Fatigue failure of a rear axle shaft of an automobile

This paper describes the failure analysis of a rear axle shaft used in an automobile which had been involved in an accident. The axle shaft was found to break into two pieces. The investigation was carried out in order to establish whether the failure was the cause or a consequence of the accident. An evaluation of the failed axle shaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that the axle shaft fractured in reversed bending fatigue as a result of improper welding.     

汽车复合制动气室复位弹簧建模及仿真研究

以汽车复合制动气室行车制动腔内的复位弹簧为研究对象,利用Pro-E与Ansys workbench软件对其进行实体建模和有限元分析,探讨模型的有限元处理方法,分析网格划分的可用性.采用Ansys workbench软件对复位弹簧进行疲劳分析,得到应力应变分布特征和疲劳寿命曲线;进一步对复位弹簧进行模态分析,求得在500kPa气压下1⁴阶复位弹簧模态响应图,找出所建模型的薄弱点;揭示了在循环载荷作用下复位弹簧失效的根源和规律,对进一步开展复位弹簧结构优化设计有重要意义.     

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.     

Failure of an idle air control valve in a gasoline-fuelled automobile

The functional failure of an idle air control valve in a gasoline-fuelled automobile in India has been analyzed. It was found that the failure was related to the formation of deposits inside the idle air control valves. The deposit was found to contain both organic and inorganic constituents. The deposits were characterized using a variety of techniques including chemical analysis, optical microscopy, FTIR spectroscopy, GC–MS, X-ray diffraction and thermal analysis. The possible source of the deposit formation has been analyzed. It is inferred that the cause of deposit formation and the subsequent failure of the idle air control valve is because of the back flow and condensation of fuel vapor combined with contamination through extraneous dirt especially silica from the air intake system.     

An air crash due to fatigue failure of a ball bearing

The failure analysis of an air crash conclusively shows that the cage of the central main bearing of the compressor region failed due to fatigue. The broken piece of the cage got struck between the bearing balls and the races and impaired the function of the bearing resulting in the crash.     

Metallurgical investigation of a prematurely failed roller bearing used in the support and tilting system of a steel making converter used in an integrated steel plant

An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting system of a basic oxygen furnace (BOF) converter used in the steel melting shop of an integrated steel plant. The converter bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance of brittle oxide and other macroinclusions. Electron probe microanalysis (EPMA) studies confirmed that the brittle oxides that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race samples contained high-carbon tempered martensite. Microhardness measurements revealed that although the core hardness of the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder than that of the inner-race. Scanning electron microscope (SEM) observations of the roller fracture surface revealed striations indicative of fatigue, and energy-dispersive spectrometric (EDS) analyses corroborated a high incidence of silicate inclusions at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.     

Failure analysis of a pressure vessel bolt in a nuclear fuel fretting wear simulator

This paper presents a failure analysis on a pressure vessel bolt of a fretting wear simulator. After 500 h tests, in an upper pressure vessel of a fretting wear simulator, one bolt among eight was fractured near the bolt neck regions. The fracture surface was examined by using a scanning electron microscope (SEM) to determine the failure initiation and failure mode. The result indicates that the fracture surface shows intergranular fracture features. Based on the mechanical property data of a bolt material, it is concluded that the exerted stress on the bolt applied by an internal pressure of the pressure vessel has a negligible effect on the major failure causes. In order to verify the mechanical properties of the fractured bolt, tensile test has been performed and its result was compared with material specification. As a result, it is thought that both excess heat treatment during the surface hardening procedure and loose parts in the thread hole have significant effects on the pressure vessel bolt failure. In this paper, the reasons for this failure were discussed by using metallographic studies of the failure surface, mechanical tests with the failed bolt and the stress distribution of the contact regions with considering loose parts by using FE analysis.     

Analysis of manufacturing effects on fatigue failure of an automotive component using finite element methods

In this study, the fatigue life of an automotive suspension component was analysed using finite element methods with regard to stamping and welding effects. Because automotive suspension components are produced by forming and welding sheet metal, there are various effects on the final product, such as uneven thickness distribution, residual stresses and weld notches. Manufacturing effects may change the mechanical performance of the automotive components; therefore, it is desirable to consider these effects in the early design stage. Residual stresses due to work hardening and thermal deformation were investigated through process simulation. The redistribution and relaxation of residual stresses in a component were investigated in fatigue life analysis under a cyclic loading condition. Various equivalent relaxation curves were investigated and one was selected after comparisons with test results. The fatigue simulation results were compared to the test results; a good correlation between the two was achieved for the residual stress effects in terms of life cycles and failure locations. The simulation results also show that welding produces more detrimental effects than stamping with regard to the fatigue life of a component.     

