Fatigue in engine connecting rod bolt due to forming laps

An analysis was performed to asses the failure root cause of an automotive diesel engine which experienced collapse only 6 month after revision. The connecting rod bolts torque disassembly was monitored and fractured parts were selected to laboratory fracture analysis. It was verified with fatigue rupture of one of the fourth connecting rod bolt. Tensile tests were performed in four of the remaining connecting rod bolts. During this procedure, it was verified another bolt with fatigue crack propagation an indication that the first fatigued bolt did not have suffer torque relaxation. A finite element analysis was performed in connection with an analytical fracture mechanics approach aiming to evaluate the relation between tightening force and fatigue crack propagation in connecting rod bolts. The engine collapse occurred due to forming laps in the grooves of the bolt shank. Finally, some design improvements were suggested for avoid future failures: a gap in the groove length at the connecting rod cap interface, enough to avoid combination of forming laps and higher stress amplitude; increase of the bolt torque assembly to reduce stress amplitude.     

不同装夹方式下的连杆疲劳寿命

目的分析大头间隙配合及大头小头均间隙配合这2种不同装夹方式,对连杆疲劳寿命的检测产生的不同的结果。方法采用MTS880±500 kN和MTS322±250 kN疲劳试验系统,采用不同的装夹方式模拟发动机工作状态,并通过"可靠性试验SAFL方法"进行数据处理。结果采用大头间隙配合装夹的连杆失效概率为5.2×10-5,大头小头均间隙配合装夹的连杆失效概率为0.87×10-5;并且2种装夹方式会影响连杆疲劳试验后的断裂位置。结论连杆疲劳试验中,采用大头小头均间隙配合装夹方式得到的连杆失效概率,较大头间隙配合装夹方式更小。进行疲劳试验需谨慎选择装夹方式。     

甲烷制冷压缩机配件断裂分析

对甲烷制冷压缩机断裂活塞杆和连杆的材料成分,力学性能,显微主其断口和应力进行了试验分析,确定了其断裂模式为疲劳断裂,调质热处理工艺不当和活塞杆连接螺纹根部应力过大是产生疲劳断裂的主要原因,提出了预防断裂的措施。     

Inverse Reconstruction of Fracture Splitting Connecting Rod and its Strength and Fatigue Life

Journal of Failure Analysis and Prevention - To investigate the strength and fatigue life of a fracture-split connecting rod, this study combines experiments on connecting rod fracture splitting,...     

Failure analysis of a special vehicle engine connecting rod

This paper presents failure analysis of the connecting rod used in a 12-cylinder diesel engine set on a special vehicle. The fracture of the connecting rod occurred during the engine test in laboratory conditions. Chemical and metallographic analysis as well as mechanical testing have confirmed that the material properties of the connecting rod met the requirements of standard specification and technical documentation. Linear finite element (FE) analysis was performed to evaluate stress state of the connecting rod under maximum load. The results of FE analysis showed that the position of the fracture is consistent with the zone of highest stress. Fractographic analysis has not been able to reveal the main cause of mechanism of fracture due to substantially damaged fracture surface.Inadequate machining, absence of polishing and highest stress in the region of fracture were identified as the main causes of failure. Finally, the engine had been working at maximum load for a longer period of time that also led to the breakage of the connecting rod.     

20CrMo连杆断裂失效分析

采用光学金相和电子探针分析仪等分析手段，对20CrMo嘉陵摩托车连杆的断裂进行了分析。结果表明：连杆在热处理过程中在其表面层形成了粗大的马氏体针状组织是造成断裂的主要原因，并在其显微断口上形成沿晶断口。     

柴油机连杆盖及连杆螺栓断裂分析

采用化学分析,金相检验及力学性能测定方法对汽车柴油机在运行中发生的连杆盖及上、下螺栓的早期断裂进行了分析。结果表明,连杆盖首先发生疲劳断裂,随后导致连杆螺栓的断裂,而这种断裂既与应力有关,又与零件的加工工艺有关。     

QT700-2球墨铸铁发动机曲轴失效分析

目的某厂生产的QT700-2球墨铸铁曲轴在路试过程中出现断裂,需寻找失效原因并提出解决措施。方法通过应用金相组织分析、化学成分分析、表面残余应力测试和力学性能测试等方法,对该曲轴的失效原因进行了分析。结果测试后分析结果表明,曲轴是在较大扭转循环载荷下,在第四连杆颈滚压圆角边缘多点萌生裂纹而导致疲劳断裂是其失效的主要原因。结论建议改进热处理工艺,保证组织的均匀性;改进加工工艺,减少应力集中;并强化表面残余应力以提高曲轴的疲劳寿命。     

