3044 t/h超超临界锅炉磨煤机拉杆断裂原因

某电厂3044 t/h超超临界锅炉磨煤机拉杆发生断裂,通过化学成分分析、断口分析、力学性能测试和金相检验等方法对拉杆断裂原因进行了分析.结果表明:该拉杆供货态热处理质量较差,导致强度和冲击韧性指标不合格,且磨损补焊处因热处理不到位而产生冷裂纹,长期在往复拉应力及弯曲应力的作用下,裂纹疲劳扩展,最终导致拉杆疲劳断裂.     

TIG与FSW焊接工艺下2219铝合金疲劳裂纹扩展性能研究

目的 研究钨极惰性气体保护焊（TIG）和搅拌摩擦焊（FSW）对2219铝合金焊接接头疲劳性能的影响,并探究这2种不同焊接技术条件下焊接接头疲劳裂纹的产生与裂纹扩展原理,了解2种焊接接头的抗裂纹扩展能力,为工程实践应用提供数据参考。方法 采用疲劳裂纹扩展试验方法,测试上述2种焊接工艺条件下焊缝金属和热影响区组织的疲劳裂纹扩展速率da/d N和阈值,使用光学显微镜和扫描电子显微镜观察并分析金相组织和疲劳断口形貌特征。结果 疲劳裂纹倾向于沿裂纹处萌生,裂纹的存在成为主要的裂纹扩展源头,有利于加速裂纹向前延伸。热影响区由于组织结构不均匀,不同位置的晶粒尺寸存在明显差异,疲劳裂纹扩展路径倾向于沿靠近焊缝一侧向靠近母材区域扩展。TIG焊接工艺下焊缝金属和热影响区的裂纹扩展速率明显低于FSW焊接工艺下的焊缝金属和热影响区,与此同时,TIG焊接接头表现出优良的抗疲劳裂纹扩展性能。结论 通过此研究,建议2219铝合金焊接接头采用TIG焊接工艺,抗疲劳裂纹扩展效果更佳。     

某仪器舱大套筒开裂失效分析

某仪器舱3个大套筒在振动试验过程中发生断裂,失效分析后认为套筒断裂性质为疲劳断裂;断裂原因是由于材料组织不均匀(存在大量条状析出相),以致套筒经硫酸阳极化后产生了表面腐蚀裂纹;断裂过程为在振动试验过程中表面腐蚀裂纹沿条状析出相扩展,导致套筒疲劳断裂.     

某井钻杆接头断裂原因

在对某油气井的钻探过程中,钻杆接头发生断裂。采用宏观观察、化学成分分析、扫描电镜分析、力学性能测试、金相检验等方法对该钻杆接头的断裂原因进行分析。结果表明：该钻杆接头的断裂性质为疲劳断裂,钻杆钻进过程中频繁发生跳钻、憋钻现象,使其瞬时载荷较大,在钻杆大端螺纹根部萌生了疲劳裂纹,在复杂的交变载荷作用下,裂纹快速扩展,最终导致钻杆接头发生断裂。     

疲劳裂纹在奥氏体/铁素体异种钢焊接接头中的扩展行为

采用三点弯曲试样研究了疲劳裂纹在奥氏体/铁素体异种钢焊接接头中的扩展行为与显微组织的关系,测得疲劳裂纹在Cr25Ni13/13CrMo44异种钢焊接接头中的扩展速率da/dN,并且讨论了疲劳裂纹扩展与显微组织之间的关系。实验结果表明,疲劳裂纹在异种钢焊接接头熔合区中扩展的路径,是接头中韧性最低的热影响区过热区,裂纹在铁素体材料侧,跟随熔合线并平行于熔合线5～25μm扩展,而马氏体层对疲劳裂纹有较大的抗力,疲劳裂纹的扩展路径主要受组织韧性的控制。疲劳裂纹在Cr25Ni13/13CrMo44异种钢接头的扩展速率为：da/dN=7.07×10-13(△K)3.863。     

