Constant Elevated Temperatures Research Articles

Axial compressive behaviour of ultra-high performance concrete filled steel tube stub columns at elevated temperatures

Ultra-high performance concrete (UHPC) has many advantages such as high strength, good plasticity and toughness, and excellent durability. However, it is more likely to experience explosive spalling at high temperatures than ordinary concrete due to its low porosity. The ultra-high performance concrete-filled steel tubes (UHPCFSTs), where steel tubes are placed outside of UHPC to reduce its high-temperature spalling, could be an effective structural type to make full use of the advantages of both steel and UHPC. Most of the existing fire protection design codes for concrete-filled steel tube structures are based on ordinary concrete, and the performance and failure mechanisms of UHPCFSTs under high temperatures are still unclear. On this basis, an experimental study on the axial compressive performance of UHPC filled circular steel tube stub columns at constant elevated temperatures was carried out. The effects of temperature, steel tube thickness and coarse aggregate content on the compressive properties and failure modes of UHPCFST members were analysed. The results have shown that the UHPCFST mainly suffered from the overall pier coarse type and drum type failure modes. The failure characteristics were highly influenced by temperature magnitude, and 400℃ was the critical temperature where the structural performance change significantly beyond this point. The constraint effect of the steel tube on UHPC increased with the increase of thickness of steel tube, but it would be weakened with the increase of temperature or the coarse aggregate content. Finally, based on the average temperature method, a formula for calculating the ultimate load bearing capacity and equivalent stress–strain relationship of an UHPCTST stub column under a constant high temperature was proposed.

Numerical and experimental analysis of the isothermal high temperature pneumoforming process

The isothermal high temperature pneumoforming process to form tubes at constant elevated temperatures by means of internal pressure is investigated. Two materials, a ferritic (X2CrTiNb18) and a martensitic stainless steel (X12Cr13) are used for the investigations. The required material characterization is performed at the temperature and strain rate of the actual process. A new method for quantifying thermal softening via the time-dependent decrease in static yield stress is presented. At a temperature of 1000 °C, the static yield stress decreases by 50% within 100 s for both materials. The numerical models are validated on the basis of the formed geometry and used to study the influence of maximum internal pressure, axial feed, holding time under load and die edge length on the final part geometry. It was observed, that with higher internal pressures and longer holding times smaller corner radii are formed for both materials. In contrast, a superimposed axial feed as well as the effective friction coefficient have a negligible influence on the formed geometry. With an increasing die edge length, smaller radii are formed with the ferritic stainless steel numerically and experimentally. By contrast, for the martensitic stainless steel, larger radii are observed numerically. Experimentally, the limited formability of these tubes weld seam becomes apparent. Based on the findings, process windows depending on the process parameters internal pressure and die edge length were derived. Numerically, forming limit curves of tubular semi-finished products under comparable conditions serve as a failure criterion. Good agreement with experiments was observed.

Activation Energy for End-of-Life Solder Bond Degradation: Thermal Cycling of Field-Aged PV Modules

The longevity of solar photovoltaic modules depends on the durability and reliability of their components, one of which is the solder bonds in interconnect ribbons. The solder joints experience stresses from thermal cycling and constant elevated temperatures (40 °C–70 °C) in regular field operation leading to thermo-mechanical fatigue and intermetallic compound formation. To study the end-of-life wear-out mechanisms and to obtain activation energy of solder bond degradation, two field-aged modules from Arizona—a 21-year-old Solarex MSX60 module (with Sn62Pb36Ag2 at the solder joints) and an 18-year-old Siemens M55 module (with Sn60Pb40 at the solder joints)—underwent 800 and 400 modified thermal cycles, respectively. Using three heating blankets, each module had three temperature zones maintained at 85, 95, and 105 °C during the 15-min hot dwell time of the thermal cycle. Cell-level series resistance data obtained from three temperature zones enabled the calculation of activation energy for solder bond degradation for the MSX60 and the M55 modules to be 0.12 eV and 0.35 eV, respectively. From each temperature zone in both modules, busbar-solder samples were obtained, imaged through SEM, and analyzed with energy-dispersive X-ray spectroscopy. In the MSX60 module with traces of Ag in the solder material, phase segregation and growth were primarily observed at high temperatures. For M55 modules without Ag in the solder material, major phase segregation was observed in all temperature zones. The IMC thickness for both modules increased with increasing module temperature. The beneficial effect of Ag in solder material on mitigating solder bond degradation is presented.

High Melatonin Conditions by Constant Darkness and High Temperature Differently Affect Melatonin Receptor mt1 and TREK Channel trek2a in the Brain of Zebrafish.

