某锅炉混合减温器入口端K型热电偶失效断裂分析

为分析研究热电偶套管断裂的原因，并对热电偶套管进行科学合理的选型，采用扫描电镜结合能谱仪、X射线衍射仪等表面分析手段对热电偶套管断口进行综合失效分析，并对套管承受振动进行模拟计算。结果表明：套管断口表面生成腐蚀产物膜，并出现疲劳辉纹。计算结果发现套管在蒸汽作用下产生的振动频率超过了本身的固有频率，远超过了安全系数。综合分析得出由于振动变工况等综合作用下腐蚀疲劳等主因导致套管环境敏感断裂。     

油气管线热电偶套管断裂原因分析

某炼油厂油气管线上的一个热电偶套管发生断裂,使用时间仅6个月。对断裂套管进行了断口形貌、材质成分、金相组织等分析。结果表明,套管断裂主要原因是管线中油气流的冲击和湍流作用,引起套管的共振．进而产生疲劳断裂。     

压缩机出口管线热电偶套管开裂失效分析

为了查明压缩机出口管线上的热电偶套管发生开裂原因,针对热电偶套管进行了检测分析。检测结果表明,热电偶套管失效属于疲劳,套管过长以及安装位置振动超标是造成套管发生疲劳开裂的主要原因。后期采取补救措施,避免了热电偶套管的疲劳失效。     

轻合金自冲铆微动磨损及疲劳性能研究

选择4组轻合金自冲铆进行疲劳实验,用扫描电子显微镜和能谱仪对其断口进行微动磨损机理分析,并系统地研究了接头疲劳寿命和失效形式的影响因素。结果表明,下板与钉腿区的微动磨损是导致下板沿纽扣断裂和铆钉断裂的主要原因,两板间的微动磨损是导致上板靠钉头断裂的主要原因;微动磨屑主要成分为金属板材氧化物,并对微动磨损起缓冲作用。增加板厚可提高接头疲劳寿命,且疲劳载荷较大时寿命提高更为显著;增加板强可提高接头疲劳寿命,且寿命提高程度受疲劳载荷影响较小。增加板厚使失效形式从上板断裂变为下板断裂,增加板强使失效形式从板材断裂变为铆钉断裂。     

热电偶套管流体激振数值分析

用于测量管内流体温度的热电偶,由于受到高温、高速流体冲击,常发生套管断裂现象。采用大涡模拟（LES）方法对一热电偶套管周围非定常流场进行数值模拟,分析高速、非定常流动引起的热电偶套管受力与振动情况,提出在套管前串列干扰装置以抑制振动、减轻受力的改进措施,并对改进后效果进行分析和验证。研究结果可为热电偶套管结构优化设计提供参考。     

ZKG223钎杆连接套管断裂原因分析

某ZKG223钎杆连接套管在使用过程中发生断裂失效。通过宏观检查、化学成分分析、金相检验、断口分析对套管断裂失效原因进行了分析。结果表明:该连接套管断裂类型为疲劳断裂。断裂起源于套管内壁螺纹损伤处,套管与钎杆装配时存在连接松动造成使用过程中螺纹发生磕碰挤压损伤,引起疲劳裂纹的萌生及扩展,最终导致套管断裂失效。     

热电偶套管断裂原因分析及应对措施

采用化学成分分析、金相分析、宏观与微观断口分析等方法,对某炼油厂常减压装置工艺管线热电偶套管的断裂原因进行了分析,针对断裂原因,对目前在此套装置内部分热电偶的使用安全性进行了分析,并提出整改的建议以减少类似事故的发生。     

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.     

TA1钛合金自冲铆接头力学性能及微动行为

本工作以TA1自冲铆接头为研究对象,基于拉伸-剪切和疲劳试验分析了接头的力学性能,并采用扫描电镜从微观层面研究了接头的拉伸-剪切失效机理、疲劳失效机理及微动行为。结果表明:拉伸-剪切失效模式为铆钉腿部从下板拉脱,铆钉颈部存在不同程度的断裂。疲劳失效模式主要为上板断裂失效,其疲劳极限约为1.18 kN。疲劳裂纹从上板与铆钉头接触部位萌生,在持续微动磨损及疲劳循环应力作用下,沿板厚和板宽方向不断扩展,直至接头疲劳断裂。微动磨损的剧烈程度直接影响接头疲劳失效模式。上板与铆钉头接触区的微动磨损源于板宽W区域,随着微动过程的不断进行,逐步向板长L区域扩展。     

