Analysis of acoustic emission during martensitic transformation of CuZnAl alloy by measurement of forces

A special composition of sample transducer is presented by which forces corresponding to AE microevents of martensitic transformation in CuZnAl alloy can be measured.     

超声双折射法测试铝合金的内部应力

声各向同性的金属材料在应力作用下表现出声各向异性,这是用声弹性法分析材料内部应力的基础。本文用偏振方向平行或垂直于应力方向的超声纯横波对LY11型铝合金进行测试。实验结果表明:材料在拉、压单轴应力作用下,偏振方向平行和垂直于应力方向的超声纯横波的声速都发生了变化。实验在分析材料声各向异性的基础上计算材料声弹性双折射系数,得到测试LY11型铝合金内部应力的理论公式,并对其内部的残余应力进行评估。实验中利用双换能器回振法测量声速,时间测试精确度可达10^-11s,可精确测量声速的微小变化量。     

Wave phenomena during creep in macrocrystalline aluminum

A wave model of plastic flow, which has been theoretically substantiated and experimentally verified under the conditions of active quasistatic loading of diverse materials, is being developed on the basis of concepts of the autocatalytic nature of elementary acts of plastic deformation. Data from the study of the evolution of distortion fields during low-temperature creep of macrocrystalline aluminum are given in order to explain the tighter relation between the parameters of plastic-deformation waves and the characteristics of the elementary processes of plastic shear. The wave nature of this evolution is emphasized and a linear correlation is found between the creep rate and the velocity of the plasticity waves. The activation volumes of the processes controlling the velocity of the plastic waves and the creep rate are shown to be correlated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 5–9, April, 1991.     

Characterization of acoustic cavitation in water and molten aluminum alloy

High-intensive ultrasonic vibrations have been recognized as an attractive tool for refining the grain structure of metals in casting technology. However, the practical application of ultrasonics in this area remains rather limited. One of the reasons is a lack of data needed to optimize the ultrasonic treatment conditions, particularly those concerning characteristics of cavitation zone in molten aluminum.The main aim of the present study was to investigate the intensity and spectral characteristics of cavitation noise generated during radiation of ultrasonic waves into water and molten aluminum alloys, and to establish a measure for evaluating the cavitation intensity. The measurements were performed by using a high temperature cavitometer capable of measuring the level of cavitation noise within five frequency bands from 0.01 to 10 MHz. The effect of cavitation treatment was verified by applying high-intense ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al–17Si alloy. It was found that the level of high frequency noise components is the most adequate parameter for evaluating the cavitation intensity. Based on this finding, it was concluded that implosions of cavitation bubbles play a decisive role in refinement of the alloy structure.     

Low-stabilty states of aluminum crystal lattice and acoustic emission under high-temperature straining

Using aluminum as an example it is experimentally established that strain accumulation under low-stability condition at the temperatures higher than 0.5 melting temperature exhibits a non-monotonic behavior and represents sharp deformation changes within a wide range of scale levels (including macroscale). The effects of this stepwise deformation are accompanied by high-amplitude acoustic emission signals. An analysis of the anomalous metal behavior and acoustic emission shows that the activation volume of an elementary deformation event under this low-stability state is exponentially increased with the deformation temperature, indicating an increased scale of cooperative atomic displacements and manifestation of low stability of the crystal lattice of this atomic configuration. The macroscopic character of this stepwise deformation provides evidence on a correlation between elementary deformation events within a volume larger than one strain band. It is assumed that the amplitude of an acoustic signal can serve a criterion of this correlation.     

低频声波的定向辐射

实现低频声波在空间中的定向传播,是一个迫切需要解决的关键科学问题。为实现低频声波的定向传播,要考虑影响声波指向性的因素：声源辐射面积、辐射面结构和所辐射声波的频率。而影响声波传播的因素有：(1)媒质的性质,例如声阻抗、声衰减;(2)边界条件,不同的边界会产生声波不同的传播模式,例如点源在不同边界附近的辐射表现(点声源在声学硬边界表现为单级子的指向性,即全指向性;点声源在声学软边界为偶极子的指向性,呈现为八字型,具有一定的指向性);(3)结构的控制,例如通过米氏共振结构来控制低频声波的传播。文章简要介绍了影响声波传播的因素和控制声波传播的方法,并主要从边界条件和结构控制两方面来说明如何实现低频声波的定向辐射。     

Study of acoustic emission signals during fracture shear deformation

We study acoustic manifestations of different regimes of shear deformation of a fracture filled with a thin layer of granular material. It is established that the observed acoustic portrait is determined by the structure of the fracture at the mesolevel. Joint analysis of the activity of acoustic pulses and their spectral characteristics makes it possible to construct the pattern of internal evolutionary processes occurring in the thin layer of the interblock contact and consider the fracture deformation process as the evolution of a self-organizing system.     

Multiscale modeling approach to acoustic emission during plastic deformation

We address the long-standing problem of the origin of acoustic emission commonly observed during plastic deformation. We propose a framework to deal with the widely separated time scales of collective dislocation dynamics and elastic degrees of freedom to explain the nature of acoustic emission observed during the Portevin-Le Chatelier effect. The Ananthakrishna model is used as it explains most generic features of the phenomenon. Our results show that while acoustic emission bursts correlated with stress drops are well separated for the type C serrations, these bursts merge to form nearly continuous acoustic signals with overriding bursts for the propagating type A bands.     