Ambient light and temperature affect reproductive function by regulating kisspeptin and gonadotrophin-releasing hormone (GnRH) in vertebrates. Melatonin and melatonin receptors, as well as the two-pore domain K+ channel-related K+ (TREK) channels, are affected by light and/or temperature; therefore, these molecules could modulate kisspeptin and GnRH against ambient light and temperature. In this study, we investigated the effect of light and temperature, which affect melatonin levels in gene expression levels of TREK channels, kisspeptin, and GnRH. We first investigated the effects of different light and temperature conditions on brain melatonin concentrations by ELISA. Fish were exposed to either constant darkness, constant light, high temperature (35°C), or low temperature (20°C) for 72 h. Brain melatonin levels were significantly high under constant darkness and high temperature. We further investigated the effects of high brain melatonin levels by constant darkness and high temperature on gene expression levels of melatonin receptors (mt1, mt2, and mel1c), TREK channels (trek1b, trek2a, and trek2b), gnrh3, and kiss2 in the adult zebrafish brain by real-time polymerase chain reaction. Fish were exposed to constant darkness or elevated temperatures (35°C) for 72 h. trek2a, kiss2, and gnrh3 levels were increased under constant darkness. High temperature decreased gene expression levels of mt1, mt2, mel1c, and gnrh3 in the preoptic area, whereas other genes remained unchanged. Melatonin receptors, TREK channels, gnrh3, and kiss2 responded differently under high melatonin conditions. The melatonin receptors and the TREK channels could play roles in the regulation of reproduction by environmental cues, especially ambient light and temperature.

Development and validation of a model for predicting survival of young larvae of Tribolium castaneum exposed to elevated temperatures during heat treatment of grain-processing facilities

Heat treatment involves raising the ambient temperature of grain-processing facilities to 50–60 °C for at least 24 h to manage stored-product insects. The young larvae (first instars) of the red flour beetle, Tribolium castaneum (Herbst), are the most heat tolerant stage at temperatures of 50–60 °C when compared to eggs, old larvae, pupae, and adults. A thermal death kinetic model was developed to predict survival of T. castaneum young larvae exposed to six constant elevated temperatures between 42 and 60 °C. The model was based on two non-linear relationships: 1) logarithmic decrease in survival of larvae as a function of time, and 2) logarithmic decrease in larval survival as a function of temperature. The model was validated with 12 time-dependent temperature datasets collected during actual heat treatments of pilot-scale flour and feed mills and a commercial grain-processing facility. The observed survival of larvae in plastic boxes/vials with flour was used to validate model predictions. The heating rate to reach 50 °C from the ambient temperature, among the 12 datasets ranged from 0.9 to 7.8 °C/h. The mean absolute deviation between observed and predicted larval survival for 10 of the 12 datasets ranged from 2.1 to 11.4%; for the other two datasets it was 16.2 and 18.3%. These results show that the thermal death kinetic model can be used to predict survival of young larvae of T. castaneum based on time-dependent temperature profile obtained at any given location during heat treatment of grain-processing facilities.

Ultimate Strength of Uniaxially Compressed Mild Steel Plate at Elevated Temperatures

Mild steel plates used in buildings and offshore platforms are prone to fire accidents. These plates being ductile are designed effectively for buckling and ultimate strength characteristics under static loads. These characteristics get drastically affected due to reduction in stiffness of the stress strain characteristics of mild steel with increase in temperatures. This paper presents a numerical study conducted on clamped plates at elevated constant temperature for the assessment of reduced buckling and ultimate strengths. Coupled Nonlinear static thermal analysis on clamped plates was performed using standard FE software ANSYS®. Both geometric and material nonlinearities are considered in the analysis. The study comprises of plates with varying aspect ratio (1 to 4) and breadth to thickness (28 to 128) at constant elevated temperatures of 0 °C, 200 °C, 400 °C, 600 °C and 800 °C. Nondimensional plate slenderness ratios based on AISC and Eurocode at elevated temperature was evaluated. Several charts showing normalised buckling stress vs temperature and normalised ultimate strength vs temperature for varied nondimensional plate slenderness ratio and plate aspect ratios are drawn. The buckling and ultimate strengths from this study are found to be underestimated in comparison to Eurocode and AISC calculations. The reduction in buckling and ultimate strength was found to be significant beyond 400 °C. It is observed that for all plate aspect ratios, the effect of plate breadth to thickness ratio is important for temperatures below 500°C and at 800°C ultimate strength of plate is only about 10% of that of at normal temperature.