某地铁受电弓拉杆球铰轴承失效原因

某地铁受电弓拉杆球铰轴承在运行过程中发生失效断裂。采用宏观分析、断口分析、金相检验、硬度测试及化学成分分析等方法对球铰轴承的失效原因进行了分析。结果表明:该受电弓拉杆球铰轴承的断裂模式属于高周低应力疲劳断裂。球铰轴承在服役过程中承受频繁的振动和交变载荷作用,折角处的微裂纹成为疲劳裂纹源区,裂纹在交变载荷作用下不断扩展,最终导致球铰轴承失效断裂。     

客车直拉杆断裂分析

通过宏观检验、化学成分分析、断口分析以及金相检验,对客车直拉杆断裂的原因进行了分析。结果表明:直拉杆的146°弯处在使用前存在微裂纹,在客车行驶过程中该直拉杆受交变应力的作用,微裂纹以疲劳的方式扩展,当扩展到一定程度,剩余截面不能承受外力作用时发生断裂。     

疲劳裂纹在奥氏体／铁素体异种钢接接头中的扩展行为

采用三点弯曲试样研究了疲劳裂纹在奥氏体 /铁素体异种钢焊接接头中的扩展行为与显微组织的关系 ,测得疲劳裂纹在 Cr2 5 Ni13/ 13Cr Mo44异种钢焊接接头中的扩展速率 da/ d N,并且讨论了疲劳裂纹扩展与显微组织之间的关系。实验结果表明 ,疲劳裂纹在异种钢焊接接头熔合区中扩展的路径 ,是接头中韧性最低的热影响区过热区 ,裂纹在铁素体材料侧 ,跟随熔合线并平行于熔合线 5～ 2 5 μm扩展 ,而马氏体层对疲劳裂纹有较大的抗力 ,疲劳裂纹的扩展路径主要受组织韧性的控制。疲劳裂纹在 Cr2 5 Ni13/ 13Cr Mo44异种钢接头的扩展速率为 :da/ d N=7.0 7× 10 - 1 3(△ K ) 3.86 3     

发动机热端部件电子束焊修复接头的高温疲劳裂纹扩展行为

研究了力学不均匀性对电子束焊接修复接头疲劳裂纹扩展行为的影响规律，结果表明，由于力学不均匀性的影响，位于修复法接头焊缝和热潮影响区的疲劳裂纹垧母材偏转扩展，力学不均匀性与裂纹偏转导致裂纹扩展的驱动力下降，使得焊接区的表现疲劳裂纹扩展速率低于母材。     

回火工艺对65Mn钢疲劳裂纹扩展行为的影响

采用中温回火和高温回火对65Mn钢进行热处理,测试其拉伸性能,并对其疲劳裂纹扩展行为进行研究,探索其疲劳断裂机制．结果表明,两种回火方案对应的裂纹扩展速率的m值相差不大,而C值差异较大,反映出在稳定扩展区的疲劳裂纹扩展对其微观组织不敏感,塑性变形消耗了裂纹扩展的能量,使裂纹扩展速率下降;随着回火温度的升高,实验钢的疲劳断裂特征由脆性逐渐变为韧性,高温回火提高了其裂纹扩展的门槛值,降低了疲劳裂纹扩展速率,65Mn钢经高温回火后有较优的综合力学性能和疲劳性能．     

18CrNiMo7-6合金钢的弯曲微动疲劳特性

对18CrNiMo7-6合金钢进行弯曲微动疲劳实验,建立弯曲微动疲劳S-N曲线,并对实验结果进行分析。结果表明:该合金钢的弯曲微动疲劳S-N曲线不同于中碳钢材料,也不同于常规弯曲疲劳,而是呈"ε"型曲线特征。随着弯曲疲劳应力的增加,微动运行区域由部分滑移区向混合区和滑移区转变,损伤区的磨损机制以剥层、磨粒磨损和氧化磨损为主。在混合区内,裂纹最易萌生和扩展,且裂纹均萌生于材料接触区次表面。受接触应力和弯曲疲劳应力影响,弯曲微动疲劳裂纹的萌生和扩展可分为三个阶段:初期,在接触应力控制下,裂纹萌生于次表面;随后,裂纹受接触应力和弯曲疲劳应力共同控制,转向更大角度方向扩展;最后,裂纹完全受弯曲疲劳应力控制而垂直于接触表面扩展,直至断裂失效。     

