Effect Of Thermal Gradient Research Articles

Evaluation and Origin of Residual Stress in Hybrid Metal and Extrusion Bonding and Comparison with Friction Stir Welding

Hybrid metal and extrusion bonding (HYB) is an emerging solid-state welding technique that was developed about ten years ago. HYB exploits the fundamental idea of the well-established friction stir welding (FSW) technology, but a filler material is employed to enhance control of the microstructure and the mechanical properties of the joint. HYB and FSW allow joining to be performed at lower temperatures than classical fusion welding methods. Still, thermal gradient effects seem impossible to be entirely avoided, thus leading to residual stress within the weld region and neighbouring material. Although the FSW-induced residual stress evaluation has been extensively studied and understood, the evaluation and interpretation of HYB-induced residual stress have not been tackled so far. In the present paper, a quantitative investigation on residual stress and its origin in HYB was carried out for the first time. Specifically, a 4 mm thick AA6082-T6 HYB and a 4 mm thick AA6082-T6 FSW butt welds were considered. For the particular case of HYB, an AA6082-T4 was used as the filler material. In both cases, the full-field longitudinal residual stress was experimentally assessed using the Contour Method. The results showed that the HYB joint yields a higher magnitude of tensile residual stress compared to that of the FSW counterpart. A physical explanation for this difference in magnitude was attributed to the lower yield stress point exhibited by the filler material. Furthermore, the analysis revealed peak values of residual stress as high as 205±25 MPa and 165±15 MPa, for the HYB and FSW joint, respectively. Despite this, a similar distribution of residual stress across the weld was observed in both cases. An additional qualitative analysis on the transverse distortion of the welds outlined a pronounced undesired “V-like” deformation of the HYB joint of approximately 1.4°. By contrast, the FSW joint seemed not to show any perceptible bend.

Post-Buckling and Panel Flutter of Pre-Heated Functionally Graded Plates

Post-buckling and panel flutter behaviors of ceramic-metal FGM plates are studied for the skins of supersonic aircrafts. The effects of asymmetric material and temperature distributions, as well as the aerodynamic loads, on the thermo-mechanical response of FGM plates are discussed using finite element simulations. The aero-thermo-elastic model is established by using the simple power law material distribution, the rule of mixture for material effective properties, the nonlinear Fourier equations of heat conduction, von-Karman strain-displacement nonlinear relations, and the piston theory for supersonic aerodynamics. The finite element equations are established using the first-order shear deformable plate elements. The thermal post-buckling equilibrium deflections are calculated from the reduced-order model, and the flutter stability is furtherly evaluated around these equilibriums. The post-buckling equilibriums are found asymmetric about the initial position of the plate, and the critical flutter speed from the post-buckling model is much higher than the linear flutter speed on the initial position. Numerical results show that the effect of thermal gradient on the postbuckling deflection of FGM plate decreases with the increase of the material gradient index. With the increase in volume fraction of ceramics (decrease in volume fraction index), the critical flutter aerodynamic pressure of the ceramic/ metal plate increases. The flutter response amplitude increases with the increase in volume fraction index.

Molecular Dynamics Simulation of the Soret Effect on Two Binary Liquid Solutions with Equimolar n-Alkane Mixtures.

Molecular dynamics is employed to simulate the Soret effect on two binary liquid solutions with equimolar mixtures: normal pentane (n-pentane, nC-5) and normal heptane (n-heptane, nC-7) molecules plus normal decane (n-decane, nC-10) and normal pentane molecules. Moreover, two coarse-grained force field (the CG-FF) potentials, which may depict inter-/intramolecular interactions fairly well among n-alkane molecules, are developed to fulfill such investigations. In addition, thermal diffusion for the mass fraction of each of these n-alkane molecules is simulated under an effect of a weak thermal gradient (temperature difference) exerting on solution systems from their hot to cold boundary sides. Finally, quantities of the Soret coefficient (SC) for two binary solutions are calculated by means of the developed CG-FF potentials, so as to improve the calculation rationality. As a result, first, it is found that molecules with light molar masses will migrate toward the hot boundary side, while those with heavy molar masses will migrate toward the cold boundary one ; second, the SC quantities indicate that they match relevant experimental determinations fairly well, i.e., trends of these SC quantities show inverse proportionality to the thermal gradient on the systems.

