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.     

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。     

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.     

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.     

缓冲器轴套夹布胶木失效分析

起落架缓冲器轴套在使用过程中发生开裂或断裂.通过外观检查、断口观察、性能测试等方法,结合缓冲器轴套夹布胶木的装配与受力情况,分析了缓冲器轴套断裂与裂纹产生的原因.实验结果表明:夹布胶木裂纹为疲劳裂纹,夹布胶木断裂也是疲劳开裂所致.疲劳裂纹与断裂的主要原因是轴套夹布胶木与铜套之间为过渡配合,二者之间存在间隙,油进入间隙后...     

In-body corrosion fatigue failure of a stainless steel orthopaedic implant with a rare collection of different damage mechanisms

A stainless steel orthopaedic (femoral) implant was examined, which had fractured inside a patient’s thigh. It had been in his leg for almost two years, with no problem in the bone reconstruction process. The plate had apparently fractured during the first few months, when the bone had not been reconstructed completely. Different investigations such as visual assessments, hardness testing, stereoscopy, metallography, quantometry, SEM fractography and EDS microanalysis were performed in order to analyze the failure mechanism and its cause(s). Different damages were observed such as crevice corrosion pitting, initiation of cracks from these pits, intergranular surface cracking inside the crevice, and also SCC-like branched cracks. But, the main failure mechanism was determined to be corrosion fatigue assisted by crevice corrosion.     

火电机组给水前置泵轴断裂原因分析

某火电厂4号机组B给水前置泵轴投运仅19h即发生断裂。经对断裂泵轴进行宏观分析、化学成分分析、常温力学性能测试、金相检验及断口扫描电镜分析,探讨并明确了泵轴断裂原因,同时提出了防范措施。结果表明：该给水前置泵轴断裂为疲劳断裂,在弯曲及扭转载荷作用下于变截面的应力集中部位的不连续及夹杂物处形成疲劳裂纹,同时大量夹杂物及沿晶分布的粗大a铁素体的存在严重降低了基体强度,使轴体所能承受的循环应力大大降低,即在较低的循环应力作用下疲劳裂纹不断扩展并最终断裂。     

Fatigue crack growth simulation in a first stage of compressor blade

A first-stage rotary compressor blade of a Model GE-F6 gas turbine failed due to vibration in early March 2008. Initial investigations showed that pitting on the pressure side of the blade caused micro cracks, leading to larger cracks due to high cycle fatigue. To assess this failure, a series of experimental, numerical, and analytical analyses were conducted. Fractography of the fractured surface of the blade indicated that two semi-elliptical cracks incorporated and formed a single crack. In this study, static and dynamic stress analyses were performed in Abaqus software. Moreover, fracture mechanics criterion was accomplished to simulate fatigue crack growth. This was carried out using a fracture analysis code for 3-dimensional problems (Franc3D) in two states. Firstly, stress intensity factors (SIFs) for one semi-elliptical surface crack and then SIFs for two semi-elliptical surface cracks were taken into account. Finally, the Paris and Forman–Newman–De Koning models were used to predict fatigue life. Since stress level and crack shape in both conditions are the same and the SIF at the crack tip reaches the fracture toughness of the blade, SIFs results indicate that insertion of a second crack has no effect on the final SIF, however, the second crack facilitates the process of reaching the critical length. So, fatigue life in two-crack condition is less than in the one-crack state.     

Defect tolerance assessment of a helicopter component subjected to multiaxial load

In this paper a simple but effective methodology for the defect tolerant assessment of a structural component is presented. The methodology is based on the experimental evidences available in literature that at the fatigue limit non-propagating cracks are observed near defects present in the metallic material. The evaluation of the threshold stress intensity range of these short cracks allows to calculate a safety factor with respect to their propagation. The methodology is applied to the tail rotor hub of a helicopter. The complex geometry and load conditions make this component very interesting for the defect tolerance assessment. The application to an engineering problem allows to describe the difficulties that arise in this process and to show the possible solutions that can be adopted.     

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.     