压缩机连杆断裂失效分析

某炼油厂焦化装置富气压缩机连杆发生断裂。采用化学成分分析、金相检验、力学性能测试和断口分析等方法对断裂连杆进行了分析。结果表明,在交变栽荷的作用下,连杆的小头孔内应力集中处产生微裂纹并成为裂纹源,然后发生裂纹扩展,最终导致连杆小头处发生疲劳断裂。     

Failure analysis of a failed connecting rod cap and connecting bolts of a reciprocating compressor

Connecting rod cap and connecting bolts of a reciprocating compressor are subjected to complex dynamic loads therefore they are of critical machine elements. The causes for the failure of connecting rod cap and connecting bolts after approximately 175,200 h in service have been investigated. To determine the failure mechanism of the connecting system and to figure out which of the connecting rod cap and connecting bolts was broken first, material characterization and numerical analysis are conducted on the connecting rod and connecting bolts. Scanning electron microscope (SEM) and optical microscope are applied to analysis the macro and microstructures. The chemical composition and metallographic structure investigation are carried out to study characteristics of the materials. Tensile tests, hardness and impact tests are performed to check the mechanical properties of the connecting rod cap and three connecting bolts. A finite element analysis of the connecting system is employed to evaluate the overall stress distribution with maximum stress criterion. Results of this investigation indicate that the reason of failure was high cycle fatigue and the initial crack location was consistent with high stress concentration at the curvature mutation position of connecting rod cap which was broken before the connecting bolts.     

连杆断裂失效分析及锻造折叠缺陷控制研究

目的 确定连杆断裂失效模式、机理和原因,找到有效预防连杆锻造缺陷的措施。方法 针对连杆断裂失效问题,在对连杆合格件和故障件进行符合性对比试验分析的基础上,对故障件进行断裂失效模式、机理和原因分析,并对连杆锻造折叠缺陷产生类型、机理、原因、条件和时段进行分析。结果 只有在连杆制坯坯料形状与尺寸不合理、制坯坯料在预锻模膛中位置不合理、预锻形状与尺寸不合理、预锻成形打击力或变形速度不合理、终锻形状与尺寸不合理、锻模导向功能失效等多种极端不利因素同时存在的条件下,才具有产生复合型锻造折叠缺陷的可能性。结论 通过连杆锻造过程控制和锻造首锻控制,可有效预防锻造折叠缺陷产生,满足锻件高优质性、高可靠性和高有效性要求。     

Failure analysis and fatigue performance evaluation of a failed connecting rod of reciprocating air compressor

A connecting rod of a reciprocating air compressor is subjected to complex dynamic loads therefore it is of a critical machine element. Failure of this type of connecting rod was reported to occur at the rounded fillet of the big connecting rod end. The present investigation is aimed to identify the cause of failure and to evaluate fatigue performance of the failed connecting rod. Factors affecting failure including structural design, type of material and dynamic loads were assessed using standard failure analysis method. This method included analysis of chemical composition, microstructural examination using optical microscopy, hardness and tensile tests, scanning electron microscopy (SEM) fractography and stress analysis. To evaluate fatigue performance, fatigue crack growth rate (FCGR) test was performed using a sinusoidal load with a constant load amplitude. Results of this investigation suggest that the cause of failure was low cycle fatigue and the initial crack location was consistent with high stress concentration, i.e. fillet radius. From metallurgical point of view, the connecting rod was made of cast steel, not forged steel, with a considerable number of non metallic inclusions such as Al₂O₃, SiO₂ and FeO. These inclusions which were present near the surface of the rounded fillet seemed to act as stress raiser and they were responsible for crack initiation. In addition, the presence of inclusions could increase fatigue crack growth rate, da/dN (in m/cycle) as indicated by a high value of Paris’ constant (n), typically of 5.2141.     

进口氢气压缩机螺栓断裂原因分析

通过宏观检验、化学成分分析、金相检验以及断口分析等方法对氢气压缩机的地脚螺栓和连接螺栓发生断裂的原因进行了分析.结果表明:由于压缩机机座安装倾斜,造成地脚螺栓受剪切应力首先断裂;连接螺栓受振动引起的剪切应力和轴向拉应力共同作用,并且其预紧应力超过最大允许预紧应力,最后发生疲劳断裂;另外螺栓本身存在的材料缺陷致使强度不足...     

