裂纹长度和位置影响下声发射信号传播特性

针对再制造毛坯闭合裂纹检测较困难,采用声发射技术,从仿真和实验两个方面研究裂纹长度和裂纹位置对声发射信号传播特性的影响。根据裂纹中波的传输理论,采用时域有限差方法,仿真两个因素对声信号的影响,实验结果表明,裂纹长度对于信号衰减影响比裂纹位置明显;裂纹长度小于3 mm时信号的能量和幅值衰减系数(分别为-0.28和-0.15)明显大于裂纹长度大于6 mm的信号衰减系数(分别为-1.48和-0.9);裂纹长度为3 mm比裂纹长度为0.5 mm时能量和幅值相对衰减率分别大0.62 d B和0.2 d B,而其比裂纹长度为15 mm的能量和幅值相对衰减分别小14.7 d B和8 d B;裂纹位置的能量相对衰减率位于-6.4~-6.6 d B,幅值相对衰减率位于-1.4~-1.7 d B。实验研究了裂纹长度对声发射信号的影响,得出裂纹长度小于2 mm时的衰减系数明显大于裂纹长度超过6 mm的衰减系数,与仿真结果相符。因此,声发射参数与裂纹长度有明确的对应关系,声发射技术可有效检测再制造毛坯中的裂纹,尤其是长度小于3 mm的微裂纹。     

Scanning electron microscope design for quantitative multicontrast

Conceptual designs of scanning electron microscopes (SEMs) using a time-of-flight electron spectrometer are presented. The procedure for making quantitative measurements with such SEMs is shown to be much simpler and versatile than using conventional SEMs. SEMs which use an electron time-of-flight spectrometer are able to operate as multicontrast analytical probes, capable of simultaneously quantifying surface topography, voltage, and material type. In addition, it is demonstrated that these SEMs can be designed to have high spatial resolution, good signal-to-noise characteristics, and to be of compact table-top size.     

基于声发射技术的水轮机裂纹检测系统研究

研究了利用声发射技术实时检测水轮机叶片的裂纹的检测系统。重点介绍了检测系统软、硬件设计中的关键问题,以硬件描述语言(VHDL)设计出FPGA芯片来实时接收声发射信号的特征参数,来实现对水轮机叶片进行实时动态的裂纹检测。     

Fatigue features study on the crankshaft material of 42CrMo steel using acoustic emission

Crankshaft is regarded as an important component of engines, and it is an important application of remanufacturing because of its high added value. However, the fatigue failure research of remanufactured crankshaft is still in its primary stage. Thus, monitoring and investigating the fatigue failure of the remanufacturing crankshaft is crucial. In this paper, acoustic emission (AE) technology and machine vision are used to monitor the four-point bending fatigue of 42CrMo, which is the material of crankshaft. The specimens are divided into two categories, namely, pre-existing crack and non-preexisting crack, which simulate the crankshaft and crankshaft blank, respectively. The analysis methods of parameter-based AE techniques, wavelet transform (WT) and SEM analysis are combined to identify the stage of fatigue failure. The stage of fatigue failure is the basis of using AE technology in the field of remanufacturing crankshafts. The experiment results show that the fatigue crack propagation style is a transgranular fracture and the fracture is a brittle fracture. The difference mainly depends on the form of crack initiation. Various AE signals are detected by parameter analysis method. Wavelet threshold denoising and WT are combined to extract the spectral features of AE signals at different fatigue failure stages.     

How fine a surface crack can you see in a scanning acoustic microscope?

Surface cracks can be detected with great sensitivity in an acoustic microscope, even when the width of the crack is much less than the acoustic wavelength. This is because of the strong excitation and scattering of Rayleigh waves. Cracks in a variety of materials, including cracks from indents in several different ceramics, have been studied by acoustic microscopy, and the results compared with images from light and scanning electron microscopes. No crack or part of a crack that was seen by the other two techniques failed to be seen by acoustic microscopy. The contrast of a crack in an acoustic microscope showed no sign of fading as the crack became thinner towards its tip, and diffraction from tips suggests that they are acoustically well-defined points on the surface. Although theoretically the acoustic contrast from a crack might be expected to diminish when the crack width is smaller than the shear-wave decay length in the fluid, in practice this is so small that for many purposes it may be considered that there is almost no limit to how fine a crack can be and still be detected in an acoustic microscope.     

Accumulated damage process of thermal sprayed coating under rolling contact by acoustic emission technique

The accumulated damage process of rolling contact fatigue (RCF) of plasma-sprayed coatings was investigated. The influences of surface roughness, loading condition, and stress cycle frequency on the accumulated damage status of the coatings were discussed. A ball-ondisc machine was employed to conduct RCF experiments. Acoustic emission (AE) technique was introduced to monitor the RCF process of the coatings. AE signal characteristics were investigated to reveal the accumulated damage process. Result showed that the polished coating would resist the asperity contact and remit accumulated damage. The RCF lifetime would then extend. Heavy load would aggravate the accumulated damage status and induce surface fracture. Wear became the main failure mode that reduced the RCF lifetime. Frequent stress cycle would aggravate the accumulated damage status and induce interface fracture. Fatigue then became the main failure mode that also reduced the RCF lifetime.     

