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 of a SAE 4340 steel locking bolt

Several SAE 4340 steel locking bolts used to assemble speed reducer housings fractured after a few hours of operation. Micrographic and macrographic analyses, scanning electron microscopy techniques, tensile, impact and hardness testing were used to fully characterize the component and material properties. Stress calculations were performed using both Neuber analysis and Finite Element Analysis (FEA) and the results were compared. Cracks nucleated at the root of the last engaged thread due to a combination of high local stresses in this region, surface defects, non-uniformity of the thread root and low toughness of the material. After nucleation, the crack propagated by fatigue until the catastrophic failure.     

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.     

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。     

Failure analysis of cylinder clamping rods in diesel engines

This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200–250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.     

High Strength Engineering Fasteners: Design for Fatigue Resistance

One of the most frequently encountered causes of failure in engineering fasteners is fatigue. Typical locations for the fatigue of fasteners are the stress concentrations represented by the head to shank fillet, the first engaged thread and the thread runout. These locations are examined in relation to design rules known for over 60 years to be effective in reducing the stress concentration effect, but seldom used in engineering bolts, despite their relative simplicity. These design rules are also examined in relation to common fastener standards and their silence with regard to fatigue considerations.     

吊车转盘连接螺栓断裂分析

吊车转盘后部的连接螺栓发生断裂,通过化学成分分析、宏观和微观检验等方法对断裂原因进行了分析。结果表明：螺栓为疲劳断裂,螺纹根部的细小裂纹是导致螺栓发生疲劳断裂的主要原因;螺栓松动后受到弯曲载荷是引起螺栓发生疲劳断裂的诱因。     

C/SiC复合材料螺栓螺牙承载能力

为了对C/SiC复合材料螺栓螺牙的承载能力进行评估,采用有限元法和刚度折减方法对C/SiC复合材料螺牙抵抗拉脱的能力进行了研究。结果表明:当齿合螺牙数大于6时,再增加齿合螺牙数已不能有效地提高螺牙的初始拉脱载荷;增大螺距会降低螺牙初始拉脱强度,因而也不能显著地提高螺牙的初始拉脱载荷;在螺距与螺栓直径之比保持常数的情况下,螺牙的初始拉脱载荷与螺栓直径的平方成正比;而螺牙的极限拉脱载荷则近似正比于齿合螺牙数、螺距及螺栓直径。     

CRACK SHAPE EVOLUTION STUDIES IN THREADED CONNECTIONS USING A.C.F.M.

Abstract— Crack shape evolution has been studied in high strength, rolled thread, bolts. It has been found that, using the Crack Microgauge, it is possible to detect and size fatigue cracks, located at the thread root, from depths of less than 0.1 mm up to fracture. This work required a special probe system to be developed and also a theoretical solution for the a.c. field distribution in a thread. Some of the work and results are briefly described in this technical note.     

铁路货车钩缓装置钩尾扁销螺栓断裂原因分析

铁路货车钩缓装置中的钩尾销螺栓在螺杆近方头一侧根部位置发生断裂。采用微观分析方法分析了该螺柱断裂原因。综合分析表明：该断裂螺栓方头一侧可能仅局部加热后锻打成四方头,并发生过热,造成在方头一侧的螺柱根部形成马氏体组织和魏氏体组织过渡的U型区域。不良组织的存在增大了钢的脆性,冷加工性能大大降低,在U型区域和螺柱表面交界处萌生了微细裂纹。当钩尾扁销螺栓在列车运行过程中受到过大的冲击时,该螺栓上的微细裂纹在螺栓靠近方头一侧存在的沿轴向的拉应力和螺栓正常紧固力的合力联合作用下张开并迅速扩展,造成螺栓断裂。     

汽轮机高压外缸结合面双头螺栓断裂分析

某电厂汽轮机高压外缸结合面双头20Cr1Mo1VTiB钢螺栓在安装时发生断裂,通过断口分析、化学成分分析、金相检验以及力学性能测试等方法对螺栓的断裂原因进行了分析。结果表明:该螺栓钢存在严重的显微组织缺陷,力学性能指标不符合技术条件要求,减弱了螺栓的承载能力,螺纹处的应力集中效应会加剧螺栓的缺口敏感性,导致螺栓在安装时于螺纹处发生脆性断裂。     

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.     

Stress distribution and performance of threaded bolts in gasketed flange joints

The performance of the flange joint depends on the relaxation of load in bolts. This in turn affects the gasket stress and control of leakage. A finite element model of flange joint with threaded bolt-nut fastener having zero helix angle is developed and analyzed for stress distribution in the threaded bolt. The stress concentration factor at the thread root region is observed to be non-uniform along the circumference due to the bending behavior of the bolt in the flange joint. The maximum stress is observed in the first engaged thread. The variation in bolt load due to internal fluid, thermal and external loads are also investigated for both single and twin-gaskets (two concentric half-sized single gaskets). The gasket material and gasket configuration are observed to be potential factors causing variation in the distribution of load in the flange joint. In a flange joint subjected to external bending load, the maximum stress concentration in the bolt is observed to be in the first thread, instead of the first engaged thread. Under external bending load, the flange joint loses its structural integrity before the traces of leakage occur in the joint.     