Modeling of temperature and humidity ratio in an automobile passenger compartment for automobile air conditioning systems

This study investigates thermal dynamic modeling of a passenger compartment in an air conditioned automobile equipped with HVAC (heating, ventilation, and air conditioning). In particular, dynamics of temperature and humidity ratio, both critical to passenger comfort, in the passenger compartment are examined. By analyzing enthalpy change during an automobile air conditioning circulation cycle on a psychrometric chart, heat exchange between outside environment and the passenger compartment can be modeled as functions of operation settings of the HVAC. With enthalpy content decomposed into sensible heat and latent heat, changes in heat content can be attributed to the changes in the corresponding temperature and humidity ratio, respectively. Consequently, impacts resulting from the mismatch between two heat characteristics on temperature and humidity ratio can be formulated accordingly: room sensible heat factor (RSHF) of the passenger compartment and apparatus sensible heat factor (ASHF) of the HVAC. A Matlab/Simulink simulation is implemented to verify the proposed model under several control policies that either maintain constant enthalpy or maintain constant temperature in the passenger compartment. With these two temperature and humidity ratio models derived, further research on designing control policies to achieve better passenger comfort for general automobile air conditioning systems can be derived and tested.     

Investigation on the deposition failure of a reactor effluent air cooler in hydrocracking unit

The deposition failure of a reactor effluent air cooler (REAC) is investigated by the technical analysis and multiphase flow simulation. The blockages of REAC tubes are mainly induced by the corrosion products come from the upstream heat exchangers, a result of the high temperature H₂S-H₂ corrosion. Meanwhile, NH₄Cl salts crystallize upstream the air coolers and enter into the REAC tubes, flowing along with the corrosion products. Through analyzing the residence times and deposition rates of salts, it is found that the corrosion products tend to deposit on two sides of the header box, the third row of tubes and the regions of low velocities. The temperatures inside the REAC tubes significantly decrease once the tubes are blocked. Fouling of NH₄HS salts occurs when the temperature falls below 30 °C. The expansion and contraction of carbon steel with large difference in temperature lead to the bucking of tubes. The regions with high risk deposition are obtained from simulation, which agree well with the actual failure phenomenon.     

Investigation of fatigue-induced socket-welded joint failures for small-bore piping used in power plants

Nuclear power plants typically experience two or three high-cycle fatigue failures of stainless steel socketwelded connections in small bore piping during each plant-year of operation. This paper discusses fatigue-induced failure in socket-welded joints and the strategy Texas Utilities Electric Company (TU Electric) has implemented in response to these failures. High-cycle fatigue is invisible to proven commercial nondestructive evaluation (NDE) methods during crack initiation and the initial phases of crack growth. Under a constant applied stress, cracks grow at accelerating rates, which means cracks extend from a detectable size to a through-wall crack in a relatively short time. When fatigue cracks grow large enough to be visible to NDE, it is likely that the component is near the end of its useful life. TU Electric has determined that an inspection program designed to detect a crack prior to the component leaking would involve frequent inspections at a given location and that the cost of the inspection program would far exceed the benefits of avoiding a leak. Instead, TU Electric locates these cracks by visually monitoring for leaks. Field experience with fatigue-induced cracks in socket-welded joints has confirmed that visual monitoring does detect cracks in a timely manner, that these cracks do not result in catastrophic failures, and that the plant can be safely shut down in spite of a leaking socket-welded joint in a small bore pipe. Historical data from TU Electric and Southwest Research Institute are presented regarding the frequency of failures, failure locations, and the potential causes. The topics addressed include 1) metallurgical and fractographic features of fatigue cracks at the weld toe and weld root; 2) factors that are associated with fatigue, such as mechanical vibration, internal pulsation, joint design, and welding workmanship; and 3) implications of a leaking crack on plant safety. TU Electric has implemented the use of modified welding techniques for the fabrication of socket-welded joints that are expected to improve their ability to tolerate fatigue.     

Analysis of premature failure of a tie bar in an injection molding machine

Premature failure of a tie bar made of AISI 4140 steel in a 150 tonne plastic injection-molding machine has been analyzed. Although the nominal tensile stress acting on the tie bars (95.5 MPa) is far lower than the yield strength of this material (750–900 MPa), the tie bars are subjected to a pulsating cyclic loading during the plastic molding process. The failure was found to occur at the root of the first thread by transverse fatigue fracture induced by a pulsating tensile stress with multiple points of high stress concentration. The high stress concentrations appear to have been introduced by a combination of improper molding parameters resulting in uneven tensions in the four tie bars and aggravated by the presence of some material defects. The material defects observed are inclusions, presence of some retained austenite and fine cracks. A hydraulic clamping mechanism rather than a toggle clamp mechanism for the mould will minimize the cyclic strain on the tie rods.     

Analysis of an automobile assembly line as a network of closed loops working in both,stationary and transitory regimes

In this work a real automobile assembly line and the correspondent preassembly lines have been analyzed as a network of closed loops of machines decoupled by intermediate buffers. This work deals with some important aspects, which have still not been investigated in earlier literature, such as: machines processing pallets, which are not related to each other and depend on an external variable in a network with closed loops of machines and intermediate buffers, machines working at different cycle times in a network of closed loops of machines and intermediate buffers, machines working in both, stationary and transitory regime and the relationships between the cycle times of the machines in the stationary working regime in order to guarantee the production rate of the system. Finally how the transient results can be used to improve the performance of the system under certain working conditions is discussed.     

