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.     

Failure Investigation of an Aircraft Crankshaft Gear Connection

Improper assembly of an aircraft crankshaft can have serious consequences. If an adequate joint clamping force is not applied to the connection between the crankshaft and crankshaft gear during assembly, relative motion in the system could create flexural loads on connection components, and cause damage such as cyclic fatigue cracking, shear overstress fracture, and plastic deformation. Many factors can contribute to insufficient joint clamping, including poor joint seating, the presence of a foreign object on the faying surface, and failure to apply proper torque during assembly. This paper reviews a case involving a crankshaft gear connection, which separated while the subject aircraft was in flight, causing the engine to fail and the aircraft to crash. To determine the root cause of the failure, a metallurgical analysis was performed.     

Failure analysis of maraging steel fasteners used in nozzle assembly of solid propulsion system

Owing to high strength and better toughness, maraging steels are being widely used for aerospace application. In one such application, maraging steel was used to fabricate fasteners for nozzle assembly of propulsion system. Few fasteners were found broken during second stage of torque application. Failure was at head/shank junction in each of the three failed fasteners. Detailed metallurgical investigation revealed that the fasteners were under assembly load and exposure to severe corrosive environment. Crack originated from the thread root, propagated intergranular mode with tree root morphology. Considering such evidences, it was inferred that the failure occurred due to stress corrosion cracking.     

Failure of Bolt Threads Exposed to Shear Stress

An agricultural grain cart rolled over during harvest when a bolt in the hitch assembly failed. The bolt failed due to high shear stress in a thread root. The bolt would not have failed under the same conditions if the threads were not present in the shear zone (different bolt with a longer shank region). The cart rolled over due to bolt failure as a result of high shear stress.     

Ermüdungseigenschaften von Schraubenverbindungen mit gefurchtem und geschnittenem Gewinde

The fatigue strength of metric steel screws is investigated with regard to the impact of the length of thread engagement and the level of preload as well as the material and type of manufacturing the mating thread. For bolted applications in aluminum with big lengths of thread engagement (t_E>2 ? d) the way of manufacturing the female thread shows a significant impact on the number of cycles to failure depending on the thread formed or cut. In general the amount of the preload is not influencing the fatigue strength in significant way for smaller engagement depths (t_E<1 ? d). Decreasing preloads lead to a higher number of cycles to failure. For screws and nuts both made of steel, the impact on fatigue life becomes dominant in case of over‐elastic tightening and fatigue testing at reduced levels of preload in the elastic range. Based on numerical analysis the load distribution in the engaged thread area is calculated. The analysis of local stresses was made to evaluate the areas of highest notch stress which is decisive for fatigue life.     

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.     

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 intermediate gearbox of a helicopter

An intermediate gearbox of a helicopter failed resulting in an accident. A systematic failure analysis was conducted to find out the cause of failure. Examination revealed that fatigue fracturing of the driving gear was responsible for the gearbox failure. The teeth of the gear were severely damaged by spalling. Fractographic study revealed multiple fatigue crack initiation at the tooth root regions. It was established that the failure was caused due to improper assembly of the gear. A detailed analysis of the failure is presented in this paper.     

某型液压泵壳体螺纹拉脱原因分析

通过断口分析、化学成分分析、硬度测试等方法,对某液压泵壳体螺纹的拉脱原因进行了分析。结果表明：液压泵壳体螺纹拉脱的失效性质为剪切疲劳断裂;循环散热导管接头装配过紧．造成了液压泵壳体螺纹损伤,是导致剪切疲劳断裂的原因;为有效避免螺纹拉脱故障．应增加管接头与液压泵壳体螺纹的有效配合长度。     

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.     

室温下C/SiC复合材料螺纹紧固件的拧紧特性

探讨了C/SiC陶瓷基复合材料螺纹紧固件在室温下的拧紧特性。试验测试了紧固件在拧紧和拧松过程中, 力矩与预紧力两者的对应关系, 记录了预紧力在短时、 长时内的减小比率, 并用显微镜观察了螺纹面的磨损情况。另外, 分析了材料非线性拉伸行为对拧紧状态所造成的影响。结果表明: 拧紧、 拧松力矩与预紧力之间近似呈线性关系, 螺纹面和支承面的平均摩擦系数分别为0.52和0.46; 随拧紧力矩增大, 螺纹面产生一定程度磨损, 螺纹之间的相互嵌入作用减弱, 因此预紧力在拧紧后的降幅减小, 稳定性提高; 对螺栓进行适当的预拉伸处理, 提高材料的弹性极限后, 可提高紧固件的抗松弛能力。     

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 analysis on high temperature superheater Inconel® 800 tube

This paper presents failure analysis on a super alloy Inconel^® 800 superheater tube in Kapar Power Station Malaysia. Visual inspection, microscopic examinations and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The failed high temperature superheater (HTSH) tube was found snapped into two parts, heavily distorted shape and bent at several points. Microstructures of the failed tube showed that creep crack initiated at both external and internal surfaces of the tube and propagated as grain boundary creep cavities coalesced to form intergranular cracks. The severe geometry of tube causing steam flow starvation is identified to have caused increasing tube metal temperature resulting in overheating of the failed tube. Creep rupture is revealed as the cause of failure of the superheater tube.     