Tilt dependence of the secondary electron emission at low excitation energy

Influence of the specimen's slope on the secondary electron emission has been experimentally studied. Strong deviations from the inverse cos law have been observed and corresponding phenomenological equation (taking into account this deviation) is suggested. The consequences of the dependence on the topography contrast of low- and high-voltage scanning electron microscopy (SEM) image, especially for three-dimensional (3-D) reconstruction, are considered.     

Unusual secondary electron emission behavior in carbon nanotube forests

Electron yield was measured from patterned carbon nanotube forests for a wide range of primary beam energies (400–20,000 eV). It was observed that secondary and backscattered electron emission behaviors in these forests are quite different than in bulk materials. This seems to be primarily because of the increased range of electrons due to the porous nature of the forests and dependent on their structural parameters, namely nanotube length, diameter and inter‐nanotube spacing. In addition to providing insight into the electron microscopy of nanotubes, these results have interesting implications on designing novel secondary electron emitters based on the structural degrees of freedom of nanomaterials. SCANNING 31: 221–228, 2009. © 2010 Wiley Periodicals, Inc.     

Application of ion-beam etching techniques to the fine structure of biological specimens as examined with a field emission SEM at low voltage

The term “etching,” in electron microscopy, refers to the removal of specimen surface layers and includes chemical, electrolytic, and ion-beam methods. The ion-beam etching process is used to remove layers of a target material by bombarding it with ionized gas molecules. Recently, the method has been applied to the field of biological specimens; however, the practical procedures for such organic materials have not been developed. In the present study, we used an apparatus in which a beam of argon ions is collimated and focused by electrostatic lenses onto an appropriate target. We demonstrated the optimum conditions to observe biological specimens that were treated with osmium tetroxide and tannic acid. The specimens were examined uncoated at low accelerating voltage using a field emission scanning electron microscope. According to our experiments, when a biological specimen was observed under high-resolution conditions at over 50,000x magnification, the optimum condition of ion-beam etching consisted of an accelerating voltage of E = 1 keV and an ion-beam dose of It = 360 ～ 400 μA. min, depending on parts of the specimens. In order to decrease overetching, we had to choose factors such as E = 1 ～ 2 keV and It = 500 μA. min.     

Design of a piezoelectric transducer cylindrical phase modulator for simulating acoustic emission signals

To conveniently carry out the pipeline leak experiment in a laboratory, leak acoustic signals are simulated by using the converse piezoelectric effect of a piezoelectric transducer (PZT) cylindrical phase modulator. On the basis of the piezoelectric equations and electromechanical equivalence principle, the transfer function of a PZT cylindrical phase modulator is delivered. A PZT cylindrical phase modulator is designed, and the numerical simulation is conducted. Results prove that the PZT cylindrical phase modulator can effectively simulate leak acoustic emission signals when the frequency is lower than 25 KHz. __________ Translated from Journal of Beijing University of Technology, 2006, 32(8): 683–687 [译自: 北京工业大学学报]     

风力机叶片多裂纹扩展声发射信号的特征识别

针对风力机叶片蒙皮多裂纹难以状态识别的问题,根据裂纹扩展释放能量的过程,推导主裂纹扩展AE信号的表达式,从而明晰了主裂纹扩展的AE信号特性及其与应力变化之间的关联。由于多裂纹扩展AE信号为卷积混合模型,提出一种对具有非平稳、非线性特性的卷积混合AE信号特征提取的方法,以输出信号的广义能量作为目标函数得到盲解卷的滤波器迭代式,采用Godard算法通过输出信号与估计值的误差调整滤波器系数,并根据相似系数选择适当的非线性函数以减少采集设备对AE信号的影响。最后在裂纹扩展试验中,预制不同尺寸的多缺陷,对叶片试件同时施加激振载荷和循环载荷,每间隔一定的循环次数采集不同状态的AE信号,同时采用具有非全局性的瞬时频率和特征尺度来识别多裂纹在不同扩展状态下的特征,从而明晰了信号特征与多裂纹生存状态的关联,形成了识别多裂纹复合材料损伤的评价机制。     

火车轮裂纹声发射源定位技术研究

针对火车轮运行过程中受变载荷作用产生的裂纹具有的特征,对现有的裂纹声发射定位技术进行了优化.在硬件上采用更适用于车轮运行环境的安装方式,并确定了前向通道的各项参数.根据模态声发射理论,即AE信号在传播过程中具有的频散现象和多模态特性,通过对同一模式波到达各传感器的时差信号的精确分析,采用时差法和面域定位相结合的方法进行源位置的计算.由于采用多种技术进行分析,结果将更为精确.     

