Thermal Strain Research Articles

Thermo-mechanical analysis of energy tunnels accounting for thermo-plasticity

Energy tunnels are typically comprised of tunnel linings that are equipped with heat transfer pipes, thereby converting them into tunnel ground heat exchangers (tunnel GHEs). This innovative technology offers renewable methods of space heating, cooling and hot water production. While still relatively uncommon, it is becoming increasingly popular due to the growing interest in sustainability and innovation, particularly in large infrastructure projects [8, 7, 10]. Additionally, the increasing need for clean and renewable sources of energy has made energy tunnels an attractive solution for addressing the global climate challenge [7, 11, 1].
 The geotechnical behaviour of energy tunnels that subject the soil to thermal loads is still not fully understood. Numerical analyses conducted in the past use simple soil constitutive models, such as the linear elastic model [2, 3, 9] and elastic-perfectly plastic Mohr-Coulomb model [6, 4, 5]. Using simple constitutive models for tunnel design may not accurately depict the complex behaviour of soil materials under different conditions, and the complex interplay among the deformation, flow and thermal models governing the soil behaviour. This can lead to calculated stresses and strains that differ from actual values, resulting in potentially unsafe designs [12, 13].
 Due to their multi-phase nature, saturated soils exhibit both reversible and irreversible thermal volumetric changes. The reversible volumetric change, resulting from thermal expansion/contraction, is typically assumed to occur within the elastic thermo-mechanical response of the soil. Conversely, the irreversible change is more appropriately treated within the elasto-plastic response regime.
 In addition to volumetric changes, thermal effects can impact the stiffness and strength of soil, ultimately influencing the load transfer mechanisms and stress distributions within the system. These changes, in turn, can lead to additional deformations in both the tunnel and the surrounding soil. It should be noted that simply applying proportional stress paths to any of the Mohr-Coulomb models will result in solely elastic responses and fail to reproduce the irrecoverability of the volumetric response. Therefore, the incorporation of an elasto-plastic framework for analysing thermo-mechanical coupling is required to obtain more accurate predictions with respect to changes in effective stress and ground displacement.
 In this study, the coupled thermo-mechanical behaviour of energy tunnels is investigated employing a thermo-elasto-plastic constitutive model developed in the framework of the critical state soil mechanics and considering incorporating thermally induced volumetric change in the model that involves adding a thermal strain component to the model. This is achieved by including a temperature-dependent parameter that accounts for the volumetric change due to change in temperature. The model is implemented in the finite element package COMSOL Multiphysics and used to study the thermo-mechanical behaviour of an energy tunnel in 2D plane strain condition (see Figure 1). The in-situ state of stress is reproduced, and the construction sequence is simulated (soil-structure convergence). The analysis focuses on the stresses and deformations that develop within and around energy tunnels due to the influence of geothermal operation.
 The proposed model is validated following several steps. First, the mechanical model is calibrated against field data from tunnels constructed in soft soils [10]. Then, thermo-mechanical effects are validated using existing monitoring data from real-scale tunnels available in literature [3,4,10,14]. For further validation, model estimations are compared with results from small-scale thermal-mechanical tests, such as thermal-triaxial tests. This comprehensive validation strategy thus ensures a rigorous and reliable assessment of our model’s performance in replicating soil behaviour near energy tunnels.

Thermal strain relaxation of GaAs overgrown on nanovoid based Ge/Si substrate

The integration of high quality III-V materials-based devices on the Si platform is considered as an enabling path towards achieving the long-standing goal of creating low-cost/high-performance devices. However, the heteroepitaxy of III-V compounds on a Si substrate generates high stress due to the difference in thermal expansion coefficient leading notably to crack generation. Here, we report on the reduction of the thermal strain of GaAs and Ge deposited on a Si substrate. The method is based on the formation of buried high density nanovoids by electrochemical etching of Ge/Si virtual substrate followed by direct epitaxial growth of Ge and GaAs. The thermal strain has been investigated for GaAs epilayer by High-Resolution X-Ray Diffraction and Raman spectroscopy showing a 30% decrease in strain. The strain reduction relies on the nanovoids-mediated thermal strain accommodation. Furthermore, photoluminescence measurements showed a 10-fold increase of the intensity, resulting in an improved optical and crystalline quality, making this virtual substrate promising for III-V based devices on Silicon.

