Thermal Stress Changes Research Articles

Effect of residual stress on resonant frequency in Nitinol based thin film resonator

This current work focuses on the study of residual stress introduced in thin surface micromachined films due to the thermal stress. A model is presented here to study the change in thermal residual stress as the temperature changes and in turn how this change affects the resonant frequency of the resonator. The model uses two design; a resonator with straight and folded cantilevers. This work also involves in analysing the resonator performance as the material properties are changed. The analyses is done on Nitinol which is a Shape Memory Alloy (SMA). The simulation is carried out on COMSOL Multiphysics.

A hybrid simulation approach to evaluate stimulation micro-seismic events and production potential of fractured geothermal reservoirs

This study presents an integrated approach to simulate fluid flow and to predict the micro seismic events during stimulation and circulation of cold water over a longer term in geothermal reservoirs. The integrated approach based on new three dimensional fully coupled thermo-poroelastic numerical model for evaluation of energy recoverable. In the presented approach, the fracture aperture due to fracture slippage is calculated by shear and dilation. The shear slippage is controlled by the concept of shear failure using linear Mohr-Coulomb criterion. The numerical model is validated against an analytical Oda’s model for permeability tensor calculation and against an analytical solution for thermo-poroelastic model. The heat transfer between the rock and fluid is modelled by using the conductive heat transfer within the reservoir rock and convective heat transfer in discrete fractures. The thermal stress changes are included in the model to be studied by using roughness induced shear displacement principle in a poro-thermo-elastic environment. The fracture aperture changes are estimated by using an analytical model based on the distributed dislocation technique. The roughness of fracture surfaces is used in the calculation of residual fracture aperture. The presented approach is used to study the potential of permeability enhancement for Habanero geothermal reservoir at a depth of 3600 m. The result show that the increasing in tensile effective stress tend to increase the fracture aperture within the zone of cooling. This increasing in fracture aperture led to significant changes in pressure distribution (decrease in impedance) and hence, increase in the flow rate.

Analysis of Vibration and Acoustic Characteristics of a Simply Supported Double-Panel Partition under Thermal Environment

Numerical studies on the vibration and acoustic characteristics of a simply supported double-panel partition under the thermal environment are presented by the modal superposition approach and temperature field theory. Many factors are considered in this theoretical research, including acoustic refraction, dynamic response of the panel under thermal and acoustic load, vibroacoustic coupling characteristic analysis, and the variation of material properties. To access the accuracy and feasibility of the theoretical model, a finite element method is proposed to calculate the natural frequencies and mode shapes. The results show that the vibration and acoustic responses change obviously with the change of thermal stress and material properties. The rise of the graded thermal environment and thermal load decreases the natural frequencies and moves response peaks to the low-frequency range. The first valley of sound transmission loss is well consistent with the mode frequency. Finally, the relation between the average sound insulation and the thickness ratio is analyzed.

Mechanical response induced by flux jump in a cylindrical superconductor

The flux jump in bulk superconductors is accompanied by a rapid change in temperature and magnetic field, which can induce change in electromagnetic bodyforce and thermal stress. It is well known that bulk superconductors are brittle and have low mechanical strength, and thus, large electromagnetic bodyforce and thermal stress can cause damage of the bulk superconductor. In this paper, an electromagnetic-thermal-mechanical multi-physics model is adopted to compute the mechanical response of a bulk superconductor during flux jump in an external magnetic field. The results indicate that the flux jump in the bulk superconductors can also lead to the jump of the average electromagnetic force, temperature, stress, and strain. Meanwhile, it can be found that the flux jump can occur more easily with a faster change in the magnetic field, a lower ambient temperature, and a large-size superconductor. The results also show that the peak value of thermal strain is much larger than the strain generated by electromagnetic bodyforce during the flux jump. In addition, the change in strain has the same trend as that of the temperature. Thus, the strain may also be used to monitor the flux jump.

