Acoustic monitoring of the cluster test facility 20 MJ magnet system: a record of disturbance emission and training

Recently, during the second major operation of the Cluster Test Facility at the Japan Atomic Energy Research Institute (JAERI), the two cluster test coils were monitored acoustically. This represents the largest magnet system (20 MJ) to date monitored using acoustic techniques.Plots of the acoustic emission (AE) amplitudes and AE counting rate versus operating current are reported for the three current excursions to 2145 A. The AE amplitude curves show the presence of some types of disturbance and reduction in the disturbance during successive runs. A strong tendency for the magnitude of the AE counting rate to rise over the high current charging region after successive runs was observed and is attributed to a change in the way energy is released during magnet training from larger single events to more numerous smaller events. An interesting sudden drop in the AE rate, lasting 5 – 10 s, was also noted during each run, and the redistribution of stress after a sudden semi-global shift in the winding is suggested as the cause of this phenomena.     

Acoustic emission monitoring of activation behavior of LaNi5 hydrogen storage alloy

Abstract

The acoustic emission technique is proposed for assessing the irreversible phenomena occurring during hydrogen absorption/desorption cycling in LaNi₅. In particular, we have studied, through a parametric analysis of in situ detected signals, the correlation between acoustic emission (AE) parameters and the processes occurring during the activation of an intermetallic compound. Decreases in the number and amplitude of AE signals suggest that pulverization due to hydrogen loading involves progressively smaller volumes of material as the number of cycles increases. This conclusion is confirmed by electron microscopy observations and particle size distribution measurements.     

Acoustic emission monitoring of activation behavior of LaNi5 hydrogen storage alloy

The acoustic emission technique is proposed for assessing the irreversible phenomena occurring during hydrogen absorption/desorption cycling in LaNi₅. In particular, we have studied, through a parametric analysis of in situ detected signals, the correlation between acoustic emission (AE) parameters and the processes occurring during the activation of an intermetallic compound. Decreases in the number and amplitude of AE signals suggest that pulverization due to hydrogen loading involves progressively smaller volumes of material as the number of cycles increases. This conclusion is confirmed by electron microscopy observations and particle size distribution measurements.     

Acoustic emission from superconducting magnets

We have shown experimentally that the acoustic emission (AE) is an effective means of monitoring the operating conditions of superconducting magnets and can be used to prevent catastrophic quenching. the signal-to-noise ratio of AE transducers at liquid helium temperature has been found to be about 18 dB higher than that at room temperature; AE originating from a superconducting magnet and those from the liquid helium shower can be clearly discriminated by observing their wave forms and frequency spectra. These results are also discussed from a theoretical view point based on an elastic medium model for the superconducting magnet.     

An effective sensor for tool wear monitoring in face milling: Acoustic emission

Acoustic Emission (AE) has been widely used for monitoring manufacturing processes particularly those involving metal cutting. Monitoring the condition of the cutting tool in the machining process is very important since tool condition will affect the part size, quality and an unexpected tool failure may damage the tool, work-piece and sometimes the machine tool itself. AE can be effectively used for tool condition monitoring applications because the emissions from process changes like tool wear, chip formation i.e. plastic deformation, etc. can be directly related to the mechanics of the process. Also AE can very effectively respond to changes like tool fracture, tool chipping, etc. when compared to cutting force and since the frequency range is much higher than that of machine vibrations and environmental noises, a relatively uncontaminated signal can be obtained. AE signal analysis was applied for sensing tool wear in face milling operations. Cutting tests were carried out on a vertical milling machine. Tests were carried out for a given cutting condition, using single insert, two inserts (adjacent and opposite) and three inserts in the cutter. AE signal parameters like ring down count and rms voltage were measured and were correlated with flank wear values (VB max). The results of this investigation indicate that AE can be effectively used for monitoring tool wear in face milling operations.     

