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.     

Relation between the amplitude of acoustic emission of an ensemble of microflaws and the deformation rate: Reconstruction of the number of AE events during a change in the dynamic range of detected signals

A model of acoustic emission from a dislocation cluster during its passage through a boundary is considered. A dependence of the amplitude of an acoustic-emission (AE) event on the deformation rate and the loading frequency has been obtained. Algorithms for reconstructing the number of lost AE events below the discrimination threshold of the instrumentation are obtained for the case of a power-law amplitude distribution. A method for correcting the parameters of the detected stream of events at various discrimination thresholds, deformation rates, and loading frequencies is described. A method for estimating the dynamic range on the basis of the average value of the detected AE-signal amplitudes is proposed. We propose using the results of this study for increasing the reliability of the AE method for diagnostics of a prebreakdown state.     

Diagnostics of critical points of the friction coefficient of a multilayer nanomodified antifriction coating by an acoustic emission technique

The dynamics of the changing parameter of the acoustic emission (AE) in the course of tests of a multilayer nanomodified antifriction coating are considered. The results of investigating the shape and the spectra of AE pulses in different regions of the experimental curve of the dependence of the friction coefficient on the test time are reported. A method is proposed for establishing the transition points from the steady-state stage to the failure and friction-without-coating stages according to the intensity of the restored AE event stream, the spectral distribution, and the amplitude of the concurrent acoustic emission in the ultrasonic-frequency range.     

Use of Invariant Combinations of Parameters Characterizing Acoustic Emission in Diagnostics of Prefracture States of Solids

The paper is devoted to the diagnostics of solids based on deviations of invariant combinations of measured acoustic-emission (AE) parameters from their stable values. Introduction of an informative AE parameter susceptible to transitions between stages of aw generation has been justified. The method of sampling of an AE flow by selecting each mth pulse yields little information because it records Erlang's flow, which is notably different from the input flow of signals.     

The influence of the coordinate errors of setting sensors of a piezoelectric antenna on the accuracy of localizing sources of acoustic-emission signals

The main factors that affect the error of determining the coordinates of an acoustic-emission (AE) signal source are considered. The influence of errors of determining coordinates ±Δx _i and ±Δy _i of piezoelectric-antenna transducers on the AE-signal localization accuracy are analyzed. It is theoretically and experimentally proven that the errors in the coordinates of the transducers substantially influence the AE-signal localization accuracy, especially at a low sampling frequency of the analog-to-digital converter of the AE system’s measuring channel and when low-frequency AE transducers are used.     

Diagnostics of the predestruction state based on amplitude and time invariants of the flow of acoustic-emission acts

The method and results of diagnostics of the predestruction state are described based on deviations of the amplitude and time invariant relationships between the parameters of the flow of acoustic- emission (AE) acts and their stable values. The relationship of the AE invariant method with the methodology of the synergetic approach to the destruction of solids is shown. Amplitude invariant relationships of AE have been estimated both by tests of standard specimens of 15Kh2NMFA hull-plate steel and by the previous results of AE tests of a VVER-1000 reactor vessel.Translated from Defektoskopiya, Vol. 40, No. 8, 2004, pp. 79–83. Original Russian Text Copyright © 2004 by Builo.     

Investigation of acoustic emission due to deformation of titanium alloy and some results of acoustic-emission diagnostics of its predestruction states

The paper presents acoustic-emission (AE) characteristics of the Ti-Al-Mo-Si alloy. The energy of one AE event has been estimated over the entire frequency band under investigation. AnS-shaped curve of the logarithm of the frequency of AE events plotted against the strain has been obtained, and the physical interpretation of such a dependence has been given. A tentative explanation of the delay of AE events on the initial stage of the material deformation with a stress concentrator has been suggested. On the base of the correlation between the service-life expectancy and the frequency of AE events, which has been established in our experiments, the degree of confidence in the diagnostic data concerning the predestruction state of gas-turbine blades in turbojet aircraft engines has been estimated.     

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.     

Using two-stage clustering of acoustic-emission signals for the detection of weld flaws

A two-stage method for the clustering of acoustic-emission (AE) signals using their parameters and digitized shapes was considered. AE signals were initially clustered according to a set of their informative parameters; the primary clusters that were obtained using these parameters were then subjected to clustering using their digitized AE signal shape. The use of two-stage clustering for the processing of AE signals that were recorded at the moment of welding allowed the calculation time to be reduced without loss of localization accuracy.     

