| Fracture splitting processing technology for engine connecting rod is based on theprinciple of fracture mechanics, stress concentration theory and crack technology.Connecting rod body and cap are made high-speed fracture separation by artificial cracksources and controlled propagation. At the same time, the three-dimensional concave andconvex shape fracture surface is obtained. This natural fracture surface is chosen as thelocating datum when connecting rod body and cap is assembled, and accurate positioningand perfect meshing are realized. Accordingly connecting rod machining process issimplified greatly, the manufacturing cost is reduced. The assembly quality of product andcarrying capacity are improved.The key process of connecting rod fracture splitting mainly includes notching,directional splitting and assembling bolt. Among them, it is required that directional splittingprocedure could not produce large plastic deformation, and it must be ensured that thefracture surface morphology is good. So the positioning accuracy and load capacity can besatisfied when the connecting rod works. While there are complex factors and incidentfracture splitting defects in directional splitting procedure. In terms of mechanics, theconnecting rod fracture splitting is a three-dimensional process, including crack initiation,crack growth, crack propagation, and fracture. The whole process is difficult to control.In the paper, based on part notched fracture and small scale yielding linear elasticfracture mechanics, connecting rod fracture process is simulated numerically using theAnsys/Ls-dyna. Through studying the changes of the mechanical field in fracture splittingprocess, crack initiation position and crack propagation path were achieved. Then, itsystematically analyzed the factors of fracture splitting process, the reason for fracturesplitting defects. The corresponding control measures and improvement measures wereproposed. The three-dimensional fracture surface morphology was reconstructed based onreverse engineering technology. The control range of fracture surface defects was giventhrough descripting the fracture surface quantitatively, which provided a scientific basis forcracking defect control and quality evaluation.Main research contents and results of the paper are as follows:(1) Based on the connecting rod fracture splitting model and fracture mechanics theory, the reason of connecting rod notch crack initiation and the crack tip stress distribution on thecondition of small scale yield were analyzed. It was determined that maximum principalstress was the criterion of connecting rod fracture splitting.Based on fracture theory of notched parts, the law of connecting rod notch crackinitiation was analyzed. Under the fracture splitting load, the micro-crack produced when thelocal high stress near the notch reached the material damage fracture stress, whichconformed to the traditional strength theory. Based on the modified linear elastic fracturetheory of small scale yield, stress field near the crack tip was analyzed. The crack tip waspassivated due to the plastic deformation during crack propagation and the stress relaxationeffect, and stress distribution was bounded.Combining the connecting rod fracture splitting pattern with fracture microstructureanalysis, it made clear that the micro fracture mechanism of notch crack initiation was thesame as the one of blunt crack propagation. The maximum principal stress was a major causeof fracture. And the maximum principal stress criterion was determined as the unifiedcriterion for crack initiation and propagation while doing numerical simulation analysis.(2) Connecting rod fracture process was researched through numerical simulation. The crackinitiation and stress, strain, displacement field during propagation, and load characteristicswere analyzed systematically. The high stress range, crack initiation position, plasticdeformation time and area were determined. A theoretical basis was provided for fracturesplitting quality control.The three-dimensional finite element analysis model of the connecting rod fracture wasconstructed. The mechanics performance parameters of high carbon micro-alloynon-quenched and tempered steel C70S6were obtained by experiment. And the stress, strain,displacement field, and the crack initiation and propagation characteristics during connectingrod fracture splitting were analyzed numerically. The results showed that under the fractureload, the notch introduced stress concentration and three-dimensional tensile stress, whichmade the connecting rod achieve quasi brittle fracture at low stress. When the fracturesplitting was beginning, the local high stress was limited in range of0.5mm from the top ofthe notch as the center; Since the three dimensional constraints between the large end freesurface of connecting rod and the thickness middle surface was different, it led to differencesin stress state. The subsurface of the top of middle surface notch where the crack initiatedwas at a higher three dimensional tensile stress state.In addition, energy was accumulated for micro-crack initiation before fracture initiation,so functioning time of fracture splitting force was relatively long, which made plasticdeformation occur intensively before crack initiation, and the plastic deformation zone was located in range of0.25mm from the top of the notch. Propagation time after crack initiationwas very short, and plastic deformation at the fracture surface edge was small. Because ofplastic deformation, out-of-roundness phenomenon appeared which meant the large end boreextended in the connecting rod axis direction and shrunk along the vertical axis.