脱氢压缩机组轴瓦失效分析

在对脱氢用离心压缩机组解体大修时发现其滑动轴承瓦面合金大面积脱落。采用材料性能测试、扫描电镜观察、金相检验和氢含量测定等方法对该轴瓦进行失效分析并进行流体动压润滑理论计算。结果表明：该型轴瓦失效的主要原因为氢气渗入材料。同时,氢鼓泡和氢致开裂的存在降低了轴瓦上巴氏合金与基体的结合强度及合金的疲劳强度,进而引起轴瓦的疲劳失效。另外,轴瓦的局部过热和转子不对中加速了轴瓦失效。     

65Mn钢弹簧垫圈开裂原因分析

通过宏观检验、化学成分分析、硬度测试、金相检验和断口分析等方法对65Mn钢弹簧垫圈的开裂原因进行了分析。结果表明:该垫圈氢含量较高,服役时大部分区域处于悬空状态,造成垫圈变形不均匀,形成应力集中。另外,垫圈凸面的防滑压花以及压花边缘的轮廓线处也存在应力集中,氢原子容易在此聚集,最终导致垫圈发生了氢致延迟脆性开裂。     

某高压锅炉水冷壁管开裂原因分析

某热电厂高压锅炉在水压试验过程中水冷壁管发生开裂,通过宏观分析、壁厚测量、化学成分分析、硬度测试、金相检验、扫描电镜分析和能谱分析等方法,对水冷壁管的开裂原因进行了分析。结果表明:该水冷壁管是垢下腐蚀氢损伤导致的脆性开裂。水冷壁管发生氢腐蚀,且在腐蚀过程中汽水反应生成的部分原子氢扩散渗入金属内部,与珠光体中的碳化物反应生成甲烷,较大的甲烷分子聚集于晶界而使晶界开裂,在内壁形成沿晶微裂纹,裂纹扩展最终导致水冷壁管发生脆性开裂。     

钻井四通开裂原因分析

对某在用钻井四通开裂事故进行了调查,采用力学试验、化学分析、金相、扫描电镜、能谱分析等手段并结合钻井四通的设计等方面寻找原因。结果表明,该四通开裂处存在大量缩孔等铸造缺陷,且该处加工制造时呈直角而造成应力集中。因此,一旦在铸造缺陷处形成裂纹就发生快速脆性断裂,最终导致该钻井四通失效。     

钻井四通开裂原因分析

对某在用钻井四通开裂事故进行了调查,采用力学试验、化学分析、金相、扫描电镜、能谱分析等手段并结合钻井四通的设计等方面寻找原因.结果表明,该四通开裂处存在大量缩孔等铸造缺陷,且该处加工制造时呈直角而造成应力集中.因此,一旦在铸造缺陷处形成裂纹就发生快速脆性断裂,最终导致该钻井四通失效.     

在扩径过程中DH36钢管开裂原因分析

海洋平台用DH36钢管在扩径过程中出现开裂,采用化学成分分析、扫描电镜分析、能谱分析、氧化分析、金相检验等方法对钢管的开裂原因进行了分析。结果表明:该DH36钢管开裂是由钢板原始缺陷引起的。钢坯在加热炉中加热时,缺陷表面再次发生氧化形成氧化圆点,轧制时缺陷被碾压变形,形成裂纹;在钢管扩径时,裂纹扩展最终导致钢管开裂。     

曲轴开裂原因

采用宏观观察、化学成分分析、力学性能测试、微观分析等方法对某开裂曲轴进行分析。结果表明：曲轴在服役时发生润滑不良,使其与铜合金轴瓦之间相互摩擦,铜合金在摩擦高温的影响下发生异种金属渗入现象,曲轴表面形成较多填充有铜合金的微裂纹,这些裂纹成为疲劳源,最终导致曲轴疲劳开裂。     

重型H型钢拉伸性能异常原因

针对重型H型钢在纵向拉伸和Z向拉伸试验中塑性低和断口出现银白色斑点等异常现象，采用宏观观察、化学成分分析、金相检验、断口分析以及去氢退火试验等方法，对造成上述异常现象的原因进行了分析。结果表明：试样断口表面上存在的银白色斑点是氢不断地向试样材料缺陷处(氢陷阱)聚集而形成的“白点”。去氢退火可以有效地使恢复材料的塑性。     

电梯用高强度螺钉断裂分析

某电梯用高强度螺钉在安装2个月后出现批量断裂现象,利用直读光谱仪、氮氢氧联合测定仪、光学显微镜、扫描电子显微镜等对螺钉断裂原因进行了分析。结果表明:该批螺钉由于镀锌后除氢不及时导致其本身氢含量较高和螺钉冷拔后未进行车削导致其在镀锌前表面就已存在大量的微小缺陷,是导致该批螺钉安装后发生断裂的两个主要原因;这些微小缺陷充当了氢陷阱并造成应力集中,在螺钉所受的静拉应力作用下,氢在缺陷处不断富集,最终导致螺钉发生氢致延迟脆性断裂。     

