Study On The Fracture Behavior Of Three-dimensional Surface Crack Based On Extended Finite Element Method

For the three-dimensional(3-D)surface cracks often encountered in engineering,the stress state of the crack tip is a three-dimensional stress state and is affected by the3-D geometrical constraint of structural component.Both the classical fracture mechanics and the conventional design codes used in engineering are generally developed based on the two-dimensional(2-D)fracture analysis.The research of 3-D surface crack needs further improvement.At present,the extended finite element method(XFEM)is widely used in crack analysis because of its advantages for discontinuity problem.Based on the material fracture performance tests,the 3-D fracture behaviors of the component with surface crack were studied by using the XFEM module of ABAQUS software.The influences of the component shape,component size,initial crack size and load mode on the fracture behavior of the component with surface crack were analyzed.It provided a useful reference for fracture safety assessment of components and three-dimensional surface crack research.The following main contents are carried out:1.For the coupler subjected to tensile load and the axle subjected to bending load,the basic mechanical properties and fracture tests of materials were carried out.The basic mechanical properties parameters such as yield strength and so on were obtained by the tensile tests of cast Grade E steel and axle steel.Then,the fracture toughness tests of cast Grade E steel were carried out by using CT specimens,and the fracture toughness K_IC value of cast Grade E steel was obtained.Moreover,the three-point bending test of axle steel with surface crack was carried out,and the load-displacement curve in the loading process was obtained.The experimental data for verification and material parameters were provided for subsequent finite element analysis.2.The fracture processes of the plate and round bar with surface crack subjected to a tensile load were simulated by the XFEM.The simulated results show that the ultimate load calculated by the material fracture toughness is lower than that by the XFEM simulation.The difference of ultimate loads obtained by the two methods increases with increasing the size of the specimen.Meanwhile,with the increase in the size of the component, the second stage of the load-displacement curve reflecting the stable crack propagation gradually shortens,and the fracture mode tends to change from the ductile fracture to brittle fracture.In addition,the shape changes of surface crack in the process of propagation under tensile load were discussed.3.The three-point bending and four-point bending fracture behaviors of circular shaft with surface cracks were simulated.Compared with the tensile case,under bending load,after crack initiation,the load increases very slowly,and after reaching the maximum load,the load will decrease slowly.While under tensile load,after reaching the maximum load,the load will decrease rapidly.From the change of crack shape in the process of crack propagation,the surface crack shape of circular shaft under three-point bending and four-point bending load are scallop shape at the initial stage of crack propagation,which is similar to that under tensile load.At the later stage of crack propagation,the front edge of surface crack under three-point bending load is straight, close to the through crack,which is obviously different from that under four-point bending and tensile loads.Moreover,three-point bending and four-point bending finite element simulations for the circular shaft with a radius of 10mm and different initial cracks were carried out.From the bending load-displacement response of circular shaft with various initial crack depths,the influence of initial crack depth on the four-point bending load of circular shaft is obviously greater than that on the three-point bending load of circular shaft.