Estimates of Reliability of Acoustic Emission Tests Taking Account of Physical and Mechanical Features of Cracking

The paper describes a method for estimating reliability of acoustic emission (AE) tests of large-scale metallic structures with due account of physical and mechanical features of cracking and resulting radiation. Amplitude spectra (A-curves) of acoustic emission pulses obtained in tests of cracking samples are given. Such spectra are used in assessing the reliability of tests of large-scale objects. The reliability of acoustic emission tests of pipelines has been analyzed.     

Tool life monitoring during the diamond turning of electroless Ni–P

Diamond tools become severely worn when machining Ni–P plating materials. Tool life monitoring is therefore essential to avoid the deterioration of workpiece quality. In this paper, in order to better detect tool life while the tools are in use, detailed investigations of the cutting force and acoustic emission (AE) measured during diamond turning process have been made. The results of this tool-life testing show that the cutting force and AE supply valuable information on tool failure; the dynamic component of the thrust force fluctuates chaotically when the tool dulls. This phenomenon can be detected using the 1/f^β power spectrum with a spectral exponent of β > 1. On the other hand, the AE amplitude a tends to rise just when chipping occurs on the cutting edge. This feature can be detected by the amplitude distribution spectrum, in which the AE event rate follows the power law a^−m with a scaling exponent of m < 2. Therefore, we can conclude that a spectral exponent of β > 1 and a scaling exponent of m < 2 can be used as the criteria to gauge tool life, because it was observed at the end of the tool life that tool corner had become worn out and that chipping had occurred on the cutting edge.     

A correlation analysis and invariant method of acoustic-emission signals in the diagnostics of predestruction states of materials

The results of the application of correlation analysis and invariant method of acoustic-emission (AE) signals during the mechanical loading of 95X18 steel samples are considered. The circuit of a controller that is intended for calculation of statistical characteristics of AE signals is given. A method for diagnostics of predestruction states based on the appearance of stably positive values of the amplitude-correlation coefficient and AE pulse-repetition intervals and deviations of invariant AE relationships from their stable values is proposed.     

Inspection of locomotive bearing rings by the acoustic-emission method

The results of testing the bearing rings (BRs) of a locomotive axle box by the acoustic-emission (AE) method are presented. The small size of the localization zone of BRs leads to a significant spread of the coordinates of flaws even at minimal errors of the time of arrival of AE signals at piezoelectric-antenna probes. To refine the testing technique, the maximum testing load necessary for initializing AE signals is determined. Deformations in BRs are measured with a strain-gauging system related to the AE system. During recording of AE signals, the mechanical stresses are measured, thus allowing rejection of parasitic signals and an increase in the localization accuracy.     

Detecting hazardous sources of acoustic-emission signals using the estimated energy of clusters

A method for detecting hazardous sources of acoustic-emission (AE) signals by the level of their energy activity using clustering by digitized signal shape was considered. The influence of the distance between a source and an AE transducer on the energy of recorded AE signals was analyzed. The energy distribution median of signals in a cluster was proposed for evaluating the hazard of an AE signal source. The application of the developed method was tested based on the example of the analysis of AE inspection data on the welding of steel specimens with incomplete fusion and a crack at the root of a weld seam. This testing confirms the results of the fractographic inspection of a defective weld seam.     

Effect of fuzzy C means technique in failure mode discrimination of glass/epoxy laminates using acoustic emission monitoring

Acoustic emission is one of the powerful techniques that can be used for in situ structural health monitoring of composite laminates. One of the main issues of AE is to characterize the different damage mechanisms from the detected AE signals. Unsupervised Pattern recognition has been one of the techniques used for the identification of a specific failure mode in composites from Acoustic emission data. Cross ply composite laminate of size 300 × 300 mm is fabricated using Vacuum bag molding. ASTM D3039 Standard tensile specimens are cut from the laminate and these specimens are subjected to uni axial tensile test under Acoustic Emission monitoring. Fast Fourier transform analysis (FFT) and Short Time Fast Fourier Transform (STFFT) analysis are performed on the Wave forms of the AE hit data obtained during the conduct of tensile test to characterize the failure modes in crossply specimens. Fast Fourier Transform enabled calculating the frequency content of each damage mechanism. In this paper Fuzzy C Mean clustering is performed for the AE parameters obtained from the test and the efficiency of this technique is being investigated using FFT AND STFFT analysis.     

