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。     

Failure analysis on fracture of worm gear connecting bolts

The worm gear connecting bolts of refueling machines of a nuclear power plant, with implementing standard of ANSI/ASME B18.3 and ASTM A574-08 and strength grade of 10.9, fractured at the thread neck position after running for about 10 years, and means such as macro examination, chemical compositions analysis, hardness testing, metallographic examination and fracture analysis, were used to analyze the fracture property and reasons of the bolts. The results show that the fracture of the bolts is due to two-way bending fatigue fracture. Surface decarburization of the bolts and stress concentration at the bolt thread neck decreased the fatigue strength of this position and resulted in the initiation of fatigue cracks. By comprehensive analysis and stress estimating, it was concluded that the main reason for fracture of the bolts is that there was a big gap between the bolts and the bolt holes, which resulted in fatigue fracture of the worm gear connecting bolts.     

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.     

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。     

蜗轮连接螺栓断裂失效分析

装卸料机上的蜗轮连接螺栓材料为35钢,强度等级为10.9级,在设备运行大约10a后发生断裂。对断裂螺栓进行宏观、化学成分、硬度、金相、能谱和断口分析后得出,该螺栓的断裂性质为双向弯曲疲劳断裂,螺栓表面的脱碳和螺纹颈部的应力集中降低了该部位的疲劳性能。通过综合分析和螺栓受力估算后得出,螺栓断裂的主要原因是螺栓和内齿轮螺栓孔之间存在较大的间隙,使螺栓的受力状态和受力大小过早地发生了变化,造成连接螺栓疲劳断裂。     

1240MPa级Ti-38644高锁螺栓的拉伸疲劳增寿机理

用扫描电镜和能谱观测分析Ti-38644高强钛合金高锁螺栓的拉伸疲劳断口,揭示了高锁螺栓的疲劳裂纹萌生和扩展的微观特征和疲劳增寿机理。结果表明,Ti-38644高强钛合金高锁螺栓的疲劳断口包括疲劳裂纹萌生区、扩展区和瞬断区:疲劳裂纹从螺栓头下圆角滚压薄弱部位表面萌生,随后在基体中呈放射性扩展;进入扩展区后裂纹的尺寸由微观扩展至宏观,以疲劳条带扩展机制为主,同时也存在解理断裂。头下圆角处的变形层对Ti-38644高锁螺栓的疲劳寿命有显著的影响,变形层使Ti-38644钛合金高锁螺栓的疲劳寿命明显提高。通过微观组织与疲劳寿命的对比,探讨了Ti-38644钛合金高锁螺栓疲劳强化的作用机理。     

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。     

挤出机摩擦离合器螺栓断裂分析

某挤出机中气动摩擦离合器与减速器端连接的螺栓在使用过程中经常发生断裂,改用另一种材料的螺栓后情况未有很好改善。采用化学成分分析、力学性能测试、断口分析和金相检验等方法,对螺栓断裂的原因进行了分析。结果表明：起裂源位于螺栓的加工刀痕、表面擦伤处及因微动疲劳所致的螺纹微裂纹处,这些部位均存在应力集中,在振动作用下,萌生的裂纹不断扩展,使有效承载面积不断减小,最终引起螺栓疲劳断裂。     

Modelling fatigue crack propagation of a cracked metallic member reinforced by composite patches

The concept of fracture for material elements at front of a crack for fatigue crack propagation was extended to the fatigue crack propagation of a cracked metallic member reinforced with a composite patch in this paper. From static mechanics and linear elastic fracture mechanics, force transfer on a cracked member through a composite patch was analyzed and a formula connecting the stress intensity factor with crack length was obtained. Thereafter, a fracture model for fatigue crack propagation of a repaired cracked metallic member was proposed. A new expression for the fatigue crack propagation rate has thus been derived. The expression was verified objectively by the test data. It is in good agreement with the test results.     

Experimental and numerical study of fatigue crack growth of aluminum alloy 2024-T3 single lap simple bolted and hybrid (adhesive/bolted) joints

In this paper, the effects of tightening torque (clamping force) on the fatigue crack growth rate and stress intensity factors in cracked single lap simple bolted and hybrid (adhesive/bolted) joints have been studied experimentally and numerically. To do so, series of fatigue crack growth tests for two different amounts of tightening torque in Aluminum alloy 2024-T3 pre-cracked joints have been carried out to record the fatigue crack growth and also the fatigue life of specimens. In the numerical part, finite element method was employed to obtain the stress intensity factors and also the effective stress intensity factor ranges for different crack lengths to explain the behavior of fatigue crack propagation. It was found that the hybrid joint has longer fatigue crack growth life compared to the simple bolted joint at a given bolt tightening torque. The results also showed that a higher bolt tightening torque provides improved fatigue crack growth life for both types of the joints.     