Design and analysis of an optical planar waveguide using Invar as core and silicon nitride as cladding layer to control birefringence

Abstract The main problem with optical planar waveguide is thermal stress which arises when layers inside the planar waveguide get fused together with different coefficient of thermal expansion (CTE) and due to variation in thermal properties, the problem of thermally induced strain and birefringence occurs inside the waveguide glass layer. In proposed work, the mathematical analysis of thermal induced stress and the thermal strain in waveguide glass layer is calculated. ‘C’ software is used for analysis of an optical planar waveguide to calculate the thermal stress. By using material properties of Invar (64 % iron-36 % nickel alloy) and silicon nitride (Si3N4) an optical planar waveguide is designed. Here when silicon nitride (Si3N4) is used as a cladding layer and Invar (64 % iron-36 % nickel alloy) as a core layer in an optical planar waveguide, the problem of birefringence get eradicated as zero thermal stress was achieved.

Investigating the effect of laser modulation and input energy on bending mechanism and bending angle in the multi-pass laser forming process through real-time monitoring

Laser forming is a non-contact type process used for bending, spatial forming, and alignment of metallic components through a controlled application of laser energy. The process involves localized rapid heating and cooling, inducing thermal strains exceeding the elastic strain of the material resulting in plastic deformation. The process is broadly governed by the temperature gradient mechanism (TGM), buckling mechanism (BM), and upsetting mechanism (UM). However, in the case of multi-pass laser forming process, heat accumulation and variation in the flow stress because of consecutive thermal cycles and strain hardening due to multiple bending cycles makes the process highly complicated. While operating laser in modulated mode may be a wise choice to handle the heat accumulation, inter-pulse cooling cycles along with variation in peak and average power with change in duty cycle affects the bending mechanism and angle significantly. Therefore, in the present study, a real-time monitoring system was developed to investigate the bending mechanism for a multi-pass laser forming process carried out in laser modulated mode. Also, the effect of average power, peak power, heat accumulation, strain hardening on bending mechanism and bending angle were investigated. Keeping the peak power constant, variation in duty cycle from 100% to 40% resulted in reduced average energy and strain hardening was found to be dominant, decreasing bending angle per pass. On contrast, variation in duty cycle by keeping the average power constant resulted in heat accumulation dominating the bending mechanism.

Cavitation impact damage of polymer: A multi-physics approach incorporating phase-field

The challenge of material surface damage and spalling, caused by high-frequency, high-pressure jets owing to cavitation, remains a substantial concern. To better understand the physical mechanisms of cavitation-induced cyclic impact, we developed a multi-field-coupling framework. This framework encapsulates polymer viscoelastic-viscoplastic deformation, thermal softening, strain softening, and damage evolution represented by phase fields, which are crucial processes in cyclic impact. Particularly, we achieved full coupling between these key physical processes by introducing appropriate functional forms. We emphasize the need to include both plasticity and viscosity in the energy-driving crack propagation, to accurately represent the accumulation of polymer fatigue damage. With this thermodynamically consistent, fully coupled model, we could simulate surface ring cracking, a phenomenon often observed experimentally under cyclical impact loading. In the context of this particular crack morphology, we also discerned distinct fatigue failure mechanisms when variables such as strength, the radial extent of the cavitation impact load, and others were altered. We propose that a key determinant of cavitation flow impact is the combined effect of thermal softening, strain softening, and phase-field degradation within the polymer. Understanding these mechanisms offers a deeper insight into cavitation damage and provides a theoretical basis for cavitation-resistant design.

Determination of single crystal thermal expansion in Uranium-6wt%Niobium shape memory alloy using in-situ diffraction and modeling of textured polycrystalline samples

In-situ neutron diffraction and ex-situ dilatometry experiments were performed on a chemically banded, quenched uranium-6wt% niobium shape memory alloy to study the impact of deformation-(detwinning-) induced texture on the evolution of thermally induced strains (and associated stresses). Thermal heating and cooling cycles between 5K and 473K were performed in-situ with neutron diffraction, and the lattice strain evolution is reported for the observed monoclinic α″ phase. Comparisons between the measured diffraction strains and macroscopic dilatometry experiments reveal relationships between micro- and macro-level thermal expansion. Softening of the texture during heating suggests that twin boundary motion can accommodate the large internal thermal strains which are approximately 2% greater than that typically observed during the 175K heating interval used to age more randomly oriented polycrystalline material. Assuming weak constraint of neighboring grains in randomly textured polycrystals, the single crystal thermal expansion tensor is extracted from measurements of lattice strains over the range 5K to 473K. Predictions of polycrystalline thermal expansion, based upon this single crystal thermal expansion tensor, are shown to compare favorably with bulk thermal expansion observations of the as quenched microstructure. However, such a lower-bound estimate is insufficient to explain all aspects of the behavior of the textured material, where the matrix is not isotropic. It is hypothesized that relaxation processes which occur within the quenched microstructure during heating are responsible for the distinct thermal expansion behavior observed during the initial heating cycle compared to cooling and subsequent cycling.