Stellite12钴基合金的疲劳性能及其断裂机理研究

采用三点弯曲疲劳法测得光滑试样和直缺口试样的S-N曲线以研究Stellite12钴基合金的疲劳性能,并通过断口形貌观察进一步探究该钴基合金的断裂过程。结果表明:光滑试样的疲劳极限为545 MPa,约为原始抗弯强度1552 MPa的25.4%;直缺口试样的疲劳极限约为101MPa,约为静态抗弯强度517.6MPa的19.1%。对于疲劳敏感性,光滑试样与直缺口试样的疲劳敏感性分别为397和31。此外发现疲劳裂纹多萌生于近表层聚集的碳化物处,同时表面缺陷也可诱发疲劳裂纹的萌生。疲劳裂纹的扩展主要表现为碳化物的穿晶断裂,钴基体在应力比R=0.1的疲劳加载条件下虽表现出一定的韧性且呈现出较多的撕裂脊,但也呈现出一定的脆性断裂模式,因此疲劳裂纹扩展模式为真疲劳与静态疲劳的混合模式。     

Failure Analysis of a Diesel Engine Connecting Rod

A failure investigation has been conducted on a diesel engine connecting rod. The fracture occurred at the small head of the connecting rod. Visual and scanning electron microscopy observations show that a lot of axial grooves appear on the internal surface close to the fracture and the fatigue cracks initiated from the axial grooves. Fractography indicates that the multiple-origin fatigue fracture is the dominant failure mechanism. The machining or assembling process was responsible for the formation of the axial grooves.     

纵肋与横隔板交叉构造细节穿透型疲劳裂纹扩展特性及其加固方法研究

纵肋与横隔板交叉构造细节是正交异性钢桥面板最易发生疲劳开裂的构造细节,通过建立有限元数值模型,采用断裂力学方法,研究栓接角钢加固方式对该处疲劳易损细节穿透型裂纹的加固效果。基于疲劳试验足尺节段模型相对应有限元模型,建立了纵肋与横隔板焊接处穿透型疲劳裂纹模型,针对栓接角钢和纵肋外侧栓接钢板两种加固技术的加固效果进行评估。研究结果表明:钢桥面板纵肋与横隔板交叉构造细节的疲劳裂纹扩展至一定长度后将发展成穿透型裂纹,裂纹面受力复杂,纵肋腹板内外侧疲劳裂纹扩展特性表现的不一样,但是随着裂纹扩展的逐步进行,裂纹尖端的开裂模式均以复合型开裂为主;栓接角钢加固方式主要抑制纵肋与横隔板交叉构造细节易损部位疲劳裂纹的I型开裂,因此能很好地抑制短裂纹的扩展,但对于该细节处以复合形式扩展的穿透型疲劳裂纹的加固效果并不显著;在纵肋外侧栓接半U形钢板的加固方法能有效改善穿透型疲劳裂纹的等效应力强度因子,并且加固之后均保持在裂纹扩展阈值以下,表明该加固方式对穿透型疲劳裂纹有良好加固效果。     

2219铝合金TIG和FSW接头力学及疲劳性能

目的 研究钨极氩弧焊(TIG)和搅拌摩擦焊(FSW)对2219铝合金(母材)力学及疲劳性能的影响。方法 通过拉伸试验,得到了母材、TIG和FSW接头的抗拉强度和伸长率;通过疲劳性能试验测试了母材、TIG和FSW接头在不同应力下相应的疲劳寿命,根据疲劳试验结果绘制了其试样的S-N曲线;使用扫描电子显微镜观察并分析了疲劳断口的形貌特征。结果 未焊接的铝合金母材抗拉强度和伸长率最高,分别为506 MPa和15.92%;TIG接头抗拉强度和伸长率分别为330 MPa和7.65%,FSW接头抗拉强度和伸长率分别为310 MPa和8.74%。母材、TIG和FSW接头等3种疲劳试样在2×10⁶次循环下的疲劳强度分别为129、108、115 MPa,其疲劳断口均可分为裂纹源区、裂纹扩展区和瞬间断裂区,疲劳裂纹分别起始于试样表面的局部变形区、第二相夹杂物和“吻接”缺陷。疲劳裂纹扩展区的主要形貌为疲劳辉纹和二次裂纹,瞬间断裂区以脆性断裂为主。结论 TIG和FSW等2种焊接工艺均导致了2219铝合金的强度、塑韧性和疲劳性能降低,其接头表面的第二相夹杂物和“吻接”缺陷促进了疲劳裂纹的萌生。     

