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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Finite-element calculation of high-temperature acoustic-emission transducers

The use of the finite-element method for studying piezoelectric high-temperature transducers was described in application to the problems of acoustic emissions. The amplitude-frequency characteristics were calculated and the character of resonances was investigated. The theoretical results were shown to be in good agreement with experimental data.     

Modeling of acoustic-emission processes in a solid

On the basis of a model for describing the processes of acoustic emission (AE) and the relationships following from it and relating the AE parameters to the structural and physical characteristics of a solid subjected to damage, AE processes are mathematically simulated. It is shown that the morphology of the AE amplitude-frequency spectrum is determined by both the crack-size distribution function and the dependences of the displacement amplitude of interphase surfaces and the difference of elastic stresses on the surfaces of radiating cracks on their effective radius.     

Design of a piezoelectric transducer cylindrical phase modulator for simulating acoustic emission signals

To conveniently carry out the pipeline leak experiment in a laboratory, leak acoustic signals are simulated by using the converse piezoelectric effect of a piezoelectric transducer (PZT) cylindrical phase modulator. On the basis of the piezoelectric equations and electromechanical equivalence principle, the transfer function of a PZT cylindrical phase modulator is delivered. A PZT cylindrical phase modulator is designed, and the numerical simulation is conducted. Results prove that the PZT cylindrical phase modulator can effectively simulate leak acoustic emission signals when the frequency is lower than 25 KHz. __________ Translated from Journal of Beijing University of Technology, 2006, 32(8): 683–687 [译自: 北京工业大学学报]     

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.     

A local method for loading a tested object during acoustic-emission diagnostics

A new acoustic-emission diagnostics method in which there is no need to load an entire structure has been studied. This method of monitoring (as a conventional acoustic-emission method) is based on making a deformation of the tested surface; however in this case, the load is applied to a preliminarily chosen site of the tested object in the most dangerous area.     

A technique for acoustic-emission testing of the wheel pairs of a freight-car carriage

A testing technique developed for determining the acceptance levels of different inspected zones of a wheel pair is considered. It has been shown that the acceptance levels are individual for different inspected zones and are set on the basis of the statistical data obtained during the tests of wheel pairs containing flaws detected beforehand. A test record sheet containing coordinates of flaws and comments on their hazard is issued during automated acoustic-emission testing of the wheel pairs of a freight-car carriage.     

Comparison of acoustic-emission tomography images to an etalon

Criteria for comparing images of acoustic-emission tomography to an etalon via numerical simulation are tested. The results of approbation of metric and correlation criteria for comparing images and the signal-to-noise ratio for 3D inhomogeneities of sinusoidal and gaussoid types are presented. The pixel-by-pixel absolute and relative differences between images are efficient for obtaining the spatial distribution of differences between images and are quite sensitive to the inhomogeneity parameters and the data-collection and tomographic-reconstruction schemes. The comparison of the reconstructed images to a gaussoid-type etalon has shown that the image-correlation coefficient and the signal-to-noise ratio are quite sensitive to the inhomogeneity parameters and the data-collection and tomographic-reconstruction schemes. The integral metric criteria (the mean absolute and rms deviations) are almost insensitive to these parameters. The comparison of the reconstructed images to a sinusoidal etalon characterized by a large constant component has shown that the metric criteria and the centered-data correlation coefficient are sensitive to changes in the inhomogeneity parameters and the data-collection and tomographic-reconstruction schemes.