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Title: Validation and verification of the acoustic emission technique for structural health monitoring
Author: Gagar, Daniel Omatsola
Awarding Body: Cranfield University
Current Institution: Cranfield University
Date of Award: 2013
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The performance of the Acoustic Emission (AE) technique was investigated to establish its reliability in detecting and locating fatigue crack damage as well as distinguishing between different AE sources in potential SHM applications. Experiments were conducted to monitor the AE signals generated during fatigue crack growth in coupon 2014 T6 aluminium. The influence of stress ratio, stress range, sample geometry and whether or not the load spectrum was of constant or variable amplitude were all investigated. Timing filters were incorporated to eliminate extraneous AE signals produced from sources other than the fatigue crack. AE signals detected were correlated with values of applied cyclic load throughout the tests. Measurements of Time difference of arrival were taken for assessment of errors in location estimates obtained using time of flight algorithms with a 1D location setup. It was found that there was significant variability in AE Hit rates in otherwise identical samples and test conditions. However common trends characteristic of all samples could be observed. At the onset of crack growth high AE Hit rates were observed for the first few millimetres after which they rapidly declined to minimal values for an extended period of crack growth. Another peak and then decline in AE Hit rates was observed for subsequent crack growth before yet another increase as the sample approached final failure. The changes in AE signals with applied cyclic load provided great insights into the different AE processes occurring during crack growth. AE signals were seen to occur in the lower two-thirds of the maximum load in the first few millimetres of crack growth before occurring at progressively smaller values as the crack length increased. These emissions could be associated with crack closure. A separate set of AE signals were observed close to the maximum cyclic stress throughout the entire crack growth process. At the failure crack length AE signals were generated across the entire loading range. Novel metrics were developed to statistically characterise variability of AE generation with crack growth and at particular crack lengths across different samples. A novel approach for fatigue crack length estimation was developed based on monitoring applied loads to the sample corresponding with generated AE signals which extends the functionality of the AE technique in an area which was previously deficient. It is however limited by its sensitivity to changes in sample geometry. Experiments were also performed to validate the performance of the AE technique in detecting and locating fatigue crack in a representative wing-box structure. An acousto-ultrasonic method was used to calibrate the AE wave velocity in the structure which was used to successfully locate the 'hidden' fatigue crack. A novel observation was made in the series of tests conducted where the complex propagation paths in the structure could be exploited to perform wide area sensing coverage in certain regions using sensors mounted on different components of the structure. This also extends current knowledge on the capability of the AE technique.
Supervisor: Irving, Phil E. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Fatigue crack ; Probability of Detection ; Diagnostics ; Prognostics