue crack propagation,the Extended Finite Element Method does not need high density refinement of crack tip mesh and mesh reconstruction,which is very suitable for the fatigue analysis of complex structures.However,the Extended Finite Element Method has been greatly simplified in ABAQUS software.For example,the lack of crack tip singularity may easily lead to computational divergence and wrong results,which is particularly serious for mixed crack propagation.Moreover,the fatigue crack propagation criterion based on the Paris law cannot consider the influence of load order on fatigue crack growth life.Therefore,it is necessary to develop a new algorithm,which can be used to evaluate the fatigue strength of ship structures under actural loading.In this paper,a new fatigue crack propagation algorithm considering the retardation effect of the over load is developed based on XFEM and Python script language.It can predict the mixed crack path and estimate the fatigue crack life under constant or variable loading for engineering structures.The proposed algorithm can solve many defects of the ABAQUS's XFEM module,and has the characteristics of reliable calculation,wide adaptability,economy and practicality.Firstly,the defects of the ABAQUS's XFEM module in crack propagation processing have been analyzed,which indicats the necessity of developing a new algorithm for fatigue analysis.The J-integral method is proposed to calculate the stress intensity factor,which considers the near crack tip asymptotic singularity.The standard three-point bending specimen,the standard compact tensile specimen and the center double slant crack with a hole are taken as examples to validate that the J-integral method has high accuracy in calculating the I-type and I-II mixedtype stress intensity factors.Secondly,a new fatigue crack propagation algorithm considering the retardation effect of the over load is developed based on XFEM and Python script language,which can be used to predict the mixed crack path and estimate the fatigue crack life under constant or variable loading for engineering structures.Moreover,the program design idea,design flow and core principles of the algorithm are introduced,which lay a foundation for fatigue crack propagation analysis.Then,the developed fatigue crack propagation module is used to predict the mixed crack propagation paths of five models,such as the modified three-point bending specimens,the Ibeam structural joint et al.The effects of various parameters such as offset hole,multi-crack and complex stress state on the calculation results are studied.Compared with the experimental data,the literature data and the simulation data by ABAQUS,the present algorithm has been validated to be suitable for the prediction of mixed crack propagation paths in different situations.Finally,the fatigue crack life under constant and variable loading of the center-cracked plate,the modified compact tensile specimen and the I-beam structural joint are calculated by the developed fatigue life estimation module.Compared with the experimental data or literature data,the developed algorithm has been validated to predict the fatigue crack life prediction under constant and variable loading with high accuracy.Meanwhile,taking the I-beam structural joint as example,the effects of overload parameters such as overload position,overload ratio,overload interval and continuous overload cycle on the fatigue crack growth life are studied systematically.