Feasibility analysis for monitoring fatigue crack in hydraulic turbine blades using acoustic emission technique

In order to investigate the feasibility of monitoring the fatigue cracks in turbine blades using acoustic emission (AE) technique, the AE characteristics of fatigue crack growth were studied in the laboratory. And the characteristics were compared with those of background noise received from a real hydraulic turbine unit. It is found that the AE parameters such as the energy and duration can qualitatively describe the fatigue state of the blades. The correlations of crack propagation rates and acoustic emission count rates vs stress intensity factor (SIF) range are also obtained. At the same time, for the specimens of 20SiMn under the given testing conditions, it is noted that the rise time and duration of events emitted from the fatigue process are lower than those from the background noise; amplitude range is 49–74 dB, which is lower than that of the noise (90–99 dB); frequency range of main energy of crack signals is higher than 60 kHz while that in the noise is lower than 55 kHz. Thus, it is possible to extract the useful crack signals from the noise through appropriate signal processing methods and to represent the crack status of blade materials by AE parameters. As a result, it is feasible to monitor the safety of runners using AE technique. Foundation item: Project(50465002) supported by the National Natural Science Foundation of China     

Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/SiC composites by acoustic emission

High toughness and reliable three-dimensional needled C/SiC composites were fabricated by chemical vapor infiltration （CVI）. An approach to analyze the tensile behaviors at room temperature and the damage accumulation of the composites by means of acoustic emission was researched. Also the fracture morphology was examined by S-4700 SEM after tensile tests to prove the damage mechanism. The results indicate that the cumulative energy of acoustic emission （AE） signals can be used to monitor and evaluate the damage evolution in ceramic-matrix composites. The initiation of room-temperature tensile damage in C/SiC composites occurred with the growth of micro-cracks in the matrix at the stress level about 40% of the ultimate fracture stress. The level 70% of the fracture stress could be defined as the critical damage strength.     

Monitoring damage evolution of steel strand using acoustic emission technique and rate process theory

Utilizing the acoustic emission（AE） technique, an experimental investigation into the damage evolution for steel strand under axial tension was described. The damage evolution model for steel stand relating the damage evolution to acoustic emission parameters was proposed by incorporating the AE rate process theory. The AE monitoring results indicate that damages occur in both elastic and plastic phases of steel strand. In elastic phase, AE signals are mainly sent out from the micro damage due to the surface friction among the wires of steel strand, while in plastic phase, AE signals emitted from the plastic deformation of wires. In addition, the AE cumulative parameters curves closely resemble the loading curve. The AE cumulative parameters curves can well describe the damage evolution process including the damage occurrence and damage development for steel strands. It is concluded that the AE technique is an effective and useful nondestructive technique for evaluating the damage characteristics of steel strand.     

含三晶交多晶体沿晶断裂数值建模方法研究

为了全面解析风电齿轮的疲劳损伤演化机理, 以某1.5 MW风力发电机高速级齿轮为研究对象, 基于Voronoi图与多晶体组织的几何相似性, 建立介观尺度下的齿面模型, 采用内聚力单元模拟晶界对基体的割裂作用, 引入双线性分离曲线作为评判损伤的依据. 采用周期跳跃的加载方式模拟齿面接触载荷的循环累积效应, 仿真预测接触疲劳裂纹的萌生位置及其疲劳寿命, 并探讨风速变化和齿面间摩擦因数对疲劳裂纹演化的影响. 结果表明, 齿轮在疲劳损伤初期的演化速率较慢, 随着损伤量的不断累积, 其损伤速率逐渐加快. 针对研究对象, 在距齿面深度0.13、0.27 mm处最先出现裂纹, 并伴随接触应力的循环加载逐渐扩展, 最终于齿面形成典型的剥落破坏. 此外, 通过对比分析还可以发现, 高风速和齿间润滑不良均显著降低齿轮的寿命.

     

Effect of hydrogen attack on acoustic emission behavior of low carbon steel

In order to investigate the effect of hydrogen attack degree on acoustic emission(AE) behavior of low carbon steel during tensiling,specimens made of low carbon steel was exposed to hydrogen gas of 18 MPa at 450 and 500℃ for 240,480 and 720h respectively,Experimental results show that with increase of the hydrogen attack degree,the totally AE activity decreases during tensiling.In addition,the count of AE signals with high amplitude for the specimens with hydrogen attack keeps a constant which is less than that without hydrogen attack.It is concluded that AE signals orginate in the specimens with hydrogen attack from intergranular fracture induced by methane blisterings or/and microcracks on grain boundaries.     

