Title: Omnidirectional interferometric strain sensors for lamb wave detection
Language: English
Authors: Agbodjan Prince, Just Kpoti 
Qualification level: Doctoral
Advisor: Keplinger, Franz
Issue Date: 2016
Citation: 
Agbodjan Prince, J. K. (2016). Omnidirectional interferometric strain sensors for lamb wave detection [Dissertation, Technische Universität Wien]. reposiTUm. https://doi.org/10.34726/hss.2016.37335
Number of Pages: 118
Qualification level: Doctoral
Abstract: 
This thesis deals with a highly responsive method for detecting guided elastic waves, which is a promising means of non-destructive monitoring of large, preferably plate-like composite structures. Aiming at operation in environments of high electromagnetic activity, a double Michelson interferometer composed of fiber-optic components forms the core of the wave strain transduction system. The investigated interferometer employs a 33 fiber-optic directional coupler for splitting and recombinating optical waves. A segment of optical fiber that is preferably attached to the surface of the monitoring body carrying the elastic wave forms the measurement arm of the interferometer while a separate segment serves as reference. The directional characteristic of the strain transduction varies between pronounced directional or omnidirectional depending on the straight or circular shape of the attached fiber segment, respectively. Multi-turn spiral coiled fiber segments enable extraordinary high strain resolution. The double interferometer technique offers pseudo-quadrature interferometer signals that enable rather uniform resolution over a wide range of fiber elongation. This research work examines the function, technical properties, and application related aspects of interferometric transducers using finite element analysis and analytical modeling. Basic system characteristics are confirmed by experiments. Fundamental questions related to the selected detection technique that will be answered by this thesis include achievable sensitivity and resolution, perturbations of the elastic waves introduced by the attached fiber segment and the distortions of the fiber-optic strain conversion by the finite length of interaction between fiber and structure. Among other results, a leading-edge strain resolution of 8 10-9 was achieved with a single-turn fiber-optic strain transducer without any optimization of the detection system. Finite element (FE) simulations proved that attached fiber coils induce very moderate reflection and mode conversion so that a 10-turn fiber coil exhibits twice the elongation of a 5-turn coil. The transducer output signal becomes temporarily deformed compared to the wave strain if the wavelength of the surface strain approximates a fraction of the coil dimension.
Keywords: Lamb wave detection; FE simulation
URI: https://doi.org/10.34726/hss.2016.37335
http://hdl.handle.net/20.500.12708/4750
DOI: 10.34726/hss.2016.37335
Library ID: AC13408699
Organisation: E366 - Institut für Sensor- und Aktuatorsysteme 
Publication Type: Thesis
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Appears in Collections:Thesis

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