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The contour method has emerged as a promising technique for residual stress measurement in relatively large, thick and complex engineering components. The method involves making a cut in the sample of interest, measuring the subsequent relaxed deformation profile of the cut surface and using this profile to back-calculate the original out-of-plane residual stress field by finite element modelling. The method is based on the theory of elasticity in that the stress relaxation during test specimen cutting is assumed to be entirely elastic. However, when measuring residual stresses of magnitude approaching the material yield strength, plasticity can occur which introduces errors in the measured stress profile. The main aim of this thesis was to develop methods of mitigating and estimating plasticity induced errors in contour method residual stress measurements. Based on the principles of fracture mechanics, an analogy was made between the stress relaxation process and a cracked plate to investigate the origin of plasticity in the contour method. It was demonstrated that that the cut tip stress intensity factor (KT) and the corresponding plastic zone parameters are the most important parameters for characterising plasticity-induced errors in the contour method. Extensive finite element analyses were carried out to understand and control the errors associated with plasticity with a view of improving the accuracy and reliability of the method. The outcomes of this research provide a valuable insight into how accumulation of plasticity for different restraining conditions affects the performance of the contour method. A novel cutting strategy that aims at mitigating plasticity-induced error by controlling the severity of the cut tip stress concentration (i.e. stress intensity factor) during the cutting process has been developed. Furthermore, procedures (correlations) are developed to estimate the plasticity-induced errors in the results of the contour method. Finally guidelines are proposed and applied to a case study for mitigating the errors associated with plasticity in a contour method residual stress measurement.

Thermosonics is a non-destructive testing method in which cracks in an object are made visible through the local generation of heat caused by friction and/or stress concentration. The heat is generated through the dissipation of mechanical energy at the crack interfaces by vibration. The temperature rise around the area close to the crack is measured by a high-sensitivity infrared imaging camera whose field of view covers a large area. The method therefore covers a large area from a single excitation position so it can provide a rapid and convenient inspection technique for structures with complex geometry and small and closed cracks. An ultrasonic horn, originally designed for welding, has generally been used for thermosonic testing. However, it is diffcult to obtain reproducible and controllable excitation with the existing horn system because of non-linearity in the coupling; surface damage can also be produced by chattering caused by loss of contact between the tip of the horn and the structure. Therefore, the general aim of the study was to develop a reliable and convenient excitation method that should excite sufficient vibration for the detection of the defects of interest at all relevant positions in the structure and must also avoid surface damage. In this thesis, a numerical and experimental study for the development of the ex- citation method for reliable thermosonic testing is presented. Successful excitation methods for the detection of delaminations in composites and cracks in metal struc- tures are described. A simple, small wax-coupled PZT exciter is introduced as a con- venient, reliable thermosonic test system in applications where relatively low strain levels are required for damage detection such as composite plates. A reproducible vibration exciter may be su cient for thermosonic testing in some metal structures such as a thin plates. However, higher strain levels are often required in metal structures, though the required strain level is dependent on the crack size. This level of strain is not easily achieved within the reproducible vibration range because of non-linearity in the contact between the exciter and the structure. Therefore, studies are conducted with an acoustic horn with high power capability to investi- gate the characteristics of the vibration produced in a real structure with complex geometry and to develop a excitation method for achieving reliable excitation in the non-linear vibration range for thermosonic testing. An excitation method for a complicated metallic structure such as a turbine blade is also investigated and the in uence of the clamping method and the excitation signal that is input to the horn on the vibration characteristics generated in the testpiece is presented. As a result, a fast narrow band sweep test with a general purpose amplifier and stud coupling is proposed as an excitation method for thermosonic testing. This method can be ap- plied to different types of turbine blades and also to other components. One typical characteristic of a thermosonic test using non-linear vibration is the lack of repeata- bility in the amplitude and the frequency characteristic of the vibration. Therefore, vibration monitoring is necessary for reliable thermosonic testing and a Heating In- dex(HI) has been proposed as a criterion indicating whether su cient vibration is achieved in a tested structure or not. The HI is calculated from different vibration records measured by different sensors and these results are compared in this thesis. A microphone can provide a cheaper and more convenient non-contacting vibration monitoring device than a laser or strain gauge and the heating index calculated by a microphone signal shows similar characteristics to that calculated from the other sensors.

Non-destructive testing (NDT) is the process of non-invasive material examination. Within this field, ultrasonic inspection is one method of examination used to detect flaws in structural and functional industrial components, to assess their structural integrity and fitness for service. Conventional NDT ultrasonic array techniques transmit on multiple elements in parallel, according to a focal law, which facilitates beam steering, focussing and scanning within the test component. Received signals are then 'stacked' to generate images of the test component. With the advent of affordable high speed computing, novel data acquisition techniques based on sequential transmission are now able to be developed, which allow images to be generated using advanced signal processing and image reconstruction algorithms. One such data acquisition technique known as Full Matrix Capture (FMC), has received considerable research attention in recent years, largely because it allows fully focused images of test components to be generated. This project provides an improved understanding of the FMC technique and associated signal processing algorithms. It achieves this through the development of novel inspection techniques and signal processing algorithms. Collectively algorithms developed within this work were termed Sequential Phased Array (SPA). Initially comparisons were made between conventional ultrasonic techniques and the SPA algorithms in terms of image quality and speed of inspection. A novel approach was then suggested to facilitate inspection through dual-layered media, separated by a refractive interface using the SPA algorithms. The use of sparse arrays to enhance the speed of inspection using the SPA algorithms was also investigated, and the trade-off between speed of inspection against image degradation was addressed. Finally a novel approach to calibration of a FMC based system was developed, in order to provide uniform image sensitivity during inspection.

Das Ziel der Arbeit bestand darin, die aus der Literatur bekannten Zusammenhänge zwischen Korrosionsaktivität von Stahl im Beton und einer galvanostatischen Pulsmessung, welche unter Laborbedingungen gute Ergebnisse liefern, auf Messungen an realen Bauwerken anzuwenden. Dazu wurden zunächst Untersuchungen an 13 Jahre alten Prüfkörpern durchgeführt und ausgewertet. Die abgeleiteten Zusammenhänge konnten anschließend an realen Bauteilen verifiziert werden. Somit stellt die galvanostatische Pulsmessung eine hilfreiche zerstörungsfreie Prüfmethode zur Interpretation des Korrosionszustandes dar. Die neuen Erkenntnisse lassen eine bessere Einschätzung des Korrosionsverhaltens in Stahlbetonbauwerken zu, als es die Potentialmessung erlaubt.

Das Ziel der Arbeit bestand darin, die aus der Literatur bekannten Zusammenhänge zwischen Korrosionsaktivität von Stahl im Beton und einer galvanostatischen Pulsmessung, welche unter Laborbedingungen gute Ergebnisse liefern, auf Messungen an realen Bauwerken anzuwenden. Dazu wurden zunächst Untersuchungen an 13 Jahre alten Prüfkörpern durchgeführt und ausgewertet. Die abgeleiteten Zusammenhänge konnten anschließend an realen Bauteilen verifiziert werden. Somit stellt die galvanostatische Pulsmessung eine hilfreiche zerstörungsfreie Prüfmethode zur Interpretation des Korrosionszustandes dar. Die neuen Erkenntnisse lassen eine bessere Einschätzung des Korrosionsverhaltens in Stahlbetonbauwerken zu, als es die Potentialmessung erlaubt.