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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Entanglement entropy of a color flux tube in (2+1)D Yang-Mills theory

We construct a novel flux tube entanglement entropy (FTE 2 ), defined as the excess entanglement entropy relative to the vacuum of a region of color flux stretching between a heavy quark-anti-quark pair in pure gauge Yang-Mills theory. We show that FTE 2 can be expressed in terms of correlators of Polyakov loops, is manifestly gauge-invariant, and therefore free of the ambiguities in computations of the entanglement entropy in gauge theories related to the choice of the center algebra. Employing the replica trick, we compute FTE 2 for SU(2) Yang-Mills theory in (2+1)D and demonstrate that it is finite in the continuum limit. We explore the properties of FTE 2 for a half-slab geometry, which allows us to vary the width and location of the slab, and the extent to which the slab cross-cuts the color flux tube. Following the intuition provided by computations of FTE 2 in (1+1)D, and in a thin string model, we examine the extent to which our FTE 2 results can be interpreted as the sum of an internal color entropy and a vibrational entropy corresponding to the transverse excitations of the string.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Retrospective comparison of outcomes and associated complications between large bore radiologically inserted gastrostomy tube types

Purpose: Multiple approaches to radiologically inserted gastrostomy (RIG) exist. The goal of this study was to compare 30-day outcomes and associated complications between large bore balloon-retained (BR), loop-retained (LR), and pull-type (PT) RIG devices. Methods: Data on 1477 patients who underwent RIG between January 1, 2005 and December 31, 2016 were collected retrospectively using a dedicated interventional radiology database and electronic medical record. Statistical analysis was performed to compare complication rates between BR, LR, and PT devices. Results: Ninety-eight percent (1477/1507) of the procedures were successfully performed. A total of 752 BR, 323 LR, and 402 PT gastrostomy tubes were placed. The overall complication rate for BR catheters was 5.7% (25 major [3.3%] and 18 minor [2.4%]). The overall complication rate for PT catheters was 3.7% (8 major [2.0%] and 7 minor [1.7%]). The overall complication rate for LR catheters was 1.6% (4 major [1.4%] and 1 minor [0.8%]). Compared to BR catheters, LR catheters had significantly fewer total complications (P = 0.01) but not minor complications (P = 0.052). There were no significant differences in the number of complications between LR and PT catheters or between BR and PT catheters. Conclusions: Use of BR, LR, and PT devices for RIG is safe with a low incidence of complications. Compared to BR catheters, primary insertion of a LR gastrostomy was associated with significantly fewer overall complications within the first 30 days. Therefore, for initial tube placement, large bore LR catheters may be preferred over BR devices.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2018

Investigation of Mild Steel Thin-Wall Tubes in Unfilled and Foam-Filled Triangle, Square, and Hexagonal Cross Sections Under Compression Load

This study has examined proposed structures with mild steel-reinforced LM30 aluminum (Al) alloy having diversely unfilled and 10 wt.% SiCp composite foam-filled tubes for improving axial compression performance. This class of material has novel physical, mechanical, and electrical properties along with low density. In the present experiment, Al alloy foams were prepared by the melt route technique using metal hydride powder as a foaming agent. Crash energy phenomena for diverse unfilled and foam-filled in mild steel thin-wall tubes (triangular, square and hexagonal) were studied as well. Compression deformation investigation was conducted at strain rates of 0.001-0.1/s for evaluating specific energy absorption (SEA) under axial loading conditions. The results were examined to measure plateau stress, maximum densification strain, and deformation mechanism of the materials. Specific energy absorption and total energy absorption capacities of the unfilled and filled sections were determined from the compressive stress–strain curves, which were then compared with each other.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced Hot Tensile Ductility of Mg-3Al-1Zn Alloy Thin-Walled Tubes Processed Via a Combined Severe Plastic Deformation

