Failure analysis of stress corrosion cracking in aircraft bolts

This research was conducted on the failure analysis of the failed clamp bolt from a helicopter engine in the RoKAF. Through the fractography, metallography, and stress analysis of the failed part, it was found that the clamp bolt was fractured by stress corrosion cracking due to the interaction of tensile residual stress and corrosive environment. The stress corrosion crack is a phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment. Therefore, it is recommended that the material of the clamp bolt should be changed to prevent similar failures.     

循环泵法兰螺栓断裂分析

通过金相检验、断口分析和化学成分分析等方法对循环泵法兰螺栓的断裂原因进行了分析。结果表明：螺栓早期断裂失效的主要原因是材料使用有误,加上受海水中的氯离子腐蚀引起奥氏体不锈钢螺栓产生应力腐蚀裂纹并扩展所致。     

汽车用U型螺栓断裂分析

某汽车用U型螺栓制成后在搬动过程中发生断裂,断裂位于U型螺栓感应加热的锻打弯曲部位。通过宏观观察、扫描电镜微观观察以及金相检验等方法对螺栓的断裂原因进行了分析。结果表明:由于该U型螺栓锻打弯曲部位感应加热温度过高导致局部过烧,使其在锻打应力的作用下沿晶开裂形成热裂纹;冷却后该裂纹在搬动过程中的低应力作用下扩展导致螺栓最终脆断。     

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.     

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.     

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。     

Fatigue performance of friction stir welded Al2024 alloy in a different corrosive environment

The study examines the corrosion fatigue behavior of friction stir welded Al2024 alloy in the corrosive medium. The fatigue tests are conducted at a stress ratio of -1 in the different corrosive medium. The decrease in the fatigue life of welded joints in the corrosive environment is attributed to an increase in the crack initiation susceptibility in the presence of corrosive media. The fractured surfaces are investigated using the scanning electron microscopy (SEM). The formation of the corrosive compounds was studied using x-ray diffractometry.     

Cracking analysis on joint lug of aluminum alloy framework of an airplane

An aluminum alloy joint lug of the framework of an airplane was found to have a crack near the bolt hole after serving for a period. To find out the failure mode and cause, macro and micro observation, microstructure examination, EDS analysis, chemical composition analysis and dimensional check were carried out. The results show that the failure mode of the joint lug is stress corrosion cracking under the co-effect of tensile stress and corrosive environment. The cracking of the surface anodic oxidation film near the bolt hole and the exposure of part of the bolt hole inner surface to air are the main causes for the stress corrosion cracking. In addition, the outer diameter of the steel sleeve inside the bolt hole exceeded the design requirement, which resulted in greater tensile stress near the bolt hole and promoted the stress corrosion cracking. The manufacturing procedure should be adjusted to avoid the cracking of anodic oxidation film, the uncovered part of the inner surface of the bolt hole should be covered, and the outer diameter of the steel inside the bolt hole should be controlled properly to meet the design requirement, so that stress corrosion cracking near the bolt hole can be avoided.     

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.     

快卸卡箍螺栓断裂原因分析

通过断口形貌观察、X射线能谱分析、金相检验和硬度检测等试验方法,对某燃油供油导管快卸卡箍螺栓的断裂原因进行了分析,并与螺栓冲击断口和氢致疲劳断口进行了比较分析。结果表明:该快卸卡箍螺栓断口特征与冲击断口和氢致疲劳断口明显不同,其断裂性质为应力腐蚀断裂,裂纹起源于螺栓光杆段的侧表面;螺栓表面加工粗糙且没有防护对裂纹的萌生有一定的影响。对螺栓表面进行防腐处理可有效避免该类故障的再次发生。     

螺栓断裂原因与抽检结果分析

采用断口宏观形貌观察、化学成分分析、显微组织检验、力学性能测试及腐蚀产物的X射线衍射分析等手段，对断裂与部分抽检的卡瓦螺栓进行了综合试验分析。结果表明，断裂螺栓为应力腐蚀断裂。而从抽检的螺栓中发现，有的化学成分不符合要求，有的热处理工艺不合理，有的存在表面淬火裂纹等，螺栓质量的不合格率约10％。在质量分析的基础上，提出相应的改进措施。     

Failure of aircraft propeller assembly

This paper analyses the causes of the incident of a Cessna trainer whose propeller was separated due to the cracking of the propeller blade hub during the take off roll. Beach marks and fatigue striations, typical of fatigue cracks, were observed on the fracture surface and corrosive oxides were detected in the center of beach marks that are considered to be the crack origin. The stress acting on the fracture surface under a corrosive environment forms corrosive oxides, such as mud cracks. By analyzing the fractography and metallography of the failed parts, it is found that the propeller blade hub nucleated stress corrosion cracking (SCC) as a result of residual stress and corrosive environment and the SCC was the cause of the fatigue crack. Moreover, a fatigue crack reaches its critical length by repeated cyclic stress, which occurs during the rotation of the propeller blade and then, the rest of the fracture occurred instantaneously.     

