起重机钢梁疲劳特性声发射监测实验研究

起重机钢梁作为主要承力部位关系到起重机的安全运行,目前仍无有效手段对其疲劳特性进行在线实时监测。采用声发射技术对钢梁材料Q345疲劳特性进行实验研究,首先通过动态弯曲疲劳实验获得材料疲劳裂纹萌生、扩展和断裂全过程的声发射检测信号;然后提取和分析声发射信号中的特征参数,经过比较各参数历程图,发现在幅值和事件历程图中可以反映材料疲劳裂纹整体演变过程,而能量、计数、上升时间和持续时间这4个特征参数更能反映出裂纹变化的重要转折点。此外对各个阶段声发射信号产生的原因进行分析和总结,为下一步采用声发射技术对钢梁材料损伤定量及寿命预测的研究提供参考依据。     

42CrMo钢疲劳裂纹扩展剩余寿命评估

以高频三点弯曲疲劳试验机为平台,进行42CrMo钢疲劳裂纹扩展试验研究,通过建立裂纹扩展剩余寿命评估模型,实现对存在裂纹的工程机械零部件剩余寿命的评估。采用显微成像测试系统实时采集并测量疲劳扩展裂纹,使用声发射系统监测整个疲劳裂纹扩展过程。结果表明:声发射幅值、能量等特征参数可以实时反应疲劳裂纹萌生、稳定扩展和失稳扩展等各个损伤阶段,并在疲劳断裂时产生急剧的突变;裂纹扩展速率的对数值与应力强度因子幅的对数值具有较高的线性相关性,建立了不同应力工况条件下裂纹扩展剩余寿命评估模型,以双排链轮轴为例进行裂纹扩展剩余寿命评估;随着疲劳应力的增加,裂纹扩展剩余寿命减小。     

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。     

模态声发射技术在构件疲劳裂纹检测中的应用

疲劳裂纹的萌生和扩展是机械零件在变载荷作用下的主要失效形式,在变载荷下出现疲劳裂纹的同时往往伴随着弹性波的扩散,以迅速释放其内部积累的应变能。使用近几年来得到迅速发展的模态声发射技术真实地获取疲劳裂纹的声发射波形,使接收到的声发射信号较完整地反映了声发射源的物理状态。在波形分析中采用参数分析法提取声发射波形特征,建立模态声发射参数和裂纹扩展速度之间的数学关系。由于综合采用了多种技术的优点进行信号分析、处理,因此根据声发射信号特征得到的结果将更加逼近实际状态。     

涂层接触疲劳损伤过程中的声发射小波分析

采集铁基合金涂层在不同接触疲劳损伤阶段的声发射信号,并采用dB10基本小波对其进行5层小波分解和重构,分析了疲劳损伤声发射信号的波形和频率特征。结果表明:裂纹萌生阶段的原始声发射信号以连续型为主,裂纹的稳定扩展阶段以混合型为主,裂纹的失稳扩展阶段以突发型为主;通过小波变换实现了将疲劳损伤声发射信号与干扰波分离,获得了高信噪比的疲劳损伤特征信息;在不同的疲劳损伤阶段,声发射信号的频率分布各不相同。随着疲劳损伤的加剧,各层的波形幅值呈增大的趋势,并且疲劳损伤频率分布范围也更加的广泛。     

制绳钢丝疲劳损伤过程声发射研究

研究了制绳钢丝在高周疲劳过程中的声发射（ＡＥ）行为．结果表明, ＡＥ过程明显分为三个阶段,分别对应钢丝塑性变形、疲劳裂纹成核和裂纹扩展裂纹孕育及成核期约占整个疲劳寿命的７０％;裂纹扩展占３０％．     

含裂纹钢试样在拉伸过程中的声发射特性

对于带有I型裂纹的SM490A钢和SUS304钢试样,通过测量其在拉伸断裂过程中的声发射特征参数及裂纹尖端特定点的温度,分析了裂纹尖端和塑性变形区域的力学特征.结果表明:声发射能量计数、振铃累积计数在裂纹扩展的各个阶段都有着明显的曲线特征,且曲线特征和屈服、强化阶段的应力-应变曲线基本吻合,因此可以用声发射能量计数、振...     

基于小波包能量谱的声发射信号处理技术

为实现材料早期结构疲劳损伤程度的监测与评价,提出采用小波包能量谱的声发射信号处理方法.首先,搭建金属疲劳损伤在线声发射监测系统,采集金属材料早期结构疲劳损伤产生的裂纹扩展与闭合的声发射信号;其次,选取适用于该声发射信号的小波函数和小波包分解层数,从而得到小波包分解后的各个频带的能量随疲劳周期数增加的变化情况的小波包能量谱图.实验表明:上述信号处理方法可获得能表征金属材料随着疲劳周期数增加的早期结构疲劳损伤程度变化的频带特征,为桥梁结构、建筑工程结构构件中常用钢材疲劳过程的声发射监测及早期疲劳损伤程度的加剧提供参考依据.     