In the current study, combined parallel tubular channel angular pressing (PTCAP) and tube backward extrusion (TBE), as a recently developed severe plastic deformation (SPD) method, were applied at 300 °C on a commercial Mg-3Al-1Zn alloy tubes to achieve an ultrafine grained structure. Then, the microstructure, hardness, tensile properties, and fractography evaluations were done at room temperature on the SPD-processed samples. Also, to study the hot tensile ductility of the SPD-processed samples, tensile testing was performed at an elevated temperature of 400 °C, and then, the fractured surface of the tensile samples was studied. It was observed that a bimodal microstructure, with large gains surrounded by many tiny ones, was created in the sample processed by PTCAP followed by TBE. This microstructure led to reach higher hardness and higher strength at room temperature and also led to reach very high elongation to failure (~ 181%) at 400 °C. Also, the value of elongation to failure for this sample was ~ 14.1% at room temperature. The fractographic SEM images showed the occurrence of predominately ductile fracture in the samples pulled at 400 °C. This was mostly due to the nucleation of microvoids and their subsequent growth and coalescence with each other.

36 MATERIALS SCIENCE↗
DOE OSTI2021

High-Speed X-Ray Stereo Digital Image Correlation in a Shock Tube

X-ray stereo digital image correlation (DIC) measurements were performed at 10 kHz on the internal surface of a jointed structure in a shock tube at a shock Mach number of 1.42 and compared with optical stereo DIC measurements on the outer, visible surface of the structure. The shock tube environment introduces temperature and density gradients in the gas through which the structure was imaged, resulting in spatial and temporal index of refraction variations. These variations cause bias errors in optical DIC measurements due to beam-steering but have minimal influence on x-ray DIC measurements. These results demonstrate the utility of time-resolved x-ray DIC measurements in complicated environments where optical measurements suffer severe errors and/or are precluded by lack of optical access.

42 ENGINEERING↗
DOE OSTI2013

Flow and axial dispersion in a sinusoidal-walled tube: Effects of inertial and unsteady flows

Dispersion in porous media flows has been the subject of much experimental, theoretical and numerical study. Here we consider a wavy-walled tube (a three-dimensional tube with sinusoidally-varying diameter) as a simplified conceptualization of flow in porous media, where constrictions represent pore throats and expansions pore bodies. A theoretical model for effective (macroscopic) longitudinal dispersion in this system has been developed by volume averaging the microscale velocity field. Direct numerical simulation using computational fluid dynamics (CFD) methods was used to compute velocity fields by solving the Navier-Stokes equations, and also to numerically solve the volume averaging closure problem, for a range of Reynolds numbers (Re) spanning the low-Re to inertial flow regimes, including one simulation at Re = 449 for which unsteady flow was observed. Dispersion values were computed using both the volume averaging solution and a random walk particle tracking method, and results of the two methods were shown to be consistent. Our results are compared to experimental measurements of dispersion in porous media and to previous theoretical results for the low-Re, Stokes flow regime. In the steady inertial regime we observe an power-law increase in effective longitudinal dispersion (DL) with Re, consistent with previous results. This rapid rate of increase is caused by trapping of solute in expansions due to flow separation (eddies). For the unsteady case (Re = 449), the rate of increase of DL with Re was smaller than that observed at lower Re. Velocity fluctuations in this regime lead to increased rates of solute mass transfer between the core flow and separated flow regions, thus diminishing the amount of tailing caused by solute trapping in eddies and thereby reducing longitudinal dispersion.

CFD↗
DOE OSTI2014

Ethanol-induced impairment of polyamine homeostasis – A potential cause of neural tube defect and intrauterine growth restriction in fetal alcohol syndrome

