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Nonlinear heat transfer can be exploited to reveal novel transport phenomena and thus enhance people's ability to manipulate heat flux at will. However, there has not been a mature discipline called nonlinear thermotics like its counterpart in optics or acoustics to make a systematic summary of relevant researches. In the current review, we focus on recent progress in an important part of nonlinear heat transfer, i.e., tailoring nonlinear thermal devices and metamaterials under the Fourier law, especially with temperature-dependent thermal conductivities. We will present the basic designing techniques including solving the equation directly and the transformation theory. Tuning nonlinearity coming from multi-physical effects, and how to calculate effective properties of nonlinear conductive composites using the effective medium theory are also included. Based on these theories, researchers have successfully designed various functional materials and devices such as the thermal diodes, thermal transistors, thermal memory elements, energy-free thermostats, and intelligent thermal materials, and some of them have also been realized in experiments. Further, these phenomenological works can provide a feasible route for the development of nonlinear thermotics.

Wellbore cooldown is often employed before well stimulation and/or hydraulic fracture stress testing in Enhanced Geothermal Systems and high-temperature petroleum reservoirs to prevent equipment from being overheated due to high temperatures. The thermo-elastic stress resulting from heat conduction during the cooling activity can have important influence on the behavior of the hydraulic fractures. A coupled numerical model has been developed to study the thermo-mechanical effect associated with pre-injection wellbore cooldown on the wellbore pressure and geometry of the hydraulic fracture (either longitudinal or transverse to the wellbore axis). The main novelty of this numerical study is the consideration of significant near-wellbore thermal stresses in the coupled non-linear problem of hydraulic fracturing initiation and propagation, which enables investigation of the thermo-mechanical effect under different fracture propagation regimes. Simulation results show earlier fracture initiation and lower breakdown pressure caused by cooling circulation. Extensive wellbore cooling also significantly alters the evolution of wellbore pressure, as evidenced by the differences observed under various cooling conditions. Most importantly, cooling promotes the transverse initiation of hydraulic fractures in situations where the initiation would have been longitudinal (i.e., in the same plane as the well) in the absence of cooling. The cases most susceptible to the complete change of fracture initiation geometry are those where the well is drilled parallel to the least compressive stress, typically horizontal wells drilled parallel to the minimum horizontal stress but also applicable to vertical wells in cases where the vertical stress is the lower in magnitude than either horizontal principal stress. These results combine to indicate a profound potential for cooling to impact hydraulic fracture initiation and early growth, and therefore needs to be considered in the planning and interpretation of stress testing and reservoir stimulation when cooling operations are necessary.HighlightsA hydraulic fracturing model coupled with heat conduction induced by cooling circulation is constructed to study the impact of wellbore cooldown.Wellbore cooldown leads to an earlier hydraulic fracture initiation with a lower breakdown pressure, as well as reduces the wellbore pressure during fracture growth.Cooling promotes the transverse initiation of the hydraulic fracture from wells drilled along the minimum in situ stress direction.

Laser peening without coating (LPwC) involves irradiating materials covered with water with intense laser pulses to induce compressive residual stress (RS) on a surface. This results in favorable effects, such as fatigue enhancement; however, the mechanism underlying formation of the compressive RS is not fully understood. In general, tensile RS is imparted on the surface of the material due to shrinkage after heating by laser irradiation. In this study, we assessed the thermo-mechanical effect of single laser pulse irradiation and introduce a phenomenological model to predict the outcome of LPwC. To validate this model, RS distribution across the laser-irradiated spot was analyzed using X-ray diffraction with synchrotron radiation. In addition, the RS was evaluated across a line and over an area, following irradiation by multiple laser pulses with partial overlapping. Large tensile RSs were found in the spot irradiated by the single pulse; however, compressive RSs appeared around the spot. In addition, the surface RS state shifted to the compressive side due to an increase in overlap between neighboring laser pulses on the line and over the area of irradiation. The compressive RSs around a subsequent laser spot effectively compensated the tensile component on the previous spot by controlling the overlap, which may result in compressive RSs on the surface after LPwC.

The article presents the results of a numerical simulation of the deformation-stress state in the rock mass around a salt cavern which is a part of a CAES installation (Compressed Air Energy Storage). The model is based on the parameters of the Huntorf power plant installation. The influence of temperature and salt-creep speed on the stability of the storage cavern was determined on the basis of the three different stress criteria and the effort of the rock mass in three points of the cavern at different time intervals. The analysis includes two creep speeds, which represent two different types of salt. The solutions showed that the influence of temperature on the deformation-stress state around the CAES cavern is of importance when considering the stress state at a distance of less than 60 m from the cavern axis (at cavern diameter 30–35 m). With an increase in cavern diameter, it is possible that the impact range will be proportionately larger, but each case requires individual modeling that includes the shape of the cavern and the cavern working cycle.