Thermal gradient effect on helium and self-interstitial transport in tungsten

First-wall materials in a fusion reactor are expected to withstand harsh conditions, with high heat and particle fluxes that modify the materials microstructure. These fluxes will create strong gradients of temperature and concentration of diverse species. Besides the He ash and the hydrogenic species, neutron particles generated in the fusion reaction will collide with the material creating intrinsic defects, such as vacancies, self-interstitials atoms (SIAs), and clusters of such point defects. These defects and the He atoms will then migrate in the presence of the aforementioned gradients. In this study, we use nonequilibrium molecular dynamics to analyze the transport of He and SIAs in the presence of a thermal gradient in tungsten. We observe that, in all cases, the defects and impurity atoms tend to migrate toward the hot regions of the tungsten sample. The resulting species concentration profiles are exponential distributions, rising toward the hot regions of the sample, in agreement with irreversible thermodynamics analysis. For both He atoms and SIAs, we find that the resulting species flux is directed opposite to the heat flux, indicating that species transport is governed by a Soret effect (thermal-gradient-driven diffusion) characterized by a negative heat of transport that drives species diffusion uphill (from the cooler to the hot regions of the sample). We demonstrate that the steady-state species profiles obtained accounting for the Soret effect vary significantly from those where temperature-gradient-driven transport is not considered and discuss the implications of such a Soret effect on the response to plasma exposure of plasma-facing tungsten.

Fast phase error calibration through radix-p optimization processing

Optical phased arrays (OPAs) have been a popular option to manipulate the spatial behavior of the free-space optical beams. Inspired by the success of the integrated circuits (IC) industry, photonic IC (PIC) based OPAs have been actively studied in the last decade. Due to the fabrication imperfection, light passing through different paths typically results in random phase errors. The phase errors can seriously deviate the interference pattern of an OPA, and radically change the spatial distribution of the optical field. This phenomenon will significantly degrade the performance for applications like image scanning and optical wireless communication (OWC). Thermally or electrically tunable phase shifters can be applied to compensate the phase errors through dedicated calibration algorithms. However, the complexity of the calibration processes quickly increases with the number of the OPA’s elements. Hence, the whole processing can be time-consuming and this significantly pushes the overall system cost. Besides, considering the potential dynamic factors such as thermal gradient and aging effects, continuous calibration may be necessary. This results in great demand for fast and efficient calibration processing for the practical applications. In this paper, novel and efficient radix-p calibration processing is proposed which can be directly combined with most of the already reported/developed optimization algorithms as a speed-up turbo. It is shown that the computation time of the proposed radix-p processing grows linearly with the number of the OPA’s elements. In comparison to the conventional particle swarm optimization (PSO) algorithm, which exhibits exponential computation time in our numerical results, the radix-p processing shows great efficiency improvement. In the numerical results, speed with more than 12 times faster is shown for the proposed radix-p processing compared to the traditional PSO algorithms.

Parametric effect on the microstructure of direct metal deposited Inconel 718

ABSTRACT Direct metal deposition (DMD) technique results in unique microstructure when compared with the traditional manufacturing processes because of rapid solidification and the combined effect of input process parameters. However, parametric effect on the microstructure evolution is not thoroughly understood. This paper deals with the investigations carried to understand the effect of process parameters on the microstructure characteristics of Inconel 718 deposits fabricated by DMD process. The microstructure observations revealed columnar dendrites at the bottom, middle portions, and equiaxed grains in the top portion of the deposits. The different grain morphologies obtained are primarily due to the interaction effect of thermal gradient and solidification velocity. Furthermore, analysis is carried on the microstructural characteristics, and cooling rates during solidification were evaluated. It was observed that the cooling rates are varying systematically with input parameters. In addition, the grain size is observed to be a function of cooling rate. EDS analysis performed on the deposits, where the elemental mapping revealed long chains of Nb-rich phases at the interdendritic regions. The microhardness measurements done using Vickers indentation method show a minimal variation with process parameters; the hardness followed a linear relationship with the reciprocal square root of grain width.