Highlights: • Polyamine pools in embryonic and extraembryonic tissues are developmentally regulated. • Alcohol administration perturbs polyamine levels in the tissues with various patterns. • Total absence of polyamines in the embryo head at 9.5 dpc is critical for development. • The deficiency is associated with reduction in endothelial cell sprouting in the head. • Retarded migration of neural crest cells may cause development of neural tube defect. - Abstract: Introduction: Polyamines play a fundamental role during embryogenesis by regulating cell growth and proliferation and by interacting with RNA, DNA and protein. The polyamine pools are regulated by metabolism and uptake from exogenous sources. The use of certain inhibitors of polyamine synthesis causes similar defects to those seen in alcohol exposure e.g. retarded embryo growth and endothelial cell sprouting. Methods: CD-1 mice received two intraperitoneal injections of 3 g/kg ethanol at 4 h intervals 8.75 days post coitum (dpc). The fetal head, trunk, yolk sac and placenta were collected at 9.5 and 12.5 dpc and polyamine concentrations were determined. Results: No measurable quantity of polyamines could be detected in the embryo head at 9.5 dpc, 12 h after ethanol exposure. Putrescine was not detectable in the trunk of the embryo at that time, whereas polyamines in yolk sac and placenta were at control level. Polyamine deficiency was associated with slow cell growth, reduction in endothelial cell sprouting, an altered pattern of blood vessel network formation and consequently retarded migration of neural crest cells and growth restriction. Discussion: Our results indicate that the polyamine pools in embryonic and extraembryonic tissues are developmentally regulated. Alcohol administration, at the critical stage, perturbs polyamine levels with various patterns, depending on the tissue and its developmental stage. The total absence of polyamines in the embryo head at 9.5 dpc may explain why this stage is so vulnerable to the development of neural tube defect, and growth restriction, the findings previously observed in fetal alcohol syndrome.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Small GTPase R-Ras participates in neural tube formation in zebrafish embryonic spinal cord

Ras related (R-Ras), a small GTPase, is involved in the maintenance of apico-basal polarity in neuroepithelial cells of the zebrafish hindbrain, axonal collapse in cultured murine hippocampal neurons, and maturation of blood vessels in adult mice. However, the role of R-Ras in neural tube formation remains unknown. Using antisense morpholino oligonucleotides (AMOs), we found that in the spinal cord of zebrafish embryos, the lumen was formed bilaterally in rras morphants, whereas it was formed at the midline in control embryos. As AMO can cause off-target effects, we generated rras mutant zebrafish lines using CRISPR/Cas9 technology. Although these rras mutant embryos did not have a bilateral lumen in the spinal cord, the following findings suggest that the phenotype is unlikely due to an off-target effect of rras AMO: 1) The rras morphant phenotype was rescued by an injection of AMO-resistant rras mRNA, and 2) a bilaterally segregated spinal cord was not observed in rras mutant embryos injected with rras AMO. The results suggest that the function of other ras family genes may be redundant in rras mutants. Previous research reported a bilaterally formed lumen in the spinal cord of zebrafish embryos with a mutation in a planar cell polarity (PCP) gene, van gogh-like 2 (vangl2). In the present study, in cultured cells, R-Ras was co-immunoprecipitated with Vangl2 but not with another PCP regulator, Pricke1. Interestingly, the interaction between R-Ras and Vangl2 was stronger in guanine-nucleotide free point mutants of R-Ras than in wild-type or constitutively active (GTP-bound) forms of R-Ras. R-Ras may regulate neural tube formation in cooperation with Vangl2 in the developing zebrafish spinal cord.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

Fuel-rich n-heptane oxidation: A shock tube and laser absorption study

The chemical kinetics of n-heptane (n-C 7 H 16 ) – an important reference compound for real fuels – oxidation are well studied at stoichiometric and lean conditions. However, there is only limited information on the n-heptane chemical kinetics in fuel-rich combustion. In order to verify the accuracy of chemical kinetic models at these conditions, the oxidation of rich n-heptane mixtures has been investigated. Combustion of n-C 7 H16/O2/Ar mixtures at equivalence ratios, φ, of 2.0 and 3.0 behind reflected shock waves has been studied at temperatures ranging from 1066 to 1502 K and at pressures ranging from 1.4 to 6.2 atm. Reaction progress was monitored by recording pressure and absorption time-histories of ethylene (C 2 H 4 ) and n-heptane at a location 2 cm from the endwall of a 14-cm inner diameter shock tube. Ethylene and n-heptane absorption time-histories were measured, respectively, using absorption spectroscopy at 10.532 μm from a tunable CO 2 laser and at around 3.4 μm from a continuous wave distributed feedback interband cascade laser (ICL). The measured absorption time-histories were compared with modeled predictions from the Lawrence Livermore National Lab (LLNL) detailed n-heptane reaction mechanism. Overall, to the best of our knowledge, current data are the first time-resolved n-heptane and ethylene concentration measurements conducted in a shock tube at these conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Predicting combustion characteristics in externally heated micro-tubes