(3) The influences of the notch geometric parameters on stress concentration, fracturestrength, fracture initiation time and plastic deformation were analyzed. The design principleof notch geometric parameters or optimal range was given.Notch was the prefabricated fracture initiation source, and design in notch parameterdirectly affected the connecting rod fracture subdivision accuracy and product quality.Numerical simulation analysis showed that notch depth and curvature radius had significanteffects on fracture splitting. With the increase of notch depth or with the decrease of thecurvature radius, the degree of stress concentration was increased, fracture splitting strengthwas reduced, fracture initiation time was to bring forward, and plastic zone range was shrunk.The impact of notch angle on the fracture splitting process was relatively weak, and whennotch angle was equal to about30°, fracture splitting quality was optimum.The design principle of notch depth was proposed to ensure the deformation of the largeend bore after fracture separation in the range of allowable tolerances (the minimum notchdepth) and met the requirement of the large end bore precision boring allowance (themaximum notch depth). Optimized parameter range was given with practice, that iscurvature radius was equal to0.1mm~0.2mm and notch angle was equal to30°~60°.Notch depth for cars connecting rod should be controlled within0.4mm to0.5mm.(4) The influence of the bolt hole and its processing position on the fracture splittingprocessing was analyzed, and the reference range of bolt hole position design was given.Drilling bolt hole was the essential procedure before fracture splitting processing. Theexistence of the bolt hole changed the in-plane constraint effect, and the influencing on theconnecting rod fracturing splitting processing had both advantage and disadvantage. On theone hand, the splitting force, the plastic deformation of the two end surface and large endbore wall was reduced, and the peak stress points appeared on the middle surface of the topof the notch, which ensured the accuracy fracture on the predetermined splitting location; Onthe other hand, the plastic zone range on the middle surface of the notch root was increased,and plastic deformation of the hole edge of fracture surface was introduced. This had anegative effect.By comprehensive analysis results, the reference range of designing bolt hole positionwas presented. The bolt hole should be processed near to the inside of big head end and under the condition that the wall thickness between the middle surface of the top of notchand the bolt hole was larger than2mm, which could guarantee the bolt hole effectivelyplayed a positive role on the fracture splitting processing.(5) The causes of the fracture surface quality defects and incident positions were analyzed,and the ameliorative measures were proposed.Three-dimensional fracture surface is the locating datum when connecting rod body andcap are assembled, and its quality directly affects the connecting rod assembly precision andperformance. In this paper, the APDL language was used to program the secondarydevelopment order in the post processing of the simulation software. The information ofcrack propagation front edge was extracted, the crack propagation path was captured and thedifferent stages of crack growth rate were calculated. The causes of fracture surface qualitydefects, such as cracking-off, step, slag and crack bifurcation, etc., and incident positionswere analyzed.Because of the confluence after cracks bypassing the bolt hole and the compressivestress in crack propagation, fracture surface quality defects appeared easily in the area fromthe bolt hole to the outer end of large end. If the outer end of large end arc shape profile (thatis the forging surface in the process of blocking) was reserved, especially under thecondition that the outer contour and the bolt hole almost formed equidistant curve whenfracturing, the risk of such defects could be reduced.(6) By studying I-II combined fracture, the reason for the fracture line deviation and therelationship between fracture line deviation and crack angle were analyzed. Permissivepartial load range of connecting rod for passenger cars was obtained, and the fracture linedeviation of control measures were proposed for production practice.Fracture line deviation is one of the common defects in fracture splitting process, whichwill influence the connecting rod bearing capacity. The basic reason for fracture linedeviation is that partial load cause shear stress appearing on the theoretical crack surface, thetype of connecting rod fracture changes from mold I fracture into I-II combined fractureunder shear stress. According to the simulation result, the control measures in practicalproduction were proposed. Then the relationship among fracture initiation angle, fractureline deviation and the crack angle when I-II complex fracture occurred was studied. Withthe decrease of the crack angle, the absolute value of crack initiation angle and the offset offracture lines increased. According it, the car connecting rod permissive partial load rangewas given. Namely, the offset between load and connecting rod axis should be controlledwithin2°range. (7) Based on fracture surface three-dimensional reconstruction software developed byreverse engineering technology, the area of fracture surface were quantitatively analyzed,specification method for fracture surface defect were proposed. Further practicalspecification and measurement method was explored by the introduction of the roughnessparameters.According to the problem that there was a lack of effective detection methods andscientific standardized basis on the fracture surface defect (fracture surface large area shedparticles) in production practice, the connecting rod fracture reconstruction software wasdeveloped based on MFC application framework by combining fracture splittingexperiments with reverse engineering technology and using OpenGL core functions. Thefracture surface three-dimensional topography reappeared numerically in this software. Thefracture surface area was calculated quantitatively. Compared with the machined surface, thearea was increased above13%. Therefore, the method of evaluating fracture surface defectssize was proposed on the premise of not reducing the connecting rod bearing capacity, andthe permission scope of connecting rod fracture surface defect size for passenger cars wasgiven, which provided a scientific basis for reasonable evaluation of fracture surface quality.To simplify the specification process of the fracture surface defects, the roughnessparameters were introduced. And the function relation between the fracture surface defectpermission range and the contour roughness was established which was convenient forformulating control standards of different connecting rod fracture surface detect and actuallyapplying in production. |
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