Ultrasonic array inspections for complex defects

• Main Author: Felice, Maria Victoria
• Published: University of Bristol 2015
• Subjects: 620.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.687061

The aim of this thesis is to develop a simulation-assisted methodology for optimising ultrasonic array inspections and to apply it to an industrial problem. In particular, the focus is on inspections for complex defects and the motivation behind the work is an aerospace engine component that may suffer from stress corrosion cracking. Unlike fatigue cracks, these cracks are typically branched and are therefore difficult to detect with ultrasonic inspections. The development of the simulation-assisted methodology consists of two key parts. The first part is to implement a computer model that correctly simulates the interaction of ultrasound with complex cracks, while being efficient enough to be used in an optimisation exercise. An efficient Finite Element method and the Kirchhoff Approximation are compared with regards to their capability for simulating the ultrasonic scattering from complex cracks, and the Finite Element method is found to be preferable. This method is combined with ray tracing to obtain a hybrid model that simulates ultrasonic array data from the cracks and the complete model is successfully validated experimentally. The second part of the development of the methodology is to establish a process of obtaining real crack shapes to input into the hybrid model. The process which is developed involves the use of X-ray Computed Tomography to obtain high-resolution images of cracked parts and then segmenting the images. The computer model and real crack shape extraction process are used together to optimise the detection capability of an ultrasonic array inspection for the aerospace engine component. The signal-to-noise ratios obtained with a selection of array designs are compared with each other and with those obtained using a single element ultrasonic probe. The sizing capability of the array inspections is also investigated and the use of the Half-Skip Total Focusing Method to size cracks is analysed in detail.