AE02.pdf

In addition to characteristics of the waveforms

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In addition to characteristics of the waveforms themselves, there is information available from the cumulative characteristics of the signals (including the amplitude distribution and the total number of signals detected) and from rate statistics (such as the rate of signal arrival or energy at the transducer). There are thus several options available for acoustic emission source characterization. An appropriate approach must be determined for each application. Characteristics of Discrete Acoustic Emission Discrete or burst type acoustic emission can be described by relatively simple parameters. The signal amplitude is much higher than the background and is of short duration (a few microseconds to a few milliseconds). Occurrences of individual signals are well separated in time. Although the signals are rarely simple waveforms, they usually rise rapidly to maximum amplitude and decay nearly exponentially to the level of background noise. The damped sinusoid in Fig. 5 is often used to represent a burst of acoustic emission. Acoustic emission monitoring is usually carried out in the presence of continuous background noise. A threshold detection level is set somewhat above the background level (Fig. 6) and serves as a reference for several of the simple waveform properties. Using this model, the waveform parameters in Fig. 7 can be identified: (1) acoustic emission hit, (2) acoustic emission count (ringdown count), (3) acoustic emission hit energy, (4) signal amplitude, (5) signal duration and (6) signal rise time. 45 Fundamentals of Acoustic Emission Testing P ART 3. Acoustic Emission Signal Characterization 1 F IGURE 5. Idealized representation of an acoustic emission burst signal. Amplitude (relative scale) Time (relative scale) F IGURE 6. Threshold setting to avoid triggering by continuous background noise. Trigger level Noise Amplitude (relative scale) Time (relative scale)
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Cumulative representations of these parameters can be defined as a function of time or test parameter (such as pressure or temperature), including: (1) total hits, (2) amplitude distribution and (3) accumulated energy. Once a specific parameter is selected, rate functions may be defined as a function of time or test parameter: hit rate, count rate or energy rate. Based on Fig. 7, several techniques for characterizing acoustic emission are described below. Waveform Parameters Emission Hits and Count Acoustic emission hits are individual signal bursts produced by local material changes. The hit count is the number of times a signal crosses a preset threshold. High amplitude hits of long duration tend to have many threshold crossings. The number of threshold crossings per unit time depends on (1) the transducer frequency, (2) the damping characteristics of the transducer, (3) the damping characteristics of the structure and (4) the threshold level.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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