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.     

三峡升船机卧倒门支铰座螺栓联结载荷特性分析与强度设计

针对三峡升船机在试运行过程中卧倒门支铰座螺栓发生断裂故障,分析得出故障的主要原因是螺栓联结的预紧力矩（1 233 N·m）过小、A2-70-M36螺栓强度不够。基于此,提出了一种提高三峡升船机下闸首卧倒门支铰座螺栓联结可靠性的方法。首先,分别分析计算了升船机卧倒门在无水关门、无水开门、有水关门和有水开门四种工况下的液压启闭机油缸的作用力及支铰座联结螺栓的最大工作载荷,其中有水关门工况考虑了0.8,0.7,0.5 m三种壅水高,有水开门工况考虑了0.8,0.5,0.3 m三种水位降。然后,根据启闭机的额定输出力,反推得出有水关门时壅水高不超过0.7 m,有水开门时水位降不超过0.3 m;最后,根据残余预紧力条件,确定了预紧力矩为2 400 N·m;强度计算结果表明,选用10.9级M36螺栓时,卧倒门支铰座螺栓联结能够满足强度要求。维修后卧倒门运行良好,可靠性提高,这为三峡升船机下闸首卧倒门安全运行提供了技术依据。     

Stress-Corrosion Cracking and Galvanic Corrosion of Internal Bolts from a Multistage Water Injection Pump

Nineteen out of 26 bolts (studs) used for assembly of multistage water pump showed severe corrosion and cracking after brief service in a severe working environment that contained saline water, CO₂, and H₂S. The failed bolts and intact nuts were supposed to be made out of a special type of austenitic stainless steel as per ASTM A 193 B8S and ASTM A 194, respectively. However, the investigation showed that bolts and nuts are made from two different alloys: an austenitic stainless steel and a nickel-base alloy. The difference in the corrosion resistance of these two alloys led to severe galvanic corrosion. The galvanic coupling between bolts and nuts in addition to the severe working environment played major role in the premature failure of bolts. The mechanisms of bolt failure were galvanic corrosion for bolts that were in direct contact with the environment and stress-corrosion cracking in the bolts remote from the severe environment. The stress-corrosion cracking was influenced by a bad fit between the bolts and nuts threads. This resulted in a crevice and the development of an aggressive chemistry between the engaged bolt/nut threads. All factors required to cause stress-corrosion cracking were available, namely, stressed bolts (bolts under tensile stress), temperatures above 60 °C, and chloride ions.     

Investigations on core basket bolts from a VVER 440 power plant

Non-destructive examinations using ultrasonic inspections were performed on core basket bolts at a VVER 440 unit. The four M12 bolts, manufactured from solution annealed Ti-stabilized stainless steel, had been replaced due to NDE indications. Destructive investigations were performed on three of these bolts. One bolt had suffered from intergranular stress corrosion cracking, enhanced by irradiation. Irradiation induced grain boundary segregation was observed in the material. The washer of the bolt with cracking had been unintentionally spot welded to the shielding plate, restricting the free movement of the components to adapt to the differences in thermal expansion during e.g. start-up. The stress situation is modeled using finite element computation, and the location and magnitude of high stresses due to spot welding is clearly shown. The reason for the NDE indications in the other bolts were incomplete filling of the flat slot of the bolt head when the bolt was welded to the washer to prevent unscrewing of the bolt during operation.     

Failure analysis of connecting bolts and location pins assembled on the plate of main-shaft used in a locomotive turbochanger

Three connecting bolts, three location pins and navel of turbo-disk fractured, which were assembled on the plate of the main-shaft used in a locomotive turbochanger. Detailed fractographic study and metallurgical analysis were focused on the trouble bolts. The fatigue fracture is the main failure mechanism of the bolts. Appearance of the surface decarburization layer in the thread tip and root regions of the three failed bolts make the hardness at the thread regions decrease intensely so that the fatigue cracks initiated form the root at the first engaged thread. Surface damage morphology with cutting, wear and plastic deformation features was found on the working flanks of the engaged threads. Other components fractured in succession after the trouble bolt fractured.     

Metallurgical failure analysis of titanium wing attachment bolts

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.     

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.     

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。     

超500 MPa级高强钢承压型螺栓连接承载力试验研究

高强度螺栓承压型连接的承载力来源于钢板与螺栓的接触行为,承载力的发展伴随着螺栓孔周围材料的应力集中和塑性变形。普通钢延性好,可以克服螺栓孔周围应力集中的不利影响,充分发展承压承载力。随着强度增高,钢材的延性降低、变形能力下降,应力集中的不利影响将更突出。因此,需对高强钢构件的承压型连接进行研究。该文以Q550D、Q690D和Q890D三种国产高强钢共制作50组单螺栓试件,其中24组双剪切面试件、16组单剪切面试件,6组孔径变化试件,并与4组Q345B单螺栓试件进行了对比。研究表明:高强钢仍具备足够的延性,能克服应力集中的不利影响,连接破坏模式与普通钢无明显差异;剪切面数和螺栓孔径对不同高强钢承压强度影响较小,但单剪切面的试件更容易出现螺栓剪断,降低连接的延性,限制承压承载力的利用。基于已有试验的结果与中国现行标准《钢结构设计规范》比较,发现螺栓连接的钢板承压强度设计取值偏保守,对于超500 MPa高强钢可适当提高其设计值,以充分利用高强度钢材的材料强度,合理减少螺栓数量。     

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。     

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 K-monel 500 (Ni–Cu–Al alloy) bolts

A bolt manufacturer used several subcontractors to fabricate K-monel 500 bolts. The completed bolts failed mechanical testing, and the manufacturer ascribed the failure to the bar stock. Testing contracted by the supplier of the bar stock showed that it met specifications. Metallographic testing described here shows that the bolts failed because of excessive grain growth and subsequent softening during heat treatment performed on the bolts after they were formed. Alternative heat treatments are discussed.     

