涂层厚度对再制造零件失效形式的影响

目的研究不同厚度涂层的再制造零件在弯曲疲劳情况下的失效形式。方法利用电弧喷涂对零件进行再制造处理,并进行四点弯曲疲劳实验,利用扫描电镜对不同厚度涂层的再制造零件断口形貌进行观察,研究不同厚度涂层的疲劳裂纹萌生、扩展以及断裂方式。结果疲劳裂纹萌生形式主要为多源疲劳失效,在疲劳裂纹的扩展过程中,不同平面内扩展的裂纹相交,形成了台阶形貌。当涂层厚度为100、200μm时,裂纹主要萌生于四点弯曲实验中应力最集中的部位,萌生于涂层和基体之间,并逐渐地向涂层和基体扩展,直至试样断裂。而涂层厚度为300、400μm时,裂纹萌生部位主要集中于四点弯曲疲劳试验中应力最大部位的两侧,并呈对称式分布,裂纹在界面处连接,使得涂层和基体产生分层现象,之后分层部位处裂纹沿着基体方向扩展,直至试样断裂。结论涂层厚度不同,整个系统的失效模式也不同。对于较薄的涂层,裂纹模式主要为垂直于涂层-基体界面的裂纹,此时拉伸失效占主导。对于较厚的涂层,界面裂纹为主要的裂纹模式,此时剪切失效占主导。     

AZ31镁合金裂纹扩展各向异性声发射特征分析

利用声发射技术对AZ31镁合金轧制方向和横向的疲劳裂纹扩展行为进行了研究。结果表明,镁合金疲劳裂纹扩展过程中产生的声信号撞击数与循环载荷的关系,以及撞击数上升率和应力强度因子幅的关系d C/d N-ΔK,分别与常规疲劳裂纹扩展试验结果相一致,裂纹失稳扩展临界应力强度因子幅与常规试验结果的误差分别为2.86%(裂纹沿轧制方向)和3.00%(裂纹沿横向);载荷一定时,裂纹沿横向扩展总是比沿着轧制方向扩展更慢一些,进入失稳扩展阶段更迟。微观组织显示,裂纹沿横向扩展边缘处的孪晶明显增加,断口处也表现出更明显的塑性,这对裂纹的扩展产生了阻碍,即材料在横向的抗裂性能要优于轧制方向,同时证明了利用声发射监测裂纹扩展行为的可靠性。     

异种钢接头热影响区裂纹扩展声发射特征信号分析

文中以P92钢与Ni基焊材焊接热影响区为研究对象,通过采集紧凑拉伸试验过程中异种钢接头热影响区启裂及裂纹扩展的声发射特性信号,分析裂纹扩展的声发射信号幅值、频率分布、能量及振铃计数等特征参数,同时分析声发射特征信号与断口形貌之间的对应关系。结果表明：裂纹扩展声发射信号均为突发型信号,频率主要集中在50～200 kHz;裂纹稳定扩展的声发射信号具有平均幅值较低、总能量和振铃计数率参数变化平缓的特点;裂纹失稳扩展的声发射信号具有平均幅值较高、总能量和振铃计数率参数呈瞬时增加的特点;裂纹稳定扩展阶段的声发射特征信号与韧性断口特征相对应;裂纹失稳扩展阶段的声发射特征信号与准解理断裂或解理断裂特征相对应。     

声发射技术在热障涂层失效机理中的研究进展及展望

热障涂层（TBCs）具有优异的高温抗氧化、高温力学和抗热腐蚀性能而备受关注,广泛应用于航空发动机和燃气轮机热端部件中。热障涂层服役环境的恶劣和涂层体系结构的复杂,极易导致涂层发生界面分层或剥落失效,因此通过对热障涂层的裂纹萌生和扩展问题进行实时监测,对于失效机理研究显得尤为重要。简述光激发荧光压电光谱（PLPS）、红外热成像（IRT）、阻抗谱（IS）的原理及其在热障涂层失效行为研究中的应用,重点介绍声发射技术在热障涂层失效机理方面的研究成果。基于声发射的热障涂层失效过程的信号分析和深度处理,结合声发射技术在热障涂层中的参数分析和波形分析,对热障涂层失效过程及失效形态进行模式识别,通过损伤程度的定量评估来进行热障涂层的寿命预测。对声发射技术在热障涂层失效预测及寿命评估指明了方向,并创新性地对未来声发射技术在热障涂层的疲劳损伤方面研究趋势提出展望。     

