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.     

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.     

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。     

The Bellevue crane disaster

This paper describes an investigation of the cause and origin of the November 16, 2006 collapse of an overhead tower crane at an office building construction site in Bellevue, WA. This accident, and several others during the same timeframe, triggered national concern regarding crane safety. The collapse was curious because the crane overturned with no load on the hook under negligible winds while the operator was in the process of shutting the crane down for the night.This paper summarizes a comprehensive investigation of the failure, and focuses on the steel framework of the crane foundation, where a large fatigue crack initiated the collapse. The foundation was an unconventional grillage of steel beams attached to an existing concrete structure that supported the crane above post-tensioned concrete parking slabs. Analysis of the crane failure embraced three phases; namely, a Global Model to calculate generalized forces and displacements, a refined local model to calculate elasto-plastic stresses at the crack initiation site, and a fatigue/fracture mechanics analysis to investigate the criteria for crack progression. A suite of metallurgical procedures, namely chemical analysis, metallography, scanning electron microscopy, hardness testing, tensile testing, and Charpy V-Notch impact testing was performed on samples from failed foundation elements.Results of these efforts confirmed origins of fatigue cracking at the cope web stiffeners in both transverse tower support beams. Allegations that faulty welding was the principal cause and origin of the critical crack(s) and resulting tower crane foundation failure were disproved. Instead, this crane accident occurred for a reason common to many construction problems - poor communication. The crane foundation designer had relied on a structural tie between the crane tower and the building core, but communication of the contractor’s decision to delay the core construction, and thereby eliminate the tie, was somehow lost.     

轮胎螺栓断裂失效分析

某汽车轮胎螺栓在使用过程中发生断裂。采用宏、微观检验和化学成分分析等方法对失效件进行了检测。结果表明，其断裂形式为弯曲疲劳断裂。疲劳裂纹的形成是由于螺纹牙底存在脱碳和折叠裂纹等缺陷，这些缺陷的存在使该材料产生了较大的应力集中，从而导致螺栓开裂失效。     

Analysis of Slide Gate Mounting Bolt Failure in Steel Ladle Due to Fatigue

The mounting bolt failed during slide gate operation of a steel ladle was investigated. Metallurgical analysis confirms the bolt as IS 1367 10.9 grade high-tensile 42CrMo₄ steel. On comparative study with a good un-fractured sample, a striation of fine banded ferrite in pearlite matrix was revealed in failed bolt under optical microscopy. The bolts failed due to fatigue and crack initiated from the surface of machined bolt threads. Comparatively lower hardness, low UTS associated with lower %Cr and %Mo content found to aggravate premature failure of bolts during ladle operation. Microstructure of un-fractured sample found with tempered bainite phase. The fatigue failure of bolts occurred due to repetitive nature of shear force development during steel pouring through slide gate system. Preventive measures to reduce fatigue failure of the mounting bolts are proposed.     

Mobile harbor crane wheel hub fatigue failure

Failure analysis of a mobile harbor crane wheel hub demonstrated that the mechanism of failure was fatigue. The wheel hub was a ductile cast iron component that had been subjected to cyclic loading during a ten-year service period. The fracture surface of the fatigue failure also contained corrosion deposit, suggesting that cracking occurred over a period of time sufficient to allow corrosion of the cracked surfaces. Replacement and alignment of the failed wheel hub is recommended along with inspection of the nonfailed wheel hubs that remain on the crane.     

Failure of rockbolts in underground mines in Australia

Over the last 15 years there has been an increasing incidence of failure in rockbolts used in underground mines in Australia. Failures have also been observed in the United Kingdom where Australian Technology rockbolting is also used. Most of the failures in the United Kingdom were found to be initiated by corrosion pits, but in Australia, the fractures were considered likely to be due to stress corrosion cracking (SCC). This paper reports a metallurgical study of 44 failed rockbolts from four different underground mines in Australia. The study confirmed that failure was generally due to SCC and showed that this was usually initiated by bending of the bolts that occurred due to lateral movement of the rock strata. It also showed that many of the failed bolts had very low toughness with Charpy impact values of 4–7 Joules.     

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.     

Premature failure of steel gantry crane wheels

A metallurgical failure analysis was performed on a set of carbon steel gantry crane wheels following observation of excessive damage to the central tread surfaces. Rolling contact fatigue was considered as a possible failure mechanism due to the presence of what appeared to be spalling. Metallographic evaluation and hardness testing revealed that portions of the wheel tread surface had not reached the specified case hardness during heat treatment, leaving the tread surface edges in a near normalized condition. Continual contact with the rail during service allowed for plastic flow of the softer materials across the surface, resulting in the observed damage.     

Failure analysis of the turret bearings wear on SYS in the Bohai Bay area

In the China Bohai bay area, a Single anchor leg yoke mooring system (SYS) was mainly used for FPSO's permanent mooring due to the shallow water. This paper investigates the SYS failure and focus on the excessive wear that occurred to underwater turret bearings. The wear mechanism of bearing is researched by using morphology analysis and energy spectrum analysis. Nonlinear finite element analysis is performed to study the contact status between turret table and axial bearings considering the operational and extreme load condition. The comparison wear-test is carried out to clarify the contributions of eccentric load on the wear performance of axial bearing. Wear-test results indicate that eccentric load has a significant impact on wear of turret bearings. Eccentric wear also produce more abrasive particles to accelerate wear rate. A step-like is formed on bearing plate surface due to excessive wear. The failed bolts are attributed to stress corrosion crack under long-term shear fore caused by vessel surge motion.     