Stress design of a laminated MoSi2/Cr coating under particle impact and high temperature environment

A protective laminated MoSi2/Cr coating was deposited on SiC coated carbon/carbon composites by plasma spraying. Under particle impact environment, the Cr interface layer could relax stress concentration and rapidly relieve impact stress by 30% in the laminated coating layer by layer, thus keeping inner layers intact according to the experiment and simulation results. Under thermal shock condition, sudden change of thermal stress induced by the Cr interface layer could also help change the crack propagation path, thus keeping the coated sample without mass loss after 30 thermal cycles between 1500 °C and room temperature. After 157 h oxidation at 1500 °C in static air, the Cr interface layer could also inhibit the formation of penetrating cracks and convert the oxidation mechanism of parabolic mass loss law in MoSi2 coating to linear mass loss law at the final oxidation stage.

Marine Heatwave Hotspots in Coral Reef Environments: Physical Drivers, Ecophysiological Outcomes, and Impact Upon Structural Complexity

A changing climate is driving increasingly common and prolonged marine heatwaves (MHWs) and these extreme events have now been widely documented to severely impact marine ecosystems globally. However MHWs have rarely recently been considered when examining temperature-induced degradation of coral reef ecosystems. Here we consider extreme, localised thermal anomalies, nested within broader increases in sea surface temperature, which fulfil the definitive criteria for MHWs. These acute and intense events, referred to here as MHW hotspots, are not always well represented in the current framework used to describe coral bleaching, but do have distinct ecological outcomes, including widespread bleaching and rapid mass mortality of putatively thermally tolerant coral species. The physical drivers of these localised hotspots are discussed here, and in doing so we present a comprehensive theoretical framework that links the biological responses of the coral photo-endosymbiotic organism to extreme thermal stress and ecological changes on reefs associated after MHW hotspots. We describe how the rapid onset of high temperatures drives immediate heat-stress induced cellular damage, overwhelming mechanisms that would otherwise mitigate the impact of gradually accumulated thermal stress. The warm environment, and increased light penetration of the coral skeleton due to the loss of coral tissues, coupled with coral tissue decay support rapid microbial growth in the skeletal microenvironment, resulting in the widely unrecognised consequence of rapid decay and degeneration of the coral skeletons. This accelerated degeneration of the coral skeletonson a reef scale hinder the recovery of coral populations and increase the likelihood of phase shifts towards algal dominance. We suggest that MHW hotspots, through driving rapid heat-induced mortality, compromise reefs’ structural frameworks to the detriment of long term recovery. We propose that MHW hotspots be considered as a distinct class of thermal stress events in coral reefs, and that the current framework used to describe coral bleaching and mass mortality be expanded to include these. We urge further research into how coral mortality affects bioerosion by coral endoliths.

Numerical modelling and transient analysis of a printed circuit heat exchanger used as recuperator for supercritical CO2 heat to power conversion systems

The paper presents a modelling methodology for Printed Circuit Heat Exchangers (PCHEs) in supercritical CO2 (sCO2) power systems. The PCHE model can be embedded in models of the full sCO2 power unit for optimisation, transient simulation and control purposes. In particular, the purpose of the study is to assess the potential and limitations of lower order models in predicting the overall heat transfer performance of PCHEs. The heat transfer processes in the channels of the PCHE recuperator are modelled in 1-D and 3-D using commercial software platforms. The results show that predictions from the two modelling approaches are in good agreement, confirming that the 1-D approach can be used with confidence for fast simulation and analysis of PCHEs. Using the 1-D approach, the model was validated against manufacturer’s data for a 630 kW PCHE recuperator, and subsequently used to simulate the performance of the heat exchanger at design and off-design operating conditions. Performance maps produced from the simulations, enable visualization of the influence of operating conditions on the heat transfer performance and pressure drops in the heat exchanger. Dynamic simulations under transient operating conditions show that the thermal expansion of the working fluid caused by a fast reduction in density and increase in pressure in the system, can be a concern, requiring careful management of the start-up process to avoid sudden changes in temperature and thermal stresses.