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。     

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 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 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.     

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。     

Nanoscopic fatigue and stress corrosion crack growth behaviour in a high-strength stainless steel visualized in situ by atomic force microscopy

In situ atomic force microscope (AFM) imaging of the fatigue and stress corrosion (SC) crack in a high‐strength stainless steel was performed, under both static and dynamic loading. The AFM systems used were (1) a newly developed AFM‐based system for analysing the nanoscopic topographies of environmentally induced damage under dynamic loads in a controlled environment and (2) an AFM system having a large sample stage together with a static in‐plane loading device. By using these systems, in situ serial clear AFM images of an environmentally induced crack under loading could be obtained in a controlled environment, such as in dry air for the fatigue and in an aqueous solution for the stress corrosion cracking (SCC). The intergranular static SC crack at the free corrosion had a sharp crack tip when it grew straight along a grain boundary. The in situ AFM observations showed that the fatigue crack grew in a steady manner on the order of sub‐micrometre. The same result was obtained for the static SC crack under the free corrosion, growing straight along a grain boundary. In these cases, the crack tip opening displacement (CTOD) remained constant. However, as the static SC crack was approaching a triple grain junction, the growth rate became smaller, the CTOD value increased and the hollow ahead of the crack tip became larger. After the crack passed through the triple grain junction, it grew faster with a lower CTOD value; the changes in the CTOD value agreed with those of the crack growth rate. At the cathodic potential, the static SC crack grew in a zigzag path and in an unsteady manner, showing crack growth acceleration and retardation. This unsteady crack growth was considered to be due to the changes in the local hydrogen content near the crack tip. The changes in the CTOD value also agreed with those of the crack growth rate. The CTOD value in the corrosive environment was influenced by the microstructure of the material and the local hydrogen content, showing a larger scatter band, whereas the CTOD value of the fatigue crack in dry air was determined by the applied stress intensity factor, with a smaller scatter band. In addition, the CTOD value in the corrosive environment under both static and dynamic loading was smaller than that of the fatigue crack; the environmentally induced crack had a sharper crack tip than the fatigue crack in dry air.     

Failure analysis of vessel propeller bolts under fastening stress and cathode protection environment

This paper describes the failure analysis of propeller blade fastening bolts made from martensitic stainless steel 0Cr16Ni5Mo, which was ruptured under service of cathode protection for 5 years. The general crack pattern of the bolts, fractographic features, hydrogen content determination and slow tensile test results are all exhibiting the characteristics of hydrogen embrittlement. Accordingly, hydrogen diffusion driven by hydrogen concentration gradient and stress concentration was identified by experiment and finite element analysis (FEA). The morphology of the crack was intergranular of initiation from bolt cap root surface, and quasi-cleavage of propagation. The hydrogen distribution indicated that the hydrogen concentration in the bolt was in gradient distribution, and the region farther away from the sea water contains less hydrogen content. This revealed that hydrogen entered the bolt top surface through sea water under cathodic protection, and diffused from top to cap. The hydrogen content of the cap where crack initiated was 7.0 ppm, which was much higher than that in bolt shaft with normal content of 1.1 ppm. Results of low tensile test together with fractographic observation showed that the brittleness of the bolt was enhanced by the effect of hydrogen. Stress distribution calculated by FEA analysis indicated that the maximal stress of the bolt was about 1016 MPa, located at cap root surface which was consistent with crack initiation sites. The stress drove hydrogen to accumulate at root surface until cracking occurred. In a sum, the failure was attributed to the hydrogen diffusion, local high stress, and the martensitic microstructure susceptible to hydrogen embrittlement. Remedial measures such as avoiding over protection potential, that increase tempering temperature were suggested. Methods to optimize stress distribution of the bolt were also suggested based on FEA calculation.     

泵体端盖螺栓失效分析

以某企业HWD100-400燃料油泵端盖的紧固螺栓为研究对象,介绍了紧固螺栓的服役条件及常见失效形式,并通过宏观形貌分析、化学成分测试、金相组织分析、硬度测试分析等分析方法,分析了该紧固螺栓断裂的原因。结果表明,紧固螺栓在安装时预紧力过大,而产生的应力集中造成该紧固螺栓失效。