线性疲劳损伤累积理论的研究

为解决两级载荷作用下的材料常在损伤值不为一时破坏的问题，在原有常用疲劳累积理论的基础上，考虑加载顺序与领域潜在损伤对材料疲劳寿命的影响，首先建立了一种线性疲劳损伤累积模型及其数学公式。然后利用两种材料疲劳实验数据，通过该模型预测其疲劳寿命。所得结果符合实际。最后得出该模型及其数学计算公式对材料进行寿命预测是可行的。     

线性疲劳损伤累积理论的研究

为解决两级载荷作用下的材料常在损伤值不为一时破坏的问题,在原有常用疲劳累积理论的基础上,考虑加载顺序与领域潜在损伤对材料疲劳寿命的影响,首先建立了一种线性疲劳损伤累积模型及其数学公式.然后利用两种材料疲劳实验数据,通过该模型预测其疲劳寿命.所得结果符合实际.最后得出该模型及其数学计算公式对材料进行寿命预测是可行的.     

Influence of chemical liquids on the fatigue crack growth of the AZ31 magnesium alloy

The fatigue crack growth behavior of an AZ31 magnesium alloy was investigated by comparing the effect of zirconate and phos-phate chemical liquids.The morphology,components,and phase compositions of the chemical depositions at the fatigue crack tip were analyzed by employing scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and X-ray diffraction(XRD),respectively.For samples with and without the chemical liquids,their stress-intensity factor values at the fatigue crack tip were compared by using a stress-strain gauge.The results demonstrated that a zirconate film(ZrxOy·ZnxOy) and a phosphate film(Zn3(PO4)2·4H2O and MgZnP2O7) could be formed on the fatigue crack-surface at the fatigue crack tip.The stress distribution was changed because of the chemical depositions and the causticity of the chemical liquids.This could decrease the stress-intensity factor value and thus effectively cause fatigue crack closure,which reduces the fatigue crack growth rate.Moreover,it was found that the fatigue crack closure effect of zirconates was more positive than that of phosphates.     

共面裂纹疲劳扩展干涉的数值模拟

在工程结构的疲劳断裂中常常存在共同多裂纹相互干涉的现象，它们之间的相互作用将直接影响到结构的疲劳寿命，运用数值模拟法对承受拉伸疲劳载荷的有限平板上三个初始半圆形共同表面裂纹在疲劳扩展过程中的相互干涉和合并过程作了较为系统的研究，得到了共面裂纹在形状变化及应力强度因子方面的相互干涉效应。数值模拟法相对于重绘法和瞬间合并法能较精确地预测结构的疲劳寿命，从而可有效地预防结构的失效。     

花岗岩-喷射混凝土梁损伤的声发射可视化表征

为研究输水隧洞围岩-喷射混凝土衬砌结构的协同抗弯损伤特性,制备两种花岗岩-喷射混凝土组合梁进行不同加载速率的四点弯曲试验,并采用数字图像相关(DIC)和声发射(AE)两种无损监测技术对花岗岩-喷射混凝土组合梁的四点弯曲试验进行监测。结果表明：在裂缝从喷射混凝土中扩展至花岗岩表面时,荷载-应变曲线在峰后下降段出现明显的拐点,该拐点可以作为花岗岩起裂的标志;通过声发射技术对喷射混凝土中的损伤类型识别发现,剪切损伤所占的比例随着应变率的提高而增加。为实现对组合梁内部损伤的可视化分析,利用声发射损伤定位数据计算了空间b值。结合损伤定位在空间中的密度分布规律,给空间b值附加系数后构建新的Rb值。与空间b值相比,Rb值的云图对不同损伤程度的区分度更高,改善了损伤可视化分析的效果。最后,将Rb值定位的损伤云图与DIC监测到的裂缝云图进行对比,验证了这种利用声发射技术对损伤进行可视化分析方法的有效性。     

