热交换器不锈钢管束断裂失效分析

采用宏观分析、金相检验、断口宏观及微观分析以及能谱分析等方法,对某炼油厂裂化车间4台热交换器不锈钢管束先后发生大面积断裂的性质及原因进行了分析。结果表明:该热交换器管束断裂是由在交变载荷和腐蚀介质作用下发生的由外向内的腐蚀疲劳开裂引起的;腐蚀疲劳裂纹起始于管外壁的点蚀坑等应力集中处,促进腐蚀疲劳裂纹扩展的管束外部介质主要是Cl^-和H₂S;裂纹先以腐蚀疲劳开裂方式扩展,而后又呈典型的应力腐蚀开裂方式继续扩展,当应力腐蚀裂纹扩展达到管束断裂强度时便发生断裂。最后提出了预防管束断裂的措施及建议。     

某涡桨发动机三级涡轮叶片失效原因分析

某型涡桨发动机涡轮叶片在长期使用过程中出现了两种类型的裂纹与断裂失效,一类是使用寿命超过4500h时在榫齿R处出现较普遍的裂纹,另一类则是使用600h以上时在榫齿与伸根R处的断裂.在断口宏、微观观察与痕迹分析的基础上,结合成分、硬度、晶粒度测定以及有限元应力分析,对这两类失效的性质与原因进行了分析研究.结果表明,使用寿命超过4500h时叶片榫齿裂纹为蠕变-疲劳裂纹,其原因是叶片材料难以满足发动机长寿命的使用要求;而榫齿与伸根R处的断裂则是大应力机械疲劳断裂,其原因可能与相邻叶片间隙偏大致使出现较高的振动弯曲应力以及表面较深的加工锉痕引起的应力集中有关.     

高压涡轮导向叶片裂纹分析

对某发动机高压涡轮导向叶裂纹的性质和产生原因进行了分析。结果表明，导向器叶片裂纹的性质属典型的热疲劳断裂失效，引起该发动机导向叶片热疲劳断裂失效的主要原因是试验温度偏高，温度场分布不均，排气边冷却效果不良也是影响叶片开裂的因素。     

伺服机构涡轮泵转子叶片断裂失效分析

某伺服液压源涡轮泵转子在测试过程中转子叶片发生断裂,通过对失效转子的观察、测试与分析认为:转子叶片的断裂性质为疲劳断裂,断裂原因是在叶片根部存在疲劳裂纹,疲劳裂纹在动静载荷作用下失稳扩展而发生疲劳断裂.分析认为转子叶片发生疲劳破坏与其组织在锻造成型工艺过程中存在工艺缺陷,导致材料疲劳寿命下降有关.     

某工程机械传动系统用主轴断裂原因分析

某工程机械传动系统用主轴的正常使用周期为0.5a(年),但是在使用70d(天)后就发生了断裂。通过宏观分析、化学成分分析、金相检验、硬度测试、微观分析及能谱分析等方法,对主轴的断裂原因进行了分析。结果表明:该主轴的断裂模式为疲劳断裂,主轴和轴套的偏磨使得主轴表面磨损严重,磨损造成的温度升高超过奥氏体化温度,冷却后形成了局部未回火马氏体的白亮区。白亮区周围的回火区域萌生的裂纹成为疲劳源,在主轴高速旋转过程中裂纹扩展直至发生疲劳断裂。     

第一代单晶高温合金中温高应力幅下的疲劳裂纹萌生行为

通过应力控制的疲劳实验探究第一代镍基单晶高温合金DD413在中温(760℃)高应力幅下的疲劳裂纹萌生行为,并使用扫描电子显微镜观察和表征了疲劳样品的断口形貌和纵截面的显微组织。结果表明：在高应力幅条件下,疲劳裂纹主要萌生于表面开裂的块状碳化物和次表面开裂的骨架状碳化物。在疲劳过程中,氧化和循环加载的共同作用使样品表面的碳化物都发生了开裂。在样品的次表面,只有位于表面微裂纹扩展路径上的碳化物发生开裂,其原因也与氧化和循环加载有关。样品表面产生的微裂纹,是次表面碳化物发生氧化所需氧气的运输通道。在疲劳的早期阶段碳化物即发生开裂并产生微裂纹,最终使样品发生疲劳断裂。     

