Christian Petrie

Recent advances in manufacturing technologies have enabled the fabrication of complex geometries for a wide range of applications, including the energy, aerospace, and civil sectors. The ability to integrate sensors at critical locations within these complex components during the manufacturing process could benefit process monitoring and control by reducing reliance on models to relate surface measurements to internal phenomena. This study investigated embedding thermocouples in a SS316 matrix using laser powder bed fusion. Under optimal processing conditions, embedded thermocouples were characterized post-building, finding good bonding to the matrix with no melt pool penetration to the sensing elements. Moreover, the embedded thermocouple performed similarly to an identical non-embedded thermocouple during thermal testing to 500 °C with only a slight difference in response time, which was attributed to the differences in mass and the associated thermal time constants.

We present embedded FBG sensors fabricated by ultrasonic additive manufacturing process as a package method for FBG strain sensors. The vibration induced dynamic strain is interrogated by a tunable VCSEL enabled high-speed interrogation system.

The change in length of an optical fiber-based Fabry-Pérot cavity (FPC) can be precisely measured using phase tracking, but the displacement range is limited by phase ambiguity. Period tracking techniques determine the absolute FPC length, but with larger uncertainties from tracking the spacing between multiple peaks. A hybrid method is demonstrated that identifies appropriate peaks for phase tracking using a coarse estimate obtained from the free spectral range to effectively maintain the high precision (∼1 nm) of phase tracking techniques to measure ∼24 µm displacements, well beyond the range limitations (typically <1 µm) of phase tracking methods.

Optical frequency domain reflectometry (OFDR) is a spectral measurement technique in which shifts in the local Rayleigh backscatter spectra can be used to perform distributed temperature or strain measurements relative to a reference measurement using ordinary single-mode optical fibers. This work demonstrates a data processing methodology for improving the resolvable range of temperature and strain by adaptively varying the reference measurement position by position, based on the time evolution of the local optical intensities and the correlation between the reference and active measurements. These methods nearly double the resolvable range of temperature and strain compared with that achieved using the traditional static reference approach.

Optical backscatter reflectometry (OBR) is an interferometric technique that can be used to measure local changes in temperature and mechanical strain based on spectral analyses of backscattered light from a singlemode optical fiber. The technique uses Fourier analyses to resolve spectra resulting from reflections occurring over a discrete region along the fiber. These spectra are cross-correlated with reference spectra to calculate the relative spectral shifts between measurements. The maximum of the cross-correlated spectra—termed quality—is a metric that quantifies the degree of correlation between the two measurements. Recently, this quality metric was incorporated into an adaptive algorithm to (1) selectively vary the reference measurement until the quality exceeds a predefined threshold and (2) calculate incremental spectral shifts that can be summed to determine the spectral shift relative to the initial reference. Using a graphical (network) framework, this effort demonstrated the optimal reconstruction of distributed OBR measurements for all sensing locations using a maximum spanning tree (MST). By allowing the reference to vary as a function of both time and sensing location, the MST and other adaptive algorithms could resolve spectral shifts at some locations, even if others can no longer be resolved.

Welding of high-strength steels can result in large tensile strains as the base metal and filler material cool from their molten state. To combat these large tensile strains, low-transformation-temperature (LTT) metal fillers have been proposed. These fillers undergo a martensitic phase transformation at a lower temperature which can ultimately reduce the tensile strain or can even introduce compressive strain adjacent to the weld metal. However, the process for optimizing the composition of the LTT material, as well as various weld parameters for each unique weld geometry, can be quite expensive, especially if the acceptance criterion requires using neutrons, x-ray beams, or destructive techniques to characterize residual stresses. This work describes a simple, low-cost method for quantifying residual stresses and phase transformations in situ during welding. Spatially distributed fiber-optic sensors were bonded to a cast iron plate, along with tack-welded thermocouples, to measure temperature and strain during multiple passes with LTT filler metals. Results show that the fiber-optic sensors can successfully resolve compressive strain adjacent to the weld region caused by the martensitic phase transformations in the LTT filler material.

Welding of high-strength steels can result in large tensile strains as the base metal and filler material cool from their molten state. To combat these large tensile strains, low-transformation-temperature (LTT) metal fillers have been proposed. These fillers undergo a martensitic phase transformation at a lower temperature which can ultimately reduce the tensile strain or can even introduce compressive strain adjacent to the weld metal. However, the process for optimizing the composition of the LTT material, as well as various weld parameters for each unique weld geometry, can be quite expensive, especially if the acceptance criterion requires using neutrons, x-ray beams, or destructive techniques to characterize residual stresses. This work describes a simple, low-cost method for quantifying residual stresses and phase transformations in situ during welding. Spatially distributed fiber-optic sensors were bonded to a cast iron plate, along with tack-welded thermocouples, to measure temperature and strain during multiple passes with LTT filler metals. Results show that the fiber-optic sensors can successfully resolve compressive strain adjacent to the weld region caused by the martensitic phase transformations in the LTT filler material.

Ultrasonic additive manufacturing (UAM) is a solid-state hybrid manufacturing technique that leverages the principles of ultrasonic welding, mechanized tape layering, and computer numerical control (CNC) machining operations to create three-dimensional metal parts. This article begins with a discussion on the process fundamentals and process parameters of UAM. It then describes metallurgical aspects in UAM. The article provides a detailed description of a wide range of mechanical characterization techniques of UAM, namely tensile testing, peel testing, and pushpin testing. The article ends with information on sensor embedding.