Here this study employs a simplified analytical model to obtain qualitative predictions of transitions between non-adiabatic combustion regimes found in non-catalytic tubular micro-flow reactors, i.e. normal stable flames, flames with repetitive extinction and ignition (FREI), and weak flames that are stabilized by wall heat transfer. Assuming large activation energy for single-step kinetics, a comparison of exact solutions for constant and variable wall temperature profiles reveals that while a non-zero wall temperature gradient introduces a temperature lag, it has no impact on the structure of the flame solution itself. This result is significant as it establishes a link to quenching characteristics within heated micro-tubes, and further facilitates a simplified micro-flow reactor analysis. Predictions for sweeps of wall temperatures are combined to S-curves, which allow for a combustion regime classification for a given combination of micro-flow reactor geometry and mixture properties. A variation of parameters such as tube diameter or mixture stoichiometry yields predictions of shifts in regime transitions, thus providing insights on interactions between heat transfer and first order effects of chemical kinetics. A comparison to experimental data illustrates that the theoretical framework, - within inherent limitations, - captures qualitatively correct trends that reflect experimental observations, thus providing valuable insights for micro-flow reactor design and operation.

42 ENGINEERING↗
DOE OSTI2024

Experimental investigation of probabilistic failure of SiC/SiC composite tubes under multiaxial loading

This paper presents an experimental investigation of the failure behavior of SiC fiber-reinforced SiC matrix (SiC/SiC) composite tubes under multiaxial loading. A new testing apparatus is designed to independently apply axial stress and internal pressure to SiC/SiC tubes. Acoustic emission (AE) is used to monitor the damage growth in the specimen. Based on the measured stress–strain response, a strain-based criterion is proposed to determine the proportional limit stress (PLS). The proposed strain-based criterion is compared with the PLS determined by the AE measurement. The PLS is determined for different loading ratios to form a multiaxial failure surface. By testing multiple replicates, the statistical variations of the PLS are determined. In conclusion, based on the experimental results, a mathematical model is developed to characterize the probability distribution of the PLS of SiC/SiC composites under multiaxial loading.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Experimental investigation of Multi-Mode heat transfer to a Free-Falling dilute particle cloud in a heated vertical tube

The development of dilute particle heat exchangers and reactors for advanced energy systems requires an understanding of the multi-mode heat transfer from a heated wall to falling particles. This study presents experimental results of the overall heat transfer coefficient for a free-falling, dilute flow of particles with solid volume fraction from 0.0005 to 0.006 corresponding to feed rates from 3.7 kg s -1 m -2 to 44 kg s -1 m -2 in a vertical, heated tube containing quiescent air at atmospheric pressure. Tube wall temperatures are varied between 300°C to 900°C while keeping the particle inlet temperature constant. The experimental results show that the overall heat transfer coefficient is a strong function of particle feed rate and surface temperature. Good agreement was obtained with prior studies conducted at comparable temperatures but lower particle feed rates (< 4 kg m -2 s -1 ). The established correlations for particle-to-wall radiation and particle-to-gas convection were used to estimate the wall-to-gas convective contribution from the measured overall heat transfer coefficient. The experimental results indicated a 4 to 6 times improvement in the wall convection in the solid-gas mixture compared to that expected from natural convection in a single-phase gas. Furthermore, the data presented here are applicable to characterize heat transfer in dilute particle heat exchangers, furnaces, and solar receivers.

14 SOLAR ENERGY↗
DOE OSTI2019

Materials-to-device design of hybrid metal-polymer heat exchanger tubes for low temperature waste heat recovery

