A537钢在盐水中疲劳裂纹尖端塑性区尺寸减小与声发射

研究了Ａ５３７低强度结构钢在空气和３．５％ＮａＣｌ中性水溶液中疲劳裂纹扩展及其相应的声发射规律。结果表明腐蚀疲劳裂纹扩展过程中声发射活动下降。由于裂纹尖端塑性区内的位错运动是疲劳与腐蚀疲劳裂纹扩展的主要声发射源，所以认为声发射活动性下降是氢使尖塑性尺寸减小所致。用激光散斑干涉法裂纹尖端应变分布，证明氢的存在使裂尖塑性区尺寸减小，氢的存在同提高裂尖局部材料的流变应力。     

发动机三级涡轮叶片断裂分析

某发动机三级涡轮转子叶片在飞机赶快冰爬升时断裂，通过断口ＳＥＭ观察，ＥＤＡＸ成分分析，叶片基材金相检验和硬度测试等分析了叶片的断裂原因和机制，结果表明，共振是叶片失稳断裂的直接原因，热疲劳萌生裂纹，高温腐蚀疲劳控制裂纹稳态扩展，材料冶金缺陷对叶片断裂过程有促进作用。     

残余应力与胶粘剂对ARALL层板疲劳裂纹扩展特性的影响

研究了粘剂性质、含量及残余应力状态对纤维－铝合金胶接层板（ＡＲＡＬＬ）疲劳裂纹扩展特性的影响，分析了裂纹扩展过程中的分层状态的变化，结果表明，ＡＲＡＬＬ层板内富胶层的剪切形变和伴随裂纹扩展的分层区越大，即这两方面耗散能量越多，则疲劳裂纺扩展速率越低；胶粘剂含量的影响不明显，给层板施加预应力极大降低了层板的疲劳裂纹扩展速率，其本质在于裂尖在同样的疲劳载荷下实际所受到的有效应力降低。     

材料疲劳恢复新途径的探索II—脉冲电流对Ti—6Al—4V合金裂纹扩展的阻滞

研究了大电流脉冲对 Ｔｉ—６ Ａｌ—４ Ｖ 合金疲劳裂纹扩展行为的影响, 结果表明, 在疲劳裂纹扩展后, 试样经大电流脉冲处理, 对疲劳裂纹的扩展有阻滞作用, 并可以延长其疲劳扩展寿命     

屈服强度对40CrMnSiMoVA超高强度钢腐蚀疲劳裂纹扩展的影响

研究了屈服强度对４０ＣｒＭｎＳｉＭｏＶＡ（ＧＣ－４）超高强度钢腐蚀疲劳（ＣＦ）裂纹扩展行为的影响，结果表明，不同屈服强度时，ＧＣ—４钢在３．５％ＮａＣｌ溶液中的ＣＦ裂纹扩展曲线上，都出现了类似于应力腐蚀的平台区，而且随屈服强度提高，平台区裂纹扩展速度显著增大。研究表明，氢脆在ＧＣ—４钢的腐蚀疲劳中起重要作用，进一步的理论分析与计算表明，屈服强度会影响高强钢氢致开裂的过程和速度．     

挤压SiCw／LD2复合材料的疲劳裂纹扩展

本文测量了挤压19％(Vol)SiCw/LD2复合材料疲劳裂纹扩展的门槛值ΔK_(th)和扩展速率da/dN。并在扫描电镜下观察了疲劳断口形貌。结果表明,复合材料在门槛值附近和中速扩展区的疲劳裂纹扩展抗力高于基体材料LD2合金。复合材料纵向试样于170℃时效后的疲劳裂纹扩展抗力高于150℃时效,疲劳裂纹扩展扩展过程包括不在一个面内的微裂纹长大和联接这些微裂纹的(Ⅰ+Ⅱ)复合型裂纹的扩展。与裂纹相垂直的晶须有效地阻碍了微裂纹的长大。     