TA1钛合金单搭自冲铆接头微动磨损机理

对TA1钛合金单搭自冲铆接头进行疲劳实验研究接头失效形式;用扫描电子显微镜和X射线能谱线扫描研究铆钉各部位微动磨损程度的差异和接头微动磨损机理;采用威布尔分布验证数据有效性。结果表明:接头疲劳失效形式主要为上板断裂,高周疲劳均为上板断裂,低周疲劳为上下板混合断裂;微动磨屑包含氧、钛、锌和锡元素,铆钉头部微动磨损程度高于铆钉腿部。微动磨损区出现严重脱层、微动磨屑堆积和微裂纹萌生等现象,随着微动磨损及剪切力共同作用导致接头断口部位出现大量微裂纹并逐步沿深度和宽度方向扩展为宏观裂纹,最终导致接头疲劳失效。     

EFFECT OF RELATIVE SLIP AMPLITUDE ON FRETTING FATIGUE OF HIGH STRENGTH STEEL

Abstract— The effect of relative slip amplitude on fretting fatigue in high strength steel was studied at various contact pressures using fretting pads of various lengths. Under a given contact pressure, the fretting fatigue life showed a minimum at a certain relative slip amplitude. Under a fixed pad length, the life also showed a minimum at a certain contact pressure. A map of fretting fatigue life versus contact pressure and relative slip amplitude was obtained using the data of this study. The map indicated that both the phenomena which showed a minimum life in relation to slip dependence and contact pressure dependence were the same, as were the underlying mechanisms. The minimum life was interpreted in terms of local stress concentration at the fretted area.     

A fracture mechanics approach to high cycle fretting fatigue based on the worst case fret concept – I. Model development

An integrated analytical and experimental approach was taken to develop a fracture mechanics-based methodology for predicting the limiting threshold stress of high-cycle fretting fatigue in structural alloys. The contact stress field for two flat surfaces under fretting was analyzed via an integral equation technique. The local fretting stress field of the uncracked body was then utilized to obtain the stress intensity factor of an arbitrarily oriented fatigue crack using a continuum dislocation formulation. The limiting threshold stress ranges for the nonpropagation of fretting fatigue cracks were predicted on the basis that the fretting fatigue cracks are small cracks that exhibit a size-dependent growth threshold and propagate at stress intensity ranges below the large-crack threshold. In part I, the development of the worst-case fret (WCF) model is described. The influence of the limiting high-cycle fatigue (HCF) threshold stress on a variety of fretting fatigue parameters such as bearing pressure, pad geometry, shear stress, mode mixity, and coefficient of friction are elucidated by parametric calculations. In part II, the WCF model is applied to treating HCF of Ti-6Al-4V where model predictions are compared against critical experiments performed on a kilohertz fretting-fatigue rig.     

Fretting fatigue in AISI 1015 steel

A small oscillatory movement between two contacting surfaces is termed as fretting and on many occasions it acts as the crack initiation site leading to catastrophic failure of the overall structure. The occurrence of fretting is observed in many engineering structures such as shaft flanges, gas turbines, steel ropes etc. An experimental facility, which can simulate the fretting fatigue in many engineering applications, is the primary requirement of the research program. A laboratory fretting fatigue test facility capable of varying many influencing parameters of fretting fatigue such as slip amplitude, frequency, contact pressure, etc is designed and developed. Preliminary investigations on plain and fretting fatigue behaviour of AISI 1015 structural steel are reported in this paper. A strength reduction factor of about 1.30 was obtained due to fretting for the test material under the present experimental conditions. Influence of contact load on fretting was also studied. Increasing fretting contact load decreased the fatigue life in the range investigated. Failure analysis showed typical stage I oblique crack growth followed by stage II straight crack perpendicular to the fretting zone.     

Multi‐axial fretting fatigue behaviour of 35CrMoA steel

The fretting fatigue behaviours in 35CrMoA steel were investigated under conditions of the various contact pressure and the same maximum equivalent stress of cyclic multi‐axial loading. The specimens were characterized by optical microscopy and scanning electron microscopy. Results showed that the fretting fatigue life has complex variation with the increase of contact pressure. The different contact pressure also played very important role in fretting wear model and various wear scars were formed. The influence of them at the fretting surface on the fretting fatigue life was finally discussed by the comparison of the experimental and numerical analysis results.     