高强铝合金沿晶断裂声发射研究

高强度铝合金沿晶界断裂过程中的声发射信号特征反映了材料沿轧制方向的宏观性能。在带有预制裂纹的双悬臂梁(DCB)试样中，裂纹失稳扩展前，先有一短距离的韧窝状穿晶撕裂的稳态裂纹扩展。稳态裂纹扩展起始于K1C并发射出许多幅值超过阈值35dB的信号。当稳态裂纹扩展至Kmax时，便出现失稳的沿晶开裂。对稳态及失稳的裂纹扩展两种情况，其累计声发射能量均与宏观断裂能呈线性关系。     

轮轴钢35CrMoA单轴微动疲劳失效机理

通过平面对平面的接触方式,研究轮轴钢35CrMoA在单轴微动疲劳下的失效机理,采用扫描电子显微镜观察其微动损伤表面的微观形貌,通过透射电镜观察微动疲劳过程中不同应力幅值时接触区位错的组态。结果表明,随着应力幅值的增加,微动疲劳寿命降低的幅度变小;微动磨损可能加速微裂纹的萌生,也可能推迟微裂纹扩展;随着轴向应力幅值的增大,形成的位错胞尺寸也较大,并且其多边形化的趋势也越明显;微动疲劳的接触压应力和循环载荷为位错的滑移切变提供了条件。     

基于声发射的Q345B钢失稳断裂模式识别

研究了一种基于声发射技术的Q345B钢材进入失稳断裂模式的识别方法。其以声发射计数率作为研究的数据对象,利用数理统计学分析计数率数据变化的规律,提出了用声发射计数率的离散系数对疲劳裂纹扩展过程的第二阶段与第三阶段进行识别,建立了Q345B钢材声发射计数率的离散系数与应力强度因子幅值的关系,可为利用声发射技术作为Q345B钢材失稳断裂预警识别提供依据。     

中碳贝氏体支承辊钢低应力牵引滚动接触下的疲劳短裂纹行为

研究了中碳贝氏体支承辊钢在低应力、水润滑和牵引滚动条件下的接触疲劳裂纹萌生与扩展特征,发现了表面起源的垂直短裂纹和棘齿短裂纹．疲劳10^4cyc时,垂直短裂纹就在接触表面大量出现,且在萌生后立即进行高速初始扩展,其后绝大多数停止长大;棘齿短裂纹出现较晚．两种短裂纹长大到一定深度时均停止扩展．在疲劳失效寿命的70％-80％时,垂直短裂纹恢复扩展,并随即加速长大．几乎同时,两种短裂纹在亚表层以转折的方式重新扩展．在表面损伤出现之前,两种短裂纹的萌生和扩展行为始终局限在近表面薄层内．     

模态声发射——声发射信号处理的得力工具

模态声发射是声发射信号处理技术的得力工具。介绍它在声发射源定位、复合材料损伤源识别、疲劳裂纹萌生和扩展的声发射监测以及日历损伤声发射评价等工程实践中的应用。     

基于声发射技术的接触疲劳失效检测应用与研究

接触疲劳是齿轮、轴承等长期承受交变载荷的重要旋转零部件的主要失效形式。采用声发射技术对接触疲劳失效过程进行监测,对损伤程度进行检测以及揭示接触疲劳失效机理具有重要的意义。随着声发射技术不断发展及先进声发射信号处理技术的出现,其在接触疲劳失效检测中的研究也越来越深入。文章回顾了声发射技术的发展现状,综述了声发射技术应用于块体零件和涂层零件的接触疲劳失效检测的研究进展与存在的问题,探讨了进一步研究的方向和解决问题的思路。     

