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Underground pipelines are used in various process piping systems to transport gasses or fluids and are usually subjected to the effects of external corrosion. Corrosion can be defined as the deterioration of a material due to a reaction with its environment or the destruction of the material by means that are not mechanical (Fontana and Greene, 1967:2). External corrosion, due to the interaction between the pipe and the soil, is generally a slow process and the corrosion rate is influenced by a variety of external factors. Some of these factors include the ambient pH and salinity, the presence of moisture and bacteria, temperature, the electrical potential difference between the pipe and other structures and the implementation of preventative measures (such as cathodic protection and wrapping). Although the external corrosion of underground pipelines is generally a slow process in mild environments, pipe degradation as a result of external corrosion remains one of the prevalent reasons for the failure of underground pipelines. As with many mechanical systems that are prone to fail at one time or the other, the high costs involved with unforeseen failure necessitate some quantitative (or qualitative) indication of the condition of the pipe system. Some of the costs that can be expected as a result of unforeseen pipeline failure are, amongst others: • costs as a result of the failure of dependent systems; • costs as a result of the loss of production; • costs as a result of the loss of product (in distribution networks); • the cost of unscheduled maintenance (logistical costs); • costs as a result of damage to public property; • fines imposed by customers (in distribution networks); • costs related to pollution control, and • the loss of life The single most important parameter associated with the condition of a system is its profitable remaining life. This is the time during which a sub-system contributes to the well-being of a larger system and the organisation. Therefore, it is necessary to determine, with reasonable accuracy, the extent of the remaining life of a system so that managerial decisions (i.e. investments, cash-flow analyses, maintenance task scheduling and replacement programmes), based on this figure, can be made. Done correctly, this can directly lead to a decrease in maintenance costs and subsequently to an increase in profit. The extent of a corrosive attack on the pipeline might be highly localised or might be fairly uniform over the length of the installation. The fact of the matter is that, since the pipe is buried, it is very difficult to quantify the external damage caused by corrosion. A variety of techniques are in use to survey pipelines and detect anomalies. However, for large pipelines, most of these techniques are either inefficient or too expensive. There will always remain some uncertainty regarding the integrity of the pipeline. The work presented in this study is explained with valid generic examples and aims: 1. to provide the reader with sufficient background information so that the need for determining the integrity of a pipeline becomes apparent; 2. to indicate why a reliability-centred approach is necessary (Chapter 1); 3. to explain the basic principles of corrosion and the electrochemical nature of corrosion (Chapter 2); 4. to indicate areas, based on the basic principles of corrosion, where severe corrosion can be expected (Chapters 2 and 7); 5. to provide and elaborate on information regarding pipe surveillance techniques that are currently available (Chapter 3); 6. to establish the criteria for pipeline failure, in the form of a limit state Junction, for pipes that are subjected to near-constant internal pressures (static failure domain) as well as for pipes subjected to varying internal pressures (fatigue domain) (Chapters 5 and 6); 7. to indicate the sensitivity of the fatigue domain solution to changes in the system variables and to indicate that a significant reduction in the system variables does not necessarily reduce the solution accuracy (Chapter 6), and 8. to integrate the above-mentioned into a practical and workable guideline that can be used to determine the remaining life of an underground pipe network (Chapter 7). / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2002. / Mechanical and Aeronautical Engineering / unrestricted

