Numerical Simulation Of Engine Con-rod Fracture Splitting Process And Analysis On Effect Factors

Fracture splitting method for engine connecting rod is a new manufacturing method the principle of which is to design and predefine crack groove at the center of the crack end bore to generate stress concentration and then make the fracture split happen by initiatively imposing load perpendicular to fractured faces. Compared with traditional methods, it has many advantages and has been treated as a great transformation for the traditional connecting rod manufacturing technology. As a new technique of connecting rod, fracture splitting has been developed gradually in 1990s.And then it was applied to the manufacture field in large-scale. Now it has already become the most important sign of the development level of connecting rod processing of a nation.Connecting rod fracture splitting is precision processing technology and the key process of which includes notching, directional splitting and assembling bolt. The most important condition for successful splitting is that the two pieces must fit together contiguously in order for the connecting rod bearing to function properly. This condition can only be fulfilled via brittle fracture with little or no plastic deformation. Plastic deformation alters the surface topography and leads to deformation and distortion of the inner bore. However, connecting fracture splitting is a three-dimensional crack propagation course which finished complicatedly and instantaneously. Therefore, the fracture quality influence factors are numerous. It has important actual meaning to carry on numerical simulation of fracture splitting process and optimizing the parameters.The research work in this dissertation focus on several respects as setting up the numerical simulation model of con-rod and dealing with the key technology problems, crack starting analyzing for the C70S6 con-rod, analyzing the three-dimensional crack propagation course of fracture splitting by numerical simulation, dimension effect research for the thickness and notch of con-rod, and loading effect and fracture quality analysis. The purpose is to optimize the key technical parameters and improve the fracture quality by numerical simulation method for the fracture splitting technology. The major research is summarized as follows:1. The finite element model of con-rod has been set up and the pivotal technical problems have been dealt with. Meshing, contact definition, boundary conditions, project-stress free, material character, fracture definition, loading case and etc. are all quite important and needful for the foundation of the numerical model. In order to improve the efficiency of the FEM method and analyze the crack tip, the crack region and the main body of con-rod are meshed by different method with different element types. And in the contact definition, they are glued together.2. The material performance has been test for C-70 steel in order to study the crack starting state and propagating process. By the tensile test, the plastic stress-strain relation has been obtained. Also, loading speed sensitivity test has been carried on for this material. By standard test, the fracture toughness of C-70 steel and the micrographs of the fracture surface and material organization have been obtained. The experimental results show that the organization of C-70 steel is pearlite and ferrite and the pearlite is netted distributed around by the ferrite. As the tests results, C-70 steel has strong crack sensitivity and it is quite suited to the fracture splitting technology of con-rod.3.The crack staring state in the process of fracture splitting has been obtained by the numerical simulation results. By the combination of the numerical simulation and the tests, the critical J-integral has been calculated which can be used to the determination of the crack starting state. The distribution of principle stress max and crack starting points on the crack groove have been analyzed. And the plastic stress field and displacement field are all obtained by the numerical simulation results.The numerical simulation results show that the upper/down section of the crack groove are on plane stress state before crack starting wile the middle section is on plane strain state. Cracking will be start at the crack groove where the stress concentrations distributed on. The dispersion of stress concentration at the plastic region results in scattering of the starting point of cracking, which is the main reason for production of nip bits, bifurcate and steps in fracture splitting.4. Subroutine and uactive elements method are used to simulate the fracture splitting process of con-rod. The numerical results give out the process of the crack propagating. The contour bands of the equivalent stress around the split line in the process of crack growing, the time history plot of the equivalent stress, the equivalent strain and the separation force on the crack tips at different crack region are all analyzed. The fracture time plots of the average 3D J-integral at crack tips are also gotten. By the numerical simulation results, distribution of the equivalent stress and displacement field in the big bore of the con-rod after fracture splitting are all clearly shown.The analysis results show that the crack propagates