Thermomechanical Anisotropy Research Articles

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement

AbstractControlling thermomechanical anisotropy is important for emerging heat management applications such as thermal interface and electronic packaging materials. Whereas many studies report on thermal transport in anisotropic nanocomposite materials, a fundamental understanding of the interplay between mechanical and thermal properties is missing, due to the lack of measurements of direction‐dependent mechanical properties. In this work, exceptionally coherent and transparent hybrid Bragg stacks made of strictly alternating mica‐type nanosheets (synthetic hectorite) and polymer layers (polyvinylpyrrolidone) were fabricated at large scale. Distinct from ordinary nanocomposites, these stacks display long‐range periodicity, which is tunable down to angstrom precision. A large thermal transport anisotropy (up to 38) is consequently observed, with the high in‐plane thermal conductivity (up to 5.7 W m−1 K−1) exhibiting an effective medium behavior. The unique hybrid material combined with advanced characterization techniques allows correlating the full elastic tensors to the direction‐dependent thermal conductivities. We, therefore, provide a first analysis on how the direction‐dependent Young's and shear moduli influence the flow of heat.

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement.

Controlling thermomechanical anisotropy is important for emerging heat management applications such as thermal interface and electronic packaging materials. Whereas many studies report on thermal transport in anisotropic nanocomposite materials, a fundamental understanding of the interplay between mechanical and thermal properties is missing, due to the lack of measurements of direction‐dependent mechanical properties. In this work, exceptionally coherent and transparent hybrid Bragg stacks made of strictly alternating mica‐type nanosheets (synthetic hectorite) and polymer layers (polyvinylpyrrolidone) were fabricated at large scale. Distinct from ordinary nanocomposites, these stacks display long‐range periodicity, which is tunable down to angstrom precision. A large thermal transport anisotropy (up to 38) is consequently observed, with the high in‐plane thermal conductivity (up to 5.7 W m−1 K−1) exhibiting an effective medium behavior. The unique hybrid material combined with advanced characterization techniques allows correlating the full elastic tensors to the direction‐dependent thermal conductivities. We, therefore, provide a first analysis on how the direction‐dependent Young's and shear moduli influence the flow of heat.

Control of thermomechanical anisotropy in high-speed microrotor with permanent magnet for energy conversion

A microturbine for energy harvesting is analyzed, and a design approach is assessed. A magnetic dipole, applied to a microrotor to convert energy of either a fluid or a gas into mechanical power, exhibits anisotropy of thermomechanical properties. The rotor dynamics, stability, and unbalance response are predicted. The coupled effect of rotation and temperature is then investigated, to detect the actual influence of the thermomechanical anisotropy of magnet, to be controlled by the contact pressure applied through an outer layer of carbon fibers. This methodology allowed a refinement of system requirements, and defining a design procedure to prevent damage.

Anisotropic thermomechanical behavior of AA6082 aluminum alloy Al–Mg–Si–Mn

Abstract Hot rolled aluminum alloy 6082 was subjected to tensile tests and coefficient of thermal expansion measurements. These tests were realized in order to investigate the effect of direction on its anisotropic behavior. Specimens for both tests were cut in three directions (long transverse, longitudinal rolling and short transverse) with respect to rolling direction. Results showed that the longitudinal rolling and long transverse directions had limited effect on thermomechanical properties as compared to the short transverse direction. It was found that the short transverse direction exhibited higher values of stress hardening capacity and thermal expansion coefficient, lower ductility and dilatation, than the longitudinal rolling and long transverse directions. Thermomechanical anisotropy was attributed to the microstructure and precipitation behavior. It was assessed by means of tensile tests, coefficient of thermal expansion measurements, optical microscopy, X-ray diffraction and differential scanning calorimetry techniques.

Thermomechanical anisotropy and flowability of talc and glass fiber reinforced multiphase polymer composites

For fillers reinforced polymer composites, the material properties of the composites are affected by the dispersion and orientation of the inclusions. In particular, high aspect ratio inclusions such as glass fibers in the polymer matrix generate the mechanical or thermomechanical anisotropy of the composites that are critical to the dimensional stability, flowability, toughness, and strength of the final products. We investigate the microstructural anisotropy of multiphase polymer composites composed of talcs and glass fibers to characterize the anisotropic thermomechanical properties of the composites. The internal structure of the composites is observed by using an X-ray microtomography to characterize fiber length distributions. The fiber length distributions are fitted to the Weibull distribution statistical function to model the ultimate tensile strength of the composites. A fiber efficiency factor is coupled with the statistical function to understand the effect of the fiber length distributions on the mechanical strength of the composites. The thermomechanical anisotropy is evaluated by using linear thermal expansion coefficients. The flowability of molten polymer composites is evaluated experimentally and numerically to evaluate the role of inclusions on rheological properties and processability of the composites.

