In situ measurement of phase transformations and residual stress evolution during welding using spatially distributed fiber-optic strain sensors*

Abstract

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.

Demonstrate embedding of sensors in a relevant microreactor component

“…The measured strain values also include the effects of temperature due to thermo-optic effects and differential thermal expansion between the stainless steel pipe and the glass fiber. Therefore, the strain values as presented in Figure 8 would require temperature compensation similar to that demonstrated in previous efforts [18]. Such compensation is not necessary for this simple demonstration, but it can easily be performed using either the embedded TCs or the cavities that were included to allow insertion of co-located fiber-optic temperature sensors.…”

“…Fiber-optics are perhaps the most challenging sensors to embed because of their small size and relative fragility. However, they also may offer the highest potential reward because of their small size, high precision, and their ability to perform spatially distributed measurements of temperature and strain [16][17][18].…”

Characterization of Embedded Sensors in Stainless Steel Test Articles and Design/Planning for MAGNET Testing

“…Fiber optic sensors are a promising technology for measuring temperature and strain distributions due to their ability to operate at high temperatures (>1,000°C) [7][8][9][10] and moderate radiation dose [9][10][11]. Furthermore, temperature and strain can both be continually measured along the length of the entire fiber [7,12]. However, for accurate strain measurements, the fiber must have adequate mechanical coupling with the component.…”

“…However, for accurate strain measurements, the fiber must have adequate mechanical coupling with the component. Ideally the fiber-optic sensors could be directly embedded and bonded within the surrounding metal matrix [12][13][14][15][16][17], thus eliminating concerns related to the degradation of adhesives at high temperatures or under irradiation. Typically, 316L and 304 stainless steels (SSs) are chosen for the core block materials because of their relatively high strength, corrosion resistance, and high commercial availability [18,19].…”

Graphical Optimization of Spectral Shift Reconstructions for Optical Backscatter Reflectometry

“…OFDR is attractive for applications that require high spatial resolution (on the order of millimeters to centimeters) over distances on the order of meters to tens of meters. For example, OFDR-based distributed sensing has been explored for monitoring strain during welding [ 2 ] and solidification [ 3 ]; temperature distributions for cryogenic applications [ 4 ] and inside catalytic reactors [ 5 ], flowing coolant loops [ 6 , 7 , 8 ], and energized transformer cores [ 9 ]; local power deposition in nuclear reactors [ 10 , 11 ]; and liquid level [ 12 , 13 ]. More recently, fiber optic sensors have been embedded into metals [ 14 , 15 , 16 , 17 ] and ceramics [ 18 , 19 ] using additive manufacturing technologies to directly monitor spatially distributed strain in harsh environments, such as those found in nuclear reactors, where fiber could be exposed to high temperatures, radiation, or chemically aggressive media.…”