Wastewater Reclamation Holds a Key for Water Sustainability in Future Urban Development of Phoenix Metropolitan Area

Global water scarcity has been a growing concern among policy makers, researchers, and the general public. Rapid urbanization with more concentrated populations and emergent patterns of climate change (e.g., soaring temperatures and 19 years of drought) added uncertainty and complexity to the risk of water shortage for the Phoenix Metropolitan Area (PMA). In this study, we used a numerical simulation tool, termed WaterSim 5.0, to assess water sustainability indicators in PMA. Four hypothetical scenarios were evaluated, viz. desert, and green landscaping, as well as urban expansion into desert or agricultural lands, bracketing plausible paths of future development in the study area. Model outputs were evaluated using four sustainability indicators and policy controls: (1) groundwater use, (2) agricultural production, (3) personal water use, and (4) the 100-year adequate water supply metric for new population growth. All four scenarios were examined using both current (19%) and complete (100%) wastewater reclamation. We found that the rate of wastewater reuse drives a reduction in groundwater dependency in urban growth; urban-agriculture conversion provides a convenient means to enhance water sustainability. In the broader context, sustainable water planning strategies need to take into account other crucial environmental and socio-economic measures such as changes in thermal stress, agricultural production, and outdoor and rural lifestyles.

Effectiveness of vegetated patches as Green Infrastructure in mitigating Urban Heat Island effects during a heatwave event in the city of Melbourne

The city of Melbourne in southeast Australia experiences frequent heatwaves and their frequency, intensity and duration are expected to increase in the future. In addition, Melbourne is the fastest growing city in Australia and experiencing rapid urban expansion. Heatwaves and urbanization contribute in intensifying the Urban Heat Island (UHI) effect, i.e., higher temperatures in urban areas as compared to surrounding rural areas. The combined effects of UHI and heatwaves have substantial impacts on the urban environment, meteorology and human health, and there is, therefore, a pressing need to investigate the effectiveness of different mitigation options. This study evaluates the effectiveness of urban vegetation patches such as mixed forest (MF), combination of mixed forest and grasslands (MFAG), and combination of mixed shrublands and grasslands (MSAG) in reducing UHI effects in the city of Melbourne during one of the most severe heatwave events. Simulations are carried out by using the Weather Research and Forecasting (WRF) model coupled with the Single Layer Urban Canopy Model (SLUCM). The fractions of vegetated patches per grid cell are increased by 20%, 30%, 40% and 50% using the mosaic method of the WRF model. Results show that by increasing fractions from 20 to 50%, MF reduces near surface (2 m) UHI (UHI2) by 0.6–3.4 °C, MSAG by 0.4–3.0 °C, and MFAG by 0.6–3.7 °C during the night, but there was no cooling effect for near surface temperature during the hottest part of the day. The night-time cooling was driven by a reduction in storage heat. Vegetated patches partitioned more net radiation into latent heat flux via evapotranspiration, with little to no change in sensible heat flux, but rather, a reduction in the storage heat flux during the day. Since the UHI is driven by the release of stored heat during the night, the reduced storage heat flux results in reductions in the UHI. The reductions of the UHI2 varied non-linearly with the increasing vegetated fractions, with lager fractions of up to 50% resulting in substantially larger reductions. MF and MFAG were more effective in reducing UHI2 as compared to MSAG. Vegetated patches were not effective in improving HTC during the day, but a substantial improvement of HTC was obtained between the evening and early morning particularly at 2100 local time, when the thermal stress changes from strong to moderate. Although limited to a single heatwave event and city, this study highlights the maximum potential benefits of using vegetated patches in mitigating the UHI during heatwaves and the overall principles are applicable elsewhere.