Modeling Dark and White Layer Formation on Elastohydrodynamically Lubricated Steel Surfaces by Thermomechanical Indentation or Abrasion by Metallic Particles

In a series of publications, the author has shown that the passage of ductile microparticles through elastohydrodynamic (EHD) contacts results in frictional heating that can greatly affect surface damage. The thermoviscoplastic numerical model built for those studies is extended in the present article. A more rigorous analysis of dynamic (strain-rate) effects is performed and a new element of heating is introduced, namely, that owed to plastic work of the surfaces being indented. The model is then quantitatively validated against experimental data on soft and hard particles extruded in rolling and rolling–sliding contacts. It is also compared to past numerical predictions of the author. Following its validation, the model is further expanded to predict the formation of dark and white tribochemical layers of overtempered and untempered martensite, respectively, on steel surfaces, caused by the particle-induced frictional heating. Such layers are well-known in machining processes of hardened steels as being the result of phase transformations and play a critical role in contact fatigue. The debris model in this article is used to predict the layer thickness and relative hardness for a variety of operating conditions. Layers of micrometric thickness are typically found and graphic examples are presented, linking their location to that of debris dents. A parametric study examines the role of particle size and hardness, Coulomb friction coefficient, and contact rolling velocity on dark and white layer thickness and relative hardness. The layers are zones of great inhomogeneity and thermomechanical anisotropy, increasing the risk of spalling by delamination as they are potential sources of crack initiation, particularly in sliding contacts. However, white layers in particular may actually be beneficial to contact fatigue in rolling contacts because of their substantially increased hardness. The conclusion of the study is that debris-driven surface indentation and abrasion should no longer be viewed from a purely mechanistic or geometrical perspective but has to consider the tribochemical or microstructural-modification factor for the correct evaluation of the remaining useful life of a dented or abraded contact.

Study of Heat Transport in Bénard-Darcy Convection with g-Jitter and Thermo-Mechanical Anisotropy in Variable Viscosity Liquids

The effects of temperature-dependent viscosity, gravity modulation and thermo-mechanical anisotropies on heat transport in a low-porosity medium are studied using the Ginzburg–Landau model. The effect of gravity modulation is to decrease the Nusselt number, Nu and variable viscosity leads to increase in Nu. The thermo-mechanical anisotropies have opposite effect on Nu with thermal anisotropy decreasing the heat transport.

ATLAS III in situ heating test in boom clay: Field data, observation and interpretation

The ATLAS III small scale in situ heating test aimed at assessing the thermo-hydro-mechanical (THM) effects on the Boom clay of the significant temperature gradients generated in this host rock as a consequence of the geological disposal of radioactive, heat-emitting wastes. This paper presents data on temperature, pore water pressure and total stress measured during the experiment and highlights several interesting observations regarding the thermal anisotropy and THM coupling in the Boom clay. The test has a simple geometry and well defined boundary conditions, which facilitates the comparison between measurement and numerical modeling studies. These studies included three dimensional coupled THM modeling of the test. The good agreement between measurement and numerical modeling of temperature and pore water pressure yields a set of THM parameters and confirms the thermo-mechanical anisotropy of the Boom clay.

A note on thermomechanical anisotropy of clays : T. Hueckel & R. Pellegrini, Engineering Geology, 41(1–4), 1996, pp 171–180

A note on thermomechanical anisotropy of clays : T. Hueckel & R. Pellegrini, Engineering Geology, 41(1–4), 1996, pp 171–180

A note on thermomechanical anisotropy of clays

Deep clays exhibit a pronounced strain anisotropy both during mechanical loading as well as during heating and cooling at constant stress in drained isotropic conditions. During mechanical loading vertical strain is larger than the horizontal one. During heating the vertical strain is larger than the horizontal one within the elastic range; the opposite is observed in the elasto-plastic range. The above described response can be interpreted adopting a consistent rotational, kinematic hardening thermo-elasto-plastic constitutive law.

Thermal Effect on the Transverse Vibration of High-Speed Rotating Anisotropic Disk

An attempt has been made to consider the thermal effect on the transverse vibration of a high-speed rotating disk in a steady-state heat conduction. The material of the disk, in this case, is assumed to be thermomechanically anisotropic. The present attempt is made with an objective to provide some theoretical studies on the problem that may serve as a base from which more detailed investigations with regard to the usage of composite material may be attempted to gain new and needed design information regarding turbine disks and thereby to reduce the chances of turbine failure. In this connection a new critical speed of disk rotation has been obtained and consequently this critical speed is found to depend on central temperature, thermomechanical anisotropy, and so forth.