Lithium Plating Heterogeneity Caused by Realistic Thermal Gradients

Fast charging is a key requirement to alleviate range anxiety and enable mainstream adoption of electric vehicles (EVs). Charging rates are limited primarily by the risk of lithium plating, a side reaction that can lead to the rapid deterioration of cell capacity and dendrite-induced thermal runaway in extreme cases. Whilst many studies have investigated lithium plating under pseudo uniform thermal conditions, much less is known about how thermal gradients impact the severity and distribution of the deposited lithium. In this work, we investigate this using a multilayer P3D physics-based model of a high energy pouch cell accounting for nonlinear diffusion. The results show that even a small thermal gradient of 2 ⁰C across the cell thickness can lead to significant plating heterogeneity. A balance between two physical effects is found to affect the relative plating rates between the cell layers: the reduced diffusion rates in the colder regions, and the increased current density in the warmer regions. It is shown that either effect can dominate depending on the average cell temperature: at moderate and higher temperatures, an increased plating rate is observed in the colder cell layers, but the trend is reversed at low temperatures. The thermal gradients also affect the overall amount of deposited lithium compared to a thermally uniform cell. At low temperatures, where the current distribution effect dominates, the average plating rate is increased. However, at moderate to high temperatures, where the diffusion limitation effect dominates, a thermal gradient can lead to a reduced average plating rate. Sensitivity studies were conducted to confirm these conclusions are likely to apply to other pouch cells. The model findings are supported through experimental validation based on both electrochemical signatures (differential voltage analysis) and post-mortem examination. The results provide insights into the effect of EV-relevant thermal gradients on lithium plating behaviour and highlight some of the limitations of 1D models in estimating the fast charging performance of real-world systems.

Effect of non-uniform temperature exposure on the out-of-plane bending performance of ordinary laminated glass panels

Among the open design issues for structural laminated glass (LG) elements, their mechanical performance during fire exposure stands as a safety concern. This issue is the focus of this paper. An experimental investigation is conducted to evaluate the effect of non-uniform thermal gradients on the out-of-plane bending capacity of standard LG specimens. The pre-fracture, cracking, and post-fracture performances of LG panels were examined using a three-point bending setup. The bending tests were carried out on un-heated LG samples and heated samples, which were either hot or cooled down. The reliability of available analytical and numerical methods to predict the out-of-plane bending performance of LG is then investigated. The accuracy of the analytical and numerical methods was found to be highly affected by the non-uniform thermal boundaries, as well as by the assigned material properties, which are highly sensitive to the fire exposure.

Effect of thermal gradient on structural relaxation and characterization of thermal stresses in dental porcelains – A finite element study

Zirconia-based materials veneered with glassy porcelain have become the material of choice for both dentists and their patients. This restoration material is now extensively used in clinical practice for the following reasons: material closely resembles tooth appearance, biofunctionality, biocompatibility and affordability. However, premature clinical failures due to the combination of subsurface flaws and tensile stresses within porcelains raise a concern about their reliability and integrity. This study evaluates structural relaxation in dental porcelain to form a suitable explanation for subsurface tensile stresses. Finite Element Method (FEM) is used for this analysis. User material subroutines, UEXPAN, and UTRS, were developed and integrated into a finite element solver to study the effects of structural relaxation on thermal stresses in veneer during final heat treatment. The predictions of the model were validated through qualitative and quantitave means. A validated model was used for Finite Element Analysis (FEA). Faster cooling rates and high veneer thickness predicted high subsurface tensile stresses due to ineffective structural relaxation. Slow cooling rates and lower veneer thickness showed desired compressive stresses in subsurface areas with visible structural relaxation. This work, emphasizing structural relaxation in veneering dental porcelain, provides a way forward for effective designing of dental restorations. Accordingly, it is useful to tailor the desired stress state for extended life in veneered dental porcelains on zirconia frameworks.

