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.     

On the information capacity of the invariant method in the analysis of resampled acoustic-emission streams

Specific features of the diagnostics of a prefracture state using the method of invariants of acoustic emission (AE) are considered. It is shown that “resampling” of a stream by measuring each mth pulse abruptly reduces the randomness of the properties of the initial AE stream. This makes it virtually impossible to determine the degree of hazard of a flaw on the basis of some statistical parameters of resampled Erlang streams (naturally, also including the AE invariant method) even at relatively small recalculation coefficients.     

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.     

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.     

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.     

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.     

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.     

Enhancement of the acoustic-emission method use for diagnostics of rolling stock cast parts

The techniques for acoustic-emission (AE) testing of cast parts (side frames and bogie bolsters) of a freight-car truck, which improve the reliability of evaluating their engineering state at different stages of the working cycle and decrease their repeated rejection, are considered. It is proposed to determine the degree of damage and the service life of cast parts on the basis of the analysis of AE-signal sources, which have the highest integral characteristics.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Methodological aspects of assessment of the strength and residual service life of pressurized vessels based on acoustic-emission diagnostics

The state-of-the-art in acoustic-emission diagnostics of pressurized vessels is analyzed. It is shown that the widely used qualitative relationship between nondestructive testing methods and the science of material strength can be replaced with a quantitative relationship based on specially organized non-destructive testing procedures carried out when the object being tested is operated. Based on a number of examples, in which the state of pressurized vessels subject to low-temperature hydrogen (hydrogen sulfide) corrosion and to pitting corrosion is assessed, the main concepts and options of the suggested methods are described. Results of the assessment of the residual service life of some vessels are presented. The conducted research shows that a new regulatory framework needs to be developed for assessing the operability of expensive equipment in oil-refining and petrochemical plants on the basis of acoustic-emission diagnostics, applied fracture mechanics, and risk analysis.Translated from Defektoskopiya, Vol. 40, No. 11, 2004, pp. 50–61.Original Russian Text Copyright © 2004 by Volkovas, Dorosevas, Elmanovich, Bagmutov.     

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.