Lead-free acoustic emission sensors

Acoustic emission (AE) sensors have been fabricated using both soft- and hard-type lead-free (Na0.5K0.5)NbO3-based ceramics. The acoustic and electromechanical properties of the ceramics have been determined using the resonance technique. The lead-free AE sensors were calibrated using a laser source and compared to a commercial sensor. A lead zirconate titanate (PZT) 5H ceramics AE sensor has also been fabricated and calibrated for comparison. It was found that the sensitivity of lead-free AE sensors is comparable to that of the lead-based PZT sensor. To evaluate the sensors for potential application, they have been used in the detection of AE in an impact test. The lead-free sensors can reproduce AE signals accurately without giving artifacts and have potential use in commercial AE systems.     

Thermal shock/fatigue evaluation of FGM by AE technique

This study has been carried out to analyze the thermal shock and fatigue characteristics of functionally gradient material (FGM). The thermal shock/fatigue tests were conducted at a specific temperature and fracture patterns were studied by SEM and AE. Also, thermal fracture behavior of plasma-sprayed FGM and conventional coating material (NFGM) was examined by acoustic emission technique under heating and cooling. Furnace cooling and air cooling tests were used to examine the effect of temperature change under various conditions. The conventional and FGM coatings were compared to heat-resistant property and fracture surface of these materials for each temperature history. Based on these results, some critical temperature at the onset of coating failure can be evaluated to characterize the thermal resistance of the materials. It turned out to be that FGM gives higher thermal property compared with NFGM by AE signal and fracture surface analysis.     

Acoustic-emission testing and diagnostic of hazardous dynamic processes in a coal seam

Information content of various measured acoustic emission (AE) parameters of coal seams is analyzed. A concept for selection of informative features of AE signals has been formulated. The emergence of delta-coupling of AE acts has been detected on early stages of destruction. The paper suggests determining the onset of a predestructive state by detecting deviations of statistical characteristics of AE pulses from their stable forms (adequate to purely random processes). A technique for diagnosing the predestructive state and for estimating the concentration and growth rate of flawed sites based on positions of extremal points on curves of AE pulse frequency is described.     

模拟磨削烧伤声发射信号的实验研究

人们一直试图运用声发射技术来在线识别磨削烧伤,但磨削烧伤的声发射信号相对较弱,容易被磨削中其它声发射信号所掩盖。为了在发现磨削烧伤声发射信号,对镍基合金和陶瓷材料热扩散的声发射信号特征进行了研究,该热扩散由激光照射引起,来模拟磨削热效应。运用短时傅里叶变换技术,在没有其它干扰因素的条件下,模拟磨削烧伤温度的声发射信号的频率特征被成功的提取出来。发现材料温度改变越大则声发射信号越强!     

Acoustic-Emission Methods for Assessing the Process of Damage Accumulation in Estimating Remaining Service Lives of Heavily Loaded Structures

The paper is dedicated to estimating parameters of the process of damage accumulation on the base of restored frequencies of acoustic emission (AE) events and invariant AE destruction criteria. Experimental investigation of relations between AE parameters and features of early stages of fracture of layered composite materials and iron–aluminum alloys are presented. The paper describes a method for identification of destruction stages and prediction of remaining service lives based on AE measurements.     

Combined thermogravimetric and acoustic-emission diagnostics of stages of thermal destruction of substances and materials

The possibilities and limits of existing methods of estimating the thermophysical stability of substances and materials are analyzed. A combined method of thermogravimetric and acoustic-emission (AE) diagnostics of stages of thermal destruction is proposed. It is shown that combination of existing thermal analysis and AE methods leads to an increase in the accuracy and reliability of data on degradation processes in materials under the action of temperature.     

On the Information Value of the Method of Invariants of Acoustic-Emission Signals in the Diagnostics of Pre-Failure State in Materials

The specific features of using the method of the invariants of acoustic emission (AE) signals for the diagnostics of a pre-failure state are considered. Results of the diagnostics of pre-failure by the amplitude and temporal invariants of AE event stream under mechanic loading of steel specimen are presented. It has been established that the measurement and analysis of amplitude and temporal informative parameters is more pictorial and should be preferred to the direct measurement of invariant relationships. Incorrect application of the method of invariants has been analyzed.     

