Demonstration of Strain Monitoring with Fiber Optics Embedded in SS304 with Ultrasonic Additive Manufacturing

Fiber optics are ideal sensors for measuring distributed temperatures and strains in harsh conditions, such as that found in nuclear microreactors, due to their resistance to ionizing radiation and high temperatures (> 1000 °C). The fibers must be bonded to their surrounding metal matrix for accurate strain measurements, and their bonding must also survive the harsh environments. In this investigation, Ultrasonic Additive Manufacturing (UAM) was employed to layer stainless steel 304 foils and embed silica fibers in a SS304 matrix. Through microstructural evaluation, a fine grain structure (< 1 µm) was observed at the layer/fiber interface as compared to the bulk, resulting in increases in hardness at/near the interfaces. Furthermore, demonstration of strain and temperature measurements were performed through controlled furnace heating. Strong correlations were observed between the fiber bonding quality and the accuracy of strain measurements, as well as the extent of microstructural deformation at the interfaces.