Processing and microstructures of fiber Bragg grating sensors embedded in stainless steel

The benefit of layered manufacturing is the ability to create physical parts with little or no restriction from shape complexity. Using a laser-assisted shape deposition manufacturing (LASDM) methodology, fiber Bragg grating (FBG) sensors were embedded into stainless steel structures. The embedding sequence included sputtering a thin metallic film as a conductive layer on the silica fiber, electroplating the fibers with a Ni protective layer, and then laser cladding with stainless steel. Microstructural studies of each layer and interface were conducted. The grain size of electroplated nickel was approximately 150 nm before laser cladding. The electroplated nickel recrystallized during and after the laser cladding. The nickel grain size gradually increased from very fine (similar to1 - to 10-mum diameter) in the immediate layer surrounding the optical fibers to approximately 200 mum near the interface with the laser-deposited stainless steel. The deposited stainless steel appeared as a coarse dendritic structure with a grain size larger than 100 mum. The Ni/fiber interface shows full contact between the silica fibers and the Ni matrix of adequate thickness before and after laser cladding. Microstructural studies were used to conclude that the degradation of the embedded sensors is due to thermally induced stress from laser deposition if a thinner Ni layer is used. Strain and temperature measurements were successfully performed using the embedded FBG sensor, thus demonstrating the viability of this manufacturing technique.