连杆断裂剖分过程数值模拟及主要裂解缺陷分析
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摘要
发动机连杆裂解加工以断裂力学、应力集中理论及裂纹技术为基础，通过人造起裂源及其可控扩展实现连杆本体与盖端的快速断裂分离，利用获得的三维凹凸断裂面实现连杆体/盖合装时的精确定位及完美啮合，从而大幅度简化连杆加工工艺流程、降低制造成本，提高产品的装配质量及承载能力。
连杆裂解加工的核心工序主要包括预制切口、断裂剖分（也称胀断）及装配螺栓。连杆裂解加工要求断裂剖分时不能产生过大的塑性变形，并保证断裂面形态良好，以满足连杆工作时的定位精度及承载能力。然而，就力学角度而言，连杆裂解是一个三维裂纹萌生、扩展直至断裂的过程，断裂瞬间完成，影响因素复杂，易发生裂解缺陷且较难控制。
本文基于切口件断裂及小范围屈服线弹性断裂理论，利用Ansys/Ls-dyna软件数值模拟连杆断裂剖分过程，通过研究断裂过程中力学量场的变化、捕捉起裂点位置及裂纹扩展路径，系统分析了连杆断裂的影响因素、裂解缺陷的产生原因，并提出相应的控制手段及改善措施；基于逆向工程技术重构断裂面三维形貌，通过对断裂面的量化分析给出断裂面缺损的控制范围，为裂解缺陷控制及质量评价提供科学依据。
论文主要研究内容及取得成果如下：
（1）基于连杆裂解断裂模式及断裂力学理论，分析连杆切口裂纹萌生的原因及小范围屈服条件下裂纹尖端应力分布特点，确定最大主应力准则为连杆断裂的判据。
基于切口件断裂理论，分析了连杆切口裂纹萌生原因。在胀断载荷作用下，当切口附近的局部高应力达到材料损伤断裂应力时微裂纹萌生，符合传统强度理论。基于小范围屈服修正的线弹性断裂理论，分析了裂纹尖端附近的应力场。裂纹扩展时因塑性变形导致裂尖钝化并引起应力松弛效应，使应力分布有界。
结合连杆裂解断裂模式分析及断口微观形貌表明，切口裂纹萌生与钝裂纹扩展的微观断裂机制相同，最大主应力是引起断裂的主要因素，依此确定最大主应力准则作为数值模拟分析时裂纹萌生与扩展的统一判据。
（2）数值模拟研究连杆断裂剖分过程，系统分析裂纹萌生与扩展中的应力、应变、位移场，及载荷作用特点，明确了高应力范围、起裂点位置、塑性变形发生时刻及区域，为裂解质量控制提供理论依据。
构建连杆断裂剖分三维有限元分析模型，实验获取高碳微合金非调质钢C70S6的力学性能参数，并对连杆断裂过程中的应力、应变、位移场，及裂纹萌生与扩展特点进行数值分析。结果表明，在胀断载荷作用下，切口引入应力集中及三向拉应力，使连杆在低应力状态下实现准脆性断裂分离，起裂时局部高应力被限制在以切口顶端为中心的0.5mm范围内；由于连杆大头自由端面与厚度中面三维约束度不同，导致应力状态差异，中面切口顶端次表层处于较高的三向拉应力状态，裂纹萌生于此。
起裂前是为微裂纹萌生积蓄能量的阶段，故载荷作用时间相对较长，导致塑性变形主要发生在裂纹萌生前，集中位于切口顶端0.25mm范围内；而裂纹扩展至断裂所需时间极短暂，产生的断裂面边缘塑性变形量微小。塑性变形的发生，使连杆大头孔产生沿连杆轴线方向伸长、垂直轴线方向收缩的失圆现象。
（3）分析切口几何对应力集中程度、裂解力、起裂时间及塑性变形的影响，给出了切口几何参数的设计原则或优选范围。