汽车发动机曲轴断裂失效分析

某厂生产的发动机曲轴在用户使用过程中,3个月内共发生了4起曲轴断裂失效事故,采用化学成分分析、金相检验、硬度测试以及断口的宏、微观形貌分析等方法对断裂曲轴进行了分析。结果表明：曲轴断裂为高周低应力弯曲疲劳断裂,导致其断裂的主要原因是在曲轴第一曲拐过渡圆角R附近的曲柄表面聚集分布着机加工刀痕,形成了应力集中;在用户行驶过程中因车况、路面等复杂因素形成的过载或冲击载荷等作用下,在第一曲拐轴颈尺附近曲柄表面应力集中的机加工刀痕处萌生疲劳裂纹,并逐步扩展直至断裂失效。     

电厂外供泵轴断裂原因分析

某外供泵在运行期间其泵轴发生断裂。通过宏观和微观检验、化学成分分析以及硬度测试等方法对泵轴断裂的原因进行了分析。结果表明:该轴的热处理没有达到要求,使各项强度指标显著降低,加上在应力集中部位键槽根部产生了疲劳裂纹,并进一步扩展,最终导致泵轴断裂。     

Thermo-mechanical Fatigue Failure of a Low-Pressure Turbine Blade in a Turbofan Engine

This paper concerns a failure analysis case study of low-pressure turbine blades in an aero-engine. The operational condition of the engine was studied, and metallurgical investigations were carried out on two fractured blades. The failure in one blade originated at the leading edge, while in another it originated at the trailing edge then propagated in the forward direction. The crack propagation region showed mixed mode fractographic characteristics before the final failure. The mixed mode region was considered indicative of a thermo-mechanical fatigue propagation mode. Surface analysis of the blades indicated oxidation of variant thicknesses including oxide-filled intergranular cracks and grain boundary thickening beneath the oxide layer. It is considered more probable that the mechanism was more oxidation and fatigue dominated as opposed to creep-related.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

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 properties of self‐piercing riveted multi‐rivet joints in steel and aluminum sheets

Self‐piercing riveting (SPR) is an important joining technology for connecting steel and aluminum sheets. In this paper, AA6111 aluminum alloy and DP780 high‐strength steel were adopted to study the influence of fatigue on remaining static strength and energy absorption properties on self‐piercing riveting multi‐rivet joints. The results showed that energy absorption capacity of the specimens decreased significantly after high cycle fatigue. Fatigue reduced the remaining static lap shear strength of riveted specimens. Scanning electron microscopy (SEM) was used to analyze the cross section of fatigue specimens fractured by static tension. The results showed that fretting wear was found at the contact area between rivet and aluminum sheets. Fatigue bands and fatigue cracks appeared in fatigue specimens after high cycle fatigue, while those with low cycle fatigue specimens did not appear. Small cracks weaken the strength of the aluminum sheet, resulting in the static tensile strength of the riveted specimen with high cycle fatigue is lower than that of other fatigue specimens.     

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。     

Fretting fatigue of a shrink-fit pin subjected to rotating bending: Experiments and simulations

Fretting fatigue initiation was studied for a shrink-fit pin at rotating bending. Eight assemblies with four different grips were manufactured from soft normalized steel and tested at loads well below bending endurance. All pins displayed rust-red fretting oxides deep into the contact and black oxidised fretting scars with fretting fatigue cracks at the rim. The slip evolution was simulated in a three-dimensional FE model including assembly, bending and sufficiently many rotations to reach a steady-state. The extension of cyclic slip agreed with the black oxidised scar. Deeper into the contact a monotonic slip developed to the positions with rust-red oxides. Asymmetric slip and traction on the interface sides together with a slight twist of the pin in the hub and the slip development process, illustrated that a three-dimensional analysis was required for the interface. Both the stress amplitude and the Findley multi-axial criterion predicted fretting fatigue of the pin although the rotating bend stress was well below the endurance limit.     

An investigation on the failed blades in a locomotive turbine

A failure investigation has been conducted on the turbine blades used in a locomotive turbochanger, which are made from K418 Ni-base superalloy. Fractography investigation on the troubled blade indicates that cracks initiated from the surface of the concave side close to the trailing edge and propagated towards to the leading edge. The multi-origin fatigue fracture is the dominant failure mechanism of the blade. Metallographic morphology typical of over-heat damage features, such as re-dissolution of the eutectic γ + γ′, melting of the local region of the grain boundary appears in the microstructure of the airfoil part of the failed blades. Appearance of over-heat damage structure in the serviced blades makes the strength of the blade material decrease intensely to initiate fatigue cracks and make one of the blades fracture first. Fragments from the blade fractured first would crash the other blades to make the blades break or bending deformation.     

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.