Comparative transcriptome analysis of genes involved in response to thermal stress and leaf colour change of Acer palmatum

Exterior leaf colour is an important trait for ornamental foliage plants. Accumulating evidence indicates that thermal stress (TS) has a significant effect on the colour of plant leaves, but the molecular mechanism underlying TS-induced change in leaf colour between red and green of Acer palmatum is poorly understood. In this study, a comparative transcriptome analysis was performed using RNA-seq, in which A. palmatum was exposed to 23 °C for 4 d (A1_23) and 8 d (A2_23) and 28 °C for 4 d (A1_28) and 8 d (A2_28). There were 20,146 (12,304 up and 7842 down) and 9570 (3700 up and 5870 down) differentially expressed genes (DEGs) in the A1_28 and A2_28 samples compared to the A1_23 and A2_23 samples, respectively. Three hundred forty-six DEGs were co-upregulated, and 640 DEGs were co-downregulated in A1_28 and A2_28. Bioinformatics analyses revealed that A. palmatum cell resistance to TS occurs through upregulated interacting Hsp90-Hsp83-Hsp20 and GST-GSH-HSPs or through small Hsps genes and sterol biosynthesis genes that regulate membrane fluidity. In addition, our results demonstrated that A. palmatum leaves reduce the expression of the negative regulator PP2C and interleukin receptor kinase to activate PP2C substrates, such as MAPK, to regulate TS signalling pathways. Moreover, our results showed that the anthocyanin and carotenoids biosynthetic related genes significantly upregulated, and the chlorophyll biosynthetic and chloroplast constitutive related genes significantly downregulated, which were the direct molecular mechanisms of TS changing leaf colour from red return to green in A. palmatum. Our findings provide key genetic resources for further investigation of the molecular mechanisms of TS changing leaf colour in A. palmatum and other related species.

Study on the propagation law of temperature field in surrounding rock of underground coal gasification (UCG) combustion cavity based on dynamic thermal parameters

Abstract The high temperature generated during underground coal gasification will not only produce high-temperature thermal stress in the surrounding rock of the combustion area, but also change the physical and mechanical parameters of the surrounding rock. The change of thermal parameters affects the expansion of the temperature field inside the surrounding rock, causing the change of mechanical properties and internal thermal stress of surrounding rock in combustion zone, and then affects the stability of the gasifier. In the past, the research on the propagation law of temperature field in surrounding rock of combustion zone based on dynamic variation of thermal parameters with temperature was very limited, which severely limited the development of the strata control technology of underground coal gasification, so it is urgent to carry out this research. In this study, an explicit finite difference method is used to establish a numerical model for calculating the temperature field in materials under the thermal parameters varying with the temperature, which is applied to the study of the temperature field expansion in the surrounding rock of the combustion area of the underground coal gasification, obtaining the following results: With time, the influence range of temperature field in the strata gradually expands, and the temperature of each point in the strata increases first and then decreases, and the temperature extreme gradually expands to the inside of the strata. By extracting the temperature extremes of each point of surrounding rocks in the combustion cavity, it is found that the propagation range of temperature field is 11.4 m in the roof, 9.3 m in the floor and 9.6 m in the coal wall on both sides. The coal wall within 3.1 m on both sides of the combustion cavity loses its bearing capacity, and the bearing capacity of the coal wall within 1.7 m decreases, which has adverse effects on the coal wall within 4.8 m. This study can play an important role in promoting the progress of surrounding rock control technology of underground coal gasification.

Thermal analysis of the dentine tubule under hot and cold stimuli using fluid-structure interaction simulation.