冷变形对国产X70管线钢硫化氢应力腐蚀开裂的影响

采用慢应变速率法SSRT(Slow Strain Rate Test)测试了在含H2S的介质中不同冷变形度条件下管线钢X70的硫化氢应力腐蚀开裂(SCCC)性能.结果表明,冷变形度是影响国产X70管线钢SSCC的重要因素之一.冷变形促进了材料局部微观缺陷内能的增加,这些缺陷所在的位置,往往是氢易被捕捉的地方,造成氢的聚集,微观变形将会促进裂纹的萌生和扩展.同时,随着冷变形度的增加,冷变形造成的位错等缺陷使强度进一步增大而韧性降低.最终氢与应力的交互作用导致X70管线钢抗SSCC性能下降.     

同轴单搭接接头破坏过程研究

在实验所得结果之基础上研究了预偏角对单搭接胶接接头破坏机制的影响,用弹塑性有限元法模型得到了不同裂纹长度时接头整体和胶层中心等效应力沿裂纹尖端的分布,并研究了外载大小、裂纹长度对应力强度因子的影响.结果表明,同轴接头和标准接头的破坏均从胶层界面开始,但同轴接头以形成较多小裂纹为主,而标准接头中裂纹沿着胶层向中部扩展,最终导致破坏;随裂纹长度的增加,接头上等效应力渐增,胶层中心峰值应力也增大,裂纹尖端的应力远高于其他部位;当裂纹扩展到临界尺寸时,接头会迅速破坏.     

Bending strength of adhesive joints in microelectronic components: Comparison of edge-bonding and underfilling

The bending strength of underfilled and edge-bonded ball grid array (BGA) microelectronic packages assembled on printed circuit boards (PCBs) was compared using double cantilever beam (DCB) specimens. All specimens with fillets of the same size and shape failed at the same load, with cracks initiating and propagating within the PCB. This was consistent with measurements of the crack initiation strain energy release rate for PCB interfacial failure, which was significantly smaller than that of cohesive failure within the adhesives. Finite element analysis (FEA) indicated that the stress state in the PCB near the PCB-fillet interface in both underfilled and edge-bonded specimens was only a function of the adhesive fillet size and shape, and independent of the extent of the adhesive layer between the PCB and the BGA, and independent of the adhesive mechanical and thermal properties over the broad range of properties of the tested adhesives. This explained why decreasing the fillet curvature in edge-bonded specimens produced a significant increase in the joint strength. The crack path in the PCB of the edge-bonded specimens was found to change with the adhesive cure temperature; however, this had a negligible effect on the failure load.     

树脂基复合材料内应力测试与分析

将多片双向应变花预埋在平行于环氧树脂基体界面的胶层的不同深度,通过电阻应变仪测定了胶层在室温下的固化应变和在一定的环境温差作用下的纵横向应变随时间的变化情况。结果表明,在固化完成之前,表层处所测得的横向应变值最低,随时间的推移逐步增加而在超过24 h后达最大值;紧邻树脂基体-金属界面处所测得的纵横应变值在固化完成后均为最低,即纵横向内应力均为最大值。将设置于表层的应变片S4测得的应变值视为自由应变值,大致估算了环氧树脂基体固化开始至180 h之间在不同深度的内应力。      

Effect of Applied Pressure on the Joining of Combustion Synthesized Ni3Al Intermetallics with Al Alloy

We focused on the surface reinforcement of ligth weight casting alloys with Ni-AI intermetallic compounds by in-situ combustion reaction to improve the surface properties of non-ferrous casting components.In our previous works,green compact of elemental Ni and Al powders were reacted to form Ni-3Al intermetallic compound by SHS （Self-propagating high temperature synthesis） reaction with the heat of molten Al alloy and simultaneously bonded with Al casting alloy.But some defects such as tiny cracks and porosities were remained in the reacted compact.So we applied pressure to prevent thermal cracks and fill up the pores with liquid Al alloy by squeeze casting process.The compressed Al alloy bonded with the Ni-3Al intermetallic compound was sectioned and observed by optical microscopy and scanning electron microscopy （SEM）.The stoichiometric compositions of the intermetallics formed around the bonded interface and in the reacted compact were identified by energy dispersive spectroscopy （EDS） and electron probe micro analysis （EPMA）. Si rich layer was formed on the Al alloy side near the bonded interface by the sequential solidification of Al alloy.The porosities observed in the reacted Ni-3Al compact were filled up with the liquid AI alloy.The Si particles from the molten Al alloy were detected in the pores of reacted Ni-3Al intermetallic compact.The Al casting alloy and Ni-3Al intermetallic compound were joined very soundly by applying pressure to the liquid Al alloy.     