Stress behavior of concrete pavement

Warping stresses and flexural stresses due to variation of thermal gradient and wheel load, respectively, cause cracking of rigid pavement. The distress in the pavement structure can be happened because of warping stresses induced by thermal gradient which causes curling effect due to temperature variation. The wheel load resulted flexural stresses which varies with the position and depends upon contact condition and repetition of load at a specific time interval. In the present study, a comparison of stresses of single layered rigid concrete pavement due to thermal load and wheel load have been done with varying slab thicknesses and modulus of subgrade reactions. The pavement has been modeled and analysed using SAP2000 finite element software. During modeling, the single layered rigid concrete pavements comprising of PQC has been considered without any joints at edges. The PQC slab has been considered with dense liquid foundation i.e. elastic. The self weight of pavement has been excluded in the analysis. It has been found that the interior thermal warping stresses are more sensitive with respect to slab thicknesses than modulus of subgrade reactions. It can also be stated that there is significant impact on stress behavior of the pavement due to combined effect of thermal gradient along with wheel load, especially for positive and negative thermal gradient considerations.

Microstructure evolution of gadolinium doped cerium oxide under large thermal gradients

The effects of large thermal gradient annealing on the microstructure of 10 mol% gadolinium doped ceria (GDC) were investigated. GDC powder was prepared by solvent deficient method and sintered at 1650 °C for 10 h to achieve dense ceramics with ~8 μm grain size. The densified GDC samples were subsequently annealed using a 60 W infrared laser at over 2100 °C for 1 h under a thermal gradient equivalent to ~0.3–0.5 °C/μm. The post-annealed samples at 2150 °C for 1 h exhibit grains with average length and width of 37 and 28 μm, respectively. Electron backscattered diffraction (EBSD) analysis revealed that the post-annealed sample at 2150 °C consists of grains oriented close to five principal directions (<4 3 10>, <0 0 1> and <13 1 14> on [0 0 1], and <7 6 20> and <7 2 7> on [0 1 0]) within a tolerance angle of ±10°, whereas the grains of the pre-annealed sample are randomly oriented. Gadolinium diffuses 20–30 μm away from the irradiated surface, with the measured composition of regions deeper than 30 μm, Ce0.86Gd0.14O1.93, is close to that of the pre-annealed sample, Ce0.87Gd0.13O1.94. Enhancement of total conductivity of the post-annealed GDC (1.1 × 10-3 S cm-1 at 500 °C, and 2.1 × 10-2 S cm-1 at 700 °C) is observed when compared to the pre-annealed GDC (3.1 × 10-5 S cm-1 at 500 °C, and 1.7 × 10-3 S cm-1 at 700 °C), and points to the decrease in the grain boundary (GB) resistivity. This could be attributed to both the change in GB area and grain alignment.

A Parametric Study of Coupled Photo-Electro-Thermal Responses of Thin Film a-Si:H Solar Cell with Embedded Nanoparticles

A parametric investigation has been performed on a thin-film hydrogenated amorphous silicon (a-Si:H) solar cell that is enhanced with various light trapping schemes through a modelling approach. The proposed model contains a novel coupling approach and various feedback routines for a more holistic modelling treatment. The proposed optical model adopts a semi-coherent method, the electrical model extends the classical drift-diffusion model to incorporate the effects of thermal gradients, and the thermal model adopts energy conservation equations from the hydrodynamic model. Based on the simulation results, it is observed that the rise in cell temperature adversely affects the electrical performance but promotes more optical absorptions due to the unique optical properties of amorphous silicon. To obtain an optimum enhancement from the inclusion of nanoparticles, their dimensions and separation distances are essential factors. The thickness of the intrinsic active absorbing layer affects the optical performance directly which then leads to various variations in electrical and thermal responses.

