Structural Health Monitoring of Composite Cryogenic Propellant Tanks Using Optical Fiber Sensors

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A reusable launch vehicle is attracting much interest as one of promising future space transportation systems. Advanced composite cryogenic tanks are necessary to achieve an affordable light-weight vehicle. It is highly demanded to establish the structural integrity and reliability of such composite tanks because failure mechanisms of composite tanks are very complicated and are not well understood. The purpose of the present study is to establish a real-time health-monitoring technology to continuously monitor the strains and temperatures using optical fiber sensors embedded in composite cryogenic tanks.First, we adhered FBG (fiber Bragg grating) sensors on CFRP tanks in an experimental reusable launch vehicle, and measured the strains and temperatures in real time using a telemetry system during a launch-return cycle. The validity of the measuring method was proven and some fundamental problems were clarified.Then, we embedded several FBG sensors in filament-wound CFRP tanks. Small-diameter optical fibers (52 μm in diameter including the coating) were used to avoid the strength reduction due to the embedment. A new FBG temperature sensor was proposed to distinguish the temperature and the mechanical strain. An optimum measurement procedure was studied to obtain reliable strains and temperatures through comparison between experiments and theoretical prediction. Continuous monitoring was conducted during the entire fabrication cycle of filament-wound CFRP tanks. Effects of various fabrication conditions on thermal residual strains were studied. Detection of transverse cracks was also successfully made during the pressuring stage of CFRP tanks through the measurement of FBG reflection spectra.