Failure Analysis of Gearbox and Clutch Shaft from a Marine Engine

This article presents metallurgical failure analysis of a gearbox shaft and a clutch shaft from a marine engine. The gearbox shaft was made of low alloy steel, and the clutch shafts were components made of carbon steel. Fracture surface examination revealed circumferential ratchet marks with the presence of inward progressive beach marks suggesting rotary-bending fatigue failure in the case of gearbox shaft. The star-shaped pattern on the clutch shaft fracture surface suggested that the failure was due to torsional overloading which might have initiated at corrosion pits visible around the fracture surface. The gearbox shaft experienced rotational bending stresses which induced fatigue failure because the fatigue strength of the alloy was too low. The fatigue failure of the gearbox shaft led to the torsional failure of the corroded clutch shaft. The sudden, high level failure load on the clutch shaft occurred when the gear box shaft failed.     

高强度螺栓疲劳缺口系数的有限元分析

螺栓球节点网架结构在悬挂吊车作用下的疲劳是工程界和学术界关注的热点,该结构的疲劳关键是高强度螺栓的疲劳。该文借助ANSYS有限元软件;选取常用的M14、M20、M24、M30、M33、M39、M52、M60共8种规格40Cr制高强度螺栓为分析对象;采用20节点的SOLID95单元类型进行三维实体建模;在重点定量探讨螺栓直径、螺纹升角、螺纹牙根圆角半径及螺栓球四个主要影响因素的基础上,建立了适用于各种规格高强度螺栓疲劳缺口系数的通用计算公式,数值区间为4.35―4.89。该文的研究结论揭示了应力集中和疲劳源的关系,为进一步以热点应力或热点应力幅为参量建立螺栓球节点网架结构疲劳计算方法奠定了理论基础。     

Failure of two overhead crane shafts

The failure analysis of two overhead crane shafts is presented: the failure of an overhead crane drive shaft and the failure of an overhead crane gearbox shaft, due to rotating-bending fatigue. The fracture of the overhead crane drive shaft originated in small radius fillet between two different diameters of the shaft. A new shaft was made with a larger-size fillet, resulting in reduced stress concentration in this region. The failure of the overhead crane gearbox shaft originated at the intersection of two stress raisers, at the change in shaft diameter and in the keyway corner. A new shaft was made with a larger-size fillet and a larger size radius of the keyways corner to minimize stress concentration in this section. In both cases the installed couplings were replaced by gear couplings in order to allow parallel and angular misalignment as well as to avoid additional load due to misalignment. The analysis shows that the fatigue life can be significantly increased with a simple change in the structural details.     

Failure analysis of a helicopter accessory drive

The investigation of the accessory drive failure of a Bell 47 G5A helicopter was completed. The bolts and dowel pins that fasten together the accessory drive gear and the accessory drive shaft failed. Failure occurred via a fatigue mechanism, with the physical root cause for failure attributable to insufficient clamping load in service, as well as other mechanical factors with respect to the connection.     

Fatigue Failure of Flanged Thermowell

A fractured flanged thermowell was received for analysis. The fracture was located at the root of “U” insertion/immersion length which was welded to the flange. The failure of flanged thermowell was attributed to the vortex-induced fatigue. The failure was initiated by the weld failure to withstand the fluctuating load resulted from vortex after years of service. This weld failure then created stress raisers, as evidenced by the ratchet marks and river marks near the outer surface.     

Failure analysis of generator rotor fan blades

The failure analysis of a generator rotor fan blade was investigated by mechanical analysis and metallurgical examination of fracture surface. Fracture took place at the airfoil root, surface examination showed that the blade had cracked by a high cycle fatigue mechanism. However, there was no evidence of material defect. A series of analytical, finite element and experimental analysis was utilized to determine the steady-state stresses and dynamic characteristic of the blade. Possibly the failure was due to aerodynamical disturbances that resulted in a state of resonant condition of vibration. The simulation of blade with final crack showed the stress intensity factor (SIF) under these condition exceed the critical SIF and final fracture could be occurred under analyzed stresses.     

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 of a diesel engine gear-shaft

A failure investigation has been conducted on a diesel-engine gear-shaft used in a truck, which is made from 45# steel. The crack initiated from the root transition region between the cylinder and the platform of the gear-shaft, and propagated toward the direction with an angle of 45° to the axial direction first, then toward the direction normal to the axial direction. Multiply-origin fatigue fracture is the dominant failure mechanism. The oil hole on the cylinder of the gear-shaft was stopped up by the overlong bolt to lead to the absence of oil lubrication between the cylinder surface and the internal circle surface of the gear so that the friction force between them increased, which is mainly responsible for the failure of the gear-shaft. The absence of the induction-hardened layer in the root transition region between the cylinder and the platform of the fixed plate of the gear-shaft makes the fatigue strength decrease in this region. The fatigue crack initiated and propagated in this region easily under the abnormal stress.     

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.