汽车发动机曲轴弯曲疲劳试验方法研究

目的研究两种常用的疲劳试验加载原理对疲劳试验结果的影响,为曲轴选材、设计和制造的各阶段开展弯曲疲劳试验提供技术支持。方法在两种加载方式下采用同批次曲轴进行弯曲疲劳试验,利用成组法得到疲劳试验结果,针对试验加载原理和试验数据开展分析和讨论。结果电液伺服加载方法的试验精度更准确,而电磁谐振加载方法可以大幅度缩短试验周期。电液伺服加载和电磁谐振加载试验的疲劳极限分别为1140.4 N·m和1189.4 N·m。两种加载方法下,曲轴的失效情况均为连杆颈辊压槽处断裂。结论两种加载方法对曲轴弯矩的加载效果相同,试验结果的相对误差为4.3%。在曲轴的仲裁试验中或有争议的情况下使用,推荐使用电液伺服加载方法。在使用电磁谐振加载方法时须严格控制标定误差。曲轴连杆颈辊压槽是曲轴的薄弱环节,应严格控制辊压槽的机加工质量。     

内燃机车用柴油机连杆螺钉断裂分析

内燃机车在检修重组后运行约一个月,其柴油机的两根连杆螺钉发生断裂。采用化学成分分析、硬度测试、断口分析和金相检验等方法,对两螺钉断裂的原因进行了分析。结果表明:两螺钉的断裂均为疲劳型断裂,其中一根螺钉先行断裂,其断裂的根本原因可能是螺钉紧固处于不合理的状态所致。     

变速箱二轴的断裂分析

汽车变速箱二轴在工作中发生多起断裂事故。采用金相检验、扫描电镜和化学成分分析等手段，对断裂变速箱二轴进行了检验分析。结果表明，轴的断口具有明显的疲劳特征，疲劳源位于二轴花键槽根部。由于渗碳淬火工艺不当，在二轴花键槽根部的组织中产生了网状碳化物和粗大的针状马氏体，在外力作用下形成沿晶显微裂纹并扩展，是导致二轴疲劳断裂的主要原因。     

Analysis of the fatigue failure of a mountain bike front shock

We present an analysis of a mountain bike front shock failure. The failure of the 1-year-old shock occurred catastrophically as the bike was ridden off of a 1-m drop. The failure was the result of fast fracture through both shock tubes at the location where the tubes were press fit into the shock upper crown. Examination of the fracture surfaces of the tubes revealed regions of fatigue crack growth that nearly penetrated the entire thickness of both tubes. An estimate of the forces during use, coupled with stress analysis, revealed three stresses near the fracture site—axial compression, bending, and hoop stresses. During operation, the axial compressive stress is negligible while the hoop and bending stresses are significant. Based on fracture mechanics, and an estimate of the bending stress from a 1-m drop, it is confirmed that the fatigue cracks present on the fracture surface were large enough to induce fast fracture. Prior to the existence of the fatigue cracks, the stresses were magnified locally near the fracture site by a significant stress concentration caused by the sharp transition from the shock tube to the crown. The fatigue cracks initiated at a circumferential location in the tube commensurate with high tensile bending stress and the stiffest region of the crown (highest stress concentration). Based on the evidence, the most probable cause of the bike shock fatigue failure was the shock design, which facilitated high local stresses during use.     

进料泵传动轴断裂分析

采用显微组织分析、扫描电镜分析和拉伸试验等方法对进料泵传动轴的断裂进行了失效分析。结果表明,泵轴断裂属疲劳失效,轴中段键槽处在制造过程中存在淬火裂纹是导致疲劳断裂的主要原因。     

某天然气压缩机组联轴器断裂故障诊断

大型往复式压缩机的曲轴-连杆-活塞结构因其产生的不平衡力,极易发生疲劳断裂,对断裂原因进行有效分析对故障消除措施的制定具有重要作用。本文针对某气田压缩机组联轴器的多发性断裂故障,首先对断裂螺栓进行理化分析,再通过仿真计算系统扭振固有频率,并对系统进行扭振及轴功率、扭矩测试,诊断出故障产生原因是系统第五阶次扭振共振且峰值扭矩超标,继而采用在压缩机组光轴段加装一个惯性飞轮对系统扭振特性进行调频处理,使螺栓断裂故障得以根治。本文将理论和测试结合,该故障诊断方法可用于其他类似问题,具有理论和工程实际意义。     

某电传操纵系统弹簧拉杆可调叉形接头断裂原因

某飞机电传操纵系统弹簧拉杆可调叉形接头在运行过程中发生断裂,通过宏微观分析和金相检验等方法对拉杆接头的断裂原因进行了分析。结果表明:拉杆接头断裂为疲劳断裂,断裂的根本原因是其显微组织不均匀的同时出现了魏氏组织,使接头的抗疲劳性能严重降低,导致疲劳裂纹的萌生与扩展;而拉杆的受力不均匀则加速了其疲劳裂纹的扩展。