Simulation Study On Multi-crack Propagation Of Cast Iron Based On Extended Finite Element Method

The extended finite element method(XFEM)researches problem in the conventional finite element framework,not only retains the advantages of conventional finite element(CFEM),but also represents higher precise at the region near crack tip.The reason is that XFEM method avoids the difficulties of meshing as crack propagation,and solve the high stress problem with higher integration strategy instead.This paper introduces the extended finite element theory and plasticity is planted to study the crack propagation in cast iron structure.The tensile properties of the cast iron specimens with cracks were analyzed by using three type damage definitions.The main research contents and conclusions are as follows;1.This paper briefly introduces the basic principle of extended finite element method,the realization of the extended finite element method program(UEL),UEL program flow chart,level set method,Unit sub division,enrichment unit integration scheme;specifies the process of the level set function describing crack and tracing crack propagation,judges the relative relationship between the unit and the crack is by the level set,strengthens the classification of units and nodes;In order to ensure the accuracy of the integral near the crack tip,the crack is processed by the preset rule in the case of the crack passing through the unit or crack tip in the unit,integrates these regions respectively which are divided into a number of triangular or quadrilateral regions.The self-compiled UEL subroutine of the extended finite element can simulate the crack tip displacement field and strain field more accurately.The UEL code for XFEM method was complied,and can represent the strain and displacement near crack tip more accurately.2.By the uniaxial tensile simulation,the stiffness of the cast iron specimens with unilateral and bilateral crack was researched,and the man conclusions are as follows:the stiffness of unilateral crack model decreases with the crack growth in nonlinear stage;The model stiffness decreases slowly with crack growth in initial stage,but the model stiffness decreases greatly at a short distance when crack growth gets longer;the stiffness is more affected with the unilateral crack than the center crack when the length crack is equal;the greater angle between the unilateral crack and the horizontal direction,the higher the stiffness will be;with the increase of the longitudinal spacing of bilateral crack,the model stiffness reduces and is greatly influenced.3.Damage variables were defined on the basis of the initial tensile stiffness and maximum load of cast iron specimens and the damage characteristics of specimens with different crack lengths were studied,it shows that the damage is not sensitive to the crack length by the initial stiffness calculation and lower than the damage with the maximum load,and it is dangerous in the prediction of the component bearing capacity,which demonstrates the method with the tensile stiffness is more dangerous to predict the residual load bearing capacity.4.The nonlinear mechanical behavior is successfully simulated combining the elastic property on XFEM mesh with UMAT and XFEM method on the zone near crack tip.The toughness of the cast iron specimens with cracks was researched.The main conclusions are as follows:for the unilateral crack,the longer the crack length is,the greater the energy ratio will be,the deformation before the fracture of the component will be greater,the crack growth rate will get slower,and the toughness of the component will be stronger;for the bilateral crack,when longitudinal spacing is equal,the longer the crack length is,the greater the energy ratio will be,the deformation before the fracture of the component will be greater,the crack growth rate will get slower,and the toughness of the component will be stronger;for bilateral crack with equal lengths,the greater the longitudinal spacing is,the greater the energy ratio will be,the deformation before the fracture of the component will be greater,the crack growth rate will get slower,and the toughness of the component will be stronger.