Evaluation of an active acoustic waveguide sensor for embedded structural monitoring
This Thesis describes research aimed towards the realisation of an embedded ultrasonic monitoring system for operation in a smart structure environment. Alien fibres embedded within carbon fibre reinforced composite plates could be utilised to guide ultrasound to strategic release points for the interrogation of the test sample, with minimal structural degradation. In particular, an array of fibre waveguides could be utilised to deliver periodic stresses to a plate-type structure to excite Lamb wave propagation. With such a system several problems need to be addressed. Firstly, identification of an appropriate mode of propagation to transport the acoustic energy along the waveguide and meet the requirements of the system is required. Theoretical analysis of elastic wave propagation in cylinders is provided to establish the characteristics of the various supported modes. Subsequently, the longitudinal (axisymmetric) modes are highlighted as being the most appropriate for the proposed system. Ideally, such a mode should be singly generated to provide increased control on the coupling of ultrasound from the waveguide to the desired Lamb wave mode. Therefore, the transducer-fibre interface utilised for efficient single mode generation in the fibre waveguides is essential. This Thesis investigates a novel, cheap, simple, and robust coupling strategy, which employs a conical polymer bond to behave as a mechanical transformer efficiently coupling the lateral displacements of a piezoceramic transducer into longitudinal displacements in the waveguide. Theoretical and experimental analysis of this technique are described and design guidelines detailed. The Thesis then goes on to provide experimental and theoretical evidence of the fibre waveguide's ability to transport acoustic energy to and from plate-type structures for subsequent Lamb wave generation and detection. Initially, a surface mount strategy is adopted, whereby the ends of several thin cylindrical waveguides are bonded to the surface of aluminium plates to behave as a line array. Generation and detection of the fundamental symmetric (So) and asymmetric (A0) Lamb wave modes is demonstrated. Following this, the Thesis details considerations concerning the realisation of an embedded fibre waveguide monitoring system. Here, the choice of waveguide material and geometry is crucial to ensure the guided ultrasound does not leak into the structure at undesirable locations and that the structural integrity of the test sample is maintainable. Consequently, various waveguide configurations are investigated to establish an arrangement capable of meeting the system requirements. An embedded fibre waveguide Lamb wave system is then presented in a hard-setting polymer plate confirming the basic design methodology. Finally, So Lamb wave generation and detection in a carbon fibre reinforced composite plate is demonstrated utilising embedded acoustic waveguides sensors.