High geo-temperature is one of the inevitable geological disasters in deep engineering such as resource extraction, space development, and energy utilization. One of the key issues is to understand the mechanical properties and failure mechanism of high-temperature rock disturbed by low-temperature airflow after excavation. Therefore, the experimental and numerical investigation were carried out to study the impact of cooling rate on mechanical properties and failure mechanism of high temperature sandstone. First, uniaxial compression experiments of high temperature sandstone at different real-time cooling rates were carried out to study the mechanical properties and failure modes. The experimental results indicate that the cooling rate has a significant effect on the mechanical properties and failure modes of sandstone. The peak strain, peak stress, and elastic modulus decrease with an increase in cooling rate, and the fragmentation degree after failure increases gradually. Moreover, the equivalent numerical model of heterogeneous sandstone was established using particle flow code (PFC) to reveal the failure mechanism. The results indicate that the sandstone is dominated by intragrain failure in the cooling stage, the number of microcracks is exponentially related to the cooling rate, and the higher the cooling rate, the more cracks are concentrated in the exterior region. Under axial loading, the tensile stress is mostly distributed along the radial direction, and the damage in the cooling stage is mostly due to the fracture of the radial bond. In addition, axial loading, temperature gradient and thermal stress mismatch between adjacent minerals are the main reasons for the damage of sandstone in the cooling stage. Moreover, the excessive temperature gradient in the exterior region of the sandstone is the main reason for the damage concentration in this region.

We discuss the time-reversal behavior of dynamic cross-couplings among various hydrodynamic degrees of freedom in liquid crystal systems. Using a standard hydrodynamic description including linear irreversible thermodynamics, we show that the distinct thermodynamic requirements for reversible and irreversible couplings lead to experimentally accessible differences. We critically compare our descriptions with those of existing standard continuum mechanics theories, where time-reversal symmetry is not adequately invoked. The motivation comes from recent experimental progress allowing to discriminate between the hydrodynamic description and the continuum mechanics approach. This concerns the dynamics of Lehmann-type effects in chiral liquid crystals and the dynamic magneto-electric response in ferronematics and ferromagnetic nematics, a liquid multiferroic system. In addition, we discuss the consequences of time-reversal symmetry for flow alignment of the director in nematics (or pretransitional nematic domains) and for the dynamic thermo-mechanical and electro-mechanical couplings in textured nematic liquid crystals.

This work develops a unified size-dependent shear deformation theory (SDSDT) to analyze the free vibration behavior of a functionally graded (FG) magneto-electro-elastic (MEE) microplate under fully simply supported conditions, open- or closed-circuit surface conditions, biaxial compression, magnetic and electric potentials, and uniform temperature changes based on consistent couple stress theory (CCST). The FG-MEE microplate is composed of BaTiO3 (a piezoelectric material) and CoFe2O4 (a magnetostrictive material). Various CCST-based SDSDTs, considering couple stress and thickness stretching effects, can be reproduced by employing a generalized shape function that characterizes shear deformation distributions along the thickness direction within the unified SDSDT. These CCST-based SDSDTs encompass the size-dependent classical plate theory (CPT), first-order shear deformation theory (SDT), Reddy’s refined SDT, exponential SDT, sinusoidal SDT, and hyperbolic SDT. The unified SDSDT is validated by comparing its solutions with relevant three-dimensional solutions available in the literature. After validation and comparison studies, we conduct a parametric study, whose results indicate that the effects of thickness stretching, material length-scale parameter, inhomogeneity index, and length-to-thickness ratio, as well as the magnitude of biaxial compressive forces, electric potential, magnetic potential, and uniform temperature changes significantly impact the microplate’s natural frequency.

This paper presents an improved level set method for topology optimization of geometrically nonlinear structures accounting for the effect of thermo-mechanical couplings. It derives a new expression for element coupling stress resulting from the combination of mechanical and thermal loading, using geometric nonlinear finite element analysis. A topological model is then developed to minimize compliance while meeting displacement and frequency constraints to fulfill design requirements of structural members. Since the conventional Lagrange multiplier search method is unable to handle convergence instability arising from large deformation, a novel Lagrange multiplier search method is proposed. Additionally, the proposed method can be extended to multi-constrained geometrically nonlinear topology optimization, accommodating multiple physical field couplings.

A representative fabrication processing of SU-8 photoresist, Ultraviolet (UV) lithography is usually composed of spin coat, soft bake, UV exposure, post exposure bake (PEB), development and optional hard bake, etc. The exposed region of SU-8 is crosslinked during the PEB process and its physical properties highly depend on UV exposure and PEB condition. This work was initiated to investigate if thermal baking after fabrication can affect the mechanical properties of SU-8 photoresist material because SU-8 is trying to be used as a structural material for MEMS operated at high temperature. Since a temperature of 95°C is normally recommended for PEB process, elevated temperatures up to 200°C were considered for the optional hard bake process. The viscoelastic material properties were measured by dynamic mechanical analyses (DMA). Also, pulling tests were performed to obtain Young’s modulus and Poisson’s ratio as a function of strain rate in a wide temperature range. From this study, the effects of temperature on the elastic and viscoelastic material properties of SU-8 were obtained.

By employing the hyperbolic tangent model of load transfer (LT), this paper establishes the thermo-mechanical (TM) coupling load transfer analysis approach for an energy pile (EP). By incorporating the control condition of the unbalance force at the null point, the method for determining the null point considering the temperature effect is enhanced. The viability of the presented method is validated through the measured outcomes from model experiments of energy piles. A parametric investigation is conducted to explore the impact of the soil shear strength parameters, upper load, temperature variation, head stiffness, and radial expansion on the axial force, strain, and displacement of the energy pile under thermo-mechanical coupling. The results suggest that the locations of the null point and the maximum axial force are dependent on the constraint boundary conditions of the pile side and the two ends. When the stiffness of the pile top increases, axial stress and displacement increase, while strain decreases. An increase in the drained friction angle leads to an increase in axial stress under thermal-load coupling, but strain and displacement decline. The radial expansion has a negligible influence on the thermo-mechanical interaction between the pile and the soil.