声发射技术监测疲劳损伤探讨

在实验室的高频疲劳试验机上,对紧凑拉伸试样进行疲劳损伤试验,用声发射技术记录声发射参数的变化.使用小波分析识别降噪方法和外触发采集卡(只在裂纹扩展时才采集数据,裂纹闭合时不采集数据)等措施,将各种噪音分离出去,最后只有试样的疲劳损伤裂纹起裂和扩展的声发射波形信号,经处理后获得的疲劳裂纹扩展长度与应力循环次数关系,接近于人工测得的疲劳裂纹扩展长度与应力循环次数关系曲线.     

涂层疲劳损伤声发射特征参数及损伤机理

使用超音速等离子喷涂设备在1045钢基体上制备了铁基合金涂层。以球盘式接触疲劳试验机为平台,研究了涂层接触疲劳损伤过程中声发射特征参数的变化规律,并分析了涂层的接触疲劳损伤机理。结果表明,在转速为2500r/min和应力水平为1.58GPa实验条件下,点蚀是涂层的主要失效形式,表现为在涂层磨痕轨迹范围内出现大量的点蚀坑,点蚀坑深度为20～30μm。涂层表面粗糙的微凸体与轴承球滚压接触产生黏着磨损,以及涂层、磨粒、滚动轴承三者形成三体磨料磨损是点蚀失效产生的主要原因。声发射幅值、有效值(Root Mean Square,RMS)、能量、计数和平均频率对涂层表面粗糙微凸体去除、弹塑性变形、裂纹萌生、裂纹稳定扩展和失稳扩展过程比较敏感,并且在不同的疲劳损伤阶段具有不同的信号反馈特点。     

Modelling fatigue crack propagation of a cracked metallic member reinforced by composite patches

The concept of fracture for material elements at front of a crack for fatigue crack propagation was extended to the fatigue crack propagation of a cracked metallic member reinforced with a composite patch in this paper. From static mechanics and linear elastic fracture mechanics, force transfer on a cracked member through a composite patch was analyzed and a formula connecting the stress intensity factor with crack length was obtained. Thereafter, a fracture model for fatigue crack propagation of a repaired cracked metallic member was proposed. A new expression for the fatigue crack propagation rate has thus been derived. The expression was verified objectively by the test data. It is in good agreement with the test results.     

Fatigue of metals – What the designer needs?

Engineering designers have to tackle various fatigue problems in their routine work. Some problems are simple and other are complex. Most of the designers have been taught only a small part of the suitable fatigue knowledge needed to successfully deal with many of these problems except for the most trivial ones. The main reason is the vast amount and complexity of the fatigue discipline, and lack of a clear integrated approach to the main fatigue problems that may be conveniently utilized by designers. An integrated approach to fatigue, that has been introduced by one of the authors in the past, is here extended, simplified and proposed as a comprehensive fatigue design tool for engineers. The whole fatigue domain is divided into six zones that include different fatigue regimes. The propagating crack length is considered as the sole parameter to evaluate safe fatigue life, including the use of an “equivalent crack propagation rate”, which averages the intense variations of CPR in the vicinity of grain boundaries. Contrary to the many unified relations to evaluate fatigue crack propagation that were proposed in the past, the current study is based on separation. For each fatigue zone a unique prediction relation is presented. Flow chart of comprehensive software for calculation of crack propagation in the whole fatigue domain is explained, and simulation results show good fit to published test results. The method is claimed to fit for use mainly by design engineers, but possibly by fatigue experts as well.     

电解充氢对堆焊熔合区疲劳裂纹扩展行为的影响

研究了阴极充氢前后堆焊熔合区疲劳裂纹扩展行为,发现堆焊熔合区对疲劳裂纹的扩展有阻碍作用,充氢对裂纹扩展速率无明显影响,但使熔合区出现大量二次裂纹,随着充氢时间的延长,二次裂纹将更加严重。     

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MECHANICAL BEHAVIOUR OF A DUPLEX STAINLESS STEEL: Part II. FATIGUE LIFE AND FATIGUE CRACK GROWTH

Abstract— The mechanical behaviour of AISI 329 steel has been investigated for ageing times up to 20,000 h at temperatures of 475, 425, 375, 325 and 275°C. The study has concentrated on the changes in the response to cyclic strains, in the low-and the high-cycle fatigue regimes, and in the resistance to fatigue crack propagation as a function of temperature and time of ageing.
It is shown that ageing increases the fatigue resistance in the high-cycle fatigue regime, but the opposite occurs in the low-cycle fatigue regime. Ageing increases the LEFM threshold stress intensity factor range for fatigue crack propagation which reaches high values in these alloys, and is influenced by the fatigue load ratio. Crack closure contributes to the LEFM threshold stress intensity factor range for crack propagation only in the annealed condition of the AISI 329 steel.     