Acoustic emission for in situ monitoring in metal-matrix composite processing

The attractive elevated-temperature properties of metal-matrix composities (MMCs) have not been exploited in commercial applications partly because of the high processing cost and lack of reliability in fabrication. In this exploratory study, the feasibility of using acoustic emission (AE) as an in-process, non-destructive quality control technique is examined. A variation of the squeeze casting technique is selected for investigation. Acoustic emission is employed with the intent of non-intrusively establishing whether complete infiltration has occurred during composite fabrication. The problems due to the background noise during AE monitoring are overcome by using transducers with different frequency responses. The acoustic signatures of machine noise, preform crushing and metal solidification are obtained by employing suitable transducers in a series of tests that systematically evaluate the individual processes that comprise infiltration casting. The results form a strong basis for the development of an in situ AE sensor for the infiltration process.     

Experiences on high current leads for superconducting magnets; seven types from 1 kA to 30 kA

In order to extend the current lead techniques to higher currents, the operational minimum flow rates of leads from 1 to 30 kA were measured and compared with the design value.     

Acoustic emission of composite materials in tensile tests at cryogenic temperatures

Tensile tests of fibre reinforced plastics are performed at cryogenic temperatures and simultaneously acoustic emission (AE) is observed to examine the microscopic deformation and fracture processes of these materials. AE behaviours at liquid helium temperature are different from those at liquid nitrogen temperature, although the mechanical behaviours are similar. From these results, correlations of the AE sources with the microscopic deformation and fracture processes are discussed.     

Acoustic emission and inclusion fracture in 7075 aluminum alloys

For slow crack growth (da/dn 0.1 µm per load cycle) in 7075-T6 aluminum, quantitative agreement was found between the amplitude distribution of burst acoustic emissions and the area size distribution of intermetallic inclusions 10 (µm)² in area as measured on thepolished fracture face. This observation permits the prediction of the amplitude distribution of acoustic emissions due to crack growth in a particular sample of 7075-T6 aluminum directly from a simple, standard metallographic observation performed on the material in question. It was also found that a reduction of the yield stress from that of 7075-T6 aluminum (503 MPa) to that of 7075-0 aluminum (103 MPa) completely eliminated burst acoustic emission activity due to crack advance in the amplitude range studied.     

Application of the acoustic emission technique to the react and wind processed Nb3Sn superconducting magnet

A technique combining acoustic emission and voltage measurement is applied to quench experiments conducted on a newly fabricated Nb₃Sn superconducting magnet. Based on the results, the training mechanism, epoxy potting effect and conductor degradation due to the electromagnetic force are discussed.     

Acoustic emission from crack growth in an advanced zirconia refractory under thermal shock

Crack initiation and growth during the thermal shock tests of a partially stabilized zirconia advanced refractory were investigated by the analysis of acoustic emission (AE) amplitudes. The growth of cracks that were detected by AE was systematically monitored by SEM observations as increasingly severe thermal shocks were applied. The measurements of strength loss after thermal cycling in the ribbon test with various applied temperature differentials correlated with continuous monitoring by acoustic emission and confirmed the effects of microcrack growth on the resistance to thermal shock damage.     

声发射测试系统的发展

综述了声发射测试系统的发展历程及现状。展望了其发展前景，给出了两种分类依据，重点介绍了几种目前先进的声发射仪．     

Heat generation from epoxy cracks and bond failures

Energy released following cracks and bond failures were measured for an EPON epoxy near 4.2 K. Crack events were monitored with an acoustic emission sensor; the energy released by each crack or bond failure was calculated from the temperature rise measured with thermocouples. Cracking was observed to be load dependent; this may account in part for the training phenomenon observed in bringing epoxy-impregnated superconducting magnets to full design field.     