The objective of this study is to compare the thermal stress changes in the tooth microstructures and the hydrodynamic changes of the dental fluid under hot and cold stimuli. The dimension of the microstructures of eleven cats' teeth was measured by scanning electron microscopy, and the changes in thermal stress during cold and hot stimulation were calculated by 3D fluid-structure interaction modeling. Evaluation of results, following data validation, indicated that the maximum velocities in cold and hot stimuli were - 410.2 ± 17.6 and + 205.1 ± 8.7µm/s, respectively. The corresponding data for maximum thermal stress were - 20.27 ± 0.79 and + 10.13 ± 0.24cmHg, respectively. The thermal stress caused by cold stimulus could influence almost 2.9 times faster than that caused by hot stimulus, and the durability of the thermal stress caused by hot stimulus was 71% greater than that by cold stimulus under similar conditions. The maximum stress was on the tip of the odontoblast, while the stress in lateral walls of the odontoblast and terminal fibril was very weak. There is hence a higher possibility of pain transmission with activation of stress-sensitive ion channels at the tip of the odontoblast. The maximum thermal stress resulted from the cold stimulus is double that produced by the hot stimulus. There is a higher possibility of pain transmission in the lateral walls of the odontoblast and terminal fibril by releasing mediators during the cold stimulation than the hot stimulation. These two reasons can be associated with a greater pain sensation due to intake of cold liquids.

Effects of phosphorus content on generation and growth of cracks in nickel–phosphorus platings owing to thermal cycling

We observed crack generation and structural changes in electroless nickel–phosphorus (Ni–P) plating layers formed on copper-metalized silicon nitride substrates both during thermal cycling from − 40 to 250 °C and during storage (not cycling) at 250 °C in order to investigate the effect of the phosphorus contents on crack generation and growth in the Ni–P platings. The used platings contained phosphorus at three different contents: 2.1 wt% [Ni–P(low)], 6.5 wt% [Ni–P(med)], and 10.9 wt% [Ni–P(high)]. The generation time and the amount of cracks were strongly dependent on their phosphorus contents. More cracks appeared after thermal cycling than after storage at 250 °C. In Ni–P(low), cracks were generated after 200 thermal cycles, whereas no cracks were observed even after 250 h of storage at 250 °C. In Ni–P(med) and Ni–P(high), both during thermal cycling and storage at 250 °C, cracks formed during or after crystallization of the amorphous layers. These results suggest that the primary factors affecting the generation of cracks in electroless Ni–P platings are crystallization of the Ni–P platings and repeated changes in thermal stress.

Effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron (CADI) grinding balls

The effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron (CADI) grinding balls were systematically investigated in this work. The microstructures were oberserved by optical metallography and analyized by X-ray diffraction. The surface residual stress measured by the cutting method is mainly composed of thermal stress and phase transformation stress. The thermal stress in grinding balls was determined by ANSYS simulation technique, and the surface phase transformation stress was obtained by subtracting the simulated surface thermal stress from the measured surface residual stress. Results show that all microstructures consist of ausferrite, white-bright zones (mixture of martensite and austenite), nodular graphite, and carbides. The distribution of ausferrite shows uniform. With the increase of austempering temperature, the volume fraction and carbon content of austenite increase, whereas the amount of white-bright zone decreases. In addition, the surface residual stress increases with the increase of austempering temperature. Only the tension exists at the austempering temperature of 200 °C, and the pressure exists at the austempering temperature of 220–260 °C. The thermal stress changes from the tension on the inside with the radius of 0–35 mm to the pressure on the outside with the radius of 35–62.5 mm, and the stress balance state presents at the radius of 35 mm. It is also found that the transformation stress is related to the content of carbon-rich austenite, and will reduce by 5.03 MPa accompanied with 1vol.% increase of the austenite. The thermal compressive stress and the transformation tensile stress on the surface both decrease with the increase of the austempering temperature.

The effect of water vapor on nanosecond laser damage resistance of optical coatings in vacuum

In this paper, the effects of water vapor on the laser-induced damage of e-beam evaporated dielectric anti-reflection coatings in vacuum were investigated. The laser-induced damage resistance of the samples was measured in 1-on-1 and R-on-1 mode in vacuum and water vapor atmosphere. The LIDT of the anti-reflection coatings in vacuum was improved after introducing water vapor. The effects of the oxygen vacancy defect and thermal conductivity and stress change were considered to explain the improved LIDT.