Single crystal sapphire optical fiber was tested at high temperatures (1500°C) to determine its suitability for optical instrumentation in high‐temperature environments. Broadband light transmission (450‐2300 nm) through sapphire fiber was measured as a function of temperature as a test of the fiber's ability to survive and operate in high‐temperature environments. Upon heating sapphire fiber to 1400°C, large amounts of light attenuation were measured across the entire range of light wavelengths that were tested. SEM and TEM images of the heated sapphire fiber indicated that a layer had formed at the surface of the fiber, most likely due to a chemical change at high temperatures. The microscopy results suggest that the surface layer may be in the form of aluminum hydroxide. Subsequent tests of sapphire fiber in an inert atmosphere showed minimal light attenuation at high temperatures along with the elimination of any surface layers on the fiber, indicating that the air atmosphere is indeed responsible for the increased attenuation and surface layer formation at high temperatures.

The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a high‐temperature gamma radiation environment. In this work, the broadband (500–2200 nm, or 0.56–2.48 eV) optical attenuation of commercially available sapphire optical fibers was monitored in situ during continuous gamma irradiation from room temperature up to 1000°C. The gamma dose rate of the irradiation facility was measured to be 370 rad/h (dose in sapphire). Results show rapid growth of an absorption band centered below 500 nm (the minimum detectable wavelength in this work) and extending as far as ~1000 nm that reached a dynamic equilibrium after approximately 4 h of irradiation at room temperature. Increasing temperature generally reduced the added attenuation, although the added attenuation came to a nonzero equilibrium at temperatures of 1000°C and below. No peaks in the added attenuation were observed above 1000 nm, and the added attenuation at 1300 and 1550 nm remained less than 0.7 dB throughout the entire experiment. Fitting the attenuation spectra to Gaussian functions at each temperature revealed a large absorption band near 3 eV that monotonically decreased both in magnitude and width with increasing temperature. A second smaller band near 1.73 eV monotonically increased in both magnitude and width with increasing temperature, suggesting that some interconversion between defects may be occurring at higher temperatures.

The purpose of this work was to determine the spectral loss of sapphire optical fibers as a function of temperature to determine the suitability of using sapphire optical fiber‐based instrumentation in extremely high‐temperature applications such as coal gasifers, or potentially in advanced high‐temperature nuclear reactors. In this work, the broadband (450–2200 nm) optical attenuation of two commercially available sapphire optical fibers was monitored in situ as the fibers were heated in steps to temperatures up to 1400°C. The results presented in this work are the first known reporting of the broadband spectral loss in sapphire fibers (as opposed to bulk sapphire) as a function of temperature. The added attenuation in the sapphire fibers seemed to reach a steady‐state at temperatures of 1300°C and below, and remained below 3.7 dB/m at all measurable wavelengths. At 1400°C, rapid increases in attenuation were observed that appeared to increase as the square root of heating time, indicating a diffusion‐limited process. Cooling to room temperature in steps resulted in recovery of a large portion of the thermally induced attenuation, suggesting that the effects caused by heating at 1400°C introduced additional attenuation that was very temperature dependent. Based on comparison with previous work and on the spectral features of the added attenuation observed in this work, heating at 1400°C could have caused high‐temperature oxidation of the sapphire fibers, which resulted in a temperature‐dependent scattering loss mechanism.

The purpose of this work was to determine the suitability of using instrumentation utilizing sapphire optical fibers in a nuclear reactor environment. In this work, the broadband (500–2200 nm, or 0.56–2.48 eV) optical transmission in commercially available sapphire optical fibers was monitored in‐situ prior to and during reactor irradiation. The sapphire fibers were irradiated at a neutron flux of 1.5 × 10¹² n/(cm²·s) and a gamma dose rate of 75 kGy/h (dose in sapphire) to a total neutron fluence of 1.1 × 10¹⁷ n/cm² and total gamma dose of on the order of 1.5 MGy. Consistent with previous gamma irradiation experiments, an absorption band centered below 500 nm (the minimum measurable wavelength using the measurement system described in this work) and extending as far as ~1000 nm reached saturation during irradiation in the gamma shut‐down field (the gamma‐ray radiation field that is present in the reactor post‐operation, as a consequence of the decay of radioisotopes that were produced during reactor operation) of the reactor prior to reactor irradiation. Beginning reactor irradiation and increasing the reactor power caused rapid increases in attenuation, followed by a linear increase with irradiation time at constant reactor power. Shutting down the reactor caused a decrease in the added attenuation; however, restarting the reactor caused the added attenuation to rapidly return to values almost identical to those observed at the end of the previous irradiation. The decrease in attenuation that was observed after the reactor was shut down shows the importance of the in‐situ nature of the measurements made in this work (previous ex‐situ attenuation measurements could not have captured this effect). A model is proposed for the experimentally obtained values of the radiation‐induced attenuation that involves three previously identified color centers including a composite V‐center, a center, and a center. The model accounts for gamma radiation‐induced ionization of pre‐existing defects, generation of new defects via displacement damage, and conversion between defect centers via ionization and charge recombination.