Uniform Temperature Change Research Articles

The influence of thermal residual stresses and thermal generated dislocation on the mechanical response of particulate-reinforced metal matrix nanocomposites

Due to thermal expansion mismatch between reinforcing particles and matrix, thermal induced dislocations are generated in metal matrix nanocomposites (MMNCs) during cooling down from the processing temperature. These dislocations have been identified as an important strengthening mechanism in particulate-reinforced MMNCs. In this study, the development of thermal residual stresses and thermal induced dislocations in MMNCs are predicted using discrete dislocation simulation, assuming the whole material is under uniform temperature change. Shear deformation is applied after the composites are cooled down to room temperature and the influence of thermal residual stresses and thermal generated dislocation on the overall response of particulate-reinforced MMNCs are investigated. The results show that the thermal residual stresses are high enough to generate dislocations and the dislocation density is higher in the interfacial region than the rest of the matrix. The predicted mechanical behavior of the MMNCs matches the experimental results better when thermal residual stresses are included in the simulations.

Rejuvenation of metallic glasses by non-affine thermal strain.

When a spatially uniform temperature change is imposed on a solid with more than one phase, or on a polycrystal of a single, non-cubic phase (showing anisotropic expansion-contraction), the resulting thermal strain is inhomogeneous (non-affine). Thermal cycling induces internal stresses, leading to structural and property changes that are usually deleterious. Glasses are the solids that form on cooling a liquid if crystallization is avoided--they might be considered the ultimate, uniform solids, without the microstructural features and defects associated with polycrystals. Here we explore the effects of cryogenic thermal cycling on glasses, specifically metallic glasses. We show that, contrary to the null effect expected from uniformity, thermal cycling induces rejuvenation, reaching less relaxed states of higher energy. We interpret these findings in the context that the dynamics in liquids become heterogeneous on cooling towards the glass transition, and that there may be consequent heterogeneities in the resulting glasses. For example, the vibrational dynamics of glassy silica at long wavelengths are those of an elastic continuum, but at wavelengths less than approximately three nanometres the vibrational dynamics are similar to those of a polycrystal with anisotropic grains. Thermal cycling of metallic glasses is easily applied, and gives improvements in compressive plasticity. The fact that such effects can be achieved is attributed to intrinsic non-uniformity of the glass structure, giving a non-uniform coefficient of thermal expansion. While metallic glasses may be particularly suitable for thermal cycling, the non-affine nature of strains in glasses in general deserves further study, whether they are induced by applied stresses or by temperature change.

Nonlinear vibration of a nanobeam elastically bonded with a piezoelectric nanobeam via strain gradient theory

Nonlinear vibration of a nanobeam (NB) coupled with a piezoelectric nanobeam (PNB) is investigated in this article based on the strain gradient theory. The two nanobeams are coupled by an enclosing elastic medium which is simulated as Pasternak foundation. The PNB is subjected to an external electric voltage in thickness direction and a uniform temperature change. Considering the Von-Kármán geometric nonlinearity and charge equation for coupling of electrical and mechanical fields, the motion equations are derived using strain gradient theory and Hamilton׳s principle. The differential quadrature method (DQM) is applied to obtain the nonlinear frequency of NB for clamped–clamped mechanical boundary condition. Research results reveal that the dimensionless frequency of NB reduction results from increasing the external electric voltage. Furthermore, at higher values of small scale parameters, the difference between the results obtained by modified couple stress and strain gradient theories become considerable.

Spatial variability in the coefficient of thermal expansion induces pre-service stresses in computer models of virgin Gilsocarbon bricks

In this paper, the authors test the hypothesis that tiny spatial variations in material properties may lead to significant pre-service stresses in virgin graphite bricks. To do this, they have customised ParaFEM, an open source parallel finite element package, adding support for stochastic thermo-mechanical analysis using the Monte Carlo Simulation method. For an Advanced Gas-cooled Reactor brick, three heating cases have been examined: a uniform temperature change; a uniform temperature gradient applied through the thickness of the brick and a simulated temperature profile from an operating reactor. Results are compared for mean and stochastic properties. These show that, for the proof-of-concept analyses carried out, the pre-service von Mises stress is around twenty times higher when spatial variability of material properties is introduced. The paper demonstrates that thermal gradients coupled with material incompatibilities may be important in the generation of stress in nuclear graphite reactor bricks. Tiny spatial variations in coefficient of thermal expansion (CTE) and Young's modulus can lead to the presence of thermal stresses in bricks that are free to expand.