55CrSi弹簧钢失效分析与研究

本文利用扫描电镜对弹簧钢的金相组织、断口及表面进行观察分析,研究其失效原因,结果表明此弹簧钢断裂为早期疲劳断裂,裂纹源萌生于并圈处,由于该处间隙过小,一则不能保证喷丸的效果,二则增加疲劳过程的接触疲劳应力及加重表面损伤,另外该处喷丸微细粒子未清洗干净,相当于表面有一夹杂在运行时遗留在并圈处表层,导致表面损伤,并诱发微裂纹而导致弹簧早期疲劳断裂。     

流量计导压管断裂失效分析

某化工厂工艺管线上的316不锈钢材质的孔板流量计导压管断裂,导致介质泄漏发生火灾。为查明其失效原因,对断裂的仪表管进行成分、硬度、金相、断口形貌和腐蚀产物分析,确认仪表管发生断裂的原因是在安装应力、震动和环境中Cl元素的共同作用下,先发生了应力腐蚀形成裂纹源,裂纹达到门槛值后又以疲劳形式扩展,最终导致开裂。     

离合器拨叉失效分析

汽车离合器拨叉在使用期内发生断裂。采用扫描电镜、能谱仪以及化学分析等方法对离合器拨叉进行了分析。结果表明,离合器拨叉断裂是疲劳断裂。铸造缺陷、杂质元素偏聚和夹杂物的存在是形成早期裂纹源的原因。在工作应力作用下,裂纹不断扩展,最后导致拨叉完全开裂失效。     

10.9级螺栓早期疲劳断裂失效分析

某10.9级螺栓经淬火、回火处理后装配到汽车座椅上,经历颠簸疲劳试验2万次后发生断裂。从化学成分、断口形貌、硬度、显微组织方面,对螺栓断裂的原因进行了分析。结果表明:螺纹牙底过渡部位粗糙不平且有缺口,易造成应力集中产生裂纹;螺纹表层产生脱碳,增加了开裂倾向;在疲劳试验中,由于交变应力的作用形成微裂纹,造成螺栓早期疲劳断裂。     

1Cr18Ni9Ti油管裂纹分析

某发动机在工作过程中连续发生3起油管漏油事故,通过对其中一根油管上的裂纹进行宏观观察、金相检验、化学成分分析以及应力测定,并打开裂纹进行断口电镜观察,确定了裂纹的性质及油管开裂的原因。结果表明：油管上的裂纹性质为起始应力较大的疲劳裂纹,疲劳裂纹起源于油管转接嘴的R外表面,油管的装配应力较大是导致疲劳裂纹萌生的主要原因,高压燃油冲击和油管本身的振动对裂纹的萌生也有一定的影响,管接嘴处焊料局部未焊合对裂纹的萌生有一定的促进作用。     

Failure Analysis of a Truck Diesel Engine Crankshaft Made from Spheroidal Cast Iron

A diesel engine crankshaft fractured in service after 13,656 km of operation. The fracture took place on the sixth, the fifth, and the fourth crankpins and the fracture surfaces have a 45° inclination with respect to the axial of crankshaft. The cracks of the sixth and the fifth crankpin are across the oil hole and a complete fracture took place at the sixth crankpin which bore the maximum torque load. On the fourth crankpin, crack is only through the thin wall side of oil hole. The results indicate that fatigue fracture is the dominant failure mechanism of the crankshaft. It was observed that the fatigue cracks in the crankpins initiated at machining dents present on the wall of oil hole. The appearance of the machining dents on the wall of oil hole suggests improper machining and these dents supplied the stress concentration site that was mainly responsible for the fatigue fracture of crankshaft.     

某电传操纵系统弹簧拉杆可调叉形接头断裂原因

某飞机电传操纵系统弹簧拉杆可调叉形接头在运行过程中发生断裂,通过宏微观分析和金相检验等方法对拉杆接头的断裂原因进行了分析。结果表明:拉杆接头断裂为疲劳断裂,断裂的根本原因是其显微组织不均匀的同时出现了魏氏组织,使接头的抗疲劳性能严重降低,导致疲劳裂纹的萌生与扩展;而拉杆的受力不均匀则加速了其疲劳裂纹的扩展。     

