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。     

腐蚀疲劳过程的声发射

利用声发射技术研究了SM50B-Zc低强结构钢在淬火及正火两种状态下在空气和流动的3.5％NaCl水溶液中的疲劳过程。试验结果表明:在声发射振铃计数率与疲劳裂纹扩展速率及应力强度因子范围之间有良好的对应关系。     

轧制AZ31B镁合金腐蚀疲劳过程中的声发射信号分析

利用全数字声发射系统研究了轧制AZ31B 镁合金腐蚀疲劳过程中的声发射信号.结果表明,轧制AZ31B镁合金在NaCl溶液中的腐蚀疲劳过程主要存在4种声发射源,其中与腐蚀相关的两种信号分别对应于阳极溶解和阴极析氢,前者属于板平面内激励源,产生扩展波信号;后者属于板平面外激励源,产生弯曲波.与载荷相关的两种信号分别对应于塑性变形的连续型信号和裂纹扩展阶段高载荷阶段出现的裂纹扩展信号.腐蚀相关的声发射信号存在于整个疲劳过程,而塑性变形信号只发生在疲劳过程中特定的应力阶段.     

多疲劳裂纹扩展的声发射特性研究

用声发射对低强度材料LD31铝材和A3钢双疲劳裂纹扩展过程进行了监测,得到了声发射参数与裂纹扩展速率的对应关系。介绍利用后处理线定位参数调整及载荷滤波降低噪声的具体方法。     

齿轮表层疲劳裂纹扩展的声发射模型

齿轮表面的疲劳磨损是齿轮故障的主要形式之一。从弹性力学的能量理论角度,推导出了齿轮齿表疲劳裂纹在扩展时的能量关系,并根据声发射信号的特征,构造出齿表疲劳裂纹在扩展时产生的声发射信号的理论模型,说明了影响声发射信号能量强度的主要因素为裂纹扩展时间、形变体积及载荷和弹性模量的变化率,从理论上证明了声发射技术用于检测疲劳磨损的可行性。     

时效时间对7075铝合金疲劳裂纹扩展速率的影响

采用透射电镜、显微硬度仪、拉伸试验和疲劳试验等研究了 7075铝合金在欠时效(120℃×8 h)、峰时效(120℃×24 h)与过时效(120℃×48 h)状态下的微观组织、硬度、拉伸性能方面与疲劳裂纹扩展速率方面的差异.结果表明:3种时效状态的合金在高低应力强度因子范围时,疲劳裂纹扩展速率呈现不同的规律.在低应力强度因子范围时,峰时效态合金的疲劳裂纹扩展速率最高,过时效态最低;在高应力强度因子范围时,过时效态的合金表现出最优的疲劳裂纹扩展阻力,而峰时效与欠时效状态的疲劳裂纹扩展阻力相近.结合疲劳损伤能量模型、强塑积与晶界析出相差异,分析了时效时间不同引起疲劳裂纹扩展速率差异的本质原因.     

焊接残余应力对7N01铝合金疲劳裂纹扩展影响

焊接残余应力作为平均应力影响裂纹扩展. 将残余应力与外载平均应力分离,通过构建典型焊接残余应力场,借助扩展有限元计算焊接残余应力场的应力强度因子. 开展了紧凑拉伸（CT）试样的疲劳扩展试验,基于Walker公式将裂纹尖端平均应力强度因子K_m（静态量）和应力强度因子幅值ΔK（动态量）分离,获得疲劳裂纹扩展速率da/dN与K_m及ΔK的非线性关系. 结果表明,不同外载荷下,应力比与裂纹长度为非线性关系;残余应力对裂纹扩展存在尺度效应:CT试样裂纹长度小于2 mm时,残余应力场明显影响疲劳裂纹扩展速率;当裂纹长度大于2 mm,外载荷为主导因素.     

2A12-T4铝合金搅拌摩擦焊多区域疲劳裂纹扩展行为

疲劳裂纹扩展速率是材料内在组织性能的宏观外在表现,显微组织和残余应力对宏观疲劳裂纹的扩展有显著的影响. 文中研究了2A12-T4铝合金搅拌摩擦焊不同区域的疲劳裂纹扩展速率. 结果表明,在焊缝区域,由于搅拌针强烈的机械和焊接热作用,材料组织发生变化,并伴有残余应力的产生,导致不同区域裂纹的扩展速率存在明显差异. 在低应力强度因子范围(ΔK)时,热影响区和垂直于焊缝方向的疲劳裂纹扩展速率基本相同,并且都高于沿焊缝方向裂纹的扩展速率;在高应力强度因子范围(ΔK)时,垂直于焊缝方向的扩展速率逐渐高于热影响区速率,此时这两个区域的裂纹扩展速率仍然高于沿焊缝方向裂纹的扩展速率.     

AZ31镁合金疲劳裂纹扩展和过载效应

研究AZ31镁合金不同方向的力学性能、疲劳裂纹扩展行为和单峰过载效应以及载荷比对疲劳裂纹扩展速率的影响。结果表明：不同方向材料的抗拉强度相当,而屈服强度差别较大：不同方向疲劳裂纹扩展行为存在差异,是由于屈服强度不同所致,归一化后差别消失：当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的降低而降低;单峰过载时,疲劳裂纹向前扩展一段距离并偏离原扩展方向,导致随后裂纹扩展迟滞,这是由于过载增大残余压应力和裂纹出现偏折的结果。     

不锈钢在循环扭转载荷下近门槛值的疲劳裂纹扩展行为

研究了带环状预裂纹不锈钢圆棒试样在循环扭转载荷下、门槛值附近的疲劳裂纹扩展行为,用应力强度因子表征了裂纹扩展开始的门槛值．随着裂纹的扩展,裂纹扩展速率由于裂纹面的滑移接触而减小．通过外插裂纹扩展速率与裂纹长度之间的关系,可近似得到裂纹长度为零时无裂纹面滑移接触影响的裂纹扩展速率．施加的应力强度因子范围可分解为推动裂纹扩展的有效值和由于裂纹面的滑移接触而屏蔽掉的两部分．预测了疲劳裂纹的萌生和断裂极限,预测值和实验值相当一致．     

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.     