铝与钢摩擦时声发射参数与正压强之间的关系

材料不同,声发射信号的特点不同。为了提高声发射技术在监测摩擦过程中的应用水平,研究不同材料的声发射信号的特点是必要的。本文研究了铝与45钢摩擦时产生的声发射信号的特点,实验研究结果表明:在铝与45钢的摩擦过程中,相对运动速度对于正压力与声发射信号之间的关系有较大的影响;低速下,正压力与声发射信号之间无规律可言;高速下,随着正压力的增加,声发射信号各参数是增加的。     

低频水下声信号的激光探测

根据表面波声光效应的原理,提出了一种低频水下声信号的激光探测技术,并建立了实验装置。在几十赫兹的低频段,对水下声源所产生的表面声波进行了探测。实验过程中,利用MATLAB软件对拍摄的衍射图样进行扫描分析,得到了衍射图样中条纹的像素差。根据波长与条纹间距的解析关系式,得到了低频液体表面声波波长,其大小在毫米量级。利用计算机编程,根据最小二乘法拟合色散关系的回归曲线,测量结果与理论色散关系吻合。该方法具有实时、非接触的特点。     

Scaling of the acoustic emission characteristics during plastic deformation and fracture

The scaling of the amplitude and time distributions of acoustic emission pulses, which reflects the self-similarity of defect structures, is revealed. The possibility of separation of independent contributions to the flow of acoustic emission events, which have substantially different scaling exponents, is shown for porous materials. The differences in the scaling exponents are related to the development of plastic deformation and fracture of the materials. The developed approach to an analysis of acoustic emission can be used to describe its predominant mechanisms during deformation.     

Least-error localization of discrete acoustic sources

We describe a new, least-error method for locating a discrete acoustic source (which generates a radially symmetric, outgoing wave) based on time-of-arrival data. This method localizes the source by minimizing the sum of the absolute values of the differences between the squares of the theoretical and actual times of arrival. The method is suited to noisy data, and whenever the errors in the data are unbiased, the more times of arrival used, the greater the expected accuracy of localization. The method is simplest for two dimensional data, requiring only elementary algebra. By means of simulations, we demonstrate the amelioration of localization with the number of times of arrival employed: the average inaccuracy falls asymptotically as the reciprocal of the square root of this number. The new method also yielded more accurate localization, on the average, than a least-square method. We make direct comparison with time-difference-of-arrival localizations, both for simulated data and for experimental data collected at a shooting range, demonstrating the favorability of the new method. We also demonstrate its facilitation of the localization of multiple, cotemporary sources: via partitioning of the data. Our method is suited to sensor networks with computationally empowered nodes.     

Characterization of acoustic streaming in water and aluminum melt during ultrasonic irradiation

It is well known that ultrasonic cavitation causes a steady flow termed acoustic streaming. In the present study, the velocity of acoustic streaming in water and molten aluminum is measured. The method is based on the measurement of oscillation frequency of Karman vortices around a cylinder immersed into liquid. For the case of acoustic streaming in molten metal, such measurements were performed for the first time. Four types of experiments were conducted in the present study: (1) Particle Image Velocimetry (PIV) measurement in a water bath to measure the acoustic streaming velocity visually, (2) frequency measurement of Karman vortices generated around a cylinder in water, and (3) in aluminum melt, and (4) cavitation intensity measurements in molten aluminum. Based on the measurement results (1) and (2), the Strouhal number for acoustic streaming was determined. Then, using the same Strouhal number and measuring oscillation frequency of Karman vortices in aluminum melt, the acoustic streaming velocity was measured. The velocity of acoustic streaming was found to be independent of amplitude of sonotrode tip oscillation both in water and aluminum melt. This can be explained by the effect of acoustic shielding and liquid density.     

Nonlinear acoustic properties of the B95 aluminum alloy and the B95/nanodiamond composite

Research results for the nonlinear acoustic properties of the B95 polycrystalline aluminum alloy and the B95/nanodiamond composite have been described. The nonlinear properties of the alloys have been studied by the spectral method that measures the efficiency of generation of the second harmonic of a bulk acoustic wave at a frequency of 2f = 10 MHz in the field of a finite-amplitude longitudinal acoustic wave at a frequency of f = 5 MHz. The results derived by this method have been compared with the results of studies of the nonlinear acoustic properties of the test alloys using the Thurston–Brugger quasi-static method.     

Effects of sediment acoustic characteristics on low-frequency acoustic field distribution in shallow water

Most of the published literatures on low-frequency underwater sound propagation are focused on the sound propagation features in the water column,while studies on sound propagation features in the sediment layer or the semi-infinite basement are rare.In this paper,based on the wave equation,a computational model for sound energy flux in the sediment layer and the basement as well as in the water column is proposed under a cylindrical coordinate system.On this basis,the effects of various sediment acoustic parameters on the sound energy distribution and the corresponding mechanisms are elaborated through numerical examples and acoustic theory.Simulation results reveal that,in a situation where sediment P-wave speed>water sound speed> sediment S-wave speed,the greater the values of density and P-wave speed in sediment,the more likely it is that the sound energy remains in the water column without leaking to the sea floor.Conversely,the influence of the variation of S-wave speed is reversed.Basement influence on the sound propagation in the fluid layer is approximately negligible if the sediment layer is sufficiently thick.     

Constructive and destructive interference in acoustic emission during fatigue crack propagation

Fatigue crack propagation monitoring with acoustic emission reveals the presence of maxima and minima in the curve of the count rate as a function of the stress intensity factor. This is explained in terms of constructive and destructive interference due to specimen geometry. A theoretical model is developed and experimental proof of the validity of the model is presented.