模态声发射技术在构件疲劳裂纹检测中的应用

疲劳裂纹的萌生和扩展是机械零件在变载荷作用下的主要失效形式,在变载荷下出现疲劳裂纹的同时往往伴随着弹性波的扩散,以迅速释放其内部积累的应变能。使用近几年来得到迅速发展的模态声发射技术真实地获取疲劳裂纹的声发射波形,使接收到的声发射信号较完整地反映了声发射源的物理状态。在波形分析中采用参数分析法提取声发射波形特征,建立模态声发射参数和裂纹扩展速度之间的数学关系。由于综合采用了多种技术的优点进行信号分析、处理,因此根据声发射信号特征得到的结果将更加逼近实际状态。     

Acoustic emission applied to glass fibre/organic thermosetting matrix interface characterization

The aim of this paper is to propose a characterization method of the glass fibre/organic matrix interface based on acoustic emission. On the basis of tensile tests, it was possible to make a discrimination between the features which are not characteristic of the fibre/matrix interface (fracture properties and modulus) because taking into account some parameters other than the interface alone on one hand, and on the other hand certain characteristic features (strain and energy at first damage) which are defined on the basis of an analysis of the acoustic emission spectra of the material.     

Acoustic emission from deformed magnesium alloy based composites

Magnesium alloys based composites were deformed at ambient temperature. The acoustic emission (AE) was detected in situ during deformation. The time dependence of the AE count rate was measured at two threshold levels. Two AE peaks were found. The peak observed at the onset of plastic deformation was ascribed to massive twinning and dislocation slip, and the second, broader maximum was caused mainly by fibre breakage and debonding at the fibre-matrix interfaces. The AE activity was significantly influenced by the load transfer from the matrix to the fibres.     

Acoustic Emission in Composite Laminates

The response of a laminated plate to internal acoustic emission events is examined in detail. The plate consists of four layers of a unidirectional fiber composite material arranged in a cross-ply configuration. The sources considered include a vertical line couple, a horizontal line couple and a line double couple without moment. The latter is produced by the combination of two equal and opposite line couples and may be associated with a shear dislocation in the plane of the laminate. The particular response presented here is that of the normal upper surface displacement. Four different orientations of the line sources relative to the core fiber direction are considered. The sources may be located at any depth within the plate and results are shown for three locations, namely, the upper interface, the midplane and the lower interface. The receiver is positioned on the upper surface at a known distance from the line of action of the source. When this distance is small relative to the depth of the plate, it is possible to associate some of the peaks and troughs present in the response with the arrival of individual rays. At larger distances, the disturbance arises from the propagating Rayleigh Lamb modes.     

Acoustic emission location on aluminum alloy structure by using FBG sensors and PSO method

Acoustic emission location is important for finding the structural crack and ensuring the structural safety. In this paper, an acoustic emission location method by using fiber Bragg grating (FBG) sensors and particle swarm optimization (PSO) algorithm were investigated. Four FBG sensors were used to form a sensing network to detect the acoustic emission signals. According to the signals, the quadrilateral array location equations were established. By analyzing the acoustic emission signal propagation characteristics, the solution of location equations was converted to an optimization problem. Thus, acoustic emission location can be achieved by using an improved PSO algorithm, which was realized by using the information fusion of multiple standards PSO, to solve the optimization problem. Finally, acoustic emission location system was established and verified on an aluminum alloy plate. The experimental results showed that the average location error was 0.010 m. This paper provided a reliable method for aluminum alloy structural acoustic emission location.     

On the acoustic emission due to the fracture of brittle inclusions

The far-field characteristics of the emission from a theoretical model for the fracture of brittle inclusions are presented in detail. The model is a circular crack growing at constant speed from zero size until it attains a prescribed size. The far-field radiation pattern is the same as that of a simple combination of force doublets, and some qualitative similarities between force doublets and acoustic dipoles are noted. The initial shape of the far-field pulses due to the growing stage and the stopping is determined, but difficulties arise in accounting for the diffraction of a surface wave on the crack faces generated by the stopping of the crack.