胶结充填体应变率与声发射特征响应规律

应变率是影响胶结充填体力学特性和声发射特征的重要因素.为研究胶结充填体应变率与声发射特征响应规律,文中进行了单轴压缩条件下胶结充填体的声发射试验,研究了应变率、声发射在加载过程中的时序特征,探讨了应变率与声发射时序特征之间的相关性,以及两者变化趋势之间的内在联系,进一步揭示了荷载作用下胶结充填体的裂纹演化规律.结果表明:胶结充填体应变率、振铃计数率在变形全过程均表现出"升→降→稳→升"的变化趋势和阶段性特征,两者相关性较高;胶结充填体内部破裂是由骨料之间的摩擦以及胶结材料的断裂演化造成,应变率对声发射特征在一定程度上起到了支配作用;基于应变率在塑性屈服和失稳破坏阶段表现出明显的突变性,可为胶结充填体破裂预测提供有效的前兆信息;结合声发射振铃计数率、平静期以及扩容现象可更加全面的反映胶结充填体破裂情况.研究结果可为胶结充填体力学特性研究奠定理论基础.     

疲劳多裂纹扩展的常规态型近场动力学分析

为改进疲劳多裂纹扩展问题的分析,建立了常规态型近场动力学的疲劳多裂纹扩展模拟方法.在常规态型近场动力学的理论框架基础上,采用近场动力学疲劳裂纹萌生和扩展模型,建立了一个可模拟疲劳裂纹萌生与扩展的平面应力近场动力学模型.针对近场动力学疲劳模型计算量庞大的问题,又引入"临界断裂数"来优化模型的计算流程,提升模型的计算速度,以单边裂纹板为研究对象,通过选取不同"临界断裂数"的值寻求计算效率与裂纹路径精准度之间的平衡.从计算结果的对比中可以发现,当采用恰当的"临界断裂数"时,计算速度得到了较大的提升,同时裂纹仍能保证精准的形状.然后,该模型被用于非共线多裂纹板的疲劳破坏模拟仿真之中,当采用不同的"临界断裂数"时,计算运行得到的疲劳裂纹扩展路径以及相应的a-N曲线均能与实验得到的结果取得良好的吻合.研究结果表明,近场动力学疲劳分析方法不需引入断裂准则和预知裂纹路径,裂纹可以任意扩展.     

Fractal analysis on the spatial distribution of acoustic emission in the failure process of rock specimens

The spatial distribution of acoustic emission (AE) events in the failure process of several rock specimens was acquired us-ing an advanced AE acquiring and analyzing system.The box counting method (BCM) was employed to calculate the fractal dimen-sion (FD) of AE spatial distribution.There is a similar correlation between the fractal dimension and the load strength for different rock specimens.The fractal dimension presents a decreasing trend with the increase of load strength.For the same kind of specimens,their FD values will decrease to the level below a relatively same value when they reach failure.This value can be regarded as the critical value,which implies that the specimen will reach failure soon.The results reflect that it is possible to correlate the damage of rock with a macroscopic parameter,the FD value of AE signals.Furthermore,the FD value can be also used to forecast the final fail-ure of rock.This conclusion allows identifying or predicting the damage in rock with a great advantage over the classic theory and is very crucial for forecasting rockburst or other dynamic disasters in mines.     

混凝土单轴压缩过程中损伤演化与声发射分析

为了研究混凝土单轴压缩状态下的损伤演化过程,利用声发射设备采集试样破坏过程的声发射数据,基于声发射信号建立声发射能量表述的损伤本构方程,对损伤程度进行量化表述,提出了损伤因子在启裂强度和损伤强度的取值区间.基于连续介质损伤力学,对微裂纹的发育过程和声发射能量数综合分析.结果表明,理论应力-应变曲线与试验曲线吻合较好,混凝土单轴压缩过程各阶段吻合度较高,混凝土单轴压缩损伤本构模型具有一定的合理性.     