Universal field dependence of magnetic resonance near zero frequency

Magnetic resonance is a widely-established phenomenon that probes magnetic properties such as magnetic damping and anisotropy. Even though the typical resonance frequency of a magnet ranges from gigahertz to terahertz, experiments also report the resonance near zero frequency in a large class of magnets. Here we revisit this phenomenon by analyzing the symmetry of the system and find that the resonance frequency ($\omega$) follows a universal power law $\omega \varpropto |H-H_c|^p$, where $H_c$ is the critical field at which the resonance frequency is zero. When the magnet preserves the rotational symmetry around the external field ($H$), $p = 1$. Otherwise, $p=1/2$. The magnon excitations are gapped above $H_c$, gapless at $H_c$ and gapped again below $H_c$. The zero frequency is often accompanied by a reorientation transition in the magnetization. For the case that $p=1/2$, this transition is described by a Landau theory for second-order phase transitions. We further show that the spin current driven by thermal gradient and spin-orbit effects can be significantly enhanced when the resonance frequency is close to zero, which can be measured electrically by converting the spin current into electric signals. This may provide an experimentally accessible way to characterize the critical field. Our findings provide a unified understanding of the magnetization dynamics near the critical field, and may, furthermore, inspire the study of magnon transport near magnetic transitions.

On vibration of bidirectional tapered triangular plate under the effect of thermal gradient

On vibration of bidirectional tapered triangular plate under the effect of thermal gradient

Surface temperature measurement of acoustic liners in the presence of grazing flow and thermal gradient

The development of turbofans with larger fan diameters, shorter inlets, and thinner walls forces the acoustic liners to be placed closer to the hot parts of engines. This experimental study investigates the combined effects of large thermal gradients, grazing flow and acoustic level on the impedance of liners. Previous studies have shown that a coupling between these three effects can exist. The objective is thus to understand the underlying coupled phenomena, in order to extract the driving parameters for a more accurate impedance modeling. In the ONERA B2A grazing flow acoustic liner facility, the flow temperature can be accurately regulated and several types of acoustic excitation can be provided. For the purposes of this study, a test section with a heating device is used to obtain a thermal gradient between the backplate and the perforated plate of the liner sample. Infrared (IR) thermography is used to measure the temperature distribution on the perforated plate. The measurement is conducted for several configurations, to determine in which conditions the coupling between thermal and acoustic dissipation effects exists. In particular, a possible control of surface temperature by high sound pressure level is highlighted.Graphic abstract

Simulation of the effects of negative thermal gradients on separation performance of gas chromatography

The effect of a gradient of solute velocity on the chromatographic separation of closely spaced solutes is investigated by usage of a simulation. The concept of the ideal basic separation (IBS), introduced by Blumberg, is used to determine the theoretical limit of a separation without any natural or artificial gradients of features of the chromatographic medium. The IBS is the best achievable separation and can therefore be used as a baseline to which other separations can be compared to. An addition of a negative velocity gradient cannot improve the separation of closely spaced solutes. The velocity gradient is realized by adding a temperature gradient to a GC separation. The simulation confirms this theoretical limit for a range of differently strong retained solutes.In a second part controlled deviations from IBS are used to show, that a velocity gradient can be beneficial in realistic, non-IBS. The addition of a negative velocity gradient can improve e.g. the separation of broad injected solute zones or counteract a positive gradient of the mobile phase velocity caused by gas decompression along the GC column. However, the improved separation cannot exceed that of a corresponding ideal basic separation. The resolution of broadly injected solutes can be increased by up to 45% of the corresponding IBS resolution by adding a negative velocity gradient. A positive velocity gradient due to gas decompression reduces the separation by up to 6%. The added negative velocity gradient, realized by a linear temperature gradient, can compensate this resolution loss by up to 2%.