Analysis of thermal-vibration coupling modeling of combined conical-cylindrical shell under complex boundary conditions

In this paper, a unified method is proposed to predict the free and forced vibration behavior of the combined conical-cylindrical shell (CCS) structure in the steady-state thermal environment. The first-order shear deformation theory (FSDT) and the thermal strain are utilized to establish the energy equation of the combined structure, and the artificial virtual spring technique is introduced to realize the coupling between the substructures and the arbitrary boundary conditions of the CCS. The displacement function of the structure is constructed by the spectral-geometry method, and the vibration equation is solved by using the Rayleigh–Ritz method. The accuracy of the present model is verified by comparing the numerical results with the finite element method. The factors that may affect the vibration behavior of the CCS under thermal environment are analyzed specifically, and the results demonstrate that point constraints on the shell surface can effectively suppress shell vibration. This paper provides a compelling reference for vibration control of CCS in practical applications.

Use of Bender Elements to Evaluate Linkages between Thermal Volume Changes and Shear Modulus Hardening in Drained and Undrained Thermal Triaxial Tests

Abstract This study investigates linkages between volume change, pore fluid drainage, shear wave velocity, and temperature of soft clays using a thermal triaxial cell equipped with bender elements, a measurement approach that has not been explored widely in past thermo-mechanical studies. Two kaolinite specimens were consolidated mechanically to a normally consolidated state and then subjected to drained and undrained heating-cooling cycles, respectively. After cooling, the specimens were subjected to further mechanical consolidation to evaluate changes in apparent preconsolidation stress. Both specimens showed net contractive thermal strains after a heating-cooling cycle and overconsolidated behavior during mechanical compression immediately after cooling. The shear wave velocity increased during drained heating, but negligible changes were observed during drained cooling, indicating permanent hardening because of thermal consolidation during the heating-cooling cycle. The shear wave velocity decreased during undrained heating because of a reduction in effective stress associated with thermal pressurization of the pore fluid but subsequently increased when drainage was permitted at elevated temperature. The shear wave velocity increased slightly during undrained cooling but decreased when drainage was permitted at room temperature. Net increases in small-strain shear modulus of 17 and 11 % after heating-cooling cycles under drained and undrained (with drainage after reaching stable temperatures) conditions, respectively, provide further evidence to the potential of thermal soil improvement of normally consolidated clays. Transient changes in shear modulus also highlight the importance of considering drainage conditions and corresponding changes in effective stress state during heating-cooling cycles.

Investigation of stress and deflection in absorber of parabolic trough solar collector for direct steam generation

The tubular absorber of a parabolic trough solar collector (PTSC) experiences deflection and thermal stress during the operation due to the adverse circumferential and longitudinal thermal gradient in the direct steam generation (DSG) process. This study presents the coupled three-dimensional thermal–structural analysis of the absorber tube by combining heat transfer and static structural analyses using ANSYS Fluent 2021R1 software. The receiver of the LS-3 solar collector unit with a length of 4.06 m is considered as a computational domain with mass flow rates of 0.4 kg/s and 0.6 kg/s, an operating pressure of 100 bar, and the direct normal irradiance (DNI) of 750 W/m2. The deformation and stress analyses have been performed for preheating, evaporation, and superheating stages of the DSG process at solar noon. Under the subjected boundary conditions, the circumferential thermal gradient at the middle of the absorber in preheating, evaporation, and superheating sections are 27 K, 13 K, and 31 K, respectively. The maximum and minimum deflections observed in the absorber tube are 17.6 mm and 7.6 mm, respectively. The maximum and minimum normal stress along the axial direction on top of the absorber are 26.0 MPa and 12.6 MPa, respectively. It is observed that normal stresses along the axial direction is maximum at the mid of the absorber. The maximum thermal strains observed in preheating, evaporation, and superheating sections are 4.6 mm/m, 5.2 mm/m, and 5.6 mm/m, respectively.

