Additional Thermal Stress Research Articles

Thermal Stress Based Model Predictive Control of Electric Drives

The reliable operation of the power electronics system of an electric drive is a critical design target. Thermal cycling of the semiconductors in the power module is one of the main stressors. Active thermal control is a possibility to control the junction temperatures of power modules in order to reduce the thermal stress. In this paper, the finite control set model predictive control is designed for thermal stress based driving of electric drives converters. The optimal switching vector is selected using a multiparameter optimization that includes the current reference error, the additional thermal stress that a specific switching vector applies to each semiconductor, the temperature spread between semiconductors in the module, overall efficiency, and device constraints. This enables relieving the stress due to thermal cycles and reducing unsymmetrical fatigue of the modules chips while avoiding unnecessary losses. The approach is derived in theory and applied in simulation and experiment.

Hepatic thyroid signaling of heat-stressed late pregnant and early lactating cows.

During the transition between late gestation and early lactation, dairy cows experience severe metabolic stress due to the high energy and nutrient requirements of the fetus and the mammary gland. Additional thermal stress that occurs with rising temperatures during the ongoing climate change has further adverse implications on energy intake, metabolism and welfare. The thyroid hormone (TH)-mediated cellular signaling has a pivotal role in regulation of body temperature, energy intake and metabolic adaptation to heat. To distinguish between energy intake and heat stress-related effects, Holstein cows were first kept at thermoneutrality at 15°C followed by exposure to heat stress (HS) at 28°C or pair-feeding (PF) at 15°C for 6 days, in late pregnancy and again in early lactation. Herein, we focused on hepatic metabolic changes associated with alterations in the hypothalamic–pituitary–thyroid axis in HS and PF animals. T3 and T4 levels dropped with HS or PF; however, in HS animals, this decline was more pronounced. Thyroid-stimulating hormone (TSH) levels remain unaffected, while plasma cholesterol concentrations were lower in HS than PF animals. Hepatic marker genes for TH action (THRA, DIO1 and PPARGC1) decreased after HS and were lower compared to PF cows but only post-partum. Proteomics data revealed reduced hepatic amino acid catabolism ante-partum and a shift toward activated beta-oxidation and gluconeogenesis but declined oxidative stress defense post-partum. Thus, liver metabolism of HS and PF cows adapts differently to diminished energy intake both ante-partum and post-partum, and a different TH sensitivity is involved in the regulation of catabolic processes.

Effect of internal stress on the electro-optical behaviour of Al-doped ZnO transparent conductive thin films

In this work, we will report on scientific efforts aimed at unraveling the quantitative effect of elastic strain on the electro-optical behaviour of Al-doped zinc oxide (AZO). AZO thin films have been deposited by reactive magnetron sputtering to thicknesses from 300 to 500 nm, both on extra-clear glass substrates as well as on oxidised Si wafers. This resulted in both cases in polycrystalline, strongly textured (002) films. During deposition, the internal stress evolution in the growing film was monitored in-situ using high resolution curvature measurements. The resulting growth-induced elastic strain, which was found to depend heavily on the oxygen partial pressure, could further be modulated by appropriately choosing the deposition temperature. The latter also induces an additional extrinsic thermal stress component, whose sign depends on the substrate used. As such, a wide range of biaxial internal stresses could be achieved, from −600 MPa in compression up to 800 MPa in tension. The resulting charge carrier mobilities, obtained independently from room temperature Hall measurements, were found to range between 5 and 25 cm2/V s. Interestingly, the maximum mobility occurred at the zero-stress condition, and together with a charge carrier concentration of about 8 × 1020 cm−3, this gave rise to a resistivity of only 300 μΩ cm. From the stress-dependent optical transmission spectra in the range of 200–1000 nm, the pressure coefficient of the optical bandgap was estimated from the corresponding Tauc plots to be 31 meV/GPa, indicating a very high strain-sensitivity as well.

Metabolic Response to Heat Stress in Late-Pregnant and Early Lactation Dairy Cows: Implications to Liver-Muscle Crosstalk.

