Durability and Mechanical Properties of Optical Fiber Strain Sensing Cable for Structural Health Monitoring in Harsh Environment

Optical fiber cables are being widely used in structural health monitoring. Thanks to their tiny dimension, the ability to perform distributed strain measurements and to withstand harsh environment, they can be also used in nuclear facilities. The durability of the sensor in such environments has therefore to be assessed. For this reason, mechanical and thermal tests on a custom strain sensing cable have been performed, before and after being irradiated up to 500 kGy. Such custom strain sensing cable includes a special fiber, resistant to radiation, put into a commercial strain sensing cable. Using a Brillouin scattering-based device (with a PPP-BOTDA technique), evaluations of the strain and temperature sensitivities are performed on the different layers of the sensing cable, separately and altogether. This allows evaluating the influence of each component on the cable performances. This includes the fiber in its primary coating, the obtained strain sensing cable and its constitutive component (FIMT, Fiber in Metal Tube). Results point out the importance of a proper calibration of the sensing system: the Brillouin strain and temperature sensitivities depend on the fiber type and on its coatings. The choice of the optimal sensor should be based on the whole sensing chain. Results show also that radiation impact remains moderate over the target sensing range (1 % in elongation). The sensitivity coefficients variation due to radiation also remains very small.