Heat exchangers for low temperature (<150 °C) waste heat recovery (WHR) are challenging due to severe constraints on materials costs, manufacturing methods, and maintenance opportunities after installation. Here, the use of polymers instead of metals, offers an avenue for cost reduction in WHR applications, provided the typical low thermal conductivity of polymers (~0.2 W m –1 K –1 ) that results in poor overall heat transfer coefficient can be overcome. Previous approaches to enhance the thermal conductivity of polymers remain problematic in terms of cost, thermal, and thermomechanical considerations. In this work, we propose a novel hybrid metal-polymer composite assembled from strips of polymer and copper. Polymer strips with metal-clad edges are wound helically and the interfaces between the strips joined to obtain a tube for a cross-flow heat exchanger. Interfaces are designed to enhance effective heat conduction across the material and increase the overall heat transfer coefficient. Through numerical simulations, we obtain metal-polymer layouts that achieve wall effective thermal conductivity ~1 W m –1 K –1 at ~23% or ~35% volume fraction of copper or aluminum, respectively. The conductivity enhancement is sufficient for typical low temperature WHR, resulting in a ~20% enhancement in the overall heat transfer coefficient compared to a similar all-polymer heat exchanger. Thermomechanical simulations revealed that the composite pipe designs can withstand >1.4 MPa internal pressure, under the normal operating conditions of the heat exchanger. Our work provides guidelines for designing macroscopic metal-polymer composites that can be adapted for specific requirements such as reduced materials costs, ease of manufacturing, and heat transfer enhancement. In conclusion, the optimized design of hybrid metal-polymer heat exchanger tubes potentially provides a scalable and cost-effective route toward harvesting waste heat from low temperature sources.

42 ENGINEERING↗
DOE OSTI2021

A fast reduced model for a shell-and-tube based latent heat thermal energy storage heat exchanger and its application for cost optimal design by nonlinear programming

Numerical simulation of latent heat thermal energy storage (LHTES) systems plays a fundamental role in studying the physical process and guiding the engineering design. Discretization of the PDEs describing the nonlinear solidification/melting process of phase change materials (PCMs) leads to a large-scale complex dynamics system, where the system behavior depends on a set of parameters. In a design setting, repeated model evaluations are required over the set of parameters results in significant computational burden. In this paper, an explicit analytic solution was built for the propagation of the solidification front in a cylindrical coordinate. The analytic solution approach is further employed to develop a low computational reduced model (RM) as a module for a shell-and-tube based LHTES heat exchanger. The levelized Cost of Energy (LCOE) is used as a design metric and the RM model is used to apply system-level constraints in the nonlinear programming formulation that facilitates efficient global optimal design of the PCM properties, flow conditions and tube geometries. The use of LCOE as the design metric prevents over design of the heat transfer rate and also establishes a fair ground for evaluation of different thermal storage technologies and their integrated applications with other systems. Optimal results showed that a higher effectiveness results in a higher LCOE; the velocity of the HTF and the length of the channel are highly correlated with each other; both larger PCM latent energy and conductivity result in lower LCOE.

25 ENERGY STORAGE↗
DOE OSTI2019

High fidelity simulation and validation of crossflow through a tube bundle and the onset of vibration

Flow induced vibrations can be detrimental to all manner of engineering applications with fluid flow. Heat exchangers with bundles of thin tubes experiencing a crossflow are especially susceptible to the unexpected onset of vibration. Heat-exchanger design necessarily involves extensive modeling and testing to ensure that significant vibrations cannot develop for any expected flow conditions. A properly validated numerical simulation can work in conjunction with physical experiments to identify problematic vibrations and allow for rapid iteration of design at relatively low expense. A high-fidelity fluid–structure interaction code has been developed by fully coupling CFD LES/DNS code Nek5000 and CSM code Diablo. The coupled code is used to simulate crossflow through a tube bundle in a geometry recreated after a physical experiment. Validation against this experiment involves comparing the amplitude and frequency spectrum for three different flow velocities. The velocities compared straddle the onset of large magnitude vibrations. The simulation accurately captures the onset of the vibrations with a marginally lower predicted frequency of vibration. When the onset velocity is compensated for the difference in natural frequency, the simulation results closely match the experiment. Finally, the continuous nature of the simulation measurements helps illustrate the fluid mechanics behind the pin motion and reveals the front to back propagation of the vibrations as the fluid velocity increases.

