结构疲劳损伤的声发射检测可靠性研究

针对声发射技术对金属结构损伤的检测可靠性问题,以金属疲劳裂纹检测试验为基础,利用声发射特征参数的趋势分析和关联分析法,首先根据贝叶斯理论,计算了声发射对一定长度疲劳裂纹的检测概率;其次,研究了检测结构起裂时对应声发射幅度参数的阈值及其对损伤判断的影响。该研究可为减少声发射检测中的金属疲劳损伤信号误判,提高检测效率,以及确定复杂航空结构疲劳寿命提供支持。     

疲劳裂纹扩展声发射信号去噪盲分离

采用声发射技术评估疲劳裂纹扩展状态时,评估结论会受到其它类型声发射信号和噪声的干扰.针对上述问题,在分析经验模态分解和独立分量分析特点的基础上,提出集合导数优化经验模态分解与独立分量分析相结合的声发射信号去噪盲分离方法,用于疲劳裂纹扩展声发射信号的处理.分别进行模拟声发射信号和疲劳裂纹扩展试验,采用上述方法对采集声发射信号进行去噪盲分离,结果表明:基于集合导数优化经验模态分解与独立分量分析的声发射信号去噪方法可有效去除噪声信号的干扰,准确分离出疲劳裂纹扩展声发射信号,为进行含裂纹结构的疲劳损伤状态评估和剩余寿命预测奠定基础.     

声发射技术在航空维修中的应用进展

本文简要介绍了声发射检测的原理和适用范围,以及谐振式声发射传感器与光纤传感器的特点。并阐述了目前声发射检测技术在航空构件疲劳裂纹监测、复合材料监测以及飞机结构健康监测等方面的应用进展。     

基于声发射技术的金属高频疲劳监测

采用声发射技术监测高频疲劳条件下金属材料裂纹的扩展。介绍了如何运用软硬件处理的方法，从采集到的信号中分离出裂纹扩展的声发射信号。从处理后的声发射信号与观察得到的裂纹扩展对比来看，声发射参数的变化能够有效地反映材料疲劳裂纹扩展的过程，并且能更早地发现试样内部微小裂纹的变化。通过试验，得出了紧凑拉伸试样在裂纹稳定扩展阶段声发射信号能量率与应力强度因子幅值之间的关系式。     

转轴裂纹声发射监测研究

声发射技术作为一种无损检测方法,可以找到声发射信号和裂纹扩展之间的关系,分析材料变形与断裂机制。介绍了声发射技术用于转轴裂纹监测的研究,分析了声发射信号与转轴裂纹扩展之间的关系。实验表明采用声发射技术可以对转轴裂纹进行监测,证明了声发射技术在转轴监测上的可行性。     

基于声发射监测的压力容器钢裂纹损伤评价系统

基于Python语言设计并开发了一种声发射特征提取与损伤评价系统,实现了声发射特征的高效处理与损伤状态的准确评估。通过开展316LN不锈钢的疲劳裂纹扩展的声发射监测试验,对疲劳过程中的声发射信号进行处理和分析,结果表明,多维度声发射参量的经历分析和聚类分析能够有效反映裂纹扩展过程的声发射活动情况。该评价系统能够为工程结构的声发射监测与损伤评价提供参考。     

金属裂纹扩展声发射信号特性研究

机械结构件中的应力集中使得结构极易产生裂纹并逐渐扩展,裂纹扩展时伴有声发射信号,因此有必要对结构件中裂纹扩展时的声发射信号进行特征研究。为研究金属板件中的裂纹声发射源特征行为,通过分析板件中的裂纹声发射源,从理论上推导了裂纹声发射的幅值和频率特征表达式。在裂纹扩展过程中,金属板件的裂纹声发射信号幅值与声发射源的开裂长度和拉伸应力的乘积成正比,频率与裂纹开裂速度成反比,且与裂纹开裂长度成正比。用声发射检测系统对预制有初始裂纹的金属板件进行拉伸状态下的声发射监测,通过对声发射信号求取功率谱密度估计,实现不同声发射信号以功率谱在频域的分布为信号特征的有效区分。实验结果与理论分析相符合,研究结果对金属板件的裂纹声发射检测技术具有重要意义。     

基于信息熵的轧辊裂纹声发射三维定位方法

在平整机的轧制过程中,轧辊表面承受巨大的接触应力、交变应力和摩擦力作用,易产生裂纹。在复杂载荷的作用下,裂纹会成为疲劳核心,其尖端的应力急剧增加并导致裂纹扩展,严重时会造成断辊事故发生。同时,裂纹的不断扩展伴随着应力波的释放,产生声发射(Acoustic emission,AE)现象。利用声发射技术对平整机轧辊裂纹进行定位,不需要全辊身进行逐点扫描,检测效率高,实时性好。针对平整机轧辊的声发射三维定位中存在的定位"多源性"干扰问题,提出基于信息熵的轧辊裂纹声发射三维定位方法。利用信息熵对声发射参数进行权重的量化,进而获得评价轧辊状态的综合指标;根据正常辊综合指标的分布,确定干扰源的指标范围;利用此范围将裂纹辊中的干扰源有效剔除,进而提高裂纹辊的定位精度。利用该方法对某钢厂冷轧平整机的正常辊和裂纹辊的试验数据进行分析,处理前后的结果验证了方法的有效性,为准确及时地检测出裂纹的位置信息、保证轧辊的安全生产提供有力的数据支持。     

声发射在桥式起重机主梁疲劳裂纹检测中的应用

起重机主梁结构复杂,体积庞大,不利于传统无损检测技术的开展。声发射检测技术可在动态条件下,对一定距离内的活性裂纹缺陷进行检测。文中利用声发射检测技术对轻量化桥式起重机主梁进行疲劳试验,尝试通过时差法对声发射源信号进行粗略定位,并将裂纹扩展情况与声发射计数及幅值进行分析,对今后的研究工作具有指导意义。     

