Simulation And Experimental Study On Fracture Of Cast Iron Specimens Based On Elastic-plastic Extended Finite Element Method

For the fracture process of most engineering materials,the plastic yield zone usually appears in the tip of original crack and sprout crack.The elastoplastic fracture analysis method can reveal the fracture mechanism of the material and describe the crack propagation process more reasonably and accurately.Experiment and simulation are used to study the plastic yield and crack propagation behavior of gray cast iron specimens with cracks in this thesis.Based on the ABAQUS platform,the UEL analysis program of elastoplastic expanded finite element method(XFEM)is developed by using Matlab and Fortran.The tensile fracture process of gray cast iron specimens with cracks is simulated.Based on the analysis of experimental data and the sensitivity simulation analysis of fracture parameters,the tensile fracture behavior of cast iron slab specimens with unilateral and bilateral collinear cracks is studied.The main contents and conclusions are as follows:1.The uniaxial tensile test of cast iron slab specimens without cracks and with unilateral and bilateral cracks of different lengths are carried out.It is found that the load-displacement curves of cast iron specimens under uniaxial tension shows significant nonlinearity,which results from a certain degree of plastic deformation in the specimens.According to SEM and EDS analysis of fracture surface,cleavage fracture is dominant and ductile fracture is secondary.With the increase of crack length,the fracture strain and ultimate strength decrease gradually.The fracture strain of the specimens without crack is 1.4%.Three kinds of damage variables are defined by the initial stiffness,tensile strength and cracking strain of load-displacement curve respectively.The comparative analysis shows that the damage variables defined by tensile strength are more sensitive to crack length.So it is suggested to be the damage characteristic evaluation index of cast iron with cracks.2.Based on the ABAQUS platform and the elastic material constitutive relation,the elastoplastic constitutive relation on the basis of linear elastic extension finite element user-defined element subroutine(UEL)is developed.Combining the displacement variables from UEL and the the equivalent stress-strain curve obtained by experiment,the Newton-Raphson iteration method is used to calculate the stress and plastic strain components at each loading time.The UEL subroutine of elastoplastic extended finite element is written by MATLAB and FORTRAN.Compared with the experimental data,the result shows that the program can calculate the stress-strain curve on each Gaussian integration point accurately,which verifies the reasonableness and high accuracy of this thesis.3.By reading the result file of ABAQUS,distinguishing the enrichment node of the extended finite element from the non-enriched node of the finite element,the displacement,stress and strain of nodes are obtained in the current increment within the integral region,the J-integral of each increment is calculated according to the equivalent integral region method.If the J-integral exceeds the critical value Jc,the task is terminated by the LSTOP function of URDFIL subroutine in ABAQUS.Then calculating the crack propagation angle,starting the calculation task of the next crack propagation,the crack propagation simulation is realized.4.The accuracy of elastic extended finite element and elastoplastic expanded finite element are systematically studied by using stress intensity factor and J integral respectively.Compared theoretical values with finite element simulation results,the result shows that the computational accuracy of the extended finite element is obviously higher than that of the finite element method under the same mesh size,the same range of enrichment elements and the same number of integral points.The precision of extended finite element still keep steady even when the mesh is larger.Using the single factor analysis method,the suggested values of the main parameters of the slab specimens with cracks are given as follows:the mesh size suggests to be 0.25-0.5mm,the radius of the enrichment unit suggests to be 0.5-0.75mm,and the number of the XFEM integral points in the triangular sub-integral region of the crack tip element suggests to be 4-7.5.Due to it is difficult to obtain the fracture toughness of cast iron by test,The fracture toughness is fitted by using the fracture stress and crack length curve of uniaxial tensile test,so the fracture toughness Jc of cast iron specimen is determined approximately.The critical value of Jc is used as the initiation criterion to simulate the fracture of specimens with different crack lengths.The demarcative line can be drawn through the critical value of crack initiation Jc,which can be used to predict the cracking load and the cracking displacement of cast iron specimens with different crack lengths.