切口系人为预制起裂源，其参数设计直接影响连杆断裂剖分精度及产品质量。数值模拟研究表明，切口深度h和曲率半径r对连杆断裂影响显著，随着切口深度h的增加或曲率半径r的减小，应力集中程度提高、裂解力降低、起裂时间提前、塑性区范围缩小；切口张角α对其影响相对较弱，当α等于30°左右时，裂解质量最佳。
结合实际提出切口深度的设计原则，即保证断裂分离后大头孔变形量在公差允许范围内（切口深度最小值）并满足大头孔精镗余量要求（切口深度最大值）；给出参数优选范围，其中r=0.1mm~0.2mm，α=30°~60°，对于轿车连杆h=0.4mm~0.5mm。
（4）分析螺栓孔及其加工位置对断裂剖分的影响，给出了螺栓孔位置设计的参考范围。
钻铰螺栓孔是连杆加工中必不可少的加工工序。研究表明，螺栓孔的存在，改变了面内约束效应，对连杆断裂过程的影响有利有弊。一方面，降低裂解力、减少两端面及大头孔壁塑性变形，并使切口顶端在中面位置上出现应力峰值点，有利于保证连杆在预定位置精确剖分；另一方面，增大了中面切口根部塑性区范围，引入断裂面孔边缘塑性变形，产生了消极影响。
综合分析结果，给出了螺栓孔位置设计的参考范围，即保证中面切口顶端至螺栓孔壁间厚度大于2mm条件下，螺栓孔偏向大头端内侧加工，可有效发挥螺栓孔对裂解加工的积极作用。
（5）研究断裂面质量缺陷的产生原因与易发生位置，提出了改善措施。
三维断裂面是连杆体/盖合装的定位基准，其质量好坏直接影响连杆装配精度及使用性能。基于断裂过程三维数值模拟分析，借助APDL进行后处理二次开发，提取裂纹扩展前缘信息，捕捉裂纹扩展路径并计算各阶段裂纹扩展速度，明确断裂面质量缺陷，如爆口、台阶、掉渣、裂纹分叉等的产生原因及易出现位置。
由于裂纹绕过螺栓孔后的重新汇合以及裂纹扩展中的压应力，使螺栓孔至连杆大头端外侧区域极易出现断裂面质量缺陷，如果保留大头端外侧圆弧状轮廓（即制坯过程中的锻造表面），尤其是在外缘轮廓与螺栓孔近乎形成等距曲线的条件下胀断，可降低断裂面质量缺陷的发生几率。
（6）研究连杆Ⅰ-Ⅱ复合型断裂过程，分析断裂线偏移的产生原因，并提出控制措施；分析断裂线偏移量与裂纹角的关系，给出轿车连杆许可偏载范围。
断裂线偏移是裂解加工中常见缺陷之一，影响连杆承载能力。数值模拟分析表明，发生断裂线偏移的根本原因是偏载导致理论裂纹面上出现剪应力，使连杆裂解由Ⅰ型断裂变为Ⅰ-Ⅱ复合型断裂，据此提出实际生产中的控制措施。进而研究偏载条件下断裂时起裂角、断裂线偏移量与裂纹角之间的关系，随着裂纹角的减小，起裂角绝对值及断裂线偏移量增大，据此给出轿车连杆允许的偏载范围，即载荷偏离连杆轴线应控制在2°范围内。
（7）基于逆向工程技术开发断裂面形貌三维重构软件，量化分析断裂面面积，提出断裂面缺损的规范方法；引入粗糙度参量，进一步探索实用化规范手段。
针对生产实践中对断裂面缺损（断裂面大面积颗粒脱落）尚缺乏科学规范依据的现状，将实验与逆向工程技术相结合，基于MFC程序框架，利用OpenGL核心函数开发了连杆断裂面重构软件，再现断裂面三维形貌，进而定量分析断裂面面积，与机加工平面相比，断裂剖分面积增幅在13%以上，据此提出以不降低连杆承载能力为前提来规范断裂面缺损尺寸，并给出轿车连杆断裂面缺损尺寸的许可范围，为合理评价断裂面质量提供科学依据。
为简化断裂面缺损规范过程，引入粗糙度参数，建立断裂面缺损许可范围与轮廓线粗糙度之间的函数关系，方便不同种类连杆断裂面缺损控制标准的制定及生产中应用。
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.
