Long-term Monitoring Method Of Fiber Grating Sensing Technology For Ship Structure

A ship is always subjected to long-term variable fatigue loads, deriving from winds, wavesand flows when sailing on the sea. Fatigue damage is going to occur within ship structures,threatening the global strength of ship. The ship structural health monitoring technology basedon Fiber Bragg Grating Sensor (FBGS for short), which monitors the loads and responses ofstructures through FBGS on real time, can predict and assess the serve safety of ship by expertsystem. It can send out alarm at the beginning of structural damage, thus avoiding the tragedy ofaccidents.Because of ship voyages in the environment of damp, high salinity, strong electrolyte andfrequent temperature alternation for a long time, the stability of FBGS will decrease in theabsence of protection, which would led to the phenomenon of large zero-drift, deviating from theactual value or unstable strain transfer. In order to ensure the long-term stability of the FiberBragg Grating sensing system, packaging methods, pasting technique, spot-welding technique,long-term protection approaches and the effect of bonding methods and protection approaches onFBGS sensing performance were intensively studied, and some conclusions were summarized asfollow:Firstly, this paper has compared and analyzed the existing packaging methods, temperaturecompensation measures, demodulation technology and multiplex technique for distributedsensing network according to the characteristics of ship structural health monitoring. And thedetail sensing properties of film-based FBGS were particularly investigated. Then, a set ofsuitable Fiber Bragg Grating sensing technology for ship structural were built, which consists ofthe film-based packaging method, the reference grating temperature compensation method, thetunable F-P filter method and a mixed distributed multiplexing technique.Secondly, factors influencing strain transfer ratio of film-based FBGS were analyzed bytheoretical derivation. Through contrast test, a set of pasting process for FBGS engineeringapplication were put forward.Thirdly, the effect of welding parameters, especially welding heat and welding time, on theFBGS welding results were derived by Joule’s law and heat balance equation. A new micro heatwelding method based on low voltage for FBGS were first time proposed. Two kinds of shipplates were taken to do the different parameters spot welding comparison test, and suitablewelding parameters were presented. After that, special spot-welding procedures for FBGS wereset up.Fourthly, protection methods, under the environment of high salinity and strong electrolytes, for FBGS were carried out. And two protection measures were creatively proposed, polyesterfilm protection method and metal protective cover method, for damp environment and fullyimmersed environment, respectively. Then, the long-term corrosion test proved that the twoprotection methods could make the FBGS maintain a long-term stable performance.Finally, the influence of metal cover’s stiffness on the FBGS measurement accuracy wasinvestigated, and a more appropriate structure of metal cover was developed by the means oftheoretical analysis and numerical simulation.