Experiment Study On Distributed Optical Fiber Sensor Based On OTDR For Crack Detection

Crack is always an indicator of the concrete structure damaging. Therefore, it's necessary to monitor cracks happened in a large civil engineering in a rapid and convenient way. Because distributed optical fiber sensors can be used to continuously monitor the cracks of concrete construct in a 3-D way, it has been got more and more attention. Recently the technique of optical fiber sensor can be used to dynamically detect cracks in real-time and online, find the position and degree of cracks in the inner of structure in time, monitor the evolution of the scathed district and then evaluate the level of the structures' health.In this thesis, the properties of distributed optical fiber sensor based on OTDR are studied, and the qualitative relation between optical loss and the width of cracks by fitting experimental data is derived. Moreover, a scheme is given, which can determine the position, width and orientation of cracks independently. The main results are the following: a reliable and applicable semi-experience calculation model for structure crack remote measurement has been obtained by analyzing the experiment results of the relation between the optical loss in fiber and the crack width. With the calculation model the fiber optical remote sensing error is less than 0.02mm comparing with the directly measured crack width, and in some key monitored points the error could be even less than 0.01mm. In the experiment an effect of optical loss saturation in the case of larger fiber-crack cross angles ( θ ≥45°) has been observed, which will be very useful in the practical engineering projects. Based on our knowledge it is the first time that a two wavelength optical source has been used for simultaneously monitoring the width and evolution tendency of cracks at a single position. And a direct 3-D scheme for constructing an embedded optical network in the potential split location has been proposed.Conclusively, these works will promote the theoretic development of technique of distributed optical fiber sensor and have great applications to monitor cracks and evaluate the health of structures.