The continuous research of lightening mechanical and aeronautical structures is leading to the adoption of innovative materials and structure designs. and this development has to be necessarily followed by research in optimising structural health monitoring techniques. The set up of a reliable and quick monitoring technique. together with the enhancement of the mechanical performances of structures, is the tUl1ung point to the usage of even lighter and longer-lifed structures. Being impossible to build up a monolithic structure, it is common sense to understand that the most critical part of a structure is the joint between two or more different parts. For this reason, research is being focused on both the improvement of existing joint configuration, and the development of automatic systems which are able to monitor the health state of these joints when the structure is in service. The main reason urging to improve the current composite fastening joint and to develop innovative in situ monitoring techniques is linked to the continuous pursuit of cost reduction. In fact, a stronger and longer lifed fastening configuration, together with a reliable. quick and real-time non-destructive monitoring system applied on in service structures, would allow a considerable cut in maintenance time, with a consequent drop of costs. The objectives of this PhD work are manifold. On one side it focuses on the development of an innovative structural coupling configuration which is able to improve the mechanical properties of the joint; on the other hand, it aims to define reliable indexes able to assess the state of health of a joint structure. More precisely. the entire part concerning the monitoring side of the problem, it focuses on the definition of different possible method capable of determining the loosening state of a bolted connection. These methods are all based on the vibration and ultrasonic based techniques. The first step consists in the development of indexes able to predict the healthy state of a joint by studying some properties related to acoustic signals passing through the part's connection. After theoretically defining the parameters involved, experimental campaigns aimed to validate the models have been carried-out. Once the indexes were found, and confirmed to work on a small simplified structure, the most reliable and accurate IS chosen to be applied for the monitoring procedure on larger and more complicated structures. Both theoretical and experimental approaches were adopted to justify the reliability of the used method. Ultimately it was proved that with the adoption of few sensors and natural frequencies based vibration, with the developed method, it is possible to assess the location of a loosening bolt also on a multibolted structure. Additional research has to be done on real larger scale structures, hut promising results based on solid theoretical principles have been achieved. With regards to the necessity of enhancing existing fastening coupling configuration, all the efforts concentrate on the adoption and optimization of hybrid joints. Hybridization joint process takes advantages of positive aspects of both composite materials and fastening joints. Most important, the rising use of composite parts contributes to the reduction of structures' weight, with all the advantages involved, such as for instance, fuel consumption reduction with a consequent decrease of running costs. For this reason the second part of the thesis will discuss possible solution able to get out the best from coupling together composite materials and fastening configuration. More precisely all the attentions will focus on how to reduce problems generated by fastening composite parts, such as low bearing and shear strength, notch sensitivity and high dependency of joint strength from the lay-up configuration. The proposed solution will be as strong as the hybrid solutions developed so far, but also lighter at the same time
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604566 |
| Date | January 2013 |
| Creators | Amerini, Francesco |
| Publisher | University of Bath |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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