微量有害元素砷,锑对合金结构钢疲劳性能影响的研究

研究了不同Ａｓ，Ｓｂ含量的３０ＣｒＭｎＳｉＡ钢的疲劳裂纹扩展速率，探讨了疲劳裂纹扩展速度随Ａｓ、Ｓｂ含量的变化规律，结果表明：随Ａｓ，Ｓｂ含量的增加，疲劳裂纹扩展速率略为增大，其临界转变温度升高，显微组织稍有粗化。     

失效分析技术及其应用——第七讲 疲劳断裂失效分析

1.l 疲劳断裂机械构件或材料由于循环负载或交变应力作用所引起的断裂现象称为疲劳断裂,其断裂表面称为疲劳断口.疲劳断裂是受循环应力、拉应力及塑性应变这三者的共同作用而发生的.若这三者中的一种不存在,则疲劳裂纹的萌生及其扩展便不会产生.疲劳断裂形成的疲劳断口具有特殊的形貌.它们可分成三个断裂区或三个断裂发展阶段:(1)第Ⅰ阶段 疲劳断裂的成核阶段,即疲劳裂纹的萌生.(2)第Ⅱ阶段 疲劳裂纹的扩展阶段,即疲劳裂纹的亚稳扩展.(3)第Ⅲ阶段 疲劳的最终断裂或瞬时断裂阶段,即疲劳裂纹的失稳扩展.     

材料疲劳恢复新途径的探索Ⅱ：脉冲电流对Ti—6Al—4V合金裂纹扩 …

研究了大电流脉冲对Ｔｉ－６Ａｌ－４Ｖ合金疲劳裂纹扩展行为的影响。结果表明，在疲劳裂纹扩展后，试样经大电流脉冲处理，对疲劳裂纹的扩展有阻滞作用。并可以延长其疲劳扩展寿命。     

疲劳载荷下工业纯铁表面与内部裂纹长度的研究初探

疲劳裂纹扩展试验中,采用表面裂纹长度作为疲劳裂纹长度进行疲劳裂纹扩展计算和疲劳裂纹扩展模型建立的过程中,会导致计算的最终结果和模型产生一定的误差。对工业纯铁板材试样进行疲劳裂纹扩展试验,分析疲劳裂纹扩展过程中裂纹长度与裂纹扩展速率的关系,以及对疲劳裂纹扩展断口形貌特征进行观察。结果表明,上述2种方法均可来确定表面裂纹长度和内部裂纹长度之间的关系。     

Acoustic Emission Monitoring the Fatigue Crack Growth in SM50B-Zc Steel

The acoustic emission(AE)characteristicsfrom the fatigue crack propagation in SM50B-Zcsteel were strongly affected by the environment me-dium.The AE feature from the fatigue process bothin air and in 3.5% NaCl aqueous solution mani-fested a periodicity which indicated the fatigue ex-tension was discontinuous.The fatigue striationand secondary cracking were the main AE sourcesduring the fatigue crack growth in air.In contrast,AE during the fatigue crack propagation in 3.5%NaCl solution was released from the intergranularfracture and quasicleavage cracking.     

Fatigue cracks nucleation on steel, acoustic emission and fractal analysis

In this paper, a new acoustic emission (AE) diagnostic technique, for the study of fatigue cracks nucleation and propagation on steel, was investigated. Using the fractal analysis, and the box-counting method (BCM) in particular, it is possible to characterize the spatial distribution of the prime AE sources through the fractal dimension (D) that evolve with the number of fatigue cycles (N) of the specimen. D–N curves were found useful to identify the condition of incipient collapse due to the nucleation and propagation of fatigue cracks on steel. It is possible to use the fractal dimension as a damage parameter. In all tested specimens, the crisis occurs within the same range of values of fractal dimension. The results suggest that it is possible to anticipate the detection of crack beginning relating to the other theoretical or experimental techniques.     