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。     

Identification of fretting fatigue crack propagation mechanisms using acoustic emission

In this study, a fretting fatigue test has been equipped with an acoustic emission (AE) device in order to identify the successive crack propagation mechanisms. The fretting fatigue crack nucleation and propagation is a complicated process. Cracks initiate and propagate firstly due to shearing (mode II) and then by tension (mode I). The crack propagation generates mechanical energy emission. Elastic waves appear and can be detected through AE. A complete analysis of the AE signals (multi-parameter analysis, location of the AE in the loading cycle and a statistical analysis) led to an identification of three different steps in the crack propagation process. The evolution of the shearing and the tension influences in the crack propagation process is recognizable separately. Therefore, the three crack propagation steps have been identified as (a) crack propagation in mode II, (b) mixed mode crack propagation and (c) pure mode I crack propagation.     

Acoustic sources from damage propagation in Ti grade 5

In this paper fatigue test results are presented for titanium grade 5 specimens subjected to uniaxial cyclic loads. The material behavior of titanium was studied by comparing the Acoustic Emission (AE) detection technique with the Infrared Thermography (IT). The AE technique consists on attaching a couple of piezoelectric sensors on the surface of the sample in order to allow real-time recording of acoustic activity occurring in the material during the test and to localize the acoustic source, based on the assumption that part of the acoustic activity depends on the crack propagation process. All typical data of AE were collected during the tests and some of them were properly post-processed by using filters or derivative functions in order to better understand the crack propagation phenomenon. At the same time, thermographic analysis was also carried out during the experiments by continuous monitoring of surface temperature of the sample. Results of the fatigue behavior of the analyzed material beside the acoustic emission track and the thermal images of the sample are analyzed and critically discussed, in order to assess the capability of each technique in predicting the imminent failure of material.     

A new methodology for predicting crack initiation life for rolling contact fatigue based on dislocation and crack propagation

A new methodology for predicting crack initiation life is presented and validated experimentally. The methodology considers that the total fatigue life is the summation of crack initiation life and crack propagation life, since fatigue failures are due to crack initiation and crack propagation. It has been established that the crack propagation life can be estimated based on a modified Paris’ law when the size of crack is larger than a certain value. However, there has been no verified method for estimating the crack initiation life with good accuracy. The proposed methodology for predicting the crack initiation life is based on a dislocation model, and the constants for the model are determined by the crack initiation lives obtained by a new approach. This new approach determines the crack initiation life by subtracting the predicted crack propagation life from the experimentally obtained total fatigue life. The developed crack initiation life model is combined with a crack propagation life model for the prediction of fatigue life. It is noted that the standard deviation in the ratios of experimental life to predicted life by the developed fatigue life model is only 14% of that by the International Standard.     

Characteristics of elastic waves generated by fatigue crack penetration and growth in an aluminum plate

The characteristics of elastic waves emanated from crack initiation in 6061 aluminum alloy subjected to fatigue loading are investigated through experiments. The objective of the study is to determine the differences in the properties of the signals generated from fatigue test and also to examine if the sources of the waves could be identified from the temporal and spectral characteristics of the acoustic emission (AE) waveforms. The signals are recorded using nonresonant, flat, broadband transducers attached to the surface of the alloy specimens. The time dependence and power spectra of the signals recorded during the tests were examined and classified according to their special features. Six distinct types of signals were observed. The waveforms and their power spectra were found to be dependent on the crack propagation stage and the type of fracture associated with the signals. The potential application of the approach in health monitoring of structural components using a network of surface mounted broadband sensors is discussed.     

一种基于裂尖损伤区的裂纹扩展模型

提出了“裂尖损伤区”的概念,通过解析闭合模型对裂尖损伤区的应力应变历程进行了分析,应用累积损伤理论建立了一个用于分析变幅载荷下疲劳裂纹扩展寿命的模型。应用本模型计算了不同载荷谱下的裂纹扩展寿命,通过与试验结果的比较,表明本文的方法具有精度高、经济实用的特点,适合工程应用。     

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.     

含缺口结构压疲劳失效研究

结构的疲劳失效与内部的应力水平有着紧密的联系。这里阐述了压应力作用下结构内部裂纹的萌生和扩展机理，并介绍了循环压应力下的裂纹扩展计算方程，结合现代疲劳研究方法的新成果新方法对压应力条件下疲劳破坏研究的未来提出展望。     

16Mn钢焊接件疲劳裂纹扩展的微观特征研究

研究了在循环载荷作用下,16Mn钢焊接件疲劳裂纹的扩展情况.通过光学显微镜观察了裂纹扩展的微观特征,发现了裂纹在焊接结构不同部位的扩展路径方式并分析了显微组织对裂纹扩展的影响.裂纹在母材处为穿晶扩展,在热影响区和焊缝金属中的扩展为穿晶和沿晶混合型.     

Fatigue crack propagation behavior at a notch for needled C/SiC composite under tension-tension loading

Fatigue test of a needled C/SiC composite with a notch under tension-tension cyclic loading was completed, and the main fatigue crack propagation curve of the needled composite was obtained by the in situ observation of the fatigue process. By analyzing the influence of the failure number and distribution on the tensile loading subjected by 0° fiber bundles, the relationship between the main fatigue crack propagation and the distribution of 0° fiber bundles in the needled composite was established. By observing the fracture microstructure (especially the distribution of 0° fiber bundles) of the needled composite through scanning electron microscopy, the reasons for the varying fatigue resistance of different notched specimens were also explained. In addition, acoustic emission (AE) was also used to analyze the AE energy characteristics during the fatigue crack propagation process of the needled composite.