This thesis documents a PhD research program undertaken at Swinburne University of Technology between the years 2000 and 2004. The research was funded by the Cooperative Research Centre for Cast Metals Manufacturing and was undertaken in collaboration with Nissan Casting Plant Australia Pty Ltd and the Ford Motor Company Australia Limited. This thesis reports on the investigation of the possibility of using an ultrasonic sensing-based, non-destructive testing system to detect gas porosity defects in aluminium die casting parts with rough surfaces. The initial intention was to develop a procedure to obtain ultrasonic signals with the maximum possible amplitude from defects within the rough surface areas of the castings. A further intention was to identify defects with the application of a suitable signal processing technique to the raw ultrasonic signal. The literature review has indicated that ultrasonic techniques have the potential to be used to detect subsurface defects in castings. The possibility of classifying very weak ultrasonic signals obtained from rough surface sections of castings through a neural network approach was also mentioned in the literature. An extensive search of the literature has indicated that ultrasonic sensing techniques have not been successfully used to detect sub-surface defects in aluminium die castings with rough surfaces. Ultrasonic inspection of castings is difficult due to the influence of microstructural variations, surface roughness and the complex shape of castings. The design of the experimental set-up used is also critical in developing a proper inspection procedure. The experimental set-up of an A-scan ultrasonic inspection rig used in the research is described in this thesis. Calibration of the apparatus used in the inspection rig was carried out to ensure the reliability and repeatability of the results. This thesis describes the procedure used to determine a suitable frequency range for the inspection of CA313 aluminium alloy castings and detecting porosity defects while accommodating material variations within the part. The results obtained from ultrasonic immersion testing indicated that focused probes operating at frequencies between 5 MHz and 10 MHz are best suited for the inspection of castings with surface roughness Ra values varying between 50 [micro milli] and 100 [micro milli]. For the purpose of validating the proposed inspection methodology, gas porosity defects were simulated through side-drilled holes in the in-gate section of selected sample castings. Castings with actual porosity defects were also used in this research. One of the conclusions of this research was that it was extremely difficult to detect defects in castings with surface roughness above 125 [micro milli]. Once the ultrasonic signal data was obtained from the sample aluminium die castings with different surface roughness values ranging from 5 [micro milli] to 150 [micro milli] signal analysis was carried out. Signal feature extraction was achieved using Fast Fourier Transforms (FFT), Principal Component Analysis (PCA) and Wavelet Transforms (WT) prior to passing the ultrasonic signals into a neural network for defect classification. MATLAB tools were used for neural network and signal pre-processing analysis. The results indicated that poor classification (less than 75%) was achieved with the WT, PCA and combination of FFT/PCA and WT/PCA pre-processing techniques for rough surface signals. However, the classification of the signals pre-processed with the combination of WT/FFT, FFT/WT and FFT/WT/PCA classifiers provided much better classification of more than 90% for smooth surface signals and 78% to 84% for rough surface signals. The results obtained from ultrasonic testing of castings with both real and simulated defects were validated with X-ray analysis of the sample castings. The results obtained from this research encourage deeper investigation of the detection and characterisation of sub-surface defects in castings at the as-cast stage. Implications for the industrial application of these findings are discussed and directions for further research presented in this thesis.

The objectives of this thesis are to develop and validate laser ultrasonic inspection methods for on-line testing of microelectronic devices. Electronic packaging technologies such as flip chips and BGAs utilize solder bumps as electrical and mechanical connections. Since they are located hidden from view between the device and the substrate, defects such as cracks, voids, misalignments, and missing bumps are difficult to detect using non-destructive methods. Laser ultrasonic inspection is capable of detecting such defects by utilizing a high power laser pulse to induce vibrations in a microelectronic device while measuring the out of plane displacement using an interferometer. Quality can then be assessed by comparing the vibration response of a known-good device to the response of the sample under inspection. The main limitation with the implementation of laser ultrasonic inspection in manufacturing applications is the requirement to establish a known-good reference device utilizing other non-destructive methods. My work will focus on developing a method to inspect flip chip devices without requiring a previously established reference. The method will automatically examine measurement data from a large sample set to identify those devices which are most similar. The selected devices can then be utilized to compose a hybrid reference signal which can be used for comparison and defect detection. Current trends in the electronic packaging industry continue to drive toward increased solder bump density, making it increasingly difficult to generate strong ultrasonic signals in these stiffer devices. To overcome this difficulty, I propose a new excitation method which places the source of ultrasound at the inspection location for each test point on the device surface. This ensures that the same power is available for each inspection location while also increasing the signal to noise ratio. The hardware implementation of this method reduces the system complexity and required automation, which can significantly reduce equipment cost and inspection time. The implementation of the proposed excitation method in conjunction with the use of a hybrid reference signal for defect detection will improve the utility of the laser ultrasonic inspection technique to on-line inspection applications where no other non-destructive methods are currently available.

This thesis was done i cooperation with SKB Clab in Oskarshamn and studies use of sonic waves for detecting voids and irregularities in the weld joints of copper capsules used for long term storage of radioactive waste. Since these could pose material failure and thereby risk radioactive contamination of ground water it is very important to find means of quality control before storage.  During the welding procedure changes occur to the integrity of the material. The homogenous metal – in this case copper – is distorted and voids appear in and around the welded volume. A non-destructive inspection method is needed to make sure that the metal holds for the strains of long term storage. These strains are not completely known at the moment and therefore the goal of this thesis is mainly to add another tool of inspection for future studies. The tests are done using ultrasonic mapping of the welded volume. This is achieved by sending ultrasonic pulse through test samples – welded copper pieces – and recording its reflection. The recorded signals are gathered in data matrices and processed using several different signal processing methods in search of irregularities and voids. To enhance the understanding of the results a graphical user interface (GUI) is developed that allows users to visualize the results.  The welded pieces, the ultrasonic mapping and its resulting data sets were delivered to this thesis and the scope of the thesis is to develop the GUI and apply known signal processing methods to the data set.  It is shown that the irregularities do appear and that ultrasonic detection and use of the processing method is useful for quality control of the material. Further field studies are needed to identify maximum number, size and perhaps shapes of irregularities that can be within tolerance levels of the storage project.