Temperature and Load effects in Modelling and Experimental Verification of Acoustic Emission Signals for Structural Health Monitoring Applications

The present study focuses on understanding the effect of load and temperature on Acoustic Emission (AE) signal propagation in an Aluminium 2024-T3 panel. In addition, the ability of an AE system to locate damage under these operational and environmental conditions was evaluated. The work was performed in two stages. In stage one, the wave group velocities of guided Lamb waves were measured for a range of temperatures from -40 ?C to 70 ?C. At each temperature level, six different static loads were applied that ranged from 0 MPa to 250 MPa in increments of 50 MPa. A mathematical analytical model for the effect of temperature on the wave group velocities was re-produced in order to verify it through experimental and FEM results. Furthermore, experimental and FEM results have been obtained for the effect of load and the combined case. It was observed that the variation of temperature and load altered the wave group velocities. The results showed that an increase in the temperature resulted in a decrease of the wave group velocity and vice versa. Furthermore, external applied loads resulted that the change group velocity varies linearly with increasing stress and has a sinusoidal dependence with the angle between sensor path and loading direction. Which meant that the wave group velocity decreased for small angles, increased for large angles and varied with a sinusoidal behaviour for the other angles between the propagation path and the loading direction. The effect of temperature and load can be super imposed onto each other. Experimentally obtained wave group velocities with temperature were within an error of 6% to the analytical solution and for the FEM results this was within 3%. In stage two, a representative AE signal, simulating a fracture phenomenon was emitted from a randomly selected point. Using values of wave velocity measured in stage one, the location of the representative AE signal under these conditions was calculated and errors were determined. It was found that the location algorithm was not sensitive to wave group velocities changes due to temperature and loads, thus providing an accurate location of the source within 1cm, for a specimen size of 65 by 60 cm. The experimentally obtained localization results were also supported with results from a numerical model that calculated location errors for many locations in and outside the array. The effects of temperature and load were taken into account in the Time Differences Of Arrival (TDOA) function for a known location. These TDOA were then fed into a location algorithm (Geiger’s method). It was found that location errors due to temperature or load were within 1.5 cm and were at some areas more significantly affected than at other locations. The reduced location accuracy of these source locations can be related to the angle dependent effect of load on wave group velocities. Source locations that have an angle between most sensors paths and loading direction that fits the largest change in wave velocity were found to be more significantly affected. The experimental results also presented problems with threshold selection during the experiments. A low AE threshold could translate into too much noise in the acquired signals, which can result in wrongfully triggering, while a high AE threshold could translate into incorrect group velocity measurements because a later part of the waveform would be used for triggering. Therefore the development of an threshold independent trigger mechanism is a recommendation for further research as well as investigating the effect of EOC in complex or composite structures.

Subject

Acoustic Emission
Structural Health Monitoring
Environmental and Operational Conditions

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master thesis

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