连杆裂解加工的核心工序主要包括预制切口、断裂剖分（也称胀断）及装配螺栓。连杆裂解加工要求断裂剖分时不能产生过大的塑性变形，并保证断裂面形态良好，以满足连杆工作时的定位精度及承载能力。然而，就力学角度而言，连杆裂解是一个三维裂纹萌生、扩展直至断裂的过程，断裂瞬间完成，影响因素复杂，易发生裂解缺陷且较难控制。
本文基于切口件断裂及小范围屈服线弹性断裂理论，利用Ansys/Ls-dyna软件数值模拟连杆断裂剖分过程，通过研究断裂过程中力学量场的变化、捕捉起裂点位置及裂纹扩展路径，系统分析了连杆断裂的影响因素、裂解缺陷的产生原因，并提出相应的控制手段及改善措施；基于逆向工程技术重构断裂面三维形貌，通过对断裂面的量化分析给出断裂面缺损的控制范围，为裂解缺陷控制及质量评价提供科学依据。
论文主要研究内容及取得成果如下：
（1）基于连杆裂解断裂模式及断裂力学理论，分析连杆切口裂纹萌生的原因及小范围屈服条件下裂纹尖端应力分布特点，确定最大主应力准则为连杆断裂的判据。
基于切口件断裂理论，分析了连杆切口裂纹萌生原因。在胀断载荷作用下，当切口附近的局部高应力达到材料损伤断裂应力时微裂纹萌生，符合传统强度理论。基于小范围屈服修正的线弹性断裂理论，分析了裂纹尖端附近的应力场。裂纹扩展时因塑性变形导致裂尖钝化并引起应力松弛效应，使应力分布有界。
结合连杆裂解断裂模式分析及断口微观形貌表明，切口裂纹萌生与钝裂纹扩展的微观断裂机制相同，最大主应力是引起断裂的主要因素，依此确定最大主应力准则作为数值模拟分析时裂纹萌生与扩展的统一判据。
（2）数值模拟研究连杆断裂剖分过程，系统分析裂纹萌生与扩展中的应力、应变、位移场，及载荷作用特点，明确了高应力范围、起裂点位置、塑性变形发生时刻及区域，为裂解质量控制提供理论依据。
构建连杆断裂剖分三维有限元分析模型，实验获取高碳微合金非调质钢C70S6的力学性能参数，并对连杆断裂过程中的应力、应变、位移场，及裂纹萌生与扩展特点进行数值分析。结果表明，在胀断载荷作用下，切口引入应力集中及三向拉应力，使连杆在低应力状态下实现准脆性断裂分离，起裂时局部高应力被限制在以切口顶端为中心的0.5mm范围内；由于连杆大头自由端面与厚度中面三维约束度不同，导致应力状态差异，中面切口顶端次表层处于较高的三向拉应力状态，裂纹萌生于此。
起裂前是为微裂纹萌生积蓄能量的阶段，故载荷作用时间相对较长，导致塑性变形主要发生在裂纹萌生前，集中位于切口顶端0.25mm范围内；而裂纹扩展至断裂所需时间极短暂，产生的断裂面边缘塑性变形量微小。塑性变形的发生，使连杆大头孔产生沿连杆轴线方向伸长、垂直轴线方向收缩的失圆现象。
（3）分析切口几何对应力集中程度、裂解力、起裂时间及塑性变形的影响，给出了切口几何参数的设计原则或优选范围。
切口系人为预制起裂源，其参数设计直接影响连杆断裂剖分精度及产品质量。数值模拟研究表明，切口深度h和曲率半径r对连杆断裂影响显著，随着切口深度h的增加或曲率半径r的减小，应力集中程度提高、裂解力降低、起裂时间提前、塑性区范围缩小；切口张角α对其影响相对较弱，当α等于30°左右时，裂解质量最佳。