铝合金疲劳裂纹扩展声发射监测

采用声发射(acoustic emission,AE)技术对7N01铝合金单边缺口三点弯曲试样不同应力比、不同峰值载荷下疲劳裂纹扩展过程中声发射信号进行了监测,建立了裂纹扩展速率、声发射计数(count)与应力强度因子之间的关系.结果表明,大部分的声发射信号主要产生于疲劳循环载荷的低应力阶段,这主要是低应力阶段的声发射活动主要与裂纹尖端的塑性变形和裂纹闭合现象有关,声发射计数与应力强度因子之间呈指数增长的关系.基于所建立的声发射计数率与裂纹扩展速率的关系,可以预测疲劳损伤结构的剩余寿命.     

Detection of fatigue crack propagation with acoustic emission

Since its inception, acoustic emission has held great promise as a tool for detection and evaluation of damage in structures. Of particular concern is the occureence of fatigue manifested by the processes of crack initiation followed by crack growth to failure. Early research related acoustic emission parameters to physical variables, including load, stress, pressure etc. causing damage. In this paper, relationships relating acoustic emission to fatigue crack growth are developed. Acoustic emission equations, similar to Paris' law, are derived which allow determination of the stress intensity factor and the crack growth rate. These principles are embodied in a second-generation system which automatically and continuously determines these parameters. The approach is applied in the laboratory as well as to structures including bridges, air frames and transmission towers for electrical power.     

Effects of temperature, slip amplitude, contact pressure on fretting fatigue behavior of Ti811 alloys at elevated temperatures

Effects of the temperature,slip amplitude,and contact pressure on fretting fatigue(FF) behavior of the Ti811 titanium alloy were investigated using a high frequency fatigue machine and a home-made high temperature apparatus.The fretting fatigue failure mechanism was studied by observing the fretting surface morphology features.The results show that the sensitivity to fretting fatigue is high at both 350 and 500 °C.The higher the temperature,the more sensitive to the fretting fatigue failure is.Creep is an i...     

钢结构疲劳损伤声发射监测

采用声发射技术对已开裂的焊缝进行监测,实现了在复杂的环境噪声中识别出裂纹活动信号。通过分析疲劳裂纹损伤产生的原因以及材料在交变载荷作用下产生声发射的机理,得到表征焊缝裂纹活动过程的信号特征。试验表明,声发射技术对钢结构裂纹损伤的活度、强度具有较大的敏感性,可以实现对裂纹损伤状态发展变化的实时监测。     

In situ characterization of fatigue damage evolution in a cast Al alloy via nonlinear ultrasonic measurements

A new methodology is described for in situ characterization of fatigue damage accumulation using nonlinear ultrasonic measurements via analysis of the feedback signal of a closed-loop ultrasonic fatigue system. In the very high-cycle fatigue regime, ultrasonic nonlinearity increases with initiation and growth of a dominant, life-limiting fatigue crack. Based on the increase in the ultrasonic nonlinearity with fatigue cycling, crack initiation, small fatigue crack growth and fast crack growth regimes have been distinguished during cycling in specimens with different pore sizes tested at various stress amplitudes. The fraction of fatigue life spent in initiation of a life-limiting fatigue crack decreases with increasing stress amplitude. For a constant stress amplitude, the initiation life also decreases with increasing pore size. The present study also demonstrates the applicability of the methodology for fatigue crack growth studies from natural defects located internally or at the surface in smooth specimens.     