Some Peculiarities of Fatigue Crack Growth at Various Stages of Its Development

The author considers some peculiarities of fatigue crack growth in metals at the stages of its initiation and initial development, and stable and unstable growth that precedes final fracture. It is shown that at the stage of initial growth of fatigue cracks, the stress state, nonlocalized fatigue damage that precedes initiation of the main fatigue crack, residual surface stresses, surface manufacturing and in-service defects, and contact interactions are the factors that determine the crack paths. Stable growth of a fatigue crack is primarily determined by the stress-strain state of a structure as a whole and by the stress-strain state at the crack tip with allowance for its variation due to crack propagation, which is evaluated by the criteria of fracture mechanics. The author also studied peculiarities of fatigue crack development in compressor blades of marine gas turbines. It is shown that for embrittled steels, when fatigue cracks develop under plane strain conditions, final fracture occurs at very small crack sizes. In this case, the characteristics of fatigue fracture toughness are appreciably lower than the static values. The paper also considers peculiarities of unstable fatigue crack propagation.     

1240MPa级Ti-38644高锁螺栓的拉伸疲劳增寿机理

用扫描电镜和能谱观测分析Ti-38644高强钛合金高锁螺栓的拉伸疲劳断口,揭示了高锁螺栓的疲劳裂纹萌生和扩展的微观特征和疲劳增寿机理。结果表明,Ti-38644高强钛合金高锁螺栓的疲劳断口包括疲劳裂纹萌生区、扩展区和瞬断区:疲劳裂纹从螺栓头下圆角滚压薄弱部位表面萌生,随后在基体中呈放射性扩展;进入扩展区后裂纹的尺寸由微观扩展至宏观,以疲劳条带扩展机制为主,同时也存在解理断裂。头下圆角处的变形层对Ti-38644高锁螺栓的疲劳寿命有显著的影响,变形层使Ti-38644钛合金高锁螺栓的疲劳寿命明显提高。通过微观组织与疲劳寿命的对比,探讨了Ti-38644钛合金高锁螺栓疲劳强化的作用机理。     

Evaluation of fatigue crack propagation by mode I and mixed mode in 1050 aluminium

The mechanism of mixed‐mode fatigue crack propagation was investigated in pure aluminum. Push‐pull fatigue tests were performed using two types of specimens. One was a round bar specimen having a blind hole, one was a plate specimen having a slit. The slit direction cut in the specimen was perpendicular or inclined 45 degrees relative to the centre of the specimen axis. In both cases, cracks propagated by mode I or by the mixed mode combining mode I and shear mode, depending on the testing conditions. In these cases the crack propagation rate was evaluated with a modified effective stress intensity factor range. Crack propagation retardation was observed in some specimens. However, it was found that the crack propagation rate could also be evaluated by the effective stress intensity factor range independent of the crack propagation mode.     

Crack closure in friction stir weldment using non‐linear model for fatigue crack propagation

The fatigue crack propagation in a friction stir‐welded sample has been simulated herein by means of two 3‐dimensional finite element method (FEM)‐based analyses. Numerical simulations of the fatigue crack propagation have been carried out by assuming a residual stress field as a starting condition. Two initial cracks, observed in the real specimen, have been assessed experimentally by performing fatigue tests on the welded sample. Hence, the same cracks have been placed in the corresponding FE model, and then a remote load with boundary conditions has been applied on the welded specimen. The material behaviour of the welded joint has been modelled by means of the Ramberg‐Osgood equation, while the non‐linear Kujawski‐Ellyin (KE) model has been adopted for the fatigue crack propagation under small‐scale yielding (SSY) conditions. Owing to the compressive nature of the residual stress field that acts on a part of the cracked regions, the crack closure phenomenon has also been considered. Then, the original version of the KE law has been modified to fully include the closure effect in the analysis. Later, the crack closure effect has also been assessed in the simulation of fatigue propagation of three cracks. Finally, an investigation of the fracture process zone (FPZ) extension as well as the cyclic plastic zone (CPZ) and monotonic plastic zone (MPZ) extensions have been assessed.