Weld root crack propagation under mixed mode I and III cyclic loading

This study examined fatigue propagation behaviour and fatigue life of weld root cracks under mixed mode I and III loading. Fatigue tests were performed on butt-welded joints with a continuous lack-of-penetration (LOP) inclined at angles of 0°, 15°, 30° or 45° to the normal direction of the uniaxial cyclic load. Branch and/or co-planar crack propagation was observed, depending on the initial mode I stress intensity factor (SIF) range. Co-planar crack propagation predominated when the SIF range was large. The fatigue crack propagation mode affected fatigue life; the life of branch crack propagation was longer than that of co-planar crack propagation. Using an initial equivalent SIF range based on a maximum strain energy release rate criterion, the results obtained from the 0°, 15°, 30°, and 45° specimens indicated almost the same fatigue lives, despite the different inclination angles.     

0Cr16 Ni5 Mo不锈钢疲劳性能研究

通过疲劳试验对 0Cr16Ni5Mo不锈钢疲劳性能和海水中的腐蚀疲劳性能进行了研究 ,并对断口特征进行了金相及SEM分析。结果表明 :钢的σ-1 、σ-1SCC 和τ-1 分别为 5 5 0MPa、40 8MPa和 2 85MPa ,σ-1 、τ-1与σb 、σ0 .2 之间符合奥金格公式。不同条件下的疲劳断口特征各异 ,纯旋转弯曲疲劳断口属正常形貌 ,而扭转疲劳断口的扩展阶段疲劳条带不明显 ,这与受力条件复杂有关。海水介质中疲劳开裂具有表面多条裂纹源特征 ,裂纹尖端的应力集中加速裂纹扩展造成腐蚀疲劳强度降低 ,裂纹扩展至断裂符合正常疲劳断裂机理。     

Fatigue crack propagation in cellular metals

Two types of cellular metals were investigated: a closed-cell aluminium foam with a cell size of about 3.5 mm and densities ranging from 0.25 to 0.40 g/cm³ and hollow sphere structures made of a stainless steel (316L) with sphere sizes of 2 and 4 mm and a density of about 0.3 g/cm³. Fatigue and fatigue crack propagation tests were performed on these materials using an electro-dynamic resonance fatigue testing machine. The crack extension was monitored by a potential drop technique. Additionally, investigations were carried out inside the scanning electron microscope (SEM) using an in situ loading device. All tests were accompanied by local deformation measurements and fracture surface analyses. From the fatigue crack propagation tests it is evident that these materials show a relatively high Paris-Exponent m in the range of 6 to 25 compared to common ductile solid metals. Additional tests were performed to estimate the influence of crack closure, crack bridging and micro cracking on the da/dN versus ΔK curve for these materials. The in situ fatigue tests and the fracture surface analyses revealed a difference in the fatigue crack propagation mechanisms between the closed-cell foam and the hollow sphere structure: in the closed-cell foam a contiguous fatigue crack can be found, where in the case of the hollow sphere structure the fatigue crack propagation is concentrated in the vicinities of the sintering necks.     

Fracture mechanics based fretting fatigue life predictions in Ti-6Al-4V

A fracture mechanics based crack propagation analysis is developed to work directly with the output of a contact mechanics stress analysis for fretting fatigue. A series of remote load fatigue tests were conducted on specimens that had previously been subjected to fretting fatigue loading conditions. The growth of these prior fretting induced cracks were monitored and compared to results from the crack propagation analysis. A combined fatigue crack formation and propagation analysis was then applied to other fretting fatigue experiments with good success. The creation of fretting fatigue stress-life curves is also demonstrated.     

Very-high-cycle fatigue crack initiation and propagation behaviours of magnesium alloy ZK60

The aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.     

Stringer-sheets fatigue cracking of helicopter-tail-boom in welding joints of 01420 AL–Li alloy

Regularities of fatigue crack appearance and growth in-service cracked stringers of helicopter Mi-26 tail-boom area were demonstrated. Statistic analyses of cracks distribution in helicopter tail-boom has shown crack appearance not only in stringers manufactured from Al–Li alloy but in frames too. Fatigue tests of complicated sheet-stringer specimens reproducing spot-welding-bonding joint assembly experienced constant cyclic loads amplitude were performed. Result of tests have shown fatigue cracking by the regular change of inter- and transgranular crack propagation. This manner of material cracking is similar case for in-service cracked stringers. The discovered mechanism of Al–Li alloy fatigue cracking is considered based on the results of fractographic investigations in scanning electron microscope. The discussed mechanism of the crack propagation reflects Li diffusion in grain boundary under stressing ahead of a crack tip during fatigue crack propagation that weakened grain boundary and accelerate crack propagation. Crack growth periods for tested specimens were estimated and compared with in-service registered crack growth duration based on regular non-destructive inspections. In-service fatigue cracking period of stringers was established and tolerance in-service duration for helicopter between two inspections has been recommended.