The effect of aspect ratio on adhesion and stiffness for soft elastic fibres

The effect of aspect ratio on the pull-off stress and stiffness of soft elastic fibres is studied using elasticity and numerical analysis. The adhesive interface between a soft fibre and a smooth rigid surface is modelled using the Dugdale–Barenblatt model. Numerical simulations show that, while pull-off stress increases with decreasing aspect ratio, fibres get stiffer. Also, for sufficiently low aspect ratio fibres, failure occurs via the growth of internal cracks and pull-off stress approaches the intrinsic adhesive strength. Experiments carried out with various aspect ratio polyurethane elastomer fibres are consistent with the numerical simulations.     

Coupled stress and energy criterion for composite failure: Pointwise versus averaged evaluation of the stress criterion

Abstract

In this work, failure loads and failure modes of single lap adhesive joints between composite laminates are investigated. To this aim, a coupled stress and energy criterion is applied and results are compared to numerical reference solutions using cohesive zone modeling and to experimental values from literature. Possible failure modes are adhesive failure along the adherend/adhesive interface, adherend failure as intralaminar failure in the first ply closest to the adhesive layer and interlaminar failure between the first and second ply. Suitable failure criteria adressing the different failure modes are implemented within the framework of the coupled criterion. The stress criterion is carried out in a pointwise or in an averaged manner, called point method or line method respectively. It is shown that two physically sound failure modes can only be predicted using the stress criterion in an averaged manner since the pointwise evaluation does not allow the formation of certain types of cracks.     

Numerical studies on crack problems in three-layered elastic media using an image method

Two-dimensional crack problems in a three-layered material are analyzed numerically under the conditions of plane strain. An image method is adopted to obtain fundamental solutions for dislocation dipoles in trilayered media. The governing equations for equilibrium cracks can be constructed by distributed dislocation technique and their solutions are sought in terms of the displacement discontinuity method (DDM). Comparisons are made with available analytical or reference solutions for several examples at various contrasts of material constants, and good agreements are found. A crack within a brittle adhesive layer joining two semi-infinite blocks can propagate in a variety of ways. In particular, crack paths in the form of sigmoidal waves within the adhesive layer are revisited to reveal the sensitivities of cracking paths to initial crack locations and directions and residual stresses. In addition, Z-shape and H-shape cracks alternating from interface to interface are re-examined to highlight the transition of failure modes and the role of the interlayer thickness.     

Fatigue failure of adhesively patched 2024-T3 and 7075-T6 clad and bare aluminium alloys

The fatigue behaviour of adhesive patches used for repairing aircraft components was investigated. Adhesive patches were simulated using single‐lap shear specimens on clad and bare 7075‐T6 and 2024‐T3 aluminium alloy substrates. Stress–life curves were generated under constant amplitude loading at three stress ratios: R=?1, 0 and 0.5. In the bare materials, failure always occurred in the adhesive itself leaving the substrates intact. At fatigue lives below about 100 000 cycles, the clad alloy specimens also failed in this manner. However, at lower stress levels, the clad alloys failed by cracks initiating in the cladding layer along the end of the lap and subsequently propagating through the substrate. The fatigue strength of the substrate, due to the adhesive patch on the clad materials, was reduced by an order of magnitude compared to the Military Handbook values.     

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...     

Finite Element Analysis of the Void Growth and Interface Failure of Ductile Adhesive Joints

Progressive failure of ductile porous adhesive joint generally includes two competitive failure modes: the void growth of ductile adhesive layer and the interface debonding between the adhesive layer and bonding plates. The damage evolution behavior and ultimate strength of ductile adhesive joint are largely dominated by their evolving interactions. However, most of the existing research failed to predict the damage evolution of these two failure modes simultaneously. After the variational weak form of dynamic equilibrium for two adhesive solids with a finite-thickness adhesive layer and two discontinuous cohesive interfaces is given, this paper studies theoretically the competition between these two failure modes using explicit finite element analysis (FEA). The finite-deformation Gurson–Tvergaard–Needleman (GTN) model is used to predict the void growth of adhesive layer, and the bilinear cohesive model as a ABAQUS module is used to simulate the interface debonding. For single-lap joint under tensile loads, effects of the cohesive strengths, the initial void volume fraction, and the thickness of adhesive layer on their interactions are explored. Besides, the ultimate strengths by FEA are also compared with analytical solutions. Numerical results show that dominating failure mode changes from the interface debonding to the failure of adhesive layer at about the cohesive strength 40 MPa and the thickness of adhesive layer 0.5 mm for FM-73 ductile adhesive joint.