Acoustic Emission during powder Compaction and its Frequency Spectral Analysis

Abstract

Acoustic emission was detected during compaction of three different pharmaceutical materials: lactose monohydrate, microcrystalline cellulose and maize starch varying compressive forces between 0 and 60 kN. Acoustic emission signals were recorded on magnetic tape with a microphone and transformed to frequency spectra by using FFT-analysis.

After rough identification of frequency components each spectrum was divided into three bands. By using integrated band powers the acoustic activity on these bands could be compared quantitatively. Many frequency peaks were observed, too, and many of them could be identified.     

Analysis of Acoustic Emission Propagation in Metal-to-Metal Adhesively Bonded Joints

Acoustic emission (AE) monitoring shows promise as one of the most effective methods for condition monitoring of adhesively-bonded joints. Previous research has demonstrated its ability to detect, locate and classify adhesive joint failure, though in these studies little attention appears to have been paid to the differences in AE wave propagation through the bonded and un-bonded sections of the specimens tested, or to the effects of the wave modes excited or the propagation distances. This paper details an experimental study conducted on large aluminium sheet specimens to identify the effects of the presence of an adhesive layer on AE wave propagation. Three specimens are considered; a single aluminium sheet, two aluminium sheets placed together without adhesive, and an adhesively-bonded specimen. A pencil lead break (PLB) is used as a simulated AE source, and is applied to the three specimens at varying propagation distances and orientations. The acquired signals are processed using wavelet-transforms to explore time-frequency features, and compared with modified group-velocity curves based on the Rayleigh–Lamb equations to allow identification of wave-modes and edge-reflections. The effects of propagation distance and source orientation are investigated while comparison is made between the three specimens. It is concluded that while the wave propagation modes can be approximated as being constant throughout all three specimens, there is a significant change in the received waveforms due to the attenuation of high-frequency components exhibited by the bonded specimen. These findings may be utilised to provide a deeper understanding of acquired AE data, improving the current abilities to identify, locate and characterise damage mechanisms occurring within adhesive joints, ultimately improving safety in the use of adhesive bonding for critical applications.     

舰船地声信号的谱能量分析及相关性研究

分析浅水环境下舰船地震波信号和水声信号的功率谱比值（SAR）与频率的关系,以及它们的能量在频率上的差异。利用频域相关系数,分析两种信号的相关性。结果表明,在地震波信号很强的低频带（5—15Hz）和声信号很强的高频带（35—50Hz）,SAR值和相关系数都很高,而在两种信号能量强弱的过渡频带内SAR值和相关系数偏低。     

Use of Spectral Kurtosis for Improving Signal to Noise Ratio of Acoustic Emission Signal from Defective Bearings

The use of acoustic emission (AE) to monitor the condition of roller bearings in rotating machinery is growing in popularity. This investigation is centered on the application of spectral kurtosis (SK) as a denoising tool able to enhance the bearing fault features from an AE signal. This methodology was applied to AE signals acquired from an experimental investigation where different size defects were seeded on a roller bearing. The results suggest that the signal to noise ratio can be significantly improved using SK.     

涂层接触疲劳损伤过程中的声发射小波分析

采集铁基合金涂层在不同接触疲劳损伤阶段的声发射信号,并采用dB10基本小波对其进行5层小波分解和重构,分析了疲劳损伤声发射信号的波形和频率特征。结果表明:裂纹萌生阶段的原始声发射信号以连续型为主,裂纹的稳定扩展阶段以混合型为主,裂纹的失稳扩展阶段以突发型为主;通过小波变换实现了将疲劳损伤声发射信号与干扰波分离,获得了高信噪比的疲劳损伤特征信息;在不同的疲劳损伤阶段,声发射信号的频率分布各不相同。随着疲劳损伤的加剧,各层的波形幅值呈增大的趋势,并且疲劳损伤频率分布范围也更加的广泛。     