金属陶瓷热冲击疲劳裂纹形成机制

研究了金属陶瓷热冲击疲劳特性, 重点探讨了裂纹的形成机制。试验结果表明, 随着循环温度的增高, 裂纹形核孕育期缩短, 裂纹扩展速率增大; 随着金属陶瓷中金属相含量的增加, 裂纹扩展速率降低。热冲击疲劳裂纹的形成与微孔洞的形核、长大和连通有关。研究还发现, 金属陶瓷热冲击疲劳断口中存在疲劳条纹。     

The interaction between pitting corrosion,grain boundaries,and constituent particles during corrosion fatigue of 7075-T6 aluminum alloy

Concomitant corrosion fatigue research was performed on 7075-T6 aluminum alloy to gain an increased understanding of how microstructure influences pit growth, pit-to-crack transition, and critical crack propagation to fracture. Two thicknesses of rolled sheet and an extrusion of 7075-T6 aluminum alloy were etched and subjected to concomitant corrosion fatigue in a 3.5% sodium chloride solution. Testing was interrupted at various intervals to obtain information on pit generation, growth, and potential cracking. Results indicated that microstructure has a significant influence on pit-to-crack transition and fatigue crack propagation. Constituent particles competed with corrosion pits as critical crack nucleation sites, with some affecting the critical crack by either nucleation of additional cracking or linkage with the main crack. Post-fracture analysis confirmed the presence of noncritical cracks within the corroded region, related to pitting and constituent particles.     

金属疲劳断裂的声发射检测技术

疲劳断裂是金属结构的主要失效形式,通过金属疲劳断裂时声发射特征参数的提取,建立了声发射特征参数和裂纹扩展速率之间的关系,由试样的三点弯曲疲劳试验,证明采用声发射技术监测疲劳裂纹的扩展,不仅与疲劳裂纹扩展的变化规律相似,而且能实时的捕捉到疲劳裂纹的产生。     

Analysis of fatigue crack behaviour based on dynamic response simulations and experiments for tensile-shear spot-welded joints

Fatigue crack initiation and propagation behaviours were studied based on the dynamic response simulation by the three‐dimensional finite‐element analysis (FEA) and dynamic response experiments for tensile‐shear spot‐welded joints. The entire fatigue propagation behaviour from the surface elliptical cracks at the initiation stage to the through thickness cracks at the final stage was taken into consideration during the three‐dimensional FEA dynamic response simulations. The results of the simulations and experiments found that the fatigue cracks of spot‐welded joint from initial detectable crack sizes to crack propagation behaviour could be described by three stages. Approximately one‐half of the total fatigue life was taken in stage I, which includes micro‐crack nucleation and the small crack growth process; 20% of the total fatigue life in stage II, in which the existing surface crack propagates through the thickness of sheet and 30% of the total fatigue life in stage III, during which the through thickness crack propagates along the direction of plate width to the final failure. According to the relationship between the crack length and depth and the dynamic response frequency during the simulated fatigue damage process, the definition of fatigue crack initiation and propagation stages was proposed. The analysis will provide some information for the fatigue life prediction of the spot‐welded structures.     

Principal Component Analysis of Acoustic Emission Signals From Landing Gear Components: An Aid to Fatigue Fracture Detection

Abstract: This work forms part of a larger investigation into fatigue crack detection using acoustic emission (AE) during landing gear airworthiness testing. It focuses on the use of principal component analysis (PCA) to differentiate between fatigue crack propagation (FCP) signals and high levels of background noise. An artificial AE fracture source was developed and additionally five sources were used to generate differing artificial AE signals. Signals were recorded from all six artificial sources in a real landing gear component subject to no load. Furthermore, artificial FCP signals were recorded in the same component under airworthiness test load conditions. PCA was used to automatically differentiate between AE signals from different source types. Furthermore, successful separation of artificial FCP signals from a very high level of background noise was achieved. The presence of a load was observed to affect the ultrasonic propagation of AE signals.     

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was >10⁷ cycles. A significant change in the slope of the S–N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10⁷ and 10⁹ cycles are >99%.