Effect of Cu Doping on Phase Transition, Thermal Strain, and Thermal Expansion Property in Polycrystalline Ni50Mn23−xCuxGa27 Alloys

The influence of Cu doping on phase transition, thermal strain, and thermal expansion property of polycrystalline Ni50Mn23−xCuxGa27(x = 1, 3, 5, 6 and 7) alloys is investigated. The results show that with increasing Cu concentration, martensitic transformation gradually gets close to Curie transition. A magnetostructural transition (MST) is observed in samples with x = 6 and 7. Such MST can enhance magnitude of transformation strain. Besides, Ni50Mn23−xCuxGa27 alloys possess an isotropic, recoverable, and thermally stable thermal strain. Excitedly, Cu doping can effectively tune the phase transition, thermal strain, and thermal expansion properties of Ni50Mn23−xCuxGa27 alloys. Adjustable coefficients of thermal expansion from positive to negative are obtained by Cu‐doped Ni50Mn23−xCuxGa27 alloys. The average linear expansion coefficient (α) between −140.44 and −207.70 ppm K−1 is observed in samples with x = 6 and 7 for a narrow temperature span of 11–14 K. When the temperature span is about 72 K (x = 6) and 73 K (x = 7), which is the largest temperature span observed in Ni–Mn–Ga‐based alloys recently, the α values decrease to about −36 ppm K−1. These findings are beneficial for manipulating the thermal expansion property of Ni–Mn–Ga–Cu alloys and their multifunctional applications.

Microstructural evolution and hot cracking prevention in direct-laser-deposited Ni-based superalloy through Hf addition

Additive manufacturing (AM) is an emerging new paradigm in the production of industrial parts since it allows the fabrication of near-net shape products directly from designs, which is impossible with conventional manufacturing techniques. However, hot cracking phenomena in AM are a critical issue with non-weldable alloys, rendering manufactured parts unusable. There are solutions to this problem, such as alloying Hf with non-weldable Ni-based superalloys to improve cracking resistibility. Although this solution was proposed a few decades ago, the mechanisms of how Hf could prevent hot cracking in Ni-based superalloys have not been clarified in detail, until now. In this study, we revealed the Hf-driven microstructural changes in direct-laser-deposited Ni-based superalloys using various characterization techniques and phase-field simulations. Moreover, the calculated thermal strain and cracking susceptibility decreased with the Hf addition. The results demonstrated that Hf induced specific solidifying processes and resulting microstructural changes that are advantageous to the prevention of hot cracking.

Acute work rate adjustments during high-intensity interval training in a hot and temperate environment.

Heart rate drifts upward over time during interval exercise and during exercise in hot conditions. As such, work rate must be lowered to maintain target heart rate. The purpose was to characterize acute work rate adjustments during high-intensity interval training based on target heart rate. Seven humans (three females) completed five study visits: a graded exercise test on a cycle ergometer to measure maximal heart rate (HRmax) in ∼22°C and four trials performed in ∼22°C (TEMP) or ∼35°C (HOT), consisting of an 8 min warm-up at 70% HRmax followed by one (15TEMP and 15HOT) or five (43TEMP and 43HOT) rounds of high-intensity interval training (one round = 4min work at 90% HRmax and 3min recovery at 70% HRmax) totaling 15min or 43min of exercise, respectively. Work rate was lowered 33 ± 20W (p=0.005) in 43TEMP and 56 ± 30W (p=0.003) in 43HOT between the first and fifth work intervals. Thermal strain (0.2°C higher rectal temperature, p=0.01) and cardiovascular strain (6 beats·min-1 larger increase in heart rate from first to fifth recovery interval, p=0.01) were greater in 43HOT versus 43TEMP. Using target heart rate during high-intensity interval training may reduce the training stimulus, especially in hot environments, but it may also limit thermal strain and enable participants to complete the prescribed workout despite the heat.