Microbial community structure changes during oyster mushroom substrate preparation

Although oyster mushroom (Pleurotus spp.) is a valuable food, cultivated worldwide on an industrial scale, still very little is known about the microbial dynamics during oyster mushroom substrate preparation. Therefore, the characterization of the microbial dynamics by chemical and biological tools was the objective of this study. During substrate preparation, enzymatic digestibility of the substrate improved by 77%, whereas the cellulose and hemicellulose to lignin ratios decreased by 9% and 19%, respectively. Fluorescein diacetate hydrolysis reached its minimum value at the temperature maximum of the process during the composting phase and exceeded the initial level at the end of the process. Fungal species played part in the initial mesophilic phase of the substrate preparation process, but they disappeared after pasteurization in tunnels at constant elevated temperatures. Changes in the microbiota showed a marked bacterial community succession during substrate preparation investigated by 16S ribosomal deoxyribonucleic acid-based terminal restriction fragment length polymorphism (T-RFLP). Mature samples represented the least variance, which indicated the effect of the standardized preparation protocol. The relation between mushroom yield and the bacterial community T-RFLP fingerprints was investigated, but the uniformity of mushroom yields did not support any significant correlation.

Damage Accumulation in Aluminum Alloys under Plastic Deformation and Creep

The accumulation of local and bulk damage in D16 AT and 1201 T1 aluminum alloys used in aircraft engineering is studied. The local damage level is calculated from data of thermoactivation analysis of the residual life of D16 AT alloy specimens after preliminary plastic deformation. The bulk damage level is determined from the elastic-modulus defect by measuring the natural frequency of 1201 T1 alloy specimens. Life tests of the specimens were performed at constant tensile loads and elevated temperatures. The dependence of the local damage on preliminary plastic strain at room temperature is obtained. The residual life of the specimens is calculated with allowance for the damage to the material in the initial stage of failure and compared with experimental results. Data are given on the kinetics of bulk-damage accumulation in various test regimes.

The physics of baking: rheological and polymer molecular structure–function relationships in breadmaking

Molecular size and structure of the gluten polymers that make up the major structural components of wheat are related to their rheological properties via modern polymer rheology concepts. Interactions between polymer chain entanglements and branching are seen to be the key mechanisms determining the rheology of HMW polymers. Recent work confirms the observation that dynamic shear plateau modulus is essentially independent of variations in MW amongst wheat varieties of varying baking performance and is not related to variations in baking performance, and that it is not the size of the soluble glutenin polymers, but the structural and rheological properties of the insoluble polymer fraction that are mainly responsible for variations in baking performance. The rheological properties of gas cell walls in bread doughs are considered to be important in relation to their stability and gas retention during proof and baking, in particular their extensional strain hardening properties. Large deformation rheological properties of gas cell walls were measured using biaxial extension for a number of doughs of varying breadmaking quality at constant strain rate and elevated temperatures in the range 25–60 °C. Strain hardening and failure strain of cell walls were both seen to decrease with temperature, with cell walls in good breadmaking doughs remaining stable and retaining their strain hardening properties to higher temperatures (60 °C), whilst the cell walls of poor breadmaking doughs became unstable at lower temperatures (45–50 °C) and had lower strain hardening. Strain hardening measured at 50 °C gave good correlations with baking volume, with the best correlations achieved between those rheological measurements and baking tests which used similar mixing conditions. As predicted by the Considere failure criterion, a strain hardening value of 1 defines a region below which gas cell walls become unstable, and discriminates well between the baking quality of a range of commercial flour blends of varying quality. This indicates that the stability of gas cell walls during baking is strongly related to their strain hardening properties, and that extensional rheological measurements can be used as predictors of baking quality.

Phase transformations in thermally exposed Au-Al ball bonds

Gold-aluminum ball bonds were thermally exposed at constant elevated temperatures, and the resultant phase transformations studied in detail. The as-bonded microstructure of a Au-Al ball bond essentially consisted of a reaction zone (termed “alloyed zone” (AZ) in the as-bonded condition) between the Au bump and the bonded Al metallization. It is the growth of the reaction zone between the Au bump and the bonded Al metallization and also the nonbonded Al metallization during thermal exposure that gave rise to the various phase transformations. Au4Al, Au8Al3, and Au2Al are the predominant phases that grew across the ball bond until the bonded Al metallization is available to take part in the interdiffusion reactions. After the complete consumption of the bonded Al metallization, the Au-Al phases reverse transformed resulting in the formation of the Au4Al phase in the entire reaction zone across the ball bond (RZ-A). The lateral interdiffusion reactions resulted in the nucleation and the growth of all of the Au-Al phases given by the phase diagram. Kidson’s analysis and Tu et al.’s treatment were extended to a five-phase binary system to explain the phase transformations in thermally exposed Au-Al ball bonds. It is possible for all of the Au-Al phases to grow across a ball bond uninhibited as long as the bonded metallization is available. However, the supply limitation of the bonded metallization gives rise to reverse transformations where Al-rich phases transform to Au-rich phases and eventually result in the formation of the Au4Al phase in the entire RZ-A. If infinite time is allowed, Au4Al would dissolve; the extent of which is dependent on the solubility of Al in Au. No supply of Au lateral to the bond causes the reverse transformation of the Au4Al phase, giving rise to the lateral growth of the remaining Au-Al phases. If infinite time is allowed, the lateral phase transformations would result in the formation of a phase that is dependant on the relative proportion of Au and Al present in the nonbonded metallization (NBM) and Au4Al below the void line. Hence, the presence of a phase in a particular location of a ball bond is dependent on the time and temperature of thermal exposure.

