Detecting acoustic-emission signals with fiber-optic interference transducers

Results of the analysis of acoustic-emission signals generated due to ultrasonic waves propagating in a polymer composite material and registered with piezoelectric and fiber-optic sensors are presented. The fiber-optic sensors were arranged into an adaptive interferometer based on using a dynamic hologram formed in a photorefractive crystal. Reducing the setpoint fading has made it possible to improve the noise immunity and sensitivity of the measurement system when using an adaptive interferometer on a photorefractive crystal. Optical fibers in the interferometer’s measurement system served as sensors of ultrasonic waves and were built into a polymer composite material when the sample was manufactured. The sample was a rectangular plate made of a multilayer fiberglass material. It has been discovered that the sensitivity of the adaptive interferometer makes it possible to detect acoustic- emission signals generated by a Hsu–Nielsen source. When determining the speed of sound in the polymer composite material, peculiarities of registering a group wave by fiber-optic sensors have been established that are due to the anisotropy of the medium the wave propagates in and the distributed character of sensor placement in the studied composite material. The wavelet transform has been used to separate the informative component of the wanted signal.     

Clustering of sources of acoustic-emission signals by the leading-edge rise rate

It is shown that the method of clustering by the leading-edge rise rate of the envelope of an acoustic-emission (AE) signal makes it possible to reduce the time of calculation of the coordinates of flaws as compared to the clustering by shape. The developed technique realizes the on-line localization of massive flows of AE signals with an accuracy sufficient for practical application. The practical parameters of using the method for operation on a steel sheet with a simulator obtained for a statically loaded duralumin specimen and for welding of steel specimens with aluminum and titanium inserts are considered.     

A source of waves simulating acoustic-emission signals

A source of acoustic waves simulating acoustic-emission signals, a technique for calculating the force acting for a short time on the surface of a tested object, and formulas for calculating the displacement amplitudes for bulk longitudinal and transverse waves excited by this source are proposed. The possibility of calculating the amplitude of the normal component of antisymmetric mode a ₀ of Lamb waves is considered. It is shown that it is desirable to use such a source in acoustic-emission testing of trunk and industrial pipelines of the fuel and energy industry.     

Estimation of time-of-arrival errors of acoustic-emission signals by the threshold method

A technique for determining the times of arrival of acoustic-emission (AE) signals at the probes of a piezoelectric antenna with simultaneous estimation of their errors is considered. To increase the localization accuracy, it is proposed to refine the times of arrival by linearly approximating the leading edge of an AE signal taking into account the influence of noise. The slope of the leading edge is calculated according to the criterion of the minimum rms deviation from the signal envelope using the moving-window method. Examples of use of the developed method for calculating the coordinates of flaws in strength tests of structural elements are considered.     

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.     

Synthesis of Butterworth filters for increasing the interference immunity of acoustic-emission signals

Increasing interference immunity in registering acoustic signals is an important engineering task for the acoustic-emission technique. Frequency filtering is an efficient and advanced filtering technique. This study shows that interference immunity increases with Butterworth filters.     

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.     

Localization of acoustic-emission sources in objects with small geometric dimensions

The problems of the localization of sources of acoustic-emission (AE) signals for objects with small geometric dimensions are considered. The dependence of the localization error for a flaw inside a piezoelectric antenna on the error in determining the difference in the times of signal arrival (DTA) at acoustic transducers was obtained. A method for determining the DTA using wavelet scalograms of AE signals was studied. Experimental studies showed the advantages of this method over the method for determining the rms deviation of AE-signal amplitudes within a “time window.”     

Analysis of errors in determining the coordinates of acoustic-emission sources in sheet-steel structures

The main factors influencing the error in determining the coordinates of acoustic-emission (AE) sources have been analyzed. A model that allows determination of the spatial distribution of errors of the coordinates of AE sources has been developed. The dependences of the errors caused by the main influencing factors—the difference of the sensitivities of AE channels, the dimensions of the acoustic antenna, and the dispersion and amplitude of AE pulses—have been determined.     

Correlations between parameters of acoustic-emission signals and corrosion damage in aluminum alloys

Results of the studies of correlations between different parameters of acoustic-emission signals and the degree of corrosion damage of an aluminum alloy exposed to two different active media are presented. The critical values of the acoustic-emission count rate and the total acoustic-emission signal that correspond to the elastic-plastic transition and subsequent active creation of microcracks are obtained for Д16M. The limiting values of the known Ivanov-Bykov criterion, which characterizes the moment when corrosion cracking starts in Д16M, are calculated. The acoustic-emission technique may be used for nondestructive testing of corrosion damage on inner surfaces of piping in poorly accessible structures.     

