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·Methods and Instrumentation
Wigner Spectrum as Powerful Tool for Description of Acoustic Emission Hit Generated from Ceramic Structures upon Bend Load
Department of Physics, Faculty of Civil Engineering, Technical University of Brno, Czech Republic
This article describes the utilisation of the Wigner Spectrum for the description of the Acoustic Emission Method on ceramic structures.
The time-frequency techniques can help to analyse acoustic emission amplitudes or parameters. Then it is possible to distribute hits to events and to determine cause of acoustic emission.
The ceramic specimens were loaded on bending. They were made from brick clay and chosen additional materials. The description of some material properties was observed by application of the Wigner Spectrum on recorded time representations of acoustic emission hits.
Introduction of the Acoustic Emission Method
The Acoustic Emission Method is relatively new Non-Destructive Testing technique. Contra most such techniques it detects active defects inside and on the material. When the structure is not loaded the acoustic emission can not detect any discontinuity - the acoustic emission does not exist. Consequently, it does not detect geometrical inaccuracy. Acoustic emission sensors collect the burst type of signals (hits). There are two basic ways of hit descriptions. Hit description by parameters is easier and does not need so many recorded values. Time recording of hit amplitude contains more information about acoustic emission phenomena, but the number of recorded values is higher then by the parameter's description.
In next parts of the article there is shown the description of time recorded acoustic emission hits by using time-frequency analyses. It is suitable to note, that it is not easy to obtain information about physical phenomena from recorded time history of acoustic emission hit. So scientists of all world endeavour to find some (mathematical) methods for easier identification and classification of physical phenomena from the hits. Transformation from time to frequency domain was the first attempt at finding this way. However it has long been known that the description in frequency domain is very useful for stationary signal but for non-stationary one it is not much suitable. Time-frequency analysis appears as powerful tool to determination non-stationary signals. This specifies frequency components in particular time moment. The article applies Wigner Spectrum (the tool from the group of time-frequency analyses) for description of acoustic emission hits.
The tested specimens were loaded upon bending to fracture (see Fig. 1). Proportions of tested rectangular samples were the length 12 cm, the width and the thickness 1 cm. They were forced middle of length by single force. The samples were kept at the distance of 1 cm from both ends. The sensors were placed on the opposite side of the holders.
Fig 1: Diagram of tested sample (S - sensor, F - force, length in cm)
Fig 2: Time history of hit amplitude
Fig 3: Wigner spectrum of the hit part in Figure 2|
The Acoustic Emission Method is very promising method to detect active defects into tested structure. Including modern mathematical methods to the Acoustic Emission Method is acquired as a very useful tool to analyse physical (technical) phenomena of tested loaded structure.
The experimental results showed that time-frequency technique (Wigner Spectrum) applied on analyses of hits is very powerful tool for classification of specimen properties. Frequency domain describes average frequency components occurred in the tested signal. However, time-frequency domain shows particular frequency components in time moments. Thus it is useful for describing signal behaviours whose spectral characteristic is time varying. This mathematical tool is better for analysis non-stationary signals than well-known Fourier analysis.
This research has been supported by Grant Agency of Czech Republic No~103/97/P140 "Study, Analysis and Evaluation of Acoustic Emission Signals Applied on Thin-Wall Systems".
PhDr. Lenka Ritickova (Technical University of Brno, Faculty of Civil Engineering, Department of Languages) made the language correctness of this article.
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