(High-Temperature Energy, Materials, & Processes Division Subhash Singhal Award) From Electrochemical to Mechanical Modeling of SOFCs and Their Experimental Validation

Due to the asymmetric structure with multiple materials, SOFCs are subjected to various modes of mechanical stress, resulting in a risk of failure. Of particular importance are thermal and chemical strains, which depend on the micro- and macroscopic distribution of electrochemical reaction sites. Therefore, SOFC modeling must include electrochemistry, mass and charge transport, thermo-fluid dynamics, and structural analysis. In addition, appropriate experiments must be performed to validate the modeling results. This research group is working to develop a comprehensive environment for modeling and validation in order to evaluate the mechanical reliability of SOFCs.Electrochemical modeling of cathode was made based on the defect chemistry and mass transport of (La,Sr)CoO3 (LSC) and (La,Sr)(Co,Fe)O3 (LSCF). An empirical equation for the surface reaction of the porous electrode was determined by electrochemical analysis of a dense film electrode taking into account the difference between the dense film and porous electrodes [1]. Anode reaction rate on a unit length of triple phase boundary of Ni-YSZ-gas was determined using circular Ni model electrodes of various size [2]. Evaluation of 3D oxygen potential distribution inside the constituent oxides was made using an in-house code called SIMUDEL where mixed ionic/electronic conduction and oxygen nonstoichiometry were taken into consideration [3,4]. It was combined with commercial software to make structural analysis. Mechanical properties used for calculation were obtained from measurements by the resonance method and the small punch method or collected from the literature. Non-linear stress-strain behaviors were observed with Ni-YSZ cermet [5] and inelastic behaviors were found in most nonstoichiometric oxide materials. Large ferro-elasticity emerged in the LSC and LSCF [6] and in Sc stabilized ZrO2 electrolyte at elevated temperatures. Chemical strain in vacancy formation was formulated with a linear function of the defect concentration as an independent factor from the thermal expansion. In the structural analysis, it was treated as the additional temperature change of the local material. Volume change of Ni-YSZ cermet on red-ox operation was a difficult factor to formulate because of its complex dependence on composition, temperature, and atmosphere. As expected, Ni-YSZ showed expansion when oxidized, but under certain conditions at an intermediate temperature range, it rather contracted on oxidation [7].In reality, the calculated stress distribution does not always match the actual behavior of the cell since there often are unconsidered factors. Thus, experimental validation is essential. For this purpose, we have developed methods for in-situ or operando measurements of cell shape and the residual stress. Long-focus laser profilometer was used to measure the warping of a planar cell during heating, operating, and cooling operations. For the change in the diameter of a tubular cell, the laser projection type dimension meter was employed. Residual stress measurements were made with a handy X-ray analyzer (μ-X360s, Pulstec Industrial. Co. Ltd.) combined with a newly developed one- or two-chamber cell holder [8,9]. Anode-supported cells from multiple sources were analyzed, showing different stress histories in red-ox operations depending on the microstructure of the anode support layer.Currently, we are developing a protocol for testing robustness of planar SOFCs. Here, temperature distribution is intentionally applied on a cell with segmented heater allays on the top and the bottom of the cell. Severe condition tests will be performed, and the results will be analyzed with the above-mentioned simulation.AcknowledgementThis study was supported by the New Energy and Industrial Technology Development Organization (NEDO) .[1] T. Kawada, Current Opinion in Electrochemistry, 21, 274-282 (2020) DOI: 10.1016/j.coelec.2020.03.016[2] M. Takeda, Master’s thesis, Graduate School of Environmental Studies, Tohoku University (2016) : M. Takeda, et al. Submitted to J. Electrochem. Soc.[3] K. Terada, et. al., ECS Trans. 35(2 part2), 923-933 DOI: 10.1149/1.3570073[4] M. Sato, et. al., Trans. Jpn. Soc. Comp. Eng. Sci. 2017, 14 (2017) DOI: 10.11421/jsces.2017.20170004[5] S. Watanabe et al., J. Mater. Sci. 55, 8679-8693 (2020) DOI: 10.1007/s10853-020-04624-4[6] Y. Kimura et al., J. Electrochem. Soc. 161(11), F3079- F3083 (2014) DOI: 10.1149/2.0131411jes[7] Y. Morishita et al. , ECS Trans., 91(1) 1979-1984 (2019). DOI: 10.1149/09101.1979ecst[8] K. Yashiro et al., submitted to J. Power Sources[9] K. Oshima et al.,ECS Trans. 103(1), 1251-1260 (2021) DOI: 10.1149/10301.1251ecst

Coupling effect of strain gradient strengthening and thermal softening on the microscale plastic behavior of metallic materials

