| Fracture splitting technology emerged in the nineties of20thcentury, it has developed formore than20years till now, and the technology has become more mature, the equipment andproduct cracking quality is improving. This Processing method saving a lot of equipment andprocesses, reducing a lot of costs of tools，and does not require any finishing treatment to thefracture surface. and, The fracture surface is uneven, so positioning accuracy and assemblyprecision is high, loading capacity is strong. Compared with the traditional processing methods,it has an unparalleled advantage. With the improvement of the world auto industrytechnological level, fracture splitting technology has developed in many sides. Now it has beenused into crankcase bearing block not only connecting rod, the crankcase bearing block materialis brittle, and many bearing located in the same axis, so the crankcase bearing is more suitablefor fracture splitting, but it will ask for high process accuracy. Based on this, the paper researchin the following aspects：1. There are many kind of crankcase material, aluminum alloy is used in few highly gradeengine, except this, most of engine are using brittle cast iron materials. This paper selects threetypical cast iron materials: the vermicular Iron RuT380, gray cast iron HT250and ductile ironQT500, the fracture limit of three materials in the tensile test is410MPa,261MPa,606MPa.2. This paper analysis that the factors of starting notch cutting by mechanical broaching orlaser cutting ways which affect the cracking load. accordding to size of the crankcase bearing ofJetta, use ABAQUS software to modeling, take1/4of the whole bearing as we used. Inaccordance with its load in the splitting process, set up contacts and constraints. Because ofstress concentration features of crack region, use part division to simulate to improve theaccuracy of calculation.3. Take RuT380materials for example,judge the load of the split time by J-integral theoryand theory of maximum tensile stress,the result is144.92KN and139.87KN, and experimentsresult is137KN,mistakes were5.78%and2.09%respectively. For mistake is low, optimize the geometry parameters of starting notch,notch depth range from0.3~0.8mm, curvature radiusrange from0.05~0.3mm, angle range from0°~90°. By analyze how geometry parameters ofstarting notch influence cracking load, we know that cracking load grow when depth increase, orcurvature radius reduce. the angle influence is not obvious. Considering the equipment cost,choose broaching notch parameters as this: h=0.5~0.6mm, α=60°~90°, r=0.1~0.2mm;choose laser cutting notch parameters as this: h=0.5~0.6mm, α=0°~20°, r=0.05~0.1mm.4. Use the same method to judge other two kinds of materials, HT250and QT500.Optimize the geometry parameters of starting notch, for gray cast iron HT250, can be treated aslinear elastic materials, caculate stress concentration factor to verify. Optimization range HT250materials is that: broaching:h=0.3~0.5mm,r=0.1~0.3mm, α=60°~90°, the laserprocessing: h=0.3~0.5mm, r=0.05~0.1mm, α=0°~20°.For the QT500materials,broaching:h=0.6~0.7mm,r=0.1~0.2mm, α=60°~90°, the laser processing: h=0.6~0.7mm,r=0.05~0.1mm, α=0°~20°.5. Analyze the microstructure of HT250and QT500materials.Graphite morphology ofHT250is typical sheet with low strength, equal to micro-cracks, the matrix structure is pearliteand cutted by sheet graphite seriously. leading to the HT250low strength, brittle. Scan fracturesurface by SEM, observed the river-like cleavage pattern and the cleavage steps on fracturesurface. We can know the splitting process is brittle. Graphite morphology of QT500is globular,the matrix structure is continuous pearlite-ferrite. Strength is high. SEM shows river-likecleavage pattern and the cleavage steps on fracture surface. the fracture surface of the substrateshowing the cleavage of the river-like pattern, explain that split through the grain boundary.Around the graphite, the fracture surface is smooth, explain that split along the grain boundary.And there are also exist many dimples and tearing edges. So prospective cleavage fracture ishappen.6. through the expansion element method to Simulate the crack initiation and crackpropagation process of the starting groove through the expansion element method（XFEM）,obtain the three-dimensional fracture surface morphology, and discuss the influenceof the problems and defects in the simulation on the fracture quality.7. Take QT500for example, analyze the influence of notch strain and bearingdeformation by the loading speed in dynamic loading process. simulation results show that whenthe loading speed increased to500KN/s or more, crack strain decreases, the displacement of thetop of bearing hole (deformation) is also reduced, indicating that the cracking performance improved significantly. |
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