Failure Investigation of Condensate Pump Shaft

The premature failure of condensate pump shaft was investigated in this paper. The shaft was attached with feed water condensate pump system in thermal power plant boiler. The shaft was fabricated from martensitic stainless steel. A detailed study was carried out to determine the root cause of the shaft failures. Visual examination, macroanalysis, chemical analysis, microstructural characterization, and fractography examinations were also carried out to ascertain the probable cause of shaft failure. Finally, it was concluded that the failure of the shaft was due to the torsional fatigue initiated by the cracks at the thread roots.     

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.     

Failure Analysis of Cap Screws in a Diesel Engine Front Gear Train

Two failures of the front gear train cap screws of a diesel engine in a marine vessel are investigated. Fractured cap screws were present in both failures. The cap screws’ strength was compared to standards. The thread engagement was also analyzed. In one failure, the cap screws used did not comply with the standard properties and in the other failure, improper thread engagement resulted in fast fracture after few load cycles.     

Failure investigation on rear water wall tube of boiler

This paper presents failure investigation on the SA210-A1 rear water wall tube by visual site inspection, tube wall thickness measurements and microscopic examination. The rear water wall tube has failed with wide open burst and was situated at the boiler nose lower bent region. On-site wall thickness measurements were performed on all the rear water wall tubes located at the same level of the ruptured tube. The tubes were observed to have experienced significant wall thinning. Microscopic examinations on the failed rupture region and some distance away region of the as-received tubes are also conducted in order to support in determining the failure mechanism and failure root cause. Failure mechanisms are discussed and the findings obtained from the site inspection, wall thickness measurements, microscopic examinations and creep analyses may finally reveal the failure mechanism and main root cause of the failure. The failure mechanism is identified as a result of the combination of the significant localized wall thinning of the rear water wall tube due to fly-ash erosion and a thermally activated process of creep problem due to increasing of temperatures.     

Corrosion Fatigue of a Pump Bearing Journal after Exposure to Two-Phase Flow

A failure of a pressure relief valve in one of the CANDU (acronym for CANadian Deuterium Uranium Reactors, a registered trademark of AECL) reactors during routine testing resulted in the Heat Transport (HT) pumps operating under a Loss Of Coolant Accident (LOCA) condition for a short time. It took 50 min of operation before the reactor could be safely shutdown. During a part of this 50 min period, HT pumps were exposed to two-phase flow conditions. When the reactor was restarted following this incident, one of the HT pumps was operating with high shaft and frame vibrations that reached the shut down limit in a very short time. The reactor was shut down and a detailed inspection of the pump showed that the pump bearing journal on one of the 4 HT pumps had failed and had a large crack extending across its entire length. This paper summarizes the results of extensive investigations carried out to establish the root cause of the journal failure. The failure analysis included a finite-element analysis of the shrink fitted journal, metallurgical analysis, scanning electron fractography of the failed journal, and a design review of the assembly fits. Based on these investigations, the journal failure was determined to be due to corrosion fatigue. The corrective actions implemented to prevent the susceptibility of this component to future failures were detailed and required significant effort and expertise to develop and implement.     

Fatigue failure of a centrifugal compressor

A single stage centrifugal type compressor failed during operation and a blade detached. Failure analysis was carried out to delineate the cause of the failure.Investigation shows that multiple fatigue cracks were generated during service from forging defects in the root area of the blade. When they grew to a critical size under (high cycle fatigue) HCF, the blade detached from the root in an over load manner. At the high rpm of the engine, the detached blade collided with other rotating part of the engine and entangled between the compressor and multiplier assembly. This resulted in the subsequent damage in the engine.     

φ165.1mm钻铤内螺纹接头失效分析

对内螺纹接头失效的钻铤进行了化学成分、力学性能和显微组织分析,并采用扫描电镜和X射线能谱仪对断口形貌及夹杂物成分进行了分析,找出了钻铤内螺纹接头失效的原因。结果表明:该钻铤内螺纹接头失效为早期疲劳失效;导致其疲劳失效的主要原因有钻铤内螺纹根部加工不光滑,使该处在交变载荷作用下应力集中严重,首先萌生微裂纹成为疲劳源;钻铤内螺纹型号和内外径尺寸不合标准要求以及材料中含有数量较多的MnS夹杂物也都大大降低了其疲劳性能,加速了疲劳裂纹的萌生和扩展。