快卸卡箍螺栓断裂原因分析

通过断口形貌观察、X射线能谱分析、金相检验和硬度检测等试验方法,对某燃油供油导管快卸卡箍螺栓的断裂原因进行了分析,并与螺栓冲击断口和氢致疲劳断口进行了比较分析。结果表明:该快卸卡箍螺栓断口特征与冲击断口和氢致疲劳断口明显不同,其断裂性质为应力腐蚀断裂,裂纹起源于螺栓光杆段的侧表面;螺栓表面加工粗糙且没有防护对裂纹的萌生有一定的影响。对螺栓表面进行防腐处理可有效避免该类故障的再次发生。     

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...     

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.     

Failure analysis of submersible pump system collapse caused by assembly bolt crack propagation by stress corrosion cracking

Mechanically fastened joints using bolts are critical components in submersible pump systems. These bolts are subjected to aggressive environments in oil wells. Tightening torque preload and motor's weight are the principal loads that bolts support mechanically plus an occasional pipe flexure. Additionally, a corrosive sulfide-rich water environment presents an extremely demanding chemical condition. A better understanding of those failure mechanisms affecting such components could provide more safety, as well as costs and time saving during the operation of pump systems in wells.An assembly bolt from a submersible pump was fractured during service. Failure was the result of stress corrosion cracking (SCC) originated by pit corrosion. Mechanical and optical tests were performed to identify property changes. SEM with EDS, XRF and OES analyses were used to characterize the material and crack propagation. The fractured bolt material specification was medium carbon steel, while the material specified by the manufacturer was Ni–Cu alloy. The origin of the crack was located on a stress concentration region, but its nucleation was a result of high corrosive conditions.     

Failure and Fracture Analysis of Bolt Assembled on the Fan Used in the Internal Combustion Engine

Three connecting bolts fractured, which were assembled on the fan used in the internal combustion engine. Detailed fractographic study and metallurgical analysis were focused on the fractured bolts. Fractographic and metallurgical studies indicate that the fracture surface and the microstructure of three bolts exhibit similar features. The fatigue fracture is the main failure mechanism of the bolts. Appearance of the micro-cracks in the thread tip of the fractured bolts makes the stress at the thread regions increase intensively so that the fatigue cracks initiated from the thread regions. Due to the presence of dynamic load, the bolts were never properly tightened during installation which should be responsible for the formation of the cracks.     

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.     

EH系统油动机螺栓断裂分析

采用宏、微观检验方法对电液压系统油动机的断裂螺栓进行了分析,确认螺栓机加工不当引起疲劳裂纹的萌生及扩展,加之螺栓受力不均匀加速了疲劳断裂过程,提出了预防螺栓疲劳开裂失效的防范措施及改进建议。     

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。     

Experimental results and finite-element predictions of the effect of nut geometry, washer and Teflon tape on the fatigue life of bolts

The effect of nut geometry, curved spring washer and a sealing material (Teflon tape) on the fatigue life of M12 and M16 ISO bolts was investigated. This was accompanied by the study of the axial and bending stress distribution in threads by numerical simulation of bolt and nut connections using the finite‐element method. The experimental results showed that the highest fatigue life is achieved for a slotted tapered nut. The presence of a spring washer also increases the fatigue life, providing that the correct tightening torque is applied to produce a pretension in the bolt. The use of Teflon tape as a filling material between engaging threads of the bolt and nut is shown to significantly increase the fatigue life. On the whole, the highest fatigue life is obtained for a slotted tapered nut using washer. Four fracture mechanisms were observed for bolt–nut connections during the fatigue tests.     

Statistical modeling of fatigue-crack growth in a nickel-base superalloy

Two fracture mechanics-based statistical models for the fatigue crack propagation of engine materials are developed. The models are applied to fatigue crack growth rate data for IN 100, a nickel-based superalloy used in F100 engine disks, at various elevated temperatures, loading frequencies and stress ratios. The lognormal and the randomized parameter models allow the incorporation of the statistical variability associated with crack growth data into the life prediction process. The statistical distributions of (i) the crack growth rate, (ii) the propagation life to reach a given crack size (iii) the crack size at any given service life have been derived. A correlation analysis is performed to compare the results of the statistical models with test data. The correlation is demonstrated to be very good.