The closed form solutions for axisymmetric modeling of thermal stress due to repetitive pulse laser heating

• Novel closed form solutions for axisymmetric modeling of thermal stress induced by repetitive pulse laser heating are obtained. • Thermal stress distributions for different radial and axial locations of material are modeled and analyzed. • Mechanisms of energy gain and stress generation for different laser irradiation regions are analyzed. • Effects of duty cycles on thermal stress distributions are investigated. Repetitive pulse laser heating is increasingly being used in the laser processing industry such as laser welding, laser cutting and laser drilling due to its energy accumulation effect. Modeling of thermo-mechanical coupling effect of repetitive pulse laser heating can enhance the understanding of its thermo physical process. In this paper, an axisymmetric modeling of thermal stress for repetitive pulse laser heating solid materials is introduced, and its closed form solutions are obtained based on a semi-analytical method. The accuracy and efficiency of the closed form solutions are verified by comparison with the existing numerical modeling software based on the finite element method. Distributions of thermal stress for different radial locations, axial locations and duty cycles are calculated, and effects of these parameters on temporal profile of the thermal stress distributions are analyzed. In the region of laser irradiation, three components of thermal stress are all shown as compressive stress. And outside the region of laser irradiation, the hoop component is shown as tensile stress. On the material surface, the change of thermal stresses is very sharp as a result of direct absorption of incident laser energy. With the increasing of the depth, the rises and decreases of thermal stresses become smooth gradually. The duty cycle has a significant effect on the profiles of thermal stresses. In the same conditions, the maximum value of thermal stresses gets higher as the duty cycle decreases. Results of this study can provide some theoretical basis for parameter inversion and optimization of repetitive pulse laser heating, as well as some corresponding experimental research.

Thermal stress and physiological changes in watermelon seeds

ABSTRACT Given that watermelon is a crop widespread around the world, there is considerable interest in verifying how its seeds physiologically behave under unfavorable temperature conditions. This study aimed to evaluate the biochemical changes mobilization and reserves degradation, as well as the enzyme activity, during seed germination and initial growth of watermelon seedlings subjected to thermal stress, using temperature, germination percentage, germination rate and relative germination frequency over the incubation time as evaluation parameters. The experimental design was completely randomized, in a 5 x 3 factorial scheme, with five temperatures (17 ºC, 20 ºC, 25 ºC, 30 ºC and 35 ºC) and three cultivars (Charleston Gray, Fairfax and Crimson Sweet), with 4 replicates of 50 seeds. Germination, seedling growth, reserve degradation and stress protection system were evaluated. Thermal stress caused deleterious effects on watermelon seeds with germination capacity at well defined temperature limits, being 25 ºC the ideal temperature, with the highest percentage of normal seedlings. Sub- (17 ºC) and supra-optimal (30 ºC) temperatures presented more than 80 % of abnormal seedlings. The α-amylase enzyme activity is intense only at the beginning of germination. Under thermal stress, the proline contents increase mainly in the cotyledons.

Measurement and prediction of the mechanical properties of a two-component food during freezing

ABSTRACTFreezing of water-rich materials inevitably leads to changes in volume and thermal stresses. Determining and modeling the thermal stresses require the mechanical properties of the food material to be properly measured or predicted during freezing. This paper investigated mechanical properties such as the Young’s modulus and Poisson’s ratio in chicken nugget crust and crumb samples at the different freezing temperatures (‒20, ‒30, and ‒40°C). Mechanical properties were measured by compression tests at seven nugget-center temperatures and a continuous video capturing method during the freezing process. Results showed that ice formation led to a sharp increase in the Young’s modulus of samples, however, a gradual decrease in Poisson’s ratio values was observed as the nuggets’ center temperature approached the freezing point. Therefore, the greatest changes were observed at the freezing point. Kinetic and fitting equations were used to model the Young’s modulus, and the Arrhenius equation was used to describe the temperature dependence. The predicted values by the models were found in close agreement with the experimental results.