Influence of contact pressure on fretting fatigue behaviour of AA 2014 alloy with dissimilar mating material

The effect of contact pressure on the fretting fatigue behaviour of 2014 Al alloy which has been solution heat treated and age hardened (T6 heat treatment) with dissimilar mating materials, was investigated. The fretting fatigue configuration involved bridge‐type contact pads on a flat fatigue specimen. Specimens were made of 2014 Al alloy and bridge‐type pads were made AISI 4140 steel. All the fatigue tests were conducted at a rotational speed of 5000 rpm with a rotating bending fatigue machine (R=?1), using S–N curves to evaluate the fatigue and fretting fatigue properties. The fretting fatigue strength of the material subject to a T6 heat treatment condition at 1 × 10⁷ cycles was dramatically reduced, as compared to that without fretting and with as‐cast. The fretting fatigue life exhibited a variable behaviour with an increase in the contact pressure. A scanning electron microscope was employed to observe the fretting scars and fracture surfaces of the specimens. This analysis showed that cracks originated at the contact surface and crack orientations were approximately ±56 ° from perpendicular to the loading direction.     

Root Cause Analysis of Cracking in Shaft End and Coupling of a High-Power Generator

Failure analyses and root cause determination were carried out on the rotor of a high-power generator of gas–diesel dual fuel which presented cracking due to torsional fatigue in its end (region of section change and coupling), after 30,000 h of service. The generator of 307 MW–3000 rpm has a rotor (shaft of 400 mm Ø) manufactured in a proprietary steel grade equivalent to ASTM A470 type, Class 7 of high hardenability. It was reported that the equipment control system showed, in service, a high level of vibrations, not admissible for continuing the operation. First, and during the equipment shutdown for inspection, the presence of cracks, slant to the rotor shaft, was detected by means of visual inspection and dye penetrant test. The failure region corresponds to the zone of coupling–shaft joint, linked by means of fixation by interference, whereas the cracking spread on two fracture planes at 45° with respect to the rotor shaft. On this zone, where cracking started, a severe fretting corrosion damage was evidenced. The characterization and identification of present damage mechanisms were conducted through macrographic, fractographic, SEM, EDS, chemical analyses, and mechanical tests. It was recognized that from the damage by fretting corrosion, fatigue micro-cracks were produced that spread due to service tensions by a mechanism of fretting fatigue. The fatigue fracture propagation was developed into two orthogonal planes at 45° from the longitudinal shaft, which reveals an inversion in the loading condition, only justifiable by torsional vibrations that were assigned to a torsional resonance typical of the system dynamics. It was considered that the torsional vibrations cause micro-movements between components, promoting fretting corrosion and the subsequent fretting fatigue that finally induced the failure by high-cycle torsional fatigue with low-stress amplitude.     

Effect of Fatigue on Proof Strength of Gas Turbine Combustor Casing

The objective of an aero-engine combustor structural design is primarily to provide the engine combustor with sufficient fatigue life and strength to permit its continuous operation until a scheduled engine overhaul and to withstand certain overload. During the service, combustor is subjected to cyclic pressure which may affect its overpressure capability. The combustor casing is considered as one of the critical parts in the engine. In this paper, the proof pressure capability of a combustor casing subjected to fatigue loading is evaluated. The effects of the fatigue damage on the proof pressure capability of the combustor casing are also established. During the testing of the casing, the pressure was continuously increased to evaluate the over pressure capability of the casing. The results of this experimental study on the casing of an aero-engine combustor are presented in this paper. A detailed post-fracture investigation of the fracture surfaces of the casing showed that two independent cracks, i.e., one along the circumference of the casing towards the front flange and the other one along the axial direction were developed. Further a detailed investigation of the fractured surface under stereo zoom microscope and scanning electron microscope showed that the presence of striations coupled with dimples indicated that the failure was initiated due to fatigue loading.     

Failure investigation of a low-pressure turbine blade

Several blades at a thermal power plant had failed during operation. Extensive visual, metallographic, and microhardness tests indicated that the initiation of the failure had been due to cracks formed due to fretting, which subsequently propagated in fatigue.     

Fretting fatigue failure mechanism of automotive shock absorber valve

Fretting fatigue is a complex mechanical failure phenomenon, in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. This study proposes a methodology to analyze the fretting fatigue failure mechanism of automotive shock absorber valve by means of experimental and numerical approaches. A servo hydraulic test set-up is used to simulate fretting fatigue under real working conditions. Moreover, a 3-D finite element model is developed to analyze the contact status and stress distribution at contact interface between connected components, i.e. washer-disc contact. The experimental test results depict that fretting damage appears at contact interface between washer and disc, which causes the initial crack nucleation and advancing the crack up to the final fracture of valve disc. Stress field, obtained by numerical simulation, is used to monitor some fretting fatigue features such as the distribution of relative slip amplitude, contact pressure and different stress fields at contact interfaces. Eventually, the crack initiation site is estimated by monitoring variation of equivalent multiaxial damage stress at contact interface.