On thermomechanical stress analysis of adhesively bonded composite joints in presence of an interfacial void

This paper deals with analytical thermomechanical stress analysis of adhesively bonded composite joints in presence of a structural imperfection in the form of an interfacial void within the adhesive layer based on the full layerwise theory (FLWT). The joints are subjected to mechanical tension, uniform temperature change, or steady-state heat conduction. The proposed adhesive joint is divided into three distinct regions along its length and a large number of mathematical plies through its thickness. Three sets of fully coupled governing equilibrium equations are derived employing the principle of minimum total potential energy. The three-dimensional nonlinear interlaminar stress distributions along the bond-line in the interior region and near the edges are obtained. The results, showing fast convergence speed and quite agreement with the finite element method, reveal that the presence of such defect within the adhesive layer can increase the stress concentrations resulting in premature failure and debonding.

Diurnal and seasonal changes in near‐surface humidity in a complex orography

AbstractChanges in near‐surface humidity are evaluated in relation to changes in other meteorological variables such as air temperature, precipitation, and sunshine duration for a 31 year period (1981–2011) using hourly time series recorded in Switzerland. Trends in meteorological variables are analyzed at the seasonal and subdaily scale, and changes in water vapor are tested for Clausius‐Clapeyron scaling. Results show a marked seasonality of climatic changes with a trend toward warmer, clearer, and drier near‐surface atmosphere strengthening from January to June. During this period considerable negative trends in relative humidity are detected. An abrupt shift in climatic trends occurs during the month of July, after which warming trends are considerably smaller and relative humidity remains constant or increases. An evaluation of reanalysis products demonstrates strong consistency with station observations and when combined with teleconnection indices supports the hypothesis that shifts in the general circulation patterns rather than local feedbacks are the principal drivers of observed seasonality of climate change. However, local feedbacks can play a role in enhancing summer humidity and convective activity (lightning and evening precipitation). Subdaily changes are significant with more pronounced daytime trends of temperature and humidity during spring and the opposite during summer. The strong seasonality and diurnal variability of changes in near‐surface meteorology over the last three decades in Switzerland warns against climate change impact assessments at local scale that consider uniform changes in air temperature and assume Clausius‐Clapeyron scaling for near‐surface humidity.

Nonlinear Dynamics of a Functionally Graded Thin Simply-Supported Plate Under a Hypersonic Flow

An analysis on the nonlinear dynamics of an FGM plate in hypersonic flow subjected to an external excitation and uniform temperature change is presented in this article. Material properties of the constituents are assumed to be temperature-dependent and the effective properties of FGM plate are graded in thickness direction according to a simple power law function in terms of the volume fractions. Based on von Karman type nonlinear strain–displacement relationship, the nonlinear governing equations of motion are derived by using Hamilton's principle. Galerkin's method is then utilized to discretize the governing partial equations to a two-degree-of-freedom nonlinear system, including the quadratic and cubic nonlinear terms. A third-order piston theory is applied for the hypersonic aerodynamic load. The Mach number and in-plane loading are used as bifurcation parameters. Numerical results including the bifurcations, waveform, phase plots, and Poincare maps are presented, for an FGM plate to show the influences of material gradient index, Mach number, and in-plane load on the nonlinear dynamics.

Thermally Induced Vibration and Stability of Initially Stressed Hybrid Composite Plates

The vibration and buckling behavior of hybrid composite plates with a general state of nonuniform initial stress in thermal environments are investigated. Governing equations including the effects of transverse shear deformation are established using the energy variation method. The initial stress is taken to be a combination of uniaxial extension and pure bending stresses. The temperature distribution in the hybrid plate is assumed to be a combined uniform and linear temperature change in the transverse direction. An example problem on an initially and thermally stressed laminated plate is solved. The effects of various parameters on the thermal induced vibration and stability of hybrid composite plates are studied. The natural frequency and buckling load are found to be sensitive to the thickness ratio of Al to CRFP layers, the state of initial stresses, and temperature rise.