Passive Heating Attenuates Post-exercise Cardiac Autonomic Recovery in Healthy Young Males.

Post-exercise heart rate (HR) recovery (HRR) presents a biphasic pattern, which is mediated by parasympathetic reactivation and sympathetic withdrawal. Several mechanisms regulate these post-exercise autonomic responses and thermoregulation has been proposed to play an important role. The aim of this study was to test the effects of heat stress on HRR and HR variability (HRV) after aerobic exercise in healthy subjects. Twelve healthy males (25 ± 1 years, 23.8 ± 0.5 kg/m2) performed 14 min of moderate-intensity cycling exercise (40–60% HRreserve) followed by 5 min of loadless active recovery in two conditions: heat stress (HS) and normothermia (NT). In HS, subjects dressed in a whole-body water-perfused tube-lined suit to increase internal temperature (Tc) by ~1°C. In NT, subjects did not wear the suit. HR, core and skin temperatures (Tc and Tsk), mean arterial pressure (MAP) skin blood flow (SKBF), and cutaneous vascular conductance (CVC) were measured throughout and analyzed during post-exercise recovery. HRR was assessed through calculations of HR decay after 60 and 300 s of recovery (HRR60s and HRR300s), and the short- and long-term time constants of HRR (T30 and HRRt). Post-exercise HRV was examined via calculations of RMSSD (root mean square of successive RR intervals) and RMS (root mean square residual of RR intervals). The HS protocol promoted significant thermal stress and hemodynamic adjustments during the recovery (HS-NT differences: Tc = +0.7 ± 0.3°C; Tsk = +3.2 ± 1.5°C; MAP = −12 ± 14 mmHg; SKBF = +90 ± 80 a.u; CVC = +1.5 ± 1.3 a.u./mmHg). HRR and post-exercise HRV were significantly delayed in HS (e.g., HRR60s = 27 ± 9 vs. 44 ± 12 bpm, P < 0.01; HRR300s = 39 ± 12 vs. 59 ± 16 bpm, P < 0.01). The effects of heat stress (e.g., the HS-NT differences) on HRR were associated with its effects on thermal and hemodynamic responses. In conclusion, heat stress delays HRR, and this effect seems to be mediated by an attenuated parasympathetic reactivation and sympathetic withdrawal after exercise. In addition, the impact of heat stress on HRR is related to the magnitude of the heat stress-induced thermal stress and hemodynamic changes.

Numerical Simulation of the Crack Formation in the Quenched Poly(l‐lactic acid) Spherulites

AbstractThe quenching process of poly(l‐lactic acid) spherulites obtained by isothermal crystallization is simulated by ANSYS software in order to study the mechanism of crack formation. Regular concentric circular or hexagon crack is found by polarizing optical microscope and scanning electron microscope. The rapid temperature variation which brings large thermal stress is considered to be the main reason for the crack, so a spherulite with diameter of 160 µm is modeled by ANSYS to compute the transient change of thermal stress. According to the temperature nephogram, it is found that the spherulites temperature almost reduces to zero during 1 s, also the thermal stress dropped by five orders of magnitude. The stress fluctuation of 35 nodes, which are equally spaced from spherulite center to outer, present regular attenuation of vibration periodically from 10−6 to 10−4 s while the peak values are different from each other. Furthermore, the differential result of the peak value shows a bell‐shaped curve while the maximum difference between two adjacent nodes reaches 0.12 MPa. Although insufficient of boundary condition will introduce error of calculation, this research is still an important attempt for simulation technique to solve problem in polymer crystallization.