A holographic technique for recording acoustic-emission signals due to deformation of solids

A promising technique for recording elastic stress waves due to deformation in solids based on the analysis of optical wave fronts using reference reflection Fourier holograms is discussed.     

The relationship of acoustic-emission signals with the processes of deformation and fracture of building structures

The parameters and spectra of acoustic-emission signals that arise during the inspection of precast concrete structures are studied. The information content of these parameters is discussed in terms of flaws that develop in structures subjected to stresses.Translated from Defektoskopiya, Vol. 40, No. 11, 2004, pp. 32–39.Original Russian Text Copyright © 2004 by Sagaidak, Elizarov.     

Analysis of variations of the dynamic characteristics of acoustic-emission sources under static loading of metal specimens

Changes in the energy of acoustic-emission (AE) signals and their cross-correlation coefficient for sources of different types have been experimentally studied for statically loaded specimens. The results allow separation of AE signals appearing during irreversible changes in the region of stress concentrators and signals of other types.     

The influence of temperature on the characteristics of piezoelectric transducers and errors of localization of acoustic-emission signals

The experimental dependences of changes in the output signals of standard and high-temperature acoustic-emission transducers (AETs) during heating and cooling are considered. It is shown that high-temperature AETs have a hysteresis that must be taken into account in high-temperature tests of various materials. The errors in localizing acoustic-emission signals from a simulator during heating and cooling of high-temperature and standard AETs are analyzed using the two-interval method.     

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.     

Diagnostics of the prefracture state of materials according to the parameters of the amplitude distribution of accompanying acoustic-emission signals

The results of an experimental and theoretical study of the form of the distribution-density function for acoustic-emission (AE) signal amplitudes at different material-destruction stages are described. On the basis of the Poisson model of the flaw-generation process, the theoretical form of the density of the acoustic-emission amplitude distribution during destruction of a solid is obtained. The conditions for the appearance and location of false amplitude maxima, which strongly reduce the reliability of the results from diagnostics of the prefracture state, are evaluated on the basis of the AE-amplitude distribution profile.     

Estimation of the strength and lifetime of technical objects using the acoustic-emission method

Estimation of the lifetime of a construction consists of forecasting the time remaining before failure and is based on extrapolation of the time dependences of the state parameters of a technical object to their critical values. The use for this purpose of simplified damageability models (e.g., a hypothesis of linear summation of flaws) and the absolute values of acoustic-emission (AE) parameters (e.g., activity, counting rate, duration of pauses) leads to a dependence of the estimate results on a large number of destabilizing factors and, as a result, to low accuracy or laboriousness of these techniques. An earlier suggested micromechanical model of AE parameters makes it possible to find a trade-off between simplicity and accuracy.     

Estimation of the strength and lifetime of welded constructions using the acoustic-emission method

Strength margins according to different criteria (static strength, low- and high-cycle fatigue, crack growth resistance), as well as the initial lifetime, accumulated long-term and cycle damages, and remaining lifetime, are classified as the main characteristics of the strength and lifetime of machines and constructions. In order to estimate the accumulated damage at the first fracture stage, it is sufficient to determine the values of parameters involved in the fracture growth equation. In order to estimate the lifetime, it is sufficient to solve this equation with regard to the accumulation time of the critical damageability based on estimation of the value of the diagnostic parameter. The possibility of estimating these characteristics using registration of AE signals and a model proposed in previous papers is shown.     

Studying the correlation of the informative parameters of acoustic-emission signals with the destruction processes in specimens with welding flaws

The results of acoustic-emission (AE) testing of flaws in steel specimens during welding and subsequent loading until destruction in MTS-50 and MTS-250 electrohydraulic machines are presented. Flaws were simulated via faulty fusions or the introduction of titanium and duralumin inserts. During loading, some specimens were broken along the weld seam and the other specimens were broken in the near-seam zone. A comparative analysis of the distributions of the informative parameters of AE signals, which are recorded during different types of specimen fracture, is presented.     

Refining visually estimated arrival times of short duration signals

Estimating the location of a radiating source from recorded sensor signals requires accurate arrival time estimates. If the sensor signals have a short time duration, generating accurate arrival time estimates is a challenging task. This paper proposes a new cross-correlation averaging algorithm which is integrated with a visual ‘peak picking’ alignment approach to improve the estimation of the arrival times. An important application that results in short time duration signals is seismic activity in mines. This seismic activity is produced by high stress faults in the vicinity of the mining operations. Locating the source of these seismic events is important for mine safety. This paper focuses on this mining application to demonstrate how the cross-correlation averaging algorithm can improve the arrival time delay estimation and thus provide better hypocenter location estimates of seismic events in mines.