Post-buckling analysis of rectangular plates comprising Functionally Graded Strips in thermal environments

Description is given of a semi-analytical finite strip method for analysing the post-buckling behaviour of functionally graded rectangular plates in thermal environments where plates are under uniform, tent-like or nonlinear temperature change across the thickness. The material properties are assumed to vary through the thickness according to the power law. The formulations are based on the classical plate theory and the concept of the principle of the minimum potential energy. The Newton–Raphson method is used to solve the equilibrium equations. A range of applications are described and the numerical results are compared to the available results, wherever possible.

Decagonal quasicrystalline elliptical inclusions under thermomechanical loading

In this work, an elegant method is proposed to derive the thermoelastic field induced by thermomechanical loadings in a decagonal quasicrystalline composite composed of an infinite matrix reinforced by an elliptical inclusion. The thermomechanical loadings include a uniform temperature change, remote uniform in-plane heat fluxes and remote uniform in-plane stresses. The corresponding boundary value problem is ultimately reduced to the solution of two independent sets of four coupled linear algebraic equations, each of which involves four complex constants characterizing the internal stress field. The solution demonstrates that a uniform temperature change and remote uniform stresses will induce an internal uniform stress field, and that uniform heat fluxes will result in a linearly distributed internal stress field within the elliptical inclusion. The induced uniform rigid body rotation within the inclusion is given explicitly.

301-nm wavelength tunable differentially driven all-polymer optical filter.

An optical filter based on all-polymer grating-assisted directional coupler is demonstrated. The wavelength tuning is differentially driven. The heater-electrodes created on the sidewalls of the polymer ridge can either blue- or red-shift the central wavelength. A total tuning range of 301 nm is achieved experimentally. The maximal local temperature gradient introduced across the coupler region is only 55°C-68°C. The filter is also insensitive to uniform ambient temperature change. Central wavelength jitter of ±3 nm is observed when the chip holder temperature cycles between 20°C and 80°C.

Experimental Study of Compliant Mechanism Considering Thermal Effect

In this paper, an ultra-precision positioning stage is introduced. The finite element method is used to establish the theoretical model, and the strain test system of experiment is established. The system includes the static strain test and the dynamic strain test. The test system contains the software and hardware system. The software system includes the static strain measurement software of DH3816, the programmable constant temperature and humidity chamber control software TEMI300, and the dynamic strain test system developed from NI Labview. The hardware system contains industrial personal computer (IPC), piezoelectric (PZT) actuator, static strain gauge of DH3816, strain amplifier of YE1817, NI USB-6009 multifunction data acquisition card, strain gauge, programmable temperature and humidity chamber, and precision positioning stage. The experimental test is carried out to examine the theoretical analysis considering thermal effect. Experimental results and analytical results indicate that whether the stresses increase or decrease linearly or nonlinearly depend on the function of the temperature to the time during the transient response same as the stresses increase or decrease linearly during the steady response under uniform temperature change. The dynamic responses of platform are affected by the temperature change, and it is essential to consider thermal effect when analyzing compliant mechanisms.

AN ANALYTICAL APPROACH FOR BUCKLING BEHAVIOR OF TEMPERATURE-DEPENDENT LAMINATED PIEZOELECTRIC FUNCTIONALLY GRADED PLATES UNDER THERMO-ELECTRO-MECHANICAL LOADINGS AND DIFFERENT END SUPPORTS

In this paper, we examine the buckling behavior of piezoelectric laminated plate made of two-layered functionally graded materials (FGMs) that are integrated with surface-bonded piezoelectric actuators and is subjected to a combined action of uniform temperature change, inplane forces, and applied actuator voltage. Temperature-dependent material properties are assumed for both the substrate FGM layer and piezoelectric layers. Employing an analytical approach, five coupled governing stability equations, which were derived based on the first-order shear deformation plate theory, are converted into a fourth-order and a second-order decoupled partial differential equations. Then, an accurate analytical solution is proposed to solve them. Parametric studies are also undertaken, and show the effects of applied actuator voltage, volume fraction exponents, plate aspect ratio, ratio of piezoelectric layer thickness to thickness of FGM layer and temperature dependency, on the buckling load of the plate.

