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.