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Thesis (M.A.). / Written for the Music Technology Area, Dept. of Theory, Schulich School of Music. Title from title page of PDF (viewed 2009/04/08). Includes bibliographical references.

Thesis (M.S.)--State University of New York at Binghamton, Thomas J. Watson School of Engineering and Applied Science, Department of Mechanical Engineering, 2007. / Includes bibliographical references.

Thesis (Masters in mechanical engineering)--Washington State University, May 2008. / Includes bibliographical references (p. 63-64).

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 188-196). Also available via the Internet.

The aim of this research work was to develop an Acoustic Emission (AE) source location method suitable for Structural Health Monitoring (SHM) of composite aircraft structures. Therefore useful key signal features and sensor configurations were identified and the proposed method was validated using both artificially generated AE as well as actual AE resulting from damage. Acoustic Emission is a phenomenon where waves are generated in stressed materials. These waves travel through the material and can be detected with suitable sensors on the surface of the structure. These stress waves are attributed to propagating damage inside the material and can be monitored while the structure is in service. This makes AE very suitable for SHM, in particular for aircraft structures. In recent years composite materials such as carbon fibre reinforced epoxy (CFRP) are increasingly being used for primary and secondary structures in aircraft. The anisotropic layup of CFRP can lead to different failure mechanisms such as delamination, matrix cracking or fibre breakage which affects the remaining life time of the structure to different extents. Accurate damage location is important for SHM systems to avoid further inspections and allows for a maintenance scheme which considers the severity of the damage, due to damage type, extent and location. This thesis presents a novel source location method which uses a small triangular AE sensor array. The method determines the origin of an AE wave by a combination of time of arrival and modal analysis. The small footprint of the array allows for a fast and easy installation in hard-to-reach areas. The possibility to locate damage outside and at a relatively far distance from the array could potentially reduce the overall number of sensors needed to monitor a structure. Important wave characteristics and wave propagation in particular in CFRP were investigated using AE simulated by an artificial source and actual damage in composite specimens.

M.Ing. (Electrical And Electronic Engineering) / A need was established for the development of a acoustical sensor in silicon. The acoustical sensor is based on the silicon pressure sensor. This sensor consists of a silicon membrane which is formed from silicon bulk material by means of etching. By transforming the movement of the membrane into a variation of capacitance, it is possible to detect a change in pressure. Signal processing is needed to insure that the : information, received from the sensor, is useable and accurate. The physical aspects of sound was investigated. An important relationship was derived between the intensity of a sound and the pressure associated with it. Because silicon was used in a mechanical environment, the mechanical properties of the material were investigated. A model was developed to simplify the design of the senor. A expression was derived for the movement of the membrane for any uniform load applied to it. The variation in capacitance was given by integrating over the area of the membrane. The condense microphone was used as a example of a design of a acoustical sensor. Arising from the example, the natural frequency of the silicon membrane was looked into. A variety of diaphragms were made to provide for a wide working area. Some of the manufacturingprocesses were individually examincd and changed to comply with the manufacturing of the diaphragms. The important process of etching was thoroughly investigated. An excellent etching agent was found with which accurate micro machining could be done. Using this etching agent, a variety of silicon and poll silicon diaphragms were made. Most of the silicon diaphragms were suceossfully formed. The. poli silicon diaphragms were however deformed because of internal stress which developed during the forming process. A signal processing circuit was developed to perform the task of transforming the variation in capacitance to a variation in voltage. The circuit utilised the current through the capacitor to establish the variation in capacitance. The current was transferred to a voltage by a differencing current amplifier. Further signal processing is done by a analogue multiplier.

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