Failure Analysis of SG Iron Collector Nozzle Holder Used in Steel Ladle

The failed collector nozzle holder (CNH) of steel ladle, made of ferrite grade spheroidal graphitic (SG) iron was investigated in the present article. The microstructure, volume fraction, and morphology of spheroidal graphite were investigated. The cause of failure was analyzed through comparative study with a good CNH, using optical and scanning electron microscopy and the data available in various sources in the literature. The effect of microstructure evolution on mechanical properties of the SG iron was analyzed. Failure analysis conducted in this article revealed that the presence of exploded and chunky graphite succeeded the distortion caused by dimensional instability at elevated temperature. The phenomena analyzed were found to be aggravated at higher carbon equivalent, lower residual magnesium, and low yield strength of the sample. The remedial measures were suggested in material and operation guidelines for failure prevention of CNH in steel ladles.     

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.     

Hydrogen-assisted stress cracking of high-strength wheel bolts

Early failures occurred with two sizes of wheel bolts used for attaching front wheels and dual rear wheels to heavy truck hubs. Failure resulted in fracture of the bolts, and was a response to the material and process specification which produced a steel microstructure highly susceptible to hydrogen-assisted stress cracking. The microstructure resulted from an alloy steel that was carburized, heat treated, and then zinc plated. This combination of material and processing produced a high-strength SAE Grade 8 bolt (equivalent to an ISO 10.9 grade) with a hard, brittle case and an anodic zinc coating. A slight misalignment of the wheel bolt coupled with a ball seat mounting design for the wheel nuts created a combined axial and bending stress that exceeded the threshold for hydrogen-assisted stress cracking.     

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.     

Tensile behaviour of anchored blind bolts in concrete filled square hollow sections

In this paper, the tensile behaviour of a new type of blind bolt, the anchored blind bolt, has been studied. This type of bolt consists of a conventional blind bolt with a headed stud extension that anchors it into the infill concrete. It has been developed for use in moment-resisting connections between I-beams and concrete-filled hollow section columns. The behaviour of these connections is highly dependent on the tensile behaviour of the anchored blind bolts. Hence, the fundamental mechanics of this behaviour has been studied here, both experimentally and in simulations using FE models. The complex interactions between the headed stud anchorage, the concrete infill, and the steel tube have been examined in detail. The experimental and numerical findings were used to understand the influence of important parameters in the tensile behaviour of the anchored blind bolts. It was found that the location of the bolt with respect to the side walls of the tube had a substantial influence on the behaviour of the anchored blind bolts. All the bolts located close to the side walls of the square hollow sections reached the ultimate tensile capacity of the equivalent structural bolts. This was due to the development of a concrete strut which transferred the load to the corner of the steel section. The bolt diameter also had a significant influence on the stiffness and strength of the anchored blind bolts.     

Investigation of a mounting bolt failure in an automobile air brake assembly

The premature fatigue failure of a mounting bolt in an air brake assembly was traced to non-adherence to the established design. The heat input from a projection weld coupled with a sharp neck radius of curvature, and the proximity of the projection weld to the bolt neck combined with a higher-than-anticipated carbon content to produce acicular martensite in a region of high stress concentration. This combination of conditions increased the susceptibility to fatigue and caused the failure.     

Behaviors of a cracked lapped joint under mixed mode loading

Fatigue failure of steel connections is a common failure mechanism, especially for structures which sustain heavy cyclic loads like steel bridges. In this paper, lapped bolted joints were modeled numerically to study the effect of a crack on the ultimate response of the joint. The site of crack initiation was located under different mixed mode loading in single and multiple bolts joints. The effects of axial to transverse loading ratio or load mixity (LM = F_x/F_y), friction coefficient (μ), and bolt diameter were analyzed. For a single bolt pin-joint, by increasing LM, the crack initiation site angle (γ) increased up to a certain value at which it became constant (γ_f) independent of LM. This value γ_f depended only on the coefficient of friction and the bolt hole diameter. Stress intensity factor and crack path of a propagated crack emanating from the predicted crack initiation site were analyzed in the lapped joints under either mode I or mixed mode loading. It was found that, for multiple-bolt joints, loaded with mixed mode loading, the crack path remained approximately horizontal like that for mode I loading. For pin joints, the crack path remained at the direction of the crack initiation. The numerical model developed was validated using existing experimental results for the initial stiffness of the bolted joint and using theoretical prediction of the stress intensity factor. A parametric study for different bolt diameters and numbers was developed to study the behavior of these connections under double and single side cracks. It was found that the crack advancement in a specific bolt hole may cause crack to initiate in other bolt holes, due to the increase of the stress concentration factor (SCF), K_t.     

Failure Analysis of a 1990 Mazda Miata Crankshaft and Timing Pulley Bolt

A Mazda Miata crankshaft and timing belt pulley bolt failed in service. This caused extensive damage to the engine. The crankshaft and bolt were analyzed to determine the cause of failure. Using visual examination and other means, it was determined that the crankshaft and bolt failed by fatigue. The crankshaft failure initiated at the keyway, while the timing belt pulley bolt initiated in the threads. Inadequate clamping force during installation of the timing belt pulley bolt is thought to have initiated failure.