Metallurgical failure analysis of fasteners in an impeller assembly

This paper examines a failure analysis of the bolts from a failed joint between an impeller blade and a rotating assembly unit. The bolts failed due to poor thread manufacture and installation. Sharpened thread roots led to high stress concentrations that favored crack initiation. An oddly shaped thread profile allowed friction between mismatched thread surfaces. Poor installation procedures allowed for the possibility of overtightening to nucleate cracks in the head-to-shank interface (which had a smaller radius and therefore a higher stress concentration) and possibly also in the thread roots. Each of these influences contributed to crack initiation in the bolts. After cracks had formed, bending fatigue then propagated the nucleated cracks to final fracture. The failure analysis also recommended using bolts with rolled threads, which allow a more complete fit between mating male and female threads, and assuring that an appropriate preload is placed on bolts during installation.     

Experimental characterization of the bending fatigue strength of threaded fasteners

The fatigue strength in bending of pre-stressed steel bolts is investigated and compared to the fatigue strength in axial tension. The strength is measured in terms of maximum engineering stress amplitude, neglecting any stress concentration in the threads. The experimental results reveal that the fatigue limit is 76% higher in bending than in axial tension. A finite element model is used to compute the stress state in the threaded region for both axial tension and bending. It allows fitting a volume based weakest link model to the experimentally observed failure probabilities. Based on the good fit of the weakest link model it is argued that randomly distributed defects in the highly stressed thread root determine the fatigue strength.     

叠层膜片联轴器螺栓断裂原因分析

采用化学成分分析、显微组织检验、硬度测定、断口宏观和微观观察及能谱分析等方法对叠层膜片联轴器断裂螺栓进行了分析.结果表明,螺栓根部的退刀槽因加工质量差造成应力集中,加上材料内部有夹杂物,同时在叠层膜片联轴器安装上有问题及机组中其他部件(如风机叶片和齿轮箱)也有损坏等各种因素的存在,该叠层膜片联轴器螺栓组使用于频繁起动的大载荷工作状态,一旦有超过该螺栓所能承受的工作应力就会引起疲劳而发生断裂,最终导致叠层膜片联轴器损坏.给出了正确设计、制造、安装和使用叠层膜片联轴器的建议.     

Failure Analysis of a Mobile Crane: A Case Study

This paper presents metallurgical failure analysis method to find the root cause of a mobile crane turret bolts failure. Focus of the metallurgical analysis was to identify the root causes of the failure of turret bolts which led to the collapse of the mobile crane. The turret bolts were made of high strength steel. Comprehensive engineering analysis suggested that the turret bolts failure was initiated with fatigue. The fatigue was most likely initiated by the corrosion at the bolt surface and accelerated by the loosening of bolts due to long servicing life and/or fluctuating tensile load. Based on the investigations, recommendations are provided to guide the operation of the mobile crane to prevent the premature failure.     

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。     

The effect of manufacturing processes on the fatigue lifetime of aeronautical bolts

Many aeronautical fastners are exposed to cyclic stresses during service. Therefore, such parts are usually designed for limited fatigue lifetime. Various combinations of process type and sequence may be employed to produce threads, each resulting in different fatigue properties. Specifications of aircraft bolts often require production of threads by heat treatment followed by rolling, in order to improve the fatigue properties. Unfortunately, these specifications are not always followed to the letter. Therefore, for either quality assurance or failure analysis purposes, it is important to be able to determine unambigiously the process by which threads were produced. The objectives of this work were to study the effect of varied thread manufacturing process type and sequence on the mechanical properties of AISI 4340 stud bolts, and to develop a laboratory procedure for distinguishing between them. Threads were produced on heat-treated and non-heat-treated stud bolts either by machining or cold-rolling. The non-heat-treated bolts were subsequently heat-treated. All bolts were then subjected to mechanical testing (static tensile, dynamic fatigue, hardness and microhardness tests), metallographic and fractographic examinations. While the fatigue properties were significantly affected by the manufacturing process used, no effects on the tensile strength of the bolt were observed. Metallographic inspection and microhardness testing, but not fractographic inspection, were found to be effective for distinguishing between different manufacturing procedures.     

Failure Analysis of a Diesel Engine Gear System Consisting of Camshaft and Crankshaft Gears

A failure investigation was conducted on a diesel engine gear system consisting of a driven camshaft and drive crankshaft gears that were used in a truck. The gears are made from a nitrided 42CrMo steel. Adjacent teeth fracture and plastic deformation regions appeared on the gears after a 400 h run test of the gear system. Fractography indicates that fatigue fracture is the dominant failure mechanism for the gear teeth. Although the appearance of needle-like nitrides in the nitrided layer and the narrow depth of the compound layer may decrease the fatigue strength of the camshaft gear, these do not suffice to lead to the premature fracture of the gear teeth. Geometrical analysis of the gears was performed and compared with an analysis of unfailed gears that had experienced a run test for 1800 h. The comparison reveals that the small fillet radius at the root area of the camshaft gear concentrated the stresses and is mainly responsible for fatigue fracture of the teeth. The camshaft gear is the component that initiated trouble in the gear system. The appearance of severe plastic deformation on the gear faces is caused by the fractured teeth crushing the teeth faces and being embedded in the grooves between teeth.