Validation and Veri cation of the Acoustic Emission Technique for Structural Health Monitoring

Abstract

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 di erent AE sources in potential SHM applications. Experiments were conducted to monitor the AE signals generated during fa- tigue crack growth in coupon 2014 T6 aluminium. The in uence of stress ratio, stress range, sample geometry and whether or not the load spectrum was of constant or variable amplitude were all investigated. Timing lters were incor- porated 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 di erence of arrival were taken for assessment of errors in location estimates obtained using time of ight algorithms with a 1D location setup. It was found that there was signi cant 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 rst 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 nal failure. The changes in AE signals with applied cyclic load provided great insights into the di erent AE processes occurring during crack growth. AE signals were seen to occur in the lower two-thirds of the maximum load in the rst 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 di erent 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 de cient. It is however limited by its sensitivity to changes in sample geometry. Experiments were also performed to validate the performance of the AE tech- nique in detecting and locating fatigue crack in a representative wing-box struc- ture. An acousto-ultrasonic method was used to calibrate the AE wave veloc- ity 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 di er- ent components of the structure. This also extends current knowledge on the capability of the AE technique.