疲劳载荷下P／M Rene95合金中夹杂物的微观行为研究

采用人工植入Al2O3夹杂的试样，通过SEM下的原位试验，动态跟踪及分析了拉－拉疲劳载荷下P／M Rene95合金中表面或近表面夹杂物的微观行为。结果表明：在所研究的夹杂物的尺度范围内，载荷波形对夹杂物的微观力学行为有很大影响。在三角波加载条件下，虽然夹杂物／基体界面更易萌生裂纹，但表面或近表面夹杂物对试样的断裂影响较小；在正弦波载荷条件下，疲劳裂纹的萌生、扩展及试样的断裂均与表面或近表面夹杂物的基本特性有关。而且，降低正弦波加载时的应力幅值可大大提高夹杂物／基体界面疲劳裂纹的萌生寿命。     

Application of a novel type Barkhausen noise sensor to continuous fatigue monitoring

A novel Barkhausen noise (BN) sensor with no need for external magnetization was tested and applied to continuous fatigue monitoring of mild steel and high strength steel specimens. This new type of sensor indicated an increase in the BN rms value under maximum tensile stress during one cycle in cyclic bending tests with increasing stress amplitude. The BN rms value under maximum compressive stress stayed, however, approximately constant. The reason for this behaviour was the stress-induced anisotropy of the BN. Bending fatigue experiments with constant stress amplitude and R=−1 were conducted at different stress levels. In addition to the BN also the acoustic emission of the specimen was measured. In the mild steel specimens the BN amplitude stayed constant after the initial saturation period, but just prior to the failure of the specimen the amplitude increased meaningfully. This increase occurred at the same time as the increase in the acoustic emission signal indicating the beginning of crack initiation and growth. In the high strength steel specimens the BN amplitude decreased after the initial saturation period. The increase of the BN signal started well before the failure of the specimen and even before the increase in the acoustic emission signal.     

十字接头恒幅和变载荷历程疲劳试验结果及预测

对十字接头分别进行恒辐轴向载荷和变载荷历程疲劳试验,接头疲劳裂纹通常开始发生在焊缝根部,但如果接头变形引起的弯曲应力较大,疲劳裂纹也可能发生在焊缝趾部。本研究试图找出影响焊接接头疲劳寿命各参数之间的相互关系,并利用作者引伸的焊接接头疲劳裂纹起始－扩展模型较精确地预测复杂焊件的疲劳寿命。该模型把疲劳裂纹起始寿命看成是疲劳裂纹萌生,早期生成并聚合成主疲劳裂纹的循环次数,利用应变控制疲劳数据和Ｐａｌｍｇ     

Influence of weld geometry and process on fatigue crack growth characteristics of AISI 304L cruciform joints containing lack of penetration defects

Abstract

The influence of weld geometry in conjunction with the gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) processes on fatigue properties of AISI 304L load carrying transverse fillet welded cruciform joints, containing lack of penetration (LOP) defects, has been studied. The fatigue lives of the joints were evaluated using conventional S -N (stress - number of cycles to failure) and fracture mechanics methods. The fatigue lives were calculated according to the two stage approach in which both the fatigue crack initiation and the crack propagation phases were considered. Constant amplitude fatigue experiments with stress ratio R=0 were carried out using a 100 kN servohydraulic Dartec universal testing machine at a frequency of 30 Hz. An automatic crack monitoring system based on crack propagation gauges was used to obtain the crack initiation and propagation data during the fatigue process. The predicted life was compared with the experimental values. It was found that the fatigue lives of the joints fabricated via GTAW were longer than those of the corresponding GMAW joints. It was also observed that the fillet geometry plays a major role in determining the failure mode and life. Test results have been compared with the BS 5400 design curve.     

应力幅对LY12CZ铝合金腐蚀疲劳应变电流响应的影响

研究了ＬＹ１２ＣＺ铝合金在应力控制下于３．５％ＮａＣｌ溶液中恒定电位下,应力幅对腐蚀疲劳（ＣＦ）应变电流行为的影响以及应变电流与腐蚀疲劳损伤相对应的关系,结果表明：应变电流能反映ＣＦ不同阶段的损伤行为,应变电流的波型随ＣＦ过程所发生的变化主要在应变电流的波幅、波峰数及其与应力波的相位差上,不同应力幅下的应变电流行为的差异主要表现在：高应力幅下,在一个循环周次内,瞬变电流在整个腐蚀疲劳过程中均出现两