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。     

Acoustic emission method for tool condition monitoring based on wavelet analysis

It is believed that the acoustic emission (AE) signals contain potentially valuable information for tool wear and breakage monitoring and detection. However, AE stress waves produced in the cutting zone are distorted by the transmission path and the measurement systems and it is difficult to obtain an effective result by these raw acoustic emission data. In this article, a technique based on AE signal wavelet analysis is proposed for tool condition monitoring. The local characterize of frequency band, which contains the main energy of AE signals, is depicted by the wavelet multi-resolution analysis, and the singularity of the signal is represented by wavelet resolution coefficient norm. The feasibility for tool condition monitoring is demonstrated by the various cutting conditions in turning experiments.     

Detecting transition to chatter mode in peakless tool turning by monitoring vibration and acoustic emission signals

Stability of a peakless tool turning on slender shafts was studied under conditions of low- and high-magnitude vibrations by registering and short-time Fourier transformation (STFT) processing of acoustic emission (AE) and vibration acceleration (VA) signals. Both VA and AE signals have been registered in three positions of the cutting tool on the workpiece and for different shaft diameters. Both amplitude- and frequency-dependent AE and VA characteristics were obtained and analyzed for overall process signal length as well as for single frames. It was shown that power spectrum characteristic could be used for monitoring the fast-occurring changes in the cutting process stability. A criterion of the cutting process stability based on the power spectrum has been offered.     

Study of nanoobjects of different nature using submillimeter laser ablation

This paper reviews the experimental work conducted at the physicochemical workstation of the Novosibirsk free electron laser (FEL) since 2005 on developing methods for studying the fractional composition of nanoobjects of different nature. It has been found that the submillimeter radiation of the FEL leads to soft ablation of biological macromolecules. Furthermore, the molecules are transferred to the aerosol phase separately, each type of molecules forms its own fraction of aerosol nanoparticles without destruction and denaturation, and their sizes correlate with molecular weight. It is suggested that submillimeter laser ablation can be used to determine the sizes of nanoobjects of different origin. The paper discusses the results of experiments with mineral clusters, nanopowders, synthetic and biological polymers carried out using traditional methods of investigation of the fractional composition of nanoscale materials (atomic force microscopy, scanning electron microscopy, dynamic light scattering, and x-ray diffractometry) and using the new method—the analysis of the aerosol products of submillimeter laser ablation. It is shown that submillimeter laser ablation with the subsequent registration of aerosol products by modern equipment is a versatile, simple, fast, and accurate method which can complement the above-mentioned methods.     

Workpiece roundness profile in the frequency domain: an application in cylindrical plunge grinding

In grinding, most control strategies are based on the spindle power measurement, but recently, acoustic emission has been widely used for wheel wear and gap elimination. This paper explores a potential use of acoustic emission (AE) to detect workpiece lobes. This was achieved by sectioning and analysing the AE signal in the frequency domain. For the first time, the profile of the ground workpiece was predicted mathematically using key frequencies extracted from the AE signals. The results were validated against actual workpiece profile measurements. The relative shift of the wave formed on the surface of the part was expressed using the wheel-workpiece frequency ratio. A comparative study showed that the workpiece roundness profile could be monitored in the frequency domain using the AE signal during grinding.     

Feature extraction of AE characteristics in offshore structure model using Hilbert-Huang transform

This paper addresses an application of recently developed signal processing tool based on the Hilbert-Huang transform (HHT) to characterize the acoustic emission (AE) signals released from the offshore structure model. The AE signals from the cracks in the welded steel nodes of the offshore structure model are collected during the tensile testing in water and the simulated AE signals are also collected in the offshore structure model. Instantaneous frequency and energy features based on local properties of the AE signals are then extracted using the Hilbert-Huang transform method. In order to demonstrate the advantage of the AE feature extraction techniques based on Hilbert-Huang transform, the conventional AE analysis is also provided side-by-side for comparison. The results verify that the method based on HHT better characterizes the AE signals than the classical AE techniques. It can be concluded that the AE signal analysis based on HHT is an effective tool to extract the features and this opens perspectives for crack recognition in offshore structures.     

Application of the Spectral Analysis of Acoustic Emission Signals to Studies of Vulnerability of TiN Coatings on Steel Substrates

A technique has been suggested for processing spectra of acoustic emission (AE) signals based on a signal classification using shapes of normalized spectral density curves. This enables one to calculate averaged spectral images of AE signal groups, which characterize specific types of AE sources, and to estimate intensities of various emission sources on various stages of deformation. The suggested technique has been used in estimating the TiN vulnerability when it is strained concurrently with the substrate.     

