For this analysis only three sources were considered

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For this analysis, only three sources were considered: plastic deformation, fatigue cracking and rubbing. Each of these sources might be further broken down. For this research, however, the focus was determining whether fatigue cracking was occurring and when. A typical problem associated with acoustic emission testing is separating the desired crack data from plastic deformation, rubbing and any extraneous noises. To minimize undesired sources, the data acquisition software recorded only waveforms over a given amplitude. This threshold eliminated unwanted low amplitude signals such as system noise. Wideband transducers from 100 kHz to 1.0 MHz were used rather than resonant transducers because the exact characteristics of the crack signals were unknown before testing. Knowing the predominant frequency of a cracking signal in this type of material may allow for less expensive and more sensitive resonant transducers, letting the test system key in on the fatigue cracking signals and filter out some unwanted noise. Six waveform characteristics are parameters used in quantifying acoustic emission test signals. 1. Energy is the measured area under the rectified signal envelope. 2. Duration is the time elapsed from when the waveform initially crosses the threshold until the waveform drops beneath the threshold again. 3. Counts are the number of times the waveform crosses the threshold while moving upward. 4. Counts to peak are the number of counts to the highest amplitude peak. 5. Amplitude is the largest voltage peak in the waveform signal. 6. Rise time is the time elapsed from when the waveform initially crosses the threshold until it reaches its peak amplitude. These parameters were used to evaluate indications and ascertain the source mechanism — fatigue cracking, plastic deformation, rubbing or extraneous noise. Acoustic Emission Test Hardware The hardware necessary for acoustic emission test analysis includes acoustic emission transducers and a computer with data acquisition capabilities. For the in-flight tests, four wideband transducers were used having a frequency response from 100 kHz to 1 MHz. In most cases, wideband transducers would not be used in an environment with so much noise; however, the main purpose of this research was to determine whether acoustic emission fatigue crack signals could be separated from other signals amidst a lot of noise. The transducers were connected to a data acquisition board in the computer. A 30 dB threshold and a 40 dB gain were used during the tests. Different filters were used. The laboratory test used no filter; the first flight test used a 10 kHz high pass filter; to reduce propeller noise, the second flight test used a 100 kHz high pass filter. Throughout the tests, the system recorded the data from the four transducers. After acquisition, the data were analyzed by playing the data back 368 Acoustic Emission Testing F IGURE 12. Flow chart of the data analysis process.
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  • Fall '19
  • Fighter aircraft, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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