at an un-equal rate in the process of fracture splitting. The critical fracture force on the middle section of the crack groove is 75.12KN which is close to the real value (75.136KN). After fracture splitting, there will be remainder stress on the middle section of the crack groove. The three-dimensional J-integral fluctuates around the critical J-integral (JIC). When fracture splitting is finished, the J-integral value will decrease to zero. By fracture splitting, the main body of con-rod is shrinking in the X direction and extended in the Y direction, which will lead to the elliptic distortion of the big bore. There are small plastic distortions both on the main body of the con-rod and the fracture surface after fracture splitting. The bolt hole plays an important part in crack propagating and the displace distribution in the fracture splitting process.5. Three geometry parameters of the crack groove have been optimized by the numerical simulation method. The influence of those parameters on the fracture force has been studied. Also, the influences of crack groove processing methods on the fracture quality have been researched.Numerical results show that the fracture force will decrease along with the deepness increasing while it will increase as the curvature radius increasing. There is no remarkable relationship between the fracture force and the opening angle. The principle of the deepness design should be guaranteeing the big bore surplus of finish machining for the distortion at least. For saloon car con-rod, the deepness is recommended in 0.4~0.5mm and 0.6mm~0.7mm for the truck con-rod. Adopting the laser method to process the crack groove, the elliptic distortion is reduced and the fracture quality is better than other methods.6. The influence of thickness of con-rod on fracture splitting technology has been studied by the combination of fracture mechanics theory and FEM method. Numerical models are set up with variable thickness values and variable fracture face shapes to analyze the stress state on the crack groove before crack starting.The numerical simulation results show that the stress concentrations are gradually gathered at the crack groove along with the thickness value increasing. Otherwise, the vertical size of the fracture surface will increase, which will lead to the decreasing of the plastic deformational area. And the difference of the principle stress max on the upon/down section will be reduced. It is quite beneficial to the crack starting, for that those sections at the crack groove will reach the critical fracture force simultaneity.7. The process of fracture splitting with back-pressure is studied by the numerical simulation method. The influence of back-pressure on the 3D J-integral, the stress concentration distribution, the distortion concentration, the displacement field and the plastic deformation are all analyzed. Otherwise, the remainder stress status on the fracture surface after fracture splitting with back-pressure is presented.The research indicate that back pressure causes the J-integral grads on the con-rod crack groove in the lengthwise direction to increase,the stress to centralize to the central section of crack groove even more rapidly. The back pressure can induce notch split and crack growing and it reduces fracture disfigurements caused by crack bifurcation and numerous joint. But the exterior force and the separation force both increase with the back pressure although the remainder stress on the fracture surface is reduced.8. The influence of loading speed on the stress field, the displacement field and the fracture quality are all analyzed by the FEM method. In order to determine the influence rule, the stress state and fracture quality are analyzed for the con-rod fracture splitting under variable loading speed.The results show that there is no relationship between the loading speed and the stress distribution on the con-rod under the middle loading speed. But the crack starting time will delay and the plastic distortion will increase as the loading speed reducing. The displacement grads will increase near the fracture surface. Otherwise, the distortion of the displacement in the main axis direction will increase and the con-rod will be elongated. But the influence of loading speed on the displacement field near the fracture surface is not obviously. While under the high loading speed, the influence is quite obvious. For that the stress concentrates at the crack groove uniformity what will shorten the crack starting time and improve the fracture quality. At the same time, the higher loading speed will cause the increase of the fracture force, and then the requisition for the facility is promoted sharply.9. Aiming at the quality optimization of fractures, a series of fracture disfigurements in splitting con-rod were enumerated and analyzed. The elliptic distortion of the big bore and the phenomenon of broken bits are also analyzed. The results show that the dispersion of stress concentration at the plastic region results in scattering of the starting point of cracking, which is the main reason for production of nip bits, bifurcate and steps in fracture splitting. Moreover, corresponding measures to prevent the diversified fracture disfigurements in fracture splitting of connecting rod are put forward as well.