通过声发射实现对钢瓶的检测

通过对CNG钢瓶无损检测常用方法特点的对比,显出声发射检测在在线式CNG钢瓶无损检测中相对于其他方法的优越性,并对声发射检测在在线式CNG钢瓶快速检测应用时所注意的问题和特点进行了描述,提出改善方法,然后在这些理论的基础上搭建一个简易的信号采集、分析处理系统,对信号进行采集和频域范围的分析与小波除噪。从结果中可以看出明显的信号与处理效果,从而说明声发射检测对于实现在线式CNG钢瓶快速检测具有可行性。     

Incoloy 825镍基合金的低周疲劳行为

为了研究Incoloy 825镍基合金的循环应力响应行为和低周疲劳行为,在室温和760℃下进行外加总应变幅控制下的疲劳实验,确定了合金在不同温度下的应变疲劳参数.利用透射电子显微镜和扫描电子显微镜分析了合金疲劳变形后的位错亚结构与断口形貌.结果表明,室温下合金呈现先循环硬化后循环软化的特征,而在760℃下合金则呈现先循环硬化后循环软化或循环稳定的特征.在不同温度下合金的塑性应变幅、弹性应变幅与断裂时的载荷反向周次之间均呈现单斜率线性关系,并分别服从Coffin-Manson和Basquin公式.在室温和760℃下合金的低周疲劳变形机制主要为平面滑移,且疲劳裂纹均以穿晶方式萌生和扩展.     

镁合金疲劳行为的研究现状及展望

简要介绍了镁合金在疲劳加载条件下的循环应力响应行为、疲劳裂纹萌生与扩展行为以及疲劳断裂的微观特征，分析了镁合金的显微组织、冶金和表面状态等材料内在因素以及环境、温度等外部因素对镁合金的疲劳强度、疲劳寿命、疲劳裂纹扩展速率的影响规律，指出了机械抛光、喷丸处理、表面滚压等表面机械加工方法以及适当的热处理均可有效地提高镁合金的疲劳强度和疲劳寿命.此外，也对镁合金疲劳行为的未来研究方向和发展趋势进行了展望.     

钢筋混凝土板裂缝对声发射波传播特征影响

目前利用声发射技术对混凝土结构进行裂缝定位或损伤检测时,并没有考虑已存在裂缝对声发射波传播特征的影响,这往往会使检测结果产生较大的偏差.为了探究裂缝参数对声发射波传播特征的影响,对3个不同保护层厚度的钢筋混凝土板试件进行了四点静力加载破坏试验和声发射测试,研究了裂缝深度、宽度、数量对钢筋混凝土板声发射波速和振幅衰减等传播特征的影响规律.试验和分析结果表明:裂缝深度对声发射传播特征的影响显著,声发射振幅衰减随裂缝深度增大而增大,声发射波速随裂缝深度增大而减小;声发射波速和振幅衰减几乎不随裂缝数量、宽度两参数变化而变化.在此基础上,提出考虑裂缝影响的修正波速时差定位法,进一步探讨了与采用传统时差定位法相比对钢筋混凝土损伤定位的影响,结果显示,基于修正波速的时差定位法对钢筋混凝土板进行损伤定位精度更高.     

Theoretical and experimental analysis on the mechanism of the Kaiser effect of acoustic emission in brittle rocks

The Kaiser effect is formally described as the absence of detectable acoustic emission （AE） events until the load imposed on the material exceeds the previous applied level and is usually used to estimate geostress. By focusing on the heterogeneity of rock material, the mechanism of the Kaiser effect under cyclic loading is analyzed based on statistic damage mechanics. Two groups of granite specimens have been cyclically loaded with two different loading paths to verify the theoretical results. The heterogeneity of rock is the real reason that causes irrecoverable damage on the Kaiser effect of acoustic emission in cyclic loading. The Kaiser effect reflects the damaged state in rocks rather than the previous stress imposed on it. Applications for using the Kaiser effect to estimate geostress were discussed here. It is shown that the commonly used uniaxial loading method for estimating geostress is not in accor- dance with the theoretical and experimental results. The analysis is of importance to use the Kaiser effect correctly for estimating geostress or in other fields. 2008 University of Science and Technology Beijing. All rights reserved.     

焊接结构疲劳破坏的若干问题

首先阐述了备受关注的钢结构疲劳破坏的危害性,从冶炼过程以及微观角度阐述钢材中疲劳裂纹的产生机理和分布的复杂性,并针对土木工程钢结构的焊接加热过程对材料的影响进行分析,阐述了焊接裂缝成为先天的"疲劳裂纹",从而加快焊接结构的疲劳破坏进程;总结前人研究得出的焊接残余应力的存在对焊接结构的疲劳强度的影响;最后提出了一些抗疲劳措施.