结合实际提出切口深度的设计原则，即保证断裂分离后大头孔变形量在公差允许范围内（切口深度最小值）并满足大头孔精镗余量要求（切口深度最大值）；给出参数优选范围，其中r=0.1mm~0.2mm，α=30°~60°，对于轿车连杆h=0.4mm~0.5mm。
（4）分析螺栓孔及其加工位置对断裂剖分的影响，给出了螺栓孔位置设计的参考范围。
钻铰螺栓孔是连杆加工中必不可少的加工工序。研究表明，螺栓孔的存在，改变了面内约束效应，对连杆断裂过程的影响有利有弊。一方面，降低裂解力、减少两端面及大头孔壁塑性变形，并使切口顶端在中面位置上出现应力峰值点，有利于保证连杆在预定位置精确剖分；另一方面，增大了中面切口根部塑性区范围，引入断裂面孔边缘塑性变形，产生了消极影响。
综合分析结果，给出了螺栓孔位置设计的参考范围，即保证中面切口顶端至螺栓孔壁间厚度大于2mm条件下，螺栓孔偏向大头端内侧加工，可有效发挥螺栓孔对裂解加工的积极作用。
（5）研究断裂面质量缺陷的产生原因与易发生位置，提出了改善措施。
三维断裂面是连杆体/盖合装的定位基准，其质量好坏直接影响连杆装配精度及使用性能。基于断裂过程三维数值模拟分析，借助APDL进行后处理二次开发，提取裂纹扩展前缘信息，捕捉裂纹扩展路径并计算各阶段裂纹扩展速度，明确断裂面质量缺陷，如爆口、台阶、掉渣、裂纹分叉等的产生原因及易出现位置。
由于裂纹绕过螺栓孔后的重新汇合以及裂纹扩展中的压应力，使螺栓孔至连杆大头端外侧区域极易出现断裂面质量缺陷，如果保留大头端外侧圆弧状轮廓（即制坯过程中的锻造表面），尤其是在外缘轮廓与螺栓孔近乎形成等距曲线的条件下胀断，可降低断裂面质量缺陷的发生几率。
（6）研究连杆Ⅰ-Ⅱ复合型断裂过程，分析断裂线偏移的产生原因，并提出控制措施；分析断裂线偏移量与裂纹角的关系，给出轿车连杆许可偏载范围。
断裂线偏移是裂解加工中常见缺陷之一，影响连杆承载能力。数值模拟分析表明，发生断裂线偏移的根本原因是偏载导致理论裂纹面上出现剪应力，使连杆裂解由Ⅰ型断裂变为Ⅰ-Ⅱ复合型断裂，据此提出实际生产中的控制措施。进而研究偏载条件下断裂时起裂角、断裂线偏移量与裂纹角之间的关系，随着裂纹角的减小，起裂角绝对值及断裂线偏移量增大，据此给出轿车连杆允许的偏载范围，即载荷偏离连杆轴线应控制在2°范围内。
（7）基于逆向工程技术开发断裂面形貌三维重构软件，量化分析断裂面面积，提出断裂面缺损的规范方法；引入粗糙度参量，进一步探索实用化规范手段。
针对生产实践中对断裂面缺损（断裂面大面积颗粒脱落）尚缺乏科学规范依据的现状，将实验与逆向工程技术相结合，基于MFC程序框架，利用OpenGL核心函数开发了连杆断裂面重构软件，再现断裂面三维形貌，进而定量分析断裂面面积，与机加工平面相比，断裂剖分面积增幅在13%以上，据此提出以不降低连杆承载能力为前提来规范断裂面缺损尺寸，并给出轿车连杆断裂面缺损尺寸的许可范围，为合理评价断裂面质量提供科学依据。
为简化断裂面缺损规范过程，引入粗糙度参数，建立断裂面缺损许可范围与轮廓线粗糙度之间的函数关系，方便不同种类连杆断裂面缺损控制标准的制定及生产中应用。
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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