42 ENGINEERING↗
DOE OSTI2019

Characterization and numerical simulation of liquid refrigerant R-134a flow emerging from a flooded evaporator tube bundle

The distribution of liquid droplets emerging from an evaporator tube bundle is characterized for refrigerant R-134a with a triangular tube arrangement with a pitch of 1.167. The purpose of this research was to improve understanding of the droplet ejection process to aid in design of evaporators typically used in larger chiller systems. A laser and camera system captured images of the evaporator headspace at varying conditions. Conventional shadowgraphy techniques were applied to recognize and match droplets for velocity calculations. The evaporator conditions varied with bundle mass fluxes of 20.3 and 40.7 kg s -1 m -2 , top-rows heat fluxes of 15.8 and 31.5 kWm -2 , and outlet saturation temperatures of 4.4 and 12.8 °C. Conditions ranged from flooded to dryout of the top rows. Droplet number, size distribution, velocity, and liquid volume fraction are presented in the headspace above the bundle. A method to numerically duplicate the droplet loading in the headspace using CFD with a Lagrangian discrete-phase model is also presented and verified, providing a powerful design tool. Liquid distribution in the headspace is found to be a strong function of all varied properties, particularly mass flux, liquid level, and saturation temperature

42 ENGINEERING↗
DOE OSTI2018

Failure behavior of SiC/SiC composite tubes under strain rates similar to the pellet-cladding mechanical interaction phase of reactivity-initiated accidents

The mechanical response of a nuclear-grade silicon carbide fiber-reinforced silicon carbide matrix (SiC/SiC) composite was investigated under mechanical loading conditions mimicking the pellet-cladding mechanical interaction (PCMI) phase of a reactivity-initiated accident (RIA). In a RIA, cladding deformation and failure can be induced by the rapid thermal expansion of the nuclear fuel. A pulse-controlled modified-burst test was used to investigate RIA-like PCMI scenarios on SiC/SiC composite samples at pulse widths from 12 to 100 ms.The strain-driven nature of the cladding sample deformation was due to the rapid internal pressurization and subsequent expansion of a secondary tube. A digital-image correlation technique was used to measure strains from the speckle-painted outer surface of the tubes. The failure strains of samples tested at slower rates, such as RIA event durations of 52 and 100 ms, showed good agreement with the literature-reported values for similar composites tested at slow strain rates. Additionally, the failure strain showed good agreement with reference expansion-due-to-compression tests at slow strain rate. However, a decrease in the failure strain was determined for the fast-rate (12 ms) tests. This indicated that the failure strain of these composites might be influenced by the strain rate during RIA-like events. In conclusion, the failure strains observed in the tests corresponded to local energy depositions of approximately 50 cal/g UO 2 from hot zero power, with an initial condition of pellet–cladding gap closure prior to the event. In-pile transient testing of these concepts that would result in hoop strain due to PCMI in the range of 0.5–1.0% is recommended.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure evolution and mechanical characterization of Nd:YAG laser beam welded titanium tubes

Highlights: • Joining of thin titanium tubes using laser beam welding. • Fusion zone exhibited coarse granular like grains with serrated and irregular boundaries. • Large spherical and irregular shape pores were observed at lower welding speeds. • Misorientation angle distribution indicated substructure boundaries along with single β grain formation from double α grains. • Fusion zone strengthened by substructure boundaries, solid solution strengthening and dislocation density. - Abstract: Grade 2 titanium tubes of outer diameter 60 mm and wall thickness 3.9 mm were effectively joined by Nd:YAG laser. The welding speed was varied in five levels (2.50, 2.75, 3.00, 3.25 and 3.50 m/min). The microstructure evolution was studied using optical microscopy and advanced characterization techniques. The bead geometry changed from parallel shape to wedge shape with increased welding speed due to changes in surface tension. The macrographs showed three zones namely fusion zone, heat affected zone (HAZ) and base metal. Coarse granular structure with irregular and serrated contours was observed in the fusion zone. Acicular α grains were observed at the extreme outer edge of the fusion zone due to rapid cooling. Spherical and irregular shaped pores were observed near the centerline of the fusion zone at lower welding speeds due to large amount of entrapped gases and downward sweeping solidification front. The grain size decreased with increased welding speed due to higher cooling rate. The misorientation angle distribution map showed three distinct regions. The increase in welding speed caused an increase in low angle boundaries and a reduction in very high angle boundaries. High density of dislocations was observed in the fusion zone due to thermal stresses caused by solidification. The microhardness of the fusion zone was recorded to be higher than base metal due to substructure boundaries, solute elements and dislocations. The increase in welding speed shifted the fracture location from fusion zone to base metal and fracture mode from brittle to ductile.

36 MATERIALS SCIENCE↗