Climate changes lead to rising temperatures during summer periods and dramatic economic losses in dairy production. Modern high-yielding dairy cows experience severe metabolic stress during the transition period between late gestation and early lactation to meet the high energy and nutrient requirements of the fetus or the mammary gland, and additional thermal stress during this time has adverse implications on metabolism and welfare. The mechanisms enabling metabolic adaptation to heat apart from the decline in feed intake and milk yield are not fully elucidated yet. To distinguish between feed intake and heat stress related effects, German Holstein dairy cows were first kept at thermoneutral conditions at 15°C followed by exposure to heat-stressed (HS) at 28°C or pair-feeding (PF) at 15°C for 6 days; in late-pregnancy and again in early lactation. Liver and muscle biopsies and plasma samples were taken to assess major metabolic pathway regulation using real-time PCR and Western Blot. The results indicate that during heat stress, late pregnant cows activate Cahill but reduce Cori cycling, prevent increase in skeletal muscle fatty acid oxidation, and utilize increased amounts of pyruvate for gluconeogenesis, without altering ureagenesis despite reduced plane of nutrition. These homeorhetic adaptations are employed to reduce endogenous heat production while diverting amino acids to the growing fetus. Metabolic adaptation to heat stress in early lactation involves increased long-chain fatty acid degradation in muscle peroxisomes, allowance for muscle glucose utilization but diminished hepatic use of amino acid-derived pyruvate for gluconeogenesis and reduced peroxisomal fatty acid oxidation and ATP production in liver of HS compared to PF cows in early lactation. Consequently, metabolic adaptation to heat stress and reduced feed intake differ between late pregnancy and early lactation of dairy cows to maintain energy supply for fetus development or milk production simultaneously reducing endogenous heat production.

Fracture behavior of soldered Al2O3 ceramic to A356 aluminum alloy and resistance of the joint to low temperature exposure

Active solder alloy 92Sn3Ag3Ti was used to join Al2O3 ceramic on cast A356 aluminum alloy substrate using an ultrasonic assisted soldering process. Due to the large discrepancy in thermal shrinkage coefficients between materials, large thermal stresses occur during cooling down to room temperature. Further cooling to −78°C and −196°C was used to inflict additional thermal stress in the samples. Shear strength, image analysis of fractured surfaces and microstructure analysis were performed to study the influence of cooling. Ceramic samples with different surface finish were used to investigate the influence of the contact between ceramic and aluminum alloy substrate on the fracture behavior. Two types of failure were observed, a detachment of the solder alloy from the ceramic surface and a cohesive failure through the solder layer. The results show that cooling the joined sample to −78°C does not significantly affect the bonding strength of the samples. However, samples that were rapidly cooled to −196°C showed reduced shear strengths even when the amount of cohesive failure area was taken into account. Many newly formed cracks in the Sn matrix were observed in the joint after the cooling which were identified as strength-determining defects.

The development of a low cost refrigerated flow-through seawater system for at-sea estimation of post-release mortality

The post-release mortality of discarded fish can represent an additional source of mortality that is important to understand for effective marine fisheries management. To best estimate these mortality rates under near-natural conditions, a refrigerated flow-through seawater system was developed for short-term holding experiments at-sea aboard commercial fishing vessels, using accessory tanks containing heat exchanging coils cooled by an ice slurry. Ice was added to these tanks in one to two-hour intervals based on the cooling requirement. This system was capable of maintaining temperatures within one degree Celsius of target conditions for 93–96% of operation, creating an environment where post-release mortality could be evaluated without additional thermal stress.

Scaling laws based metamodels for the selection of the cooling strategy of electromechanical actuators in the early design stages

The integration of electromechanical actuation systems for aerospace applications requires additional thermal stress considerations during the preliminary design, which were not necessary within the hydraulic technology. The favored models during this design phase are simple, continuous and explicit mathematical expressions to enable easy analytical manipulations or implementations in worksheets or optimization loops. The paper shows how such models can be obtained for the desired applications, illustrating the example of a Limited Angle Torque Motor (LATM) with different cooling strategies. The used method for gathering the models is a regression process on data obtained by finite element simulations. The originality of the method stands in the physical insight of the regression function which is based on scaling laws and dimensional analysis. An example of preliminary design of a LATM illustrates the use of the obtained models for the comparison of two possible architectures intended to minimize the mass of the actuator by implementing different cooling strategies.

Web-post buckling of fully and partially protected cellular steel beams at elevated temperatures in a fire

Web-post buckling behaviors of fully and partially protected cellular steel beams at elevated temperatures in a fire were investigated through verified finite element models. The partially protected cellular steel beam represented a protected cellular steel beam with the hole edge left unprotected. The temperature distribution in the web-post was non-uniform, even for the unprotected CSB and the fully protected CSB. The fire resistance time of a CSB increased linearly with the increase in fire coating thickness. However, for the partially protected cellular steel beam, the temperature gradient in the web-post becomes greater with the increase in the fire protection thickness. And with the increase in the fire protection thickness, the increment in the fire resistance time decreased. Additional thermal stress occurred due to the non-uniform thermal strain in the web-post. When web-post buckled, the principle compression stress in the web-post decreased suddenly. The stress at the center of the web-post could change from compression to tension due to the membrane action caused by the large buckling deformation.