Solutions of dynamic and acoustic responses of a clamped rectangular plate in thermal environments

This work presents an investigation of vibration and acoustic response characters of a clamped rectangular thin plate in thermal environments. The general form of governing equation of plate flexural vibration with considering thermal loads is established, and the influence of uniform temperature change on response characters is studied in detail. A set of double sinusoidal mode shape functions for fully clamped boundaries is used to describe the displacement distribution. The governing equation is solved with Fourier series expansion to discuss the natural vibration and dynamic responses of a clamped plate in thermal environments. Accordingly, acoustic radiation characteristics are obtained based on Rayleigh integral. Results show that natural frequencies decrease with the increment of plate temperature, and the first natural frequency is much more sensitive to thermal environment changes. Response curves of plate vibration and radiated sound power shift toward lower frequency range, and the response amplitudes of the first resonant peak of the two responses present opposite variation tendencies. These phenomena are verified with finite element and boundary element simulations. Thermal loads reduce the radiation efficiency of the plate obviously below the critical frequency, but the maximum value almost remains unchanged.

Semianalytic Stress-Function Variational Approach for the Interfacial Stresses in Bonded Joints

AbstractHigh interfacial stresses near the edges of bonded joints are responsible for their debonding failure. This paper reports a new semianalytic stress-function variational approach to the interfacial stresses of a bonded joint, which is made of a straight tension bar covered with a reinforcing patch and subjected to mechanical loads and/or uniform change of temperature. The process introduces two interfacial shear and normal stress functions, which are correlated via the approximately same radius of curvature of the slender adherends. All the stress components in the joint are expressed in terms of the interfacial stress functions based on the classic Euler-Bernoulli beam theory and equilibrium equations of elasticity. Deformation compatibility of the joint is satisfied by minimizing the complementary strain energy, which leads to a fourth-order ordinary differential equation (ODE) of the interfacial shear stress function. The interfacial shear and normal stresses are determined explicitly and compar...

Determination of thermal expansion coefficients for unidirectional fiber-reinforced composites

In the present work, the coefficients of thermal expansion (CTEs) of unidirectional (UD) fiber-reinforced composites are studied. First, an attempt is made to propose a model to predict both longitudinal and transverse CTEs of UD composites by means of thermo-elastic mechanics analysis. The proposed model is supposed to be a concentric cylinder with a transversely isotropic fiber embedded in an isotropic matrix, and it is subjected to a uniform temperature change. Then a concise and explicit formula is offered for each CTE. Finally, some finite element (FE) models are created by a finite element program MSC. Patran according to different material systems and fiber volume fractions. In addition, the available experimental data and results of other analytical solutions of CTEs are presented. Comparisons are made among the results of the cylinder model, the finite element method (FEM), experiments, and other solutions, which show that the predicted CTEs by the new model are in good agreement with the experimental data. In particular, transverse CTEs generally offer better agreements than those predicted by most of other solutions.

Departure from Clausius-Clapeyron scaling of water entering the stratosphere in response to changes in tropical upwelling

Water entering the stratosphere ([H2O]entry) is strongly constrained by temperatures in the tropical tropopause layer (TTL). Temperatures at tropical tropopause levels are 15–20 K below radiative equilibrium. A strengthening of the residual circulation as suggested by general circulation models in response to increasing greenhouse gases is, based on radiative transfer calculations, estimated to lead to a temperature decrease of about 2 K per 10% change in upwelling (with some sensitivity to vertical scale length). For a uniform temperature change in the inner tropics, [H2O]entry may be expected to change as predicted by the temperature dependence of the vapor pressure, referred here as “Clausius-Clapeyron (CC) scaling.” Under CC scaling, this corresponds to ∼1 ppmv change in [H2O]entry per 10% change in upwelling. However, the change in upwelling also changes the residence time of air in the TTL. We show with trajectory calculations that this affects [H2O]entry, such that [H2O]entry changes ∼10 % less than expected from CC scaling. This residence time effect for water vapor is a consequence of the spatiotemporal variance in the temperature field. We show that for the present-day TTL, a little more than half of the effect is due to the systematic relation between flow and temperature field. The remainder can be understood from the perspective of a random walk problem, with slower ascent (longer path) increasing each air parcel's probability to encounter anomalously low temperatures. Our results show that atmospheric water vapor may depart from CC scaling with mean temperatures even when all physical processes of dehydration remain unchanged.

