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.     

声发射试验的中碳钢构件疲劳裂纹的仿真模型

通过Abaqus建立中碳钢构件的有限元模型,在给定初始裂纹的基础上,利用扩展有限元法(XFEM),仿真得到裂纹扩展过程中塑性应变能和弹性能的变化曲线。结合中碳钢试件的声发射试验,处理试验数据得到裂纹扩展过程中声发射信号的绝对能量曲线,以及通过红外热像仪的记录得到裂纹扩展过程中温度变化曲线。仿真得到的弹性能与试验中得到的能量曲线具有相同数量级,同时根据仿真得到的塑性应变能反向推导中碳钢最终断裂时温度变化显著的区域尺寸,与红外热像仪记录的温度变化显著的区域尺寸相符。试验得到的结果与仿真得到的结果基本吻合,这就为中碳钢疲劳寿命预测的声发射试验提供了理论依据。     

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。     

机体飞行载荷疲劳试验中的声发射特性分析

机体飞行载荷疲劳试验是全尺寸飞机疲劳试验的最关键部分,主要目的是暴露机体结构疲劳薄弱部位和疲劳损伤关键部位,确定这些位置疲劳裂纹的扩展特征。论文介绍利用声发射技术对应力集中且可达性差的关键部位进行实时监控的细节,通过分析声发射特性,及时得到监测区域内裂纹萌生扩展的起始时间和位置的信息。详细叙述在高背景噪声环境下如何获取有用声发射信号的方法,包括使用谐振式传感器,采用空间滤波技术、基于统计平均的特征参数趋势分析和基于幅度的相关参数趋势分析的相互验证,以及采用载荷滤波、幅度滤波和空间滤波相结合的综合滤波方法。结果表明,这种声发射信号快速处理方法成功的实时监测到螺栓孔周围裂纹的萌生和扩展过程。     

利用声发射技术监测低温环境下轴承钢的损伤

在轴承的重载接触区域经常出现疲劳损伤，深入了解疲劳损伤的起因和发展对于预测轴承零件的寿命具有重大意义。选择用声发射法监测低温下接触疲劳试验中的裂纹起源和扩展过程，通过分析试验中的声发射信号即可确定显微疲劳的起源和扩展过程。     

风力机叶片疲劳裂纹AE信号的小波变换优化方法

为实现风力机叶片的及时有效地监测和维护,使用声发射技术采集疲劳裂纹信号,从而提取裂纹特征。而声发射信号的突发性和冲击性需要具有时频分析能力的信号处理方式来提纯和降噪,小波变换方法作为常用的时频处理方式油漆有效性,但是现有的小波基函数不足以适应该信号的分析。提出基于Shannon熵理论计算疲劳裂纹扩展的声发射信号的小波基函数带宽参数,得到最适合此裂纹声发射信号的Morlet小波基函数,计算优化基函数的小波,获得风力机疲劳裂纹特征成分在时间尺度平面的高幅值能量分布。实验研究表明,优化小波基的方法具有很好的时频聚集性和抗噪能力,实现了风力机叶片裂纹声发射信号的时频特征清晰准确的提取。     

声发射信号降噪及在裂纹检测中的应用研究

针对采用声发射技术评估疲劳裂纹断裂状态时存在噪声干扰,以及采用小波包阈值降噪方法或集合经验模态分解降噪方法处理信号时会受到阈值选取、模态分量等因素的影响等问题,对上述两种降噪方法的工作原理进行了分析。提出了将两者相结合的声发射信号降噪方法,根据声发射信号处理流程可分为IMF-WPT降噪方法、WPT-EEMD降噪方法和EEMD-WPT降噪方法;利用以上3种降噪方法对同一模拟含噪声疲劳断裂声发射信号进行了降噪处理,分别计算了降噪处理后信号的信噪比和失真度;分别进行了金属材料和焊接结构疲劳裂纹扩展试验,利用WPT-EEMD降噪方法对采集声发射信号进行了降噪处理。研究结果表明:WPT-EEMD降噪方法降噪后的信号信噪比为10.32,失真度为0.31%,降噪效果最好;该方法可以有效去除高频噪声的干扰,提高分析结果的可靠性。     

风力机叶片裂纹的动态扩展特性研究

首先推导了风力机叶片裂纹尖端塑性区的应力强度因子,确定了影响裂纹扩展的主要因素,然后根据裂纹扩展时的声发射信号特性,探讨了声发射信号的特征提取方法。在试验中分析了不同试件的裂纹形貌特性,并针对采集的裂纹扩展声发射信号,比较了不同小波基函数对声发射信号特征提取的影响,提取了裂纹动态扩展的特征频率,从而建立了声发射特征参数与声发射源之间的关联机制,为风力机叶片状态监测和早期预警提供理论依据。     

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。     

声发射技术在疲劳裂纹检测中的应用

主要讨论了声发射技术应用于疲劳裂纹的检测.根据疲劳裂纹的信号特征及测试场合,选用合适的疲劳裂纹检测方法,结合板簧疲劳裂纹试验机上的实验分析,得出声发射技术应用于疲劳裂纹检测的优势.     

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.     

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。     

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.     

