| The fracture splitting of connecting rod, which broke through the traditional design idea of separating by machining process,positioning and assembling parts, can make the cap separate from the connecting rod by the controlled directional fracture with fewer procedures and equipment investment, lower costs, better-quality products, etc. The fracture surface of the connecting rod and cap after fracture splitting can provide the higher assembly precision and the stronger loading capacity, as a result, it can improve the engine performance. The fracture splitting processing of connecting rod has advantage over the other traditional methods.During fracture splitting connecting rod, the most important thing is to make the cap brittle fracture split from the connecting rod with little or no plastic deformation and achieve appropriate three-dimensional fracture face. Because plastic deformation can alter the fracture surface topography and lead to deformation and distortion of the inner bore so that it can not make the connecting rod and the cap after fracture splitting contiguously and accurately fit together to achieve subsequent finishing processes. As a result it would affect assembly precision and loading capacity of connecting rod in the engine. Fracture splitting connecting rod is a complicated and instantaneous three-dimensional crack propagation course and is influenced by many factors. The elastic-plastic fracture mechanics theory which could be used to study fracture splitting connecting rod is now developing. Therefore the parameters of fracture splitting were always determined by experiences from the past. So supported by the national natural science foundation(50375066) and based on the fracture nature of fracture splitting connecting rod, the relevant theoretical models of fracture splitting connecting rod were established and by these models, the theoretical analysis and FEA numerical simulations were carried out and the relevant parameters of fracture splitting were determined. And testes were done to validate these parameters. The major research is summarized as follows:1. The mechanics and fracture properties of the connecting rod material before fracture splitting were determined by the uniaxial tensile and the linear-elastic plane-strain fracture toughness tests. The effect of the bearing hole plastic deformation after fracture splitting of the connecting rod on roundness accuracy,assembly precision,subsequent finish machining and engine working were analyzed. Based on these testing data and analysis, according to the related theories of line-elastic fracture mechanics and elastic-plastic fracture mechanics, this paper concluded that connecting rod before fracture splitting is a elastic-plastic structure with two side starting-notches, that the load for fracture splitting is dynamic and impactive, and that the nature of fracture splitting connecting rod is a mode I fracture course after small scale yielding.2. Elastic-plastic fracture characteristics were analyzed. The crack body of elastic-plastic fracture was defined. J-integral of the non-linear crack body is equal to the total energy flowing into the crack body, which drives the crack body elastic-plastic deformation and fracture. The elastic energy dissipation of elastic-plastic crack body after fracture is defined. When the fracture is in the critical point, the critical J-integral of the non-linear crack body is equal to the total critical energy driving the crack body fracture after elastic-plastic deformation. Because J-integral of elastic-plastic crack body is sum of linear-elastic J-integral and plastic J-integral, so when the fracture happen, the critical linear-elastic J-integral is converted to the energy for forming the new crack surfaces and the crack body elastic energy dissipation, and the critical plastic J-integral is converted to plastic deformation energy of the crack body. According to this, the physical meaning of J-integral of linear elastic and elastic-plastic fracture can be consistent. When the fracture of the elastic-plastic crack body was starting, the elastic-plastic crack body relatively plastic was defined, which could be used as a measure of the scale of crack body yielding. Based on the crack body relatively plastic, the limit of load for fracture after small scale yield was calculated by fracture mechanics theory. As a reference and by FEA numerical simulations of fracture splitting connecting rod, the theory design load of fracture splitting connecting rod was identified as half of the overall yield load of the effective cross-section of bearing load. The theory design load was verified by the tensile test of Jetta EA113 engine connecting rod with V-starting-notches.3. According to Freterman's theory on joint strength, the fracture mechanism after the large-scale or overall yielding is different from after small scale yielding. So Mises equivalent stress fracture criterion after the large-scale fracture and the overall yield fracture and the maximum tensile stress fracture criterion after the small scale yielding fracture were presented and verified by the experiment and finite element calculations. Referring to the design load of fracture splitting and by reasonably simplifying the V-notch by broaching and the U-notch by wire cut electrical discharge and establishing the connecting rod geometric models for fracture splitting, the fracture finite element numerical simulation were carried out. The parameters and tolerances of the V-notch root radius and the U-notch depth were determined by the maximum tensile stress fracture criterion after the small scale yielding fracture.4. As for the starting-notch by laser processing, the microscopic geometric, the composition and the micro-hardness distribution of local metal structure surrounding the notch were analyzed. Referring to the energy method of linear elastic fracture mechanics and to avoid dealing with the complex geometry of the starting-notch by laser, the paper defined the equivalent critical stress intensity factor and the equivalent critical J-integral for the starting-notch by laser and estimated the equivalent critical J-integral, which provided a reference to determine the depth parameter and the tolerance of the starting-notch by laser for fracture splitting connecting rod.5. There is the different depth of starting-notches at both sides of connecting rod. And the structure of oblique incision connecting rod is asymmetric. The difference depth and the asymmetric structure affect on the order and the time lag of fracture splitting at both sides of connecting rod. The time lag of fracture splitting is the longer at both sides of connecting rod, and there is the bigger bending deformation at the side of later fracture, and there is the more plastic deformation during fracture splitting connecting rod. The FEA models of one side fracture splitting of connecting rod were established and the FEA simulation were done to study the role of clamping the connecting cap during fracture splitting. The results showed that clamping the connecting cap could restrict the connecting cap rotation during fracture splitting and weaken the influence of asymmetric factors on fracture splitting. The FEA models of flat and oblique incision connecting rod with different crack depth on both sides were established and the FEA simulation were done. The experiments of fracture splitting connecting rod were carried out by electrometric method. Based on the results of the above simulations and the experiments, the paper obtained the quantitative effects of the asymmetric starting-notch depth on fracture splitting the flat and the oblique incision connecting rod. When fracture splitting the flat incision connecting rod, the bearing hole plastic deformation after fracture splitting might be limited by controlling the ratio of depth difference to shallower depth of the two starting-notches of flat incision connecting rod in the reasonable range. When fracture splitting the oblique incision connecting rod, the optimization of the depth parameters on both sides of starting-notch could reduce the effect of the asymmetric structure on fracture splitting of the oblique incision connecting rod.6. The wedge block mechanism with self-locking function was chose to achieve clamping the connecting cap. The load of fracture splitting equipment of connecting rod were obtained by the wedge block which was driven and could duplicate the load direct by the hydraulic cylinder. The relation between the load of fracture splitting equipment and the theory design load was determined. The design method of the load of fracture splitting equipment was proposed. By this method, the loads of fracture splitting EA113 engine(1.6L) connecting rod of Jetta car, 8.6L diesel engine 36D connecting rod of the truck and 13L diesel engine 53D connecting rod of the heavy truck and special vehicles were calculated. Based on these loads, the equipments for fracture splitting the connecting rod for the engine of car,light and heavy truck were made. These equipments have already applied mass production of connecting rod and achieved good economic benefits.
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