Omnidirectional interferometric strain sensors for lamb wave detection / von Just K. Agbodjan Prince
VerfasserAgbodjan Prince, Just Kpoti
Begutachter / BegutachterinKeplinger, Franz
ErschienenWien, 2016
Umfangxii, 118 Seiten : Illustrationen, Diagramme
HochschulschriftTechnische Universität Wien, Dissertation, 2016
Zusammenfassung in deutscher Sprache
Schlagwörter (EN)Lamb wave detection / FE simulation
URNurn:nbn:at:at-ubtuw:1-91803 Persistent Identifier (URN)
 Das Werk ist frei verfügbar
Omnidirectional interferometric strain sensors for lamb wave detection [44.43 mb]
Zusammenfassung (Englisch)

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.