Post-Buckling Analysis of Laminated Composite Doubly Curved Panel Embedded with SMA Fibers Subjected to Thermal Environment

In this article, thermal post-buckling of laminated composite panel with SMA subjected to uniform temperature change is investigated. The panel model is derived taking the geometric nonlinearity in Green-Lagrange sense based on the HSDT and then it is extended to incorporate the material nonlinearity arises due to temperature in the SMA via marching technique. The panel is discretized using nonlinear finite element. The system equations are developed by minimizing the total potential energy of the system and solved by a direct iterative method. The effects of different parameters are examined in detail and the results are compared with the published results.

Hydrophilic–hydrophobic copolymer nano-sized particle gels: Swelling behavior and dependence on crosslinker chain length

Poly N-isopropylacrylamide-co-ethylacrylate [P(NIPAM-co-EA)], Poly N-isopropylacrylamide-co-2-hydroxyethylmethacrylate [P(NIPAM-co-HEMA)], and Poly N-isopropylacrylamide-co-2-hydroxyethylacrylate [P(NIPAM-co-HEA)] nano-sized particle copolymer hydrogels were synthesized to investigate their volume phase transition behavior. Increasing the hydrophobic or hydrophilic monomer content of hydrogels led to uniform changes in transition temperature and swelling ratio. Different statistical thermodynamic models are presented to describe the swelling behavior of copolymer gels. The classical interaction energy parameter in the thermodynamic model, which is not suitable for accurately representing the equilibrium swelling of a copolymer hydrogel, was modified semi-empirically in order to reflect the individual contribution of each hydrogel constituent. In addition, we investigated the volume phase transition of PNIPAM gels as a function of crosslinker chain length. The crosslinkers used in this study were N,N′-methylenebisacrylamide (BIS) and two different molecular weights of Poly(ethyleneglycol) diacrylate (PEGDA, 575 and 700). Interestingly, the use of a long chain crosslinker resulted in a decreased volume change compared to that of a short chain crosslinker without any significant changes in transition temperature. To understand this unexpected result, we used a molecular dynamics simulation for BIS and PEGDA 575 to demonstrate the micro-scale conformation change of crosslinker chain mixed with water or ethanol. The proposed thermodynamic models represent the swelling behavior of the copolymer hydrogels and PNIPAM gels with different length crosslinkers.

Data-based modelling and environmental sensitivity of vegetation in China

Abstract. A process-oriented niche specification (PONS) model was constructed to quantify climatic controls on the distribution of ecosystems, based on the vegetation map of China. PONS uses general hypotheses about bioclimatic controls to provide a "bridge" between statistical niche models and more complex process-based models. Canonical correspondence analysis provided an overview of relationships between the abundances of 55 plant communities in 0.1° grid cells and associated mean values of 20 predictor variables. Of these, GDD0 (accumulated degree days above 0 °C), Cramer–Prentice α (an estimate of the ratio of actual to equilibrium evapotranspiration) and mGDD5 (mean temperature during the period above 5 °C) showed the greatest predictive power. These three variables were used to develop generalized linear models for the probability of occurrence of 16 vegetation classes, aggregated from the original 55 types by k-means clustering according to bioclimatic similarity. Each class was hypothesized to possess a unimodal relationship to each bioclimate variable, independently of the other variables. A simple calibration was used to generate vegetation maps from the predicted probabilities of the classes. Modelled and observed vegetation maps showed good to excellent agreement (κ = 0.745). A sensitivity study examined modelled responses of vegetation distribution to spatially uniform changes in temperature, precipitation and [CO2], the latter included via an offset to α (based on an independent, data-based light use efficiency model for forest net primary production). Warming shifted the boundaries of most vegetation classes northward and westward while temperate steppe and desert replaced alpine tundra and steppe in the southeast of the Tibetan Plateau. Increased precipitation expanded mesic vegetation at the expense of xeric vegetation. The effect of [CO2] doubling was roughly equivalent to increasing precipitation by ~ 30%, favouring woody vegetation types, particularly in northern China. Agricultural zones in northern China responded most strongly to warming, but also benefited from increases in precipitation and [CO2]. These results broadly conform to previously published findings made with the process-based model BIOME4, but they add regional detail and realism and extend the earlier results to include cropping systems. They provide a potential basis for a broad-scale assessment of global change impacts on natural and managed ecosystems.