Wechselbiegeapparatur für die in situ-Untersuchung von Zerrüttungsvorgängen im Rasterelektronenmikroskop

An SEM stage (Fig. 1 and 2) is described which allows bending fatigue of metallic specimens in the vacuum of the specimen chamber. With the aid of inductive displacement pick-ups the angle of bending can be measured (Fig. 3) as well as the momentum of bending (Fig. 4). Thus the mechanical hysteresis can be registrated and the fatigue of the specimen can be controlled. Fig. 5 shows the geometry of the fatigue specimen, which has an electrolytically polished surface area of about 6 × 3 mm² on which the development of deformation topography can be studied during the fatigue experiment. The main advantages of the new fatigue apparatus are:

• 1 No interruption of cyclic straining is necessary for the investigation of the specimen surface.
• 2 The specimen is in vacuum during the complete experiment so that environmental influences are reduced.
• 3 The development of surface topography can be monitored continuously.
• 4 Differences of the topography (e. g. fatigue cracks) in the phase of tension and compression can be detected.

In the case of fatigue of polycrystalline metals, fatigue cracks often start at the surface of the specimen in slipbands produced by the cyclic straining. Fig. 6 shows typical fatigue slipbands with extrusions and intrusions for pure Al. A slipband crack in a later stage of fatigue is shown in Fig. 7. For the investigation of the distribution of fatigue cracks at the surface the possibility of watching the surface in different phases of bending is a great help. The cyclic crack opening – which depends on the crack depth – can be seen by eye on the TV-screen and can be recorded on video tape when the frequency of bending is about 1 cycle/s or can be photographed from the record display at very low frequencies of bending. Fig. 8 gives an example of the difference in crack opening in the phase of tension and compression of the specimen surface respectively.     

基于小波能谱系数的风力机疲劳裂纹特征

提出了风力机叶片裂纹扩展声发射信号的优化小波重分配尺度谱及小波能谱系数相结合的分析法。基于Shannon熵理论计算裂纹扩展声发射信号的重分配尺度谱小波基函数带宽参数,得到最适合裂纹声发射信号的Morlet小波基函数。用优化后的小波基函数计算重分配尺度谱,获得裂纹扩展特征成分在时间尺度平面的高幅值能量分布,利用特征能谱系数表征其重分配尺度谱的特征。实验结果表明,该方法有良好的时频聚集性和抗噪能力,实现了风力机叶片裂纹扩展声发射信号的时频特征提取,得到了能谱系数作为特征向量表示信号特征。该方法可用来实现风力机叶片在复杂环境中的模式识别。     

基于图像处理的疲劳裂纹扩展长度在线测量方法

为了在金属疲劳裂纹扩展试验中采用直观、准确、可靠的方法获得高精度裂纹长度尺寸,提出了基于图像处理技术的疲劳裂纹扩展长度在线测量方法。首先,采用大尺寸黑白全帧面阵CCD和微距显微镜头对疲劳裂纹扩展过程中的图像进行采集,分析疲劳裂纹的图像形态特征,采用亚像素边缘定位方法对预制裂纹边缘进行检测,求出疲劳裂纹起点位置。然后,对裂纹扩展区域进行图像子区划分,对图像子区采用基于灰度统计数据的自适应分割方法进行裂纹分割,采用单区域增长算法进行二值化裂纹图像连接得到裂纹主干。最后,进行了系统标定和疲劳裂纹长度在线测量试验。试验结果表明：本方法测量精度高、抗干扰能力强、数据稳定可靠,裂纹长度测量精度为0.03mm,满足裂纹扩展试验国际标准的要求。     

铝合金点焊拉剪接头疲劳性能及寿命分析

研究了铝合金AA5754点焊拉剪接头的疲劳性能,获得了不同厚度试件的载荷寿命曲线。研究了铝合金焊点的疲劳失效模式,讨论了焊点疲劳裂纹的扩展形式,并测量了裂纹扩展路径与点焊熔核界面之间的角度。分析了点焊拉剪试件在同时承受I型和II型载荷时,疲劳裂纹的扩展方向,并与测量值进行了比较。利用疲劳破坏后沿铝板厚度方向的实际裂纹长度修正了裂纹尖端的局部应力强度因子,提出了评价焊点寿命的疲劳参量K(i),并对试验数据进行了分析比较。结果证明K(i)可以有效关联试件尺寸效应和焊点疲劳寿命,能够用于预测焊点疲劳寿命。     

低载强化后汽车结构件疲劳裂纹扩展速度研究

为了从微观上揭示汽车结构件经过低载强化后疲劳强度和疲劳寿命提高的原因和机理,本文利用扫描电镜,对某汽车前轴低载强化前后试样断口的疲劳裂纹扩展速度进行了对比研究。研究结果表明,经过低载强化后的汽车结构件断口试样,其贝纹线的增长趋势减弱,疲劳辉度间距变小。这些微观变化有效地延缓了疲劳裂纹萌生,降低了疲劳裂纹扩展速率,使汽车结构件的疲劳强度和疲劳寿命都得到提高。     

A Comparative Study of Residual Stress Distribution Induced by Hard Machining Versus Grinding

This study investigates the residual stress distribution induced by hard machining and grinding and compares its impact on fatigue parameters. The residual stress distribution below hard turned and ground surfaces is investigated after a thermally damaged layer is removed. Fatigue parameters are computed based on the residual stress distribution to compare the impact of the residual stress distribution on the fatigue performance. Rolling contact fatigue tests are then performed to substantiate the computations. The effect of residual stresses on crack initiation depth is shown to be significant for the ground specimen. The maximum shear stress at crack initiation depth of the hard turned specimen is smaller than that of the ground specimen. Due to a significant increase in crack initiation life, the predicted rolling contact fatigue life of the hard turned specimen is longer than that of the ground specimen. The overall average in the ratios of predicted life to experimental life for the hard turned specimen is closer to 1 than that for the ground specimen. The results demonstrate that the hard turned specimen shows better rolling contact fatigue performance and better accuracy in the fatigue life prediction.