HXD1机车牵引电机转轴组件断裂失效分析

HXD1型电力机车的牵引电机转轴和小齿轮轴采用圆锥过盈配合传动结构(下称转轴组件),使用中该组件出现了早期断裂失效.本文通过理化检测、断口和配合面宏/微观形貌观察等失效分析技术对失效组件进行了分析.结果表明,材料成分、组织和显微硬度正常,小齿轮轴和电机转轴的失效形式分别为高周疲劳断裂和微动疲劳断裂.造成组件失效的原因和过程是,小齿轮轴近齿端油槽-油孔交界线处有较大的结构应力集中,油槽底部周向加工刀痕造成附加应力集中,在应力集中和旋转弯曲疲劳载荷作用下油孔边两个应力集中点萌生了疲劳裂纹并扩展;随小齿轮轴裂纹的不断扩展转轴组件结构刚度减小,继而诱发了与小齿轮轴匹配的电机轴配合面的微动疲劳,电机轴疲劳裂纹萌生于微动区的边缘处;电机转轴先于小齿轮轴完全断裂.基于本文的分析结果提出了提高组件抗疲劳断裂的技术措施.     

两种含氢钛合金的疲劳断裂特性

对含氢的两种新型钛合金Ti 4Al 2V和Ti 2Al 2 .5Zr拉 拉疲劳断裂特征进行了金相和断口观察。当材料承受大应力疲劳加载时 ,两种钛合金断口形貌除可见到疲劳弧线外 ,与静拉伸断口类似 ,氢含量不同对疲劳断口形貌的影响很小。充氢以后 ,疲劳载荷越低 ,裂纹源越多。不充氢的角裂纹通常以穿晶方式萌生 ,而充氢的角裂纹一般以准解理方式形核。Ti 4Al 2V在裂纹扩展区可见大量清晰的疲劳辉纹 ,而Ti 2Al 2 .5Zr的疲劳辉纹很少 ,且疲劳辉纹的间距比Ti 4Al 2V小得多。面裂纹一般在氢化物位置形核 ,然后穿晶扩展 ,具有沿晶和穿晶混合形貌。氢化物的含量越大 ,萌生面裂纹的几率越大     

Analysis of the fatigue failure of a mountain bike front shock

We present an analysis of a mountain bike front shock failure. The failure of the 1-year-old shock occurred catastrophically as the bike was ridden off of a 1-m drop. The failure was the result of fast fracture through both shock tubes at the location where the tubes were press fit into the shock upper crown. Examination of the fracture surfaces of the tubes revealed regions of fatigue crack growth that nearly penetrated the entire thickness of both tubes. An estimate of the forces during use, coupled with stress analysis, revealed three stresses near the fracture site—axial compression, bending, and hoop stresses. During operation, the axial compressive stress is negligible while the hoop and bending stresses are significant. Based on fracture mechanics, and an estimate of the bending stress from a 1-m drop, it is confirmed that the fatigue cracks present on the fracture surface were large enough to induce fast fracture. Prior to the existence of the fatigue cracks, the stresses were magnified locally near the fracture site by a significant stress concentration caused by the sharp transition from the shock tube to the crown. The fatigue cracks initiated at a circumferential location in the tube commensurate with high tensile bending stress and the stiffest region of the crown (highest stress concentration). Based on the evidence, the most probable cause of the bike shock fatigue failure was the shock design, which facilitated high local stresses during use.     

离合器压盘断裂的失效分析

采用化学成分分析，宏、微观检验等方法对某型轿车离合器中的断裂压盘进行了检测。结果表明，其断裂方式属于疲劳失效，疲劳裂纹萌生于支承耳根部等应力集中处；所用材料无质量问题，过旱失效起因于恶劣的工况。     

Analysis of a crankshaft fatigue failure

The reason of the crankshaft fracture of the air compressor has been analyzed through the chemical composition, mechanical properties, macroscopic feature, microscopic structure and theoretical calculation methods. The analysis results show that the crankshaft which has obvious fatigue crack belongs to fatigue fracture. The fatigue crack initiated from the fillet region of the lubrication hole because of the high bending stress concentration which is caused by both the small fillet and the misalignment of main journals. The crankshaft fatigue fracture was only attributed to the initiation and propagation of the fatigue cracks on the lubrication hole under cyclic bending and torsion. The high bending loading bending level is the root cause of the failure.     

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 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.