Experimental Study on the Influence of Temperature on Shale Mechanical Properties under Conventional Triaxial Compression

High temperature conventional triaxial compression test of shale are carried out by the MTS815 servo-controlled testing machine, based on the experimental results, the relationships between temperature and shale peak strength, elastic modulus, Poisson's ratio, cohesion, internal friction angle are investigated. Although the experimental results are discrete comparatively, the general law is obvious. When the confining pressure imposed on shale is constant and the temperature changes form 25°C to 120°C, with the increasing of the temperature, the triaxial compression strength, shear strength gradually increase, while average elastic modulus, Poisson's ratio has a slightly decrease. The thermal stress generated by the high temperature plays a role to accommodate the deformation and the function of preventing crack propagation, thus the bearing capacity of shale samples are strengthened. But the influence of temperature on shale mechanical properties mutates when the temperature is at 80°C. Shale peak strength dramatically decreased, average elastic modulus decreased slightly, and Poisson's ratio also increased slightly, which indicated that at 80°C, different thermal expansivity of mineral particles of shale may cause cross-grain boundary thermal expansion incongruous, creating additional thermal stress, thus the sample’s bearing capacity decreased.

On the discrepancy between theoretical and experimental thermal stress in chromia oxide films

Growth and residual stresses have been determined, thanks to Raman spectroscopy in chromia oxide films thermally grown in air from a Ni33at %Cr alloy between 700 and 900 °C. Due to the good sensibility of the in situ experimental device, the growth stress kinetics even at short oxidation times has been recorded and has been shown to be microstructure sensitive. At different oxidation times, the corresponding residual stress at room temperature has also been investigated and the experimental thermal stress has been deduced. The comparison between the respective magnitudes of stress variations induced by creep relaxation during isothermal oxidation, and by the additional thermal stress occurring upon cooling, has led to the definition of a stress threshold finally proposed to explain the residual stress values and the difference between theoretical and experimental thermal stress components.

Microstructure Influence on Residual Stresses in Growing Chromia Oxide Films as Determined by <i>In Situ </i>High Temperature Raman Spectroscopy

In this work, growth stresses have been investigated in relation with the microstructure in the case of α-Cr2O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range [700°C-900°C]. The low acquisition time necessary to obtain a Raman spectrum allows to follow the chromia growth kinetic with sufficient accuracy. It is demonstrated that the growth stress in such oxide films can attain more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetic - subsequent establishment and relaxation - are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress to start.

Growth stresses in α-Cr2O3 thermal oxide films determined by in situ high temperature Raman spectroscopy

Growth stresses have been investigated in relation with the microstructure in the case of α-Cr2O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range (700−900 °C). It is established that the compressive growth stress in such oxide films can reach more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetics—subsequent establishment and relaxation—are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia mean grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress level to start.

Temperature tolerance of Penaeus setiferus postlarvae exposed to ammonia and nitrite

The critical thermal maximum (CTM) method was used to determine the acute effect of ammonia, nitrite and the combined effect of both toxicants on the thermal tolerance of white shrimp ( Penaeus setiferus) postlarvae. Ammonia did not affect the temperature response of shrimp. Nitrite exposure significantly decreased the temperature tolerance of postlarvae. Coexposure to ammonia and nitrite mixtures modified the CTM; an inverse relationship between ammonia/nitrite toxicity ratios and the CTM was observed. In single exposure, ammonia and nitrite concentrations were related to the mortality rate. No relation was observed between shrimp mortality and the thermal tolerance. Our results suggest that exposure to these nitrogenous compounds can decrease the ability of white shrimp postlarvae to tolerate additional thermal stress.

Stratification of heat carrier in horizontal tube with inflow of heat carrier at a different temperature

Inflow Of heat carrier of different temperature occurs on account of the influence of natural convection, and leads to temperature stratification of the flow. In practice, this effect may appear when cold heat carrier is fed along a horizontal tube to a tank with hot heat carrier, for example, in the operation of the emergency-cooling system of a watercooled, water-moderated reactor. As result of temperature stratification of the heat carrier, additional thermal stress develops in the supply tube, and must be taken into account in design calculations.

The Effect of Impermeable Clothing on Weight Loss, Heart Rate, and Core Temperature in Wrestlers

Abstract This study compared heart rate, core temperature, and weight loss responses in wrestlers dehydrating in vapor barrier sweatsuits with the same responses while they were dehydrating in light clothing. The purpose of the investigation was to determine if the vapor barrier suit induced hidromeiosis to sufficient extent to off set additional weight loss associated with augmented central and peripheral drive for sweating. Weight loss was achieved with a combination exercise and thermal stress. University varsity wrestlers exercised at workloads between 600 and 900 kpm/min in an environment with Tdb = 35 ± 1° C and Twb = 26.5–28.5° C, respectively. Relative humidity was 50–64%. Subsequent additional thermal stress at rest was provided by sauna exposures at Tdb = 77–89° C. Ten 3-min exercise periods applied alternately with 1-min rest periods were followed by two 10-min sauna exposures alternated with rest. The subjects lost 30% more body weight with than without the sweatsuit—2.17 vs 1.67 kg. The diffe...