基于声发射信号的外圆切入磨削去除率监测

针对精密外圆切入磨削加工的在线监测需求,提出一种采用声发射信号实现轴类零件材料去除率在线监测的方法。根据声发射信号强度与磨削力之间的联系,建立了声发射信号均方根曲线的预测模型,利用该预测模型研究了砂轮进给阶段和驻留阶段磨削系统时间常数的理论计算方法,推导了声发射信号均方根曲线与工件材料去除率的关系;编写了在线监测软件,利用声发射传感器实现了精密外圆切入磨削的材料去除率预测。实验证明,所建立的声发射信号均方根曲线模型具有良好的预测精度,基于该模型能够实现磨削系统时间常数在线评估,并实现精密轴类零件材料去除率的实时在线监测。     

Real-time detection of the breakage of small diameter drills with wavelet transform

This paper presents a real-time tool breakage detection method for small diameter drills using acoustic emission (AE) and current signals. Using the transmitted properties of the AE signal, apparatus for detecting the AE signal for tool breakage monitoring was developed for a machine centre. The features of tool breakage were obtained from the AE signal using typical signal processing methods. The continuous wavelet transform (CWT) and the discrete wavelet transform (DWT) were used to decompose the spindle current signal and the feed current signal, respectively. The tool breakage features were extracted from the decomposed signals. Experimental results show that the proposed monitoring system possessed an excellent real-time capability and a high success rate for the detection of the breakage of small diameter drills using combined AE and current signals.     

A thermal design method for the performance optimization of multi-stream plate-fin heat exchangers

An optimization design method based on field synergy principle is developed for Multi-stream plate-fin heat exchangers (MPHEs) with a segmented differential model. The heat exchanger is divided into a number of sub-exchangers along the main stream, and each sub-exchanger consists of N passages along the height of the exchanger. Compared with the traditional heat exchanger design, this method allows temperature and pressure fields to be obtained via coupling calculation with consideration of variable physical properties and the axial heat loss of the heat exchanger. Finally, the heat exchanger is optimally designed using a temperature-difference uniformity optimization factor based on field synergy principle. This design model can provide an accurate temperature field and pressure field, because the stream properties are determined by the mean temperature and pressure of each local sub-exchanger. Optimum results indicate that the temperature distribution on the cross section of the heat exchanger is relatively uniform and that the temperature difference of heat transfer for each stream is always a small value. These characteristics prove the feasibility and effectiveness of this design model. In this paper, a case of five stream plate-fin heat exchangers for an ethylene plant is designed under a practical cold box operating condition with the proposed model, the structure and heat transfer of which are optimally determined. The design model and optimization method proposed in this work can provide theoretical and technical support to the optimization design of MPHEs.

     

Investigation on the effect of EDM process variables and environments on acoustic emission signals

Abstract

The performance of electrical discharge machining (EDM) primarily depends on the spark quality generated in the inter-electrode gap (IEG) between the tool and workpiece. A method for obtaining accurate information about the spark gap is required to effectively monitor the EDM process. The rise and fall of thermal energy in the discharge zone at a rapid rate during the dielectric breakdown produces high-pressure shock waves. This work explores the suitability of using acoustic emission (AE) generated from these shock waves and the elastic AE waves released on the workpiece due to the induced stress to monitor the performance and spark gap in EDM. The information content of the AE signals acquired at various machining conditions was extracted using AE RMS, spectral energy and peak amplitude. These features were able to well discriminate the machining condition, tool material, workpiece material, flushing pressure, current density, the initial surface roughness of the tool. Additionally, the AE signal features had a good and consistent correlation with the performance parameters, including material removal rate, surface roughness (Ra and Rq) and tool wear. The findings lay the groundwork to develop an effective, non-intrusive in-situ AE-monitoring system for performance and IEG condition in EDM.     

Acoustic-emission testing and diagnostics of the dissolution kinetics of crystalline components

The dynamics of changes in acoustic emission (AE) parameters during the dissolution of a crystal were considered. The temperature dependence of the dissolution time constant was experimentally established. A method for estimating the crystal dissolution time constant and dynamics from the data of AE measurements was proposed.