Precast prestressed concrete pavement (PPCP): Effect of thermal gradient on curling deflection and stress

In this study, a three-dimensional (3-D) finite element (FE) model was developed and validated to evaluate the effect of thermal gradient on behavior of Precast Prestressed Concrete Pavement (PPCP) system. The FE model developed was then utilized to perform a numerical analysis for determination of the curling deflections and curling stresses by changing the important pavement parameters (i.e., prestress forces, concrete property, and subgrade stiffness). Results indicated that the 3-D FE model developed for PPCP system was capable of providing reliable predictions in consideration of the effect of thermal gradient on change in curling deflection and stress.

Stress waves in thick porous graphene-reinforced cylinders under thermal gradient environments

In this paper, the propagation of stress waves in thick porous graphene-reinforced cylinders (TPGRCs) is investigated using a meshless solution based on an axisymmetric model and moving least squares (MLSs) interpolation functions. The cylinders are assumed under a steady state thermal gradient effect along with an initial deflection response obtained from the static solution of the same cylinder subjected to a static internal pressure. To improve the design of such TPGRCs, functionally graded (FG) profiles are considered for the distribution of graphene nanofillers along their radial direction. Moreover, pores are non-uniformly embedded inside the cylinders to decrease the weight of TPGRCs. The material properties of porous and nanocomposite materials are calculated as functions of temperature using a closed cell model and a modified Halpin-Tsai (HS) technique, respectively. Considering structural damping for the system, the influences of embedding porosity, the temperature of each surface, reinforcing with graphene and the thickness of cylinder on the stress wave responses of TPGRCs have been investigated. The results reveal that the increase of porosity volume inside TPGRCs significantly increases the amplitudes and stationary time of deflections; however, it reduces the values of stationary deflections and stresses, along with the reduction of propagation speeds.

Oxygen inhibition of free-radical polymerization is the dominant mechanism behind the "mold effect" on hydrogels.

Hydrogel surfaces are of great importance in numerous applications ranging from cell-growth studies and hydrogel-patch adhesion to catheter coatings and contact lenses. A common method to control the structure and mechanical/tribological properties of hydrogel surfaces is by synthesizing them in various mold materials, whose influence has been widely ascribed to their hydrophobicity. In this work, we examine possible mechanisms for this “mold effect” on the surface of hydrogels during polymerization. Our results for polyacrylamide gels clearly rule out the effect of mold hydrophobicity as well as any thermal-gradient effects during synthesis. We show unequivocally that oxygen diffuses out of certain molding materials and into the reaction mixture, thereby inhibiting free-radical polymerization in the vicinity of the molding interface. Removal of oxygen from the system results in homogeneously cross-linked hydrogel surfaces, irrespective of the substrate material used. Moreover, by varying the amount of oxygen at the surface of the polymerizing solutions using a permeable membrane we are able to tailor the surface structures and mechanical properties of PAAm, PEGDA and HEMA hydrogels in a controlled manner.

Advanced computer model for analysis of reinforced concrete and composite structures at elevated temperatures

abstract: This paper is concerned on the development of a computational model based on finite element procedures for advanced analysis capable of estimating the behavior of reinforced concrete and composite steel-concrete plane structures exposed to fire. The program implemented is called NASEN, the specific thermo-structural module is used to analyze structures under fire conditions. The effects of geometric nonlinearity, material nonlinearity and nonlinear thermal gradients are incorporated into the model, as well as the degradation of material properties with increased temperature. The methodology applied for the solution is based on the unidirectional coupling of the thermal and mechanical solutions. The cross-sections of the structural elements are discretized with two-dimensional meshes for thermal analysis, while the structural analysis uses a one-dimensional beam-column element. Numerical examples are presented to demonstrate the accuracy of the computational code developed in relation to the numerical and experimental solutions found in the literature. In summary, the program adequately predicted the response of the studied structures.