Acoustic emission testing at elevated temperatures using zinc oxide thin film broadband sensors
Creators: name: Ghouri, Aamir Ali
Publisher: name: Zenodo
Year: 2024

description: This research is focused on the detection and location of simulated AE sources in plate-like structures using zinc oxide (ZnO) based thin film sensing devices. Typically, thin film piezo devices have higher center frequencies from MHz-GHz. To use them for low frequency spectrum (less than 1 MHz) was the main question in this work. It was expected that the sensing devices would become less sensitive with a reduced signal to noise ratio when operate off- the- resonance. The signal conditioning hardware was built in-house to achieve reasonably improve SNR of the acquired data. For the deposition of zinc oxide thin films at micro-scale, a physical vapour deposition method was used. Thin films were sputtered on aluminium foil substrate using DC magnetron sputtering system under the optimised conditions. The film structures and crystal orientations were investigated using X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). To validate the performance of the bespoke thin film devices, the linear and triangulation AE source detections and localisation were performed. For simulated AE sources, both the pencil lead break and ball drop methods were used. For benchmark purposes, the perfo1mance of commercial AE sensors (UT-1000 from Physical Acoustic) was examined against the bespoke sensing devices. Data acquired using thin film devices for linear source location were suggested dominated anti­symmetric Lamb wave mode (Ao), using the velocity of the Ao wave mode the known and assessed source location was found in good commitment. For triangulation source location, the acquired data from thin film devices suggested a mean distance vector error of 0.013 m, the error value was found about 0.01 m smaller relative to the commercial AE sensing devices. The lower error in measurements with thin film sensors is possibly due to the difference in piezoelectric properties between the thin film and bulk PZT elements. Thin film devices were examined for detecting simulated AE sources at elevated temperatures up to 300 °C, the results demonstrated that the sensing competence of the bespoke sensors with anti-symmetric zero-order Lamb wave mode (Ao). It was found that the velocity of the mode decreased with the increment in temperature, in other words, the mode arrived later time as the temperature increased. The amplitude of the acquired signals deteriorated gradually with an increase in temperature. Two possible reasons for this trend would be (i) poor acoustic coupling and designed assembly deterioration (ii) the disparity in the coefficients of thermal expansions (CTEs) of the ZnO thin film and the test specimen. For future work, it is recommended to implement research outcomes in a commercial environment. This opportunity gives more confidence about the performance of developed AE systems with integration to bespoke thin film AE sensors and various data analysis approaches. Also, to utilise the flexibility feature of the thin film AE sensors, it will be useful to examine test geometries other than the plate, such as pipes and pressure vessels, which were not covered in the scope of this feasibility work.