Thermal spray to embed optical fibers for the monitoring and protection of metallic structures

Abstract

In the framework of using fiber optics (FO) for structural health monitoring, a true challenge is to fix the fiber onto structures guaranteeing both protection for the former and an effective adhesion on the latter. This work proposes a method to obtain such result via thermal spray technique on metallic structures, allowing its use in the most severe conditions of corrosion and wear. Since the transmission medium between the structure and the sensitive part of the optical fiber is represented by the fiber coating, three differently coated fibers were used on C-40 steel substrate: polyacrylate, polyimide and ORMOCER. In addition, the use of a primer to improve the bond on the substrate was evaluated. The adhesion between FO and metallic coating is evaluated through optical microscopy (OM) and scanning electrons microscopy (SEM) analysis. The functionality is also verified with both thermal and mechanical tests to calibrate the measuring accuracy. The results indicate that the best combination is that of the polyimide fiber, a zinc primer and aluminum coating. The proven qualities are the adhesion at the interface between the metallic coating and the fiber optics, and the preservation of the structural integrity of the fiber itself and its coating, and a precise measurement of strain acquired by fiber Bragg grating sensors (FBGs). The use of the thermal spray process is thus proved to be a solution for the optical fiber and substrate interaction, since it preserves the integrity of the optical fiber, due to the low temperature of the process, adding the protection that the metallic coating offers as well.