裂纹尖端塑性区的声发射特性研究

对裂纹尖端形成塑性区时产生声发射信号的累计数与应力强度因子的关系进行研究。运用线弹性断裂理论和弹塑性断裂理论进行了分析,得出累计数在小范围屈服时与应力强度因子的4m次方成正比,在大范围屈服时与应力强度因子的2m次方成正比。对碳钢和不锈钢的单边切口试样进行了拉伸声发射监测实验,实验测得大范围屈服时,比例系数分别为0.333和0.183,m值为0.5。     

A7N01铝合金复合加载下的疲劳裂纹扩展行为

利用CTS试样,研究了A7N01P-T4铝合金母材在I-II型复合加载下,不同加载角度时疲劳裂纹的扩展行为,利用有限元数值计算复合加载下裂纹尖端的应力强度因子(SIF, stress intensity factor)得到了加载角度与裂纹开裂方向的关系,并与由最大周向应力准则导出的关系进行了对比,二者吻合良好;根据疲劳试验和有限元计算的结果,并引入当量应力强度因子,分析了不同加载角下疲劳裂纹的扩展速率. 结果表明,经当量化处理后,各加载角下的裂纹扩展速率曲线基本重合,并且满足Paris公式.     

Primary water stress corrosion cracking in alloy 600 and intergranular crack growth rate

The effects of the stress intensity factor and dissolved hydrogen on the crack growth rate in primary water reactor environments was investigated with alloy 600 with continuous carbide films at grain boundaries. A primary water stress corrosion cracking of intergranular fracture mode was observed. The intergranular crack growth rate decreased with a decreasing stress intensity factor, and the dissolved hydrogen content influenced the crack growth rate. Such results are consistent with those reported previously.     

喷涂层接触疲劳损伤的声发射研究

用超音速等离子喷涂设备在45钢基体上制备了铁基合金涂层。以球盘式疲劳试验机为平台,研究了涂层的接触疲劳损伤行为,探测并分析了涂层在不同应力水平下疲劳损伤的声发射反馈信号。结果表明,涂层的接触疲劳损伤过程主要包括弹塑性变形、裂纹萌生和缓慢扩展、裂纹亚临界扩展、裂纹失稳扩展4个阶段。裂纹萌生和缓慢扩展阶段是决定疲劳寿命长短的主要阶段。接触应力越大,裂纹亚临界扩展时间和失稳扩展时间越短。涂层的最终失效模式可以根据裂纹失稳扩展阶段声发射幅值的最大值来判断,发生点蚀失效时幅值最大值约为82.4 dB,剥落失效时幅值最大值约为90.2 dB,分层失效时幅值最大值约为91.3 dB。     

Using acoustic emission to monitor stress-corrosion cracking

In-situ monitoring of the condition of materials has the potential for reducing the risk of failures, improving maintenance scheduling and costs, and allowing predictions of component life. Stress-corrosion cracks have been shown to emit acoustic signals, and correlations between crack area and the number of acoustic emission events have been developed in laboratory tests. On-line monitoring will require the ability to discriminate between background and crack growth signals. Acoustic emission characteristics such as event rate, amplitude and rise time can be used to discriminate between valid and invalid events. A summary of acoustic emission from stress-corrosion cracking initiation and long crack growth, acoustic emission characteristics, and potential applications is given in this article.     

Monitoring Sub-Critical Crack Growth due to Stress Corrosion or Hydrogen Embrittlement by Acoustic Emission

Abstract

Acoustic emission ring-down counting has been used to monitor sub-critical crack growth by stress corrosion of mild steel in disodium hydrogen phosphate, -brass in sodium nitrite and magnesium–7% aluminium in potassium chromate/sodium chloride mixtures, and by hydrogen embrittlement in mild steel and a 7179 T651 aluminium alloy. The level of acoustic emission activity varies widely between different alloy/environment systems and is dependent upon microstructure, fracture path and crack growth rate. The practical significance of the results is discussed.     

Application of Paris’s law to thermal cycling-induced failure in semiconductor device patterns

The present work provides a semi-empirical model to show that the growth rate of thermal displacement-induced cracks in semiconductor devices depends on pattern thickness. Paris’s law is adopted to characterize the growth rate of cracks during thermal-cycling. The crack propagation rate is estimated from the semi-empirical relation (dc/dN)_v=C(ΔK)ⁿ, where ΔK indicates the range of an applied stress intensity factor, and C is a scaling constant. The applied stress intensity factor is related to the initial crack length as ΔK=YΔσ(πc)^1/2, where σ represents the thermal displacement-induced normal stress, c describes the pre-existing crack length, and Y is a geometrical factor. The resulting crack growth rate can be expressed as a function of device pattern thickness: dc/dN∞(1/t)^m, where t describes the pattern thickness, and m is another constant. The present semi-empirical results showing the relationship between the crack growth rate and pattern thickness indicate that if a semiconductor device pattern becomes thinner by 68%, its susceptibility to thermal cycling-induced damage will be enhanced by 76%.