不同小波基在碳钢材料声发射信号分析中的应用

为研究不同小波基对同一声发射信号分析结果之间的可比性,以碳钢在塑性变形和裂纹扩展时产生的声发射信号为研究对象,用db5和sym5两种常用小波基对检测信号进行了分析对比.结果表明,对碳钢在塑性变形和裂纹扩展时产生声发射信号,2种小波变换在信号能量分布趋势上分析结果一致,在主要信号峰值对应频率上分析结果接近,平均相对偏差仅为1.53%,而在各级信号占能量比例的具体值和信号峰值上偏差较大,平均相对偏差在10%左右,可比性较差.     

Microstructure of Monoplacophora (Mollusca) shell examined by low-voltage field emission scanning electron and atomic force microscopy

The shell of Micropilina arntzi (Mollusca: Monoplacophora), a primitive molluscan class, was examined by using field emission scanning electron microscopy (FESEM) at low voltage and atomic force microscopy (AFM). The use of these two techniques allowed the observation of fine details of Micropilina arntzi shell and contributed to bring new features concerning the study of molluscan shell microtexture. Imaging with low-voltage FESEM provided well-defined edge contours of shell structures, while analyzing the sample with AFM gave information about the step height of stacked internal structures as well as the dimension of the particles present in their surface at a nanometric level. The shell microstructure of Monoplacophora species presents different patterns and may be a taxonomic implication in the systematic studies of the group.     

Simple modifications for accurate high-temperature gas exposures using scanning electron microscopy

Relatively low-cost modifications to standard commercial scanning electron microscopes (SEM) that allow accurate exposure of sample(s) to noncorrosive gases at ambient and high temperatures are outlined. Energy-dispersive spectroscopic analysis of sample(s) exposed to noncorrosive gases at high temperatures is demonstrated. Gas exposure is limited to pressures of less than 10^?4 torr (1.33 × 10^?2 Pa) as a result of limitations on SEM system operation and SEM safety interlocks. Gases are limited to noncorrosive types as a result of potential damage to system detection devices and internal mechanical parts.     

针对不连续带洞结构的声发射源定位方法研究

由于兰姆波在舷窗等带孔结构表面的传播路径不再是线性路径,传统方法难以解决撞击的声源定位问题。 本文针对该问题,提出了一种基于声发射的网格映射概率成像定位方法。 该方法通过建立网格映射模型解决了信号传播的非线性路径难题,并通过对网格节点的信号最快传播路径计算得到相应的理论时间差值。 之后利用自适应能量阈值方法与 AIC 方法计算信号的实际到达时间差,通过比较理论值与实际值的偏差构建概率图像以实现定位解算。 通过有限元模拟分析了带洞结构对兰姆波传播路径的影响,给出信号传播路径的确定依据。 通过对 32 个撞击点的定位实验验证了定位方法的可行性,平均绝对误差为 1. 15 cm。     

弹簧钩断裂失效分析

弹簧在使用过程中,要承受交变应力的作用,故而对材料疲劳强度有着很高的要求。不当的热处理和加工工艺会严重降低弹簧的疲劳强度,从而使弹簧过早地产生疲劳断裂失效。本文使用扫描电子显微镜对弹簧断口进行观察分析,找出断裂原因,为改进工艺提供依据。     

Methods for characterizing plant fibers

The effectiveness of different microscopy techniques for measuring the dimensions of ultimate fibers from harakeke (Phormium tenax, New Zealand flax) was investigated using a factorial experimental design. Constant variables were geographical location, location of specimens along the leaf, season (winter), individual plant, a fourth leaf from a north-facing fan, age of plant, and cultivars (two). Experimental variables were microscopy techniques and measurement axis. Measurements of width and length of harakeke ultimate fibers depended on the microscopic preparation/technique used as well as the cultivar examined. The best methods were (i) transverse sections of leaf specimens 4 microm thick, embedded in Paraplast and observed using light microscopy, and (ii) nonfixed ultimate fibers observed using scanning electron microscopy.     

运用声发射技术监测金属塑性成型过程中润滑状态的研究

为监测金属塑性成型过程中的润滑状态,采用运用声发射技术,通过对无润滑和有润滑时金属塑性变形过程和摩擦过程的监测及对比,分别研究了金属摩擦声发射信号和塑性变形声发射信号.结果表明,不同材料摩擦产生的声发射信号数值上大小不同,同种材料摩擦声发射信号数值上小于塑性变形声发射信号;采用声发射技术,基于实时波形和声发射信号参数的平均值都能监测金属塑性成型时的润滑状态.