Daniel C. Sweeney

This study presents a novel confined hot rolling technique for embedding optical fiber sensors into wrought metallic components, with the goal of enabling real time strain monitoring in extreme environments such as advanced nuclear reactors. Finite element analysis was used to investigate the stress–strain response of embedded fiber assemblies under various rolling conditions. Multi-pass rolling was found to reduce stress concentrations, shear gradients and roll separation forces compared to single-pass deformation. Guided by simulation results, experimental trials were conducted on a 316 stainless steel (SS316) plate with precision-machined channels housing sapphire optical fibers (compatible with higher temperature processing) within the tubes. The assemblies were hot rolled at 900 °C to achieve a 40% total reduction. Microstructural analysis confirmed progressive channel closure and bonding at both tube-matrix and fiber-tube interfaces, with no significant interfacial defects. For lower temperature applications, copper (Cu)-coated silica fibers were embedded into aluminum alloy 6061 (AA6061) plates using the same method. Optical backscatter reflectometry during thermal cycling of the rolled sample showed minimal optical attenuation and compressive residual fiber stresses after rolling. The integrated computational and experimental approach demonstrates a scalable and robust pathway for embedding optical sensors into structural metals for high temperature structural health monitoring.

A typical structural health monitoring technique involves measuring the vibrational characteristics of components or systems to detect signs of degradation or damage. Many industrial applications require engineered systems to safely operate under extreme, high-temperature environments that pose challenges not only to materials but also to sensors that would be used for structural health monitoring. In this study, miniaturized optical Fabry–Perot cavities (FPCs) were developed and tested as a means of measuring the resonant frequencies of metal components that are most relevant to extreme-environment applications. Two of the three candidate FPC designs tested up to 800 °C provided accurate measurements (validated by theoretical models and laser Doppler vibrometry) of the fundamental vibrational mode of the specimen to which each was bonded, although both sensors failed during thermal cycling. An analysis of the reflected optical spectrum from the FPC and x-ray computed tomography revealed two opportunities to improve the sensor reliability. First, the Cu optical fiber coating that was used could either be replaced with a more oxidation-resistant material or protected with commercially available films. Second, the adhesives used to bond the fibers to metal capillaries and establish the FPC could be replaced with a more robust solution, although the Resbond 907TS adhesive appeared to outperform Resbond 907.

Optical frequency domain reflectometry (OFDR) is a technique for interrogating optical fiber sensors to generate relative, quasi-distributed measurements. Although OFDR is increasingly being adopted for aerospace, energy production, and structural monitoring applications, the quantification of uncertainty for OFDR measurements has not been developed beyond sparse empirical relationships. To address this knowledge gap, an uncertainty metric for OFDR measurements was developed. This uncertainty metric was applied to weight the edges between OFDR measurements on directed correlation graphs and analyzed to minimize the cumulative uncertainty. This work is the first to propose an uncertainty metric for OFDR and provides a generalized mathematical framework for optimizing OFDR measurements using a combination of hardware selection, optical fiber sensor selection, and postprocessing strategy.