Size effects and thermal effects together determine the microscale plastic response of metallic structures in high-integrated microelectromechanical systems (MEMS) at different temperatures. To investigate the microscale plastic behavior of metallic materials at different temperatures, a thermo-mechanical coupled microscale plasticity model is developed. In the mechanical part of this model, the framework of conventional plasticity theory is maintained, and the plastic strain gradient is introduced as an internal variable to increase the tangential hardening modulus without the introduction of higher-order stress and higher-order boundary conditions. The influence of temperature on the mechanical parameters (e.g. the yield strength) is calibrated by experiment results. In the thermal part of this model, the heat generation in the plastic deformation stage is calculated by the difference between the plastic work and the hardening stored energy influenced by the plastic strain gradient. Due to the strong nonlinearity of the coupled equations, a finite element solution algorithm that combines the radial return method and the staggered solution scheme is proposed. The effectiveness of this model and its solution algorithm is verified by comparisons with the experiment results and a numerical benchmark example. Finally, taking the tensile behavior of a plate with a hole in its center as an example, the coupling effect of strain gradient strengthening and thermal softening on the microscale plastic behavior of metallic materials is investigated. The results show that the microscale plastic behavior of metallic materials at high temperatures depends on the competition between thermal softening and strain gradient strengthening. Our study provides a theoretical basis and a reliable simulation method for the design of MEMS at different temperatures.

Solidification crack elimination in TIG welding of Al7075: Experimental investigation and modified RDG modelling

Although solidification crack susceptibility modelling has been a point of interest for many years in fusion welding of aluminum alloys, the underlying mechanism of solidification crack elimination is not yet fully understood. To contribute in closing this knowledge gap, the present study investigates and proposes a new modified Rappaz-Drezet-Gremaud (RDG) physical model in autogenous and heterogeneous TIG welding of Al7075 sheets. The core novelty of the new model is that it includes the shrinkage strain rate, which allows to better capture the effects of capillary pressure, coherency point, as well as final dendrite or grain size in the fusion zone. In agreement with the experimental results and previous models, the new model shows that the defining mechanism of eliminating solidification cracks in heterogeneous joints is the capacity of nanoparticles to initiate heterogeneous grain nucleation, altering the fusion zone grain morphology from dendritic to fine equiaxed, with only little impact of the resulting grain size. This leads to two main effects causing reduced solidification crack susceptibility: (1) a reduction in coherency point by about 15 % from 902 K to 879 K, which reduces strain accumulation on the coherent solid network; (2) a considerable increase in capillary pressure across the entire mushy zone, allowing to maintain the total pressure at void/liquid interfaces in the positive range, which prevents the growth of potentially formed voids between grains at any temperature during solidification. In contrast, dendrite widths below 68 μm are required to prevent solidification cracks in autogenous joints without TiC nanoparticles, indicating a greater role of grain refinement. Overall, the proposed model highlights the more determinant role of the temperature-dependent solidification shrinkage strain rate in solidification crack susceptibility as it is an order of magnitude greater compared to the temperature-independent thermal contraction strain rate that has been the main or sole focus of previous models.

A new cellular automata model for hot deformation behavior of AZ80A magnesium alloy considering topological technique

In this paper, the CA model of grain topological deformation technique (T-CA) and the physical constitutive model considering strain are introduced to study the hot deformation behavior of extruded AZ80A magnesium alloy. The isothermal compression test of extruded AZ80A magnesium alloy was carried out on the Gleeble-3800 thermal simulator. The deformation temperature and strain rate were 598 K∼723 K and 0.001 s−1∼1 s−1, respectively. According to the experimental results, a phenomenological constitutive model of flow stress considering strain was constructed. The reliability of the model was verified by comparing the experimental and predicted flow stress. The influence of thermal deformation parameters, i.e., deformation temperature, strain rate, and strain, on the DRX kinetic curve and grain size evolution was studied based on the CA model with the introduction of grain topological (T-CA) deformation technology. Meanwhile, the influence of initial grain size on dynamic recrystallization was studied using the conventional CA model (C-CA) and the CA model with topological deformation (T-CA) technology. Finally, the accuracy of the T-CA model was verified by combining physical experiments and electron backscatter diffraction (EBSD) technology. At the temperature of 598 K, the strain rate of 0.1 s−1, and the strain of 0.91, the error between the average grain size simulated by the topological CA (T-CA) model and the experimental results is 7.6 %, which is consistent with the experimental results. When the initial grain size is 43 µm, compared with the conventional CA simulation method (C-CA), the CA simulation method with the introduction of grain topological deformation can promote the occurrence of recrystallization, improving the simulation speed and accuracy. The error between the percentage of recrystallization and the experiment is 4.5 % and 3.2 %, respectively, indicating that the simulation method with the introduction of grain topological deformation (T-CA) can accurately predict the dynamic recrystallization behavior of AZ80A magnesium alloy.