Metallurgical failure analysis of cold cracking in a structural steel weldment: Revisiting a classic failure mechanism

Cold cracking of structural steel weldments is a well-characterized, well-documented, and well-understood failure mechanism. Extensive effort has been put forth to recognize the welding and materials selection parameters that are conducive to cold cracking; however, these engineering efforts have not fully eliminated the occurrence of such failures. This article describes cold cracking failure specifically related to the construction industry. This particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked. These actions potentially prevented a serious catastrophic event that could have occurred have occurred either later in the construction process or possibly during the use of the building. Individual welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure, and a number of human factors relating to the actual fabrication practices also contributed to the failure process.     

Investigation of failure in main fuel pump of an aeroengine

There was an accident to a fighter aircraft. Investigation revealed that the accident was caused due to loss of power in the engine as a result of failure in the main fuel pump (MFP). The MFP was multi-plunger type. On dis-assembly, the MFP was found severely damaged and there were fractures in one plunger and four springs. Through systematic metallurgical investigation and analysis, the sequence of events leading to the failure in the MFP was established. The primary failure in the MFP was the fatigue fracture of springs. The fatigue crack initiation could be attributed to pitting corrosion on the surface of the springs. Because of multiple fractures in one of the springs, there was impact load on the corresponding plunger, which resulted in generation of an overload crack. This crack had further propagated progressively by fatigue mechanism culminating in fracture and loss of material from the side wall of the plunger. Subsequently, there was fuel leakage internally in the MFP with the resultant reduction in the fuel delivery pressure. Due to insufficient fuel supply, there was winding down of the engine RPM leading to loss of thrust. After establishing the sequence of failure in the MFP, investigation was carried out to identify the cause for the corrosion on the surface of the springs. It was established that the raw material (wires) used for the manufacture of the springs had developed corrosion pits on the surface due to improper storage.     

Fatigue failure of an aircraft wings due to fitting error in hi-locks

Two hi-locks were fractured during a flight of a carrier aircraft, causing the leakage of fuel in the right wing section. The failed and a new hi-lock were subjected to failure investigation.Material of the hi-locks was AMS-6270. The hardness of both hi-locks was also comparable to each other. The investigations entailed that the failed hi-locks were loosened or improperly preloaded. Thence the fastened sections exerted bending fatigue loads on the hi-locks. Therefore, two propagating beach mark fields were observed on the fracture surface, which were advancing towards each other under high cycle fatigue (HCF). When the crack reached at the threshold level, the hi-lock(s) failed catastrophically.     

Improvement of very high cycle fatigue properties in an AA7075 friction stir welded joint by ultrasonic peening treatment

The present paper aims to investigate the effect of ultrasonic peening treatment on the very high cycle fatigue resistance of an AA7075 friction stir welded joint. Microscopy observation, microhardness and X‐ray diffraction measurements were carried out to characterize the treated surface of peened specimens. Fatigue crack initiation sites were investigated through scanning electron microscope, and the role of enhanced surface on fatigue resistance was analyzed. The results indicate that a sensible fatigue strength improvement can be obtained through application of ultrasonic peening treatment and that fatigue cracks can initiate from the interior of the specimen. To clarify the fatigue failure mechanism, we analyzed the microstructure characteristics, compressive residual stress profile and intermetallic inclusion distribution in the surface layers, and we discussed the capability of ultrasonic peening treatment to hinder the surface crack initiation.     

Catastrophic failure of a fan in a diesel engine cooler

The fan used to cool a diesel engine fractured catastrophically after approximately 100 h of operation. The fan failed at a spider, which was resistance spot welded to a shim placed between two circular spiders of 3 mm thickness. The detailed analysis of the fracture indicated that the premature failure of the fan was due to inadequate bonding between the sheets at the weld nugget. The fracture was initiated from the nugget-plate interface. The inadequate penetration and lack of fusion between the steel sheets during resistance spot welding led to poor weld strength and the fracture during operation. The propensity to crack initiation and failure was accentuated by improper cleaning of the surfaces prior to welding and to inadequate nugget-to-sheet edge distance.     

An MILP model and a hybrid evolutionary algorithm for integrated operation optimisation of multi-head surface mounting machines in PCB assembly

This paper focuses on an operation optimisation problem for a class of multi-head surface mounting machines in printed circuit board assembly lines. The problem involves five interrelated sub-problems: assigning nozzle types as well as components to heads, assigning feeders to slots and determining component pickup and placement sequences. According to the depth of making decisions, the sub-problems are first classified into two layers. Based on the classification, a two-stage mixed-integer linear programming (MILP) is developed to describe it and a two-stage problem-solving frame with a hybrid evolutionary algorithm (HEA) is proposed. In the first stage, a constructive heuristic is developed to determine the set of nozzle types assigned to each head and the total number of assembly cycles; in the second stage, constructive heuristics, an evolutionary algorithm with two evolutionary operators and a tabu search (TS) with multiple neighbourhoods are combined to solve all the sub-problems simultaneously, where the results obtained in the first stage are taken as constraints. Computational experiments show that the HEA can obtain good near-optimal solutions for small size instances when compared with an optimal solver, Cplex, and can provide better results when compared with a TS and an EA for actual instances.