基于声发射和双谱分析的铝合金损伤原位监测研究

以列车车体材料7N01铝合金为研究对象,基于声发射和数字图像技术对7N01铝合金三点弯曲损伤过程进行了监测,采用传统的声发射参数与双谱法分析了7N01铝合金裂纹萌生及失稳扩展的声发射特征。结果表明:声发射能量和质心频率可有效预报7N01铝合金微裂纹的萌生。声发射信号的双谱等高线图显示两个频率成分之间的耦合关系,使得识别7N01铝合金三点弯曲过程中的不同阶段变得相对容易。7N01铝合金试样缺口尖端的损伤演变过程的数字图像监测结果,验证了声发射能量和质心频率对裂纹萌生的预测。实验结果显示声发射监测技术为裂纹演变行为的预测提供了依据。     

基于自相似性的混凝土损伤分析及声发射机理

声发射是一种能够动态反应材料微观结构变化的弹性波.对混凝土的声发射机理进行研究是混凝土损伤分析的有效方法.基于自相似方法对混凝土的声发射机理进行分析,研究了混凝土在轴向加载方式下的损伤过程.通过对比混凝土声发射的自相似特征值与应力之间的变化关系,将混凝土的破坏细化为一个内部结构不断被压密破坏的循环往复过程.研究结果表明,混凝土裂纹扩展处微裂纹的开闭导致混凝土应力呈现周期性的变化,使混凝土的声发射参数无法准确反映混凝土应力变化,在研究声发射参数与力学参量的定量关系时,应考虑微裂纹开闭对应力变化的影响.     

Vortex Reconnection and Acoustic Emission by the Numerical Analysis of the Gross-Pitaevskii Equation

Reconnection of quantized vortices and concurrent acoustic emission are studied numerically by analyzing the Gross-Pitaevskii equation. Although vortex reconnection was studied earlier by Koplik and Levine, the present work first investigates in more detail how the singular vortex cores reconnect. Second, acoustic emission, i.e., the emission of the density waves of the condensate, is studied when two vortices annihilate or reconnect. The local drastic change of the condensate density is found to work as an acoustic source to emit the density waves. This phenomenon may be related to the vortex tangle decay observed at mK temperatures and the eddy viscosity which is thought to be an intrinsic dissipative mechanism in a vortex tangle.     

GTAW焊接过程声音信号的分析

研究分析钨极氩弧焊（GTAW）时焊接气流,焊接速度,电弧长度和声音信号采集角度对采集到的声压信号的影响。结果表明：速度对声压影响最小;焊接声音随气体流增加而增加,当流量大于15L／min,声压信号趋于恒定;电弧长度在3．5—5mm之间时,声压随弧长变化显著,当超过5mm时变化不明显;声压随麦克风采集角的增加而增加,当大于60°时声压值变化不大,焊接声压强度在空间的分布近似于声源模型中的偶极子模型。     

Spectral Analysis of Pulsations in Flow Velocity Measured by an Acoustic Anemometer

The velocities of a gas-air flow measured by an acoustic anemometer, understood as a random process, are investigated. Such random components of the anemometer error as the measurement error and turbulent pulsations of the air flow are considered and their spectral composition is investigated. An experimentally obtained transient response of the acoustic anemometer along with a theoretical expression of the response in an approximation of the unit-step function by an inertial unit are presented.     

高频声表面波材料及器件制备工艺的研究

研究了磁控溅射和电子束蒸镀工艺对在LiNbO3基片上沉积铝膜的结构、表面形貌的影响。采用匀胶、曝光和反应离子刻蚀工艺，制备出中心频率在920MHz纵向谐振耦合换能器结构的声表面波滤波器。结果表明，电子束蒸镀的铝膜与LiNbO3单晶间具有较好的结合力，具有适合SAWF器件制备所需的微观结构，在随后的叉指换能器制备过程中表现出良好的加工性能和声传播特性。     

金刚石刀具研磨声发射信号的非高斯性特征分析

为确保金刚石刀具的制备效率和质量,采用声发射信号作为监测手段对金刚石刀具的精密研磨辅助进行过程调控.然而,该声发射信号的有效特征不能通过传统的信号处理方法辨识,实际研磨中仅能通过人工监听和操作经验定性判断.因此,本文从砂轮磨粒与刀具研磨面的微观接触特点出发,采用泊松过程模型对金刚石刀具研磨中声发射信号的激励机制进行了分析,阐明该信号具有非高斯性特征,采用线性系统理论阐明这一特征可被不失真地传输和接收,同时也给出了该信号高斯化近似的约束条件.最后通过工艺实验验证了该声发射信号的非高斯性特征在金刚石刀具研磨过程中的存在性,也证实了这一特征不受研磨速度和研磨压力等工艺参数的影响,而是与刀具研磨面的耐磨方向存在固有关联关系,从而为声发射信号监测在金刚石刀具研磨自动化系统中的合理应用奠定了方法基础.     

