MATERIAL CHARACTERISATION AND NONDESTRUCTIVE TESTING USING MICROWAVE TECHNIQUE

Abstract

Thermal barrier coatings (TBCs) and Glass Fibre Reinforced Polymer (GFRP) have been widely used as insulation coatings in many industries due to their highly favorable material properties. However, due to aging and cyclic process, the performance of these materials are affected by various defects such as cracks on metal substrate undercoating, delamination between coatings and metal substrate and porosity changes in the coating. Hence, numerous non-destructive testing (NDT) techniques have been employed to identify these defects, for example, eddy current testing, ultrasonic testing, thermography, X-ray inspection, acoustic emission, and liquid penetrant inspection. NDT plays a significant part in the maintenance of structural integrity of the system by enabling timely detection before critical damage occurs. Although each of these techniques has their advantages on their own application, there are certain limitations in inspecting dielectric coatings such as TBCs and GFRP. Microwave NDT appears to be one of the most promising inspection tools in detecting the presence defects in coatings. Microwave signals, unlike ultrasound signals, are capable of penetrating dielectric materials and interacting with their inner structure, with sensitivity to changes at boundary interfaces, without suffering from high attenuation, making them useful for defect detection underneath coatings. Microwave NDT techniques are based on studying the magnitude or phase data of the transmitted and/or reflected waves to inspect a sample under test. Changes in the data can reveal abnormalities, or changes in material characteristics, inside or at the boundary of the materials under inspection. This thesis focuses on the development of microwave NDT inspection technique using open-ended rectangular waveguide (OERW) for two main NDT applications; material characterization and defect imaging. The novel microwave NDT inspection system combined features of time and swept frequency domain architectures and signal processing to fulfil the research challenge. Theoretical background related to this research is employed in the initial stage of the thesis to provide the scientific discussion of proposed method. The first application is material characterization, particularly evaluating the porosity changes of in-service thermal barrier coated turbine blades using WR-90OERW. This is the first time porosity changes of cyclic coated turbine blade has been evaluated theoretically and experimentally using microwave NDT technique. The findings have been validated with relative permittivity measurement of TBCs using a waveguide technique and scanning electron microscope (SEM) image. The second application reported in this thesis is defect detection and imaging of crack-on-metal surface undercoating and delamination. The results show that new broad frequency sweep and time domain analysis using OERW is capable to detect and image the defects and reveal the depth of defects with an acceptable degree of accuracy. The results reported in this thesis prove that the proposed novel combined time and frequency domain, microwave NDT inspection technique provides a simple and effective tool for coating inspection that is vital for numerous industrial applications. Moreover, the proposed methods are capable of delivering an in-situ microwave NDT system and could form a vital part of quality control in manufacturing as well as a portable solution for service inspection.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robin Sloan (Supervisor) & Emad Alsusa (Supervisor)