A Study of Tensile and Fatigue Loading Effects on the Performance of Metal-Packaged FBG Strain Sensor Developed for Cryogenic Applications

Strain measurements are the fundamental task for structural health monitoring of critical equipment for cryogenic applications. However, the failure of strain sensors has been inevitable due to cyclic loading and/or over-loading. Therefore, a durable metal-packaged FBG sensor for strain measurements at cryogenic temperatures is successfully developed by combining magnetron sputtering and electroplating processes. Tensile and fatigue loading effects on the performance of the sensors are evaluated for the first time by uniaxial tensile testing at constant cryogenic temperatures down to 80 K and tension-tension fatigue testing at 133 K to verify their durability and reliability. The sensors which have high-quality multilayer metallic layers and good interfacial bonding exhibit markedly higher sensitivity than that of the bare FBG, with good linearity, stability, and repeatability and without obvious hysteresis. Fatigue failure of the sensors may occur due to fatigue fracture of the optical fibers with the gratings at a certain high strain level and due to zero shift caused by interface debonding or cracking at a relatively lower strain level. As the strain level is lowered, the sensor fatigue life is increased. There is at least one substantially lower strain level at which the sensors with good linearity, repeatability, and stability do not fail at the maximum number of test cycles. These results indicate that the metal-packaged FBG sensors are promising candidates for long-term measurement of strains, and thus monitoring structural health of critical equipment at cryogenic temperatures.