Design and performance evaluation of a multi-disc magnetorheological fluid brake for A00-class minicars

This paper presents the design and performance evaluation of a multi-disc magnetorheological fluid (MRF)-based brake (MRB) for A00-class minicars. The braking performance of the MRB is studied by means of theoretical analysis and experimental verification. Firstly, the MRB is designed according to the shear model of the MRF, and the structure optimization is carried subsequently. Secondly, multi-physical simulations of the MRB are conducted to investigate the transient temperature field, thermal stress and thermal strain distribution of the MRB under different braking models; Finally, a performance evaluating testbed is built to experimentally assess the braking performance of the MRB. The results indicate that the theoretical braking torque of the MRB fulfills the target value. The thermal strain-induced deformation of the disc is minimal and has a negligible effect on the torque output. In addition, the MRB is experimentally validated to exhibit excellent braking performance in terms of sufficient torque output capacity, rapid response, low temperature rise characteristic, as well as favorable velocity following property.

Indoor overheating influences self-reported symptoms and mood-state in older adults during a simulated heatwave: Effects of mid-day cooling centre use

Public health agencies recommend that older adults without home air-conditioning visit cooling centres to mitigate physiological strain from high ambient temperatures during heat waves. However, there is little evidence regarding their influence on self-reported environmental symptoms and mood-state after returning to the heat. MethodsForty adults (64–79 years) underwent a daylong laboratory-based indoor overheating simulation (9-hours, heat index: 37 °C) with (cooling, n = 20) or without (control, n = 20) a 2-hour air-conditioning intervention (hours 5–6). Mean skin and core temperature areas under the curve (AUC, hours 0–9) were used to assess cumulative thermal strain. Group differences in total symptom scores and subjective heat illness (68-item environmental symptoms questionnaire) as well as total mood disturbance and energy index (40-item profile of mood states questionnaire) were evaluated at end-heating (adjusted for pre-exposure scores). ResultsCooling reduced mean skin and core temperature AUCs by 4.0 [0.1, 0.8] and 1.6 [0.4, 2.8] °C·hour compared to control (both p < 0.048). However, at end-heating neither mean skin nor core temperatures differed between groups (both p > 0.999). Total symptom scores and subjective heat illness were 0.58-fold [0.44, 0.77] and 0.56-fold [0.40, 0.78] lower in the cooling compared to control group (both p < 0.001). Mood disturbance was 0.91-fold [0.83, 0.99] lower for cooling than control (p = 0.036), although energy index was not different between groups (p = 0.141). ConclusionCooling centres can have sustained positive effects on perceived thermal strain and mood-state in older adults after returning to the heat. However, continued vigilance and use of appropriate countermeasures to mitigate physiological strain from indoor overheating should be encouraged as body temperatures can rapidly return to pre-cooling levels.

Asymptotic solutions for large fractures growing parallel to a free plane

Straight 2D and disc-like cracks situated close to a parallel free plane are considered and asymptotic solutions for the stress intensity factors (SIFs) and crack opening are developed. Two asymptotic terms with respect to the small ratio of distance from the surface to the crack length/radius are derived. The derivation is based on considering the material between the crack and the free surface as an elastically supported beam or circular plate. The crack is considered to be opened either by uniform tensile stress or a pair of concentrated forces or moments representing the action of central defects (these represent for instance the crack growth from initial inclined crack or pore under compression acting parallel to the free surface). A crack with non-interacting surfaces (the conventional crack) and a crack with linear links between the surfaces are considered. Interaction with the free surface is shown to considerably increase the crack opening and SIFs, while the presence of the elastic links can reduce these crack parameters. In particular, the opening and SIFs of a crack with linear links under a pair of concentrated forces/moments exponentially vanish with the crack dimensions tending to infinity. The obtained asymptotic solutions provide a simple and accurate tool for investigating mechanisms of surface delamination and spallation (including thermal spallation and strain rockbursts).

Effect of Final Electromagnetic Stirring on Internal Thermal Stress, Strain and Cracking of Continuous Casting Bloom

Effect of Final Electromagnetic Stirring on Internal Thermal Stress, Strain and Cracking of Continuous Casting Bloom