Determination of the parameters of the dispersion curves of lamb waves with the use of the Hough transform of the spectrogram of an AE signal

A method for the recognition of dispersion curves on the spectrogram of acoustic-emission signals has been developed based on the Hough transform. The method was successfully tested in an experiment using a Hsu-Nielsen source. It has been determined that the suggested method can determine the distance from the source of a signal, even when the signal arrives only at one acoustic emission transducer. This enables one to perform AE testing only with one-sided access to any dimensional object or when there is a sufficiently large distance between transducers, when an AE signal reaches only one of them.     

A method for identifying deformation and destruction stages by location of critical points of a reconstructed flow of acoustic-emission events

A method for identifying the stages of deformation and destruction processes in materials on the basis of detection and location of the critical points (local extrema and inflection points) of the recorded parameters of the reconstructed flow of acoustic-emission events. The issues of the applicability of this method to both the flaw-formation processes and the phenomenon of Poisson flow of events accompanying the AE model. A method for reconstructing the parameters of the initial flow of AE events inside a material based on the recorded AE signals is described. It is shown that the reconstruction procedure appreciably increases the accuracy and reliability of the results of AE diagnostics of the stages observed in flaw-formation processes.     

Vibration impact acoustic emission technique for identification and analysis of defects in carbon steel tubes: Part B Cluster analysis

Current steel tubes inspection techniques are invasive, and the interpretation and evaluation of inspection results are manually done by skilled personnel. Part A of this work details the methodology involved in the newly developed non-invasive, non-destructive tube inspection technique based on the integration of vibration impact (VI) and acoustic emission (AE) systems known as the vibration impact acoustic emission (VIAE) technique. AE signals have been introduced into a series of ASTM A179 seamless steel tubes using the impact hammer. Specifically, a good steel tube as the reference tube and four steel tubes with through-hole artificial defect at different locations were used in this study. The AEs propagation was captured using a high frequency sensor of AE systems. The present study explores the cluster analysis approach based on autoregressive (AR) coefficients to automatically interpret the AE signals. The results from the cluster analysis were graphically illustrated using a dendrogram that demonstrated the arrangement of the natural clusters of AE signals. The AR algorithm appears to be the more effective method in classifying the AE signals into natural groups. This approach has successfully classified AE signals for quick and confident interpretation of defects in carbon steel tubes.     

Monitoring Strategy of Thermal-Induced Convex Deformation of Workpiece in Surface Grinding

In surface grinding, the presence within the workpiece of a high temperature gradient causes a large thermal bending moment, which can cause over-grinding due to the induced convex deformation of the workpiece by the bending moment. The average grinding forces and acoustic emission (AE) are used to predict the over-grinding, based on the facts that the up-grinding force is larger than the down-grinding force, and the AE is larger in up-grinding. Hence, over-grinding can be detected between passes from the mean values of the grinding forces and AE during grinding. Because of the positional separation between the bottom of the workpiece and the dynamometer, the signal transmission from the contact zone to the transducer is reduced, and so the AE does not satisfactorily indicate the amount of over-grinding. The strategy for monitoring the over-grinding by measuring the grinding force is satisfactorily verified in this study.     

钢板声发射时间反转聚焦增强定位方法

声发射检测方法具有实时动态监测优点,应用越来越广泛,但是对声源的定位始终没有更大的突破。在钢板声发射检测中,提出一种基于时间反转理论的声发射源准确定位的方法。由于声发射检测是一种被动检测技术,结合时间反转聚焦理论,推导出对声源信号实现时间反转聚焦增强处理方法,可增强检测信号中声源幅值,提高信噪比;然后根据声源信号到达时间推算出声源聚焦时刻,利用弹性波传播理论对传感器监测区域重建信号传播波动图,显示出声源位置和区域;最后通过实验测试对该方法进行验证,结果表明该方法能有效提高损伤声源信号的能量,对检测区域的信号重建和定位显示准确地给出损伤声源位置。     

Electron beam damage of amorphous synthetic polymers

The electron beam damage of amorphous polymers (PVC, PVDC, PTFE, PVDF, Hypalon, PES, Horizon and PDMS) has been measured by the elemental loss using EDX analysis. An attempt has been made to identify and eliminate the experimental error which would influence the measured decay curves. Only Cl and F in some polymers were found to be volatile. The PVC-Cl decay curves were compared with UV and IR spectra at different stages of irradiation and it has been concluded that some Cl is trapped in a more stable structure with conjugated double bonds. The formation of isolated double bonds and their destruction has been revealed by a staining technique for PVC, PMMA, PS and PC. It is likely that the mechanism of formation of isolated double bonds and their conversion to a more stable structure with conjugated double bonds is valid generally for all polymers. The staining peak can therefore be used as a universal technique for the beam sensitivity measurement.