基于声发射信号参数分析的耐火材料损伤聚类研究

耐火材料声发射信号可分为单一突发型和叠加突发型两种,通过分析其波形特征,将上升时间、持续时间、幅度、能量、计数和质心频率作为区分这两种信号的特征参数,利用K均值聚类方法,实现耐火材料不同损伤类型的声发射信号分类。利用该方法对MgO-C耐火材料受压损伤信号进行分析,研究结果表明:单一突发型信号主要频率成分单一集中,信号主要呈现50～60kHz和150～160kHz两种频率段,叠加突发型信号主要频率成分为上两种频率成分的叠加;将信号分为3类,第1类和第2类信号反映了具有计数少、持续时间短和能量小特点的单一突发型信号,第2类信号的损伤强度大于第1类,第3类信号反映了具有计数多、持续时间长和能量大特点的叠加突发型信号。     

金属声发射信号三种时频域分析的比较研究

本文首先介绍了短时傅立叶变换、小波变换、二次型时频分布等三种时频域分析方法,并利用这三种分析方法分别对金属裂纹声发射信号进行了分析处理,对比研究了这三种时频域分析方法对金属裂纹声发射信号处理上的不同特点.     

温压技术及其应用

概述了温压技术的发展和致密化机制 ,并介绍了用温压技术制成的材料的应用。在此基础上 ,指出温压技术的技术关键以及在国内开展该技术研究的重要意义     

三维编织复合材料测试中声发射信号的分析与研究

论述了声发射技术在三维编织复合材料弯曲过程中的应用及实验方法,给出了声发射在三维编织复合材料中弯曲过程的特征.为了精确地知道材料的损伤部位,声发射源信号的采集非常重要.试验结果表明,考虑到声发射源定位研究的准确性,建议采用波形分析法.在声发射信号的采集过程中比较了参数分析法和波形分析法.根据信号的衰减程度,选择较准确的声发射源信号.利用小波变换的不同算法得到了三维编织复合材料较准确的声发射信号.     

频谱分析中用于相位和频率校正的相位差校正法

提出了一种对连续时域信号分前后两段作傅里叶变换,利用其对应离散谱线的相位差校正出谱峰处的准确频率和相位的新校正方法——相位差校正法,通过窗谱函数的公式还可以校正其幅值,以解决离散频谱分析中由于谱峰谱线没有对正峰顶时所带来的较大误差。该方法原理简单,通用性好,运算速度快,校正精度高,可以在不知道窗谱函数表达式的情况下,直接用其相位差进行求解。仿真研究表明,对单频率成分的频率、相位、幅值进行校正,频率误差小于0.0002个频率分辨率,相位误差小于0.1 度,幅值误差小于0.02% 。     

不同应变率下岩石冲击破坏的声发射特性研究

为了研究在动载荷下岩石破裂的声发射特性,采用霍普金森(SHPB)实验系统对三种岩石进行不同应变率下的冲击载荷破坏实验,同步采集破坏过程的声发射参数,从应力—应变、幅值分布、振铃计数变化以及峰值频率分布等方面进行了分析与研究。实验结果表明:力学特性方面,随着应变率增大,三种岩石的动态强度随之增大,极限应变也随之增大,且岩石试样的破碎程度随之增大,表现为碎块的尺寸减小、块数增加、碎屑增多。在声发射特性方面,三种岩石的峰值频率主要在550 k Hz以下,且随着应变率的增加,三种岩石的振铃计数都随之增大,低幅值信号比例增加,峰值频率在100 k Hz以下的低频信号随之减少,100~200k Hz以及400~550 k Hz的中高频信号随之增多,说明峰值频率有向中高频移动的趋势。