Extensional Rheology and Stability of Gas Cell Walls in Bread Doughs at Elevated Temperatures in Relation to Breadmaking Performance

ABSTRACTThe rheological properties of gas cell walls in bread doughs are considered to be important in relation to their stability and gas retention during proof and baking. Large deformation rheological properties of gas cell walls were measured using biaxial extension for a number of doughs of varying breadmaking quality at constant strain rate and elevated temperatures of 25–60°C. Strain hardening and failure strain of cell walls both decreased with temperature, with cell walls in good breadmaking doughs remaining stable and retaining their strain hardening properties at higher temperatures (60°C), while the cell walls of poor breadmaking doughs became unstable at lower temperatures (45–50°C) and had lower strain hardening. Strain hardening measured at 50°C gave good correlations with baking volume, with the best correlations achieved between rheological measurements and baking tests that used similar mixing conditions. As predicted by the Considere failure criterion, a strain hardening value of 1 defines a region below which gas cell walls become unstable, and discriminates well between the baking quality of a range of commercial flour blends of varying quality. This indicates that the stability of gas cell walls during baking is strongly related to strain hardening properties, and that extensional rheological measurements can be used as indicators of baking quality.

Thermoplasticity of Saturated Clays: Experimental Constitutive Study

Experimental results obtained from thermomechanical tests on three clays are analyzed in light of the constitutive equations of soil thermoplasticity presented in a companion paper. Heating and cooling drained tests at constant isotropic stress show a strong dependence of the elastic domain on temperature. Thermal sensitivity of elastic domain was found to be different in overconsolidated and in normally consolidated clays. Thermoplastic strain hardening builds up to compensate for thermal softening in normally consolidated clays at plastic compression during drained heating, if the constant stress is imposed. Triaxial compression tests at constant elevated temperatures show an increase in ductility and a decrease in dilatativity at high temperatures. Undrained heating tests show a significant water pressure buildup. At constant principal stress difference, the water pressure growth leads to an effective stress drop and an eventual failure at the critical state line. A temperature‐rate‐dependent non‐assoc...

Strength and reliability of ceramic modules soldered to flexible cables

A study of the strength and reliability of soldered interconnections between module pins and flexible cables is presented. A quantitative measure of the strength of the connection is developed. Individual pins were mechanically tested by measuring the force required to push them through the cable. Both force and failure mode were recorded. The push-through test was used to determine optimum solder volume for maximum initial strength and also to measure degradation following exposure to environmental stresses. The first reliability stress was to store units at constant elevated temperatures of 80, 100, and 125 degrees C for up to 8500 h. Analysis of these results was done with a nonlinear regression model. Since multiple failure modes were present, the data was deconvolved by treating it as multiply censored and analyzing it using maximum-likelihood techniques. The temperature dependence of the degradation rate was assumed to be Arrhenius. For the dominant mode, the results of this model are shown. The results give an effective activation energy of 0.6 eV. The second reliability test was to thermally cycle units between -40 and +100 degrees C for up to 1000 cycles. There was no degradation. A simple first-order estimate indicates that this is equivalent to about five lifetimes. >

Behavior of Concrete as a Barrier Material for Nuclear Waste Disposal

SummaryThe foundation necessary for a constitutive model to simulate the response of concrete at elevated temperature has been presented. A model including temperature dependent creep, material property degradation, and cracking is needed to evaluate the long term functional requirements of concrete as a barrier material for nuclear waste disposal. Since the stress due to thermal load is proportional to the modulus, the degradation of the modulus with time even at constant elevated temperatures requires continual redistribution of load. Furthermore, since this degradation is not recoverable, the response of the material at elevated temperatures exhibits a complex dependence not only on the temperature distribution, but on the prior thermal history of the structure.This constitutive model for the response of concrete at elevated temperatures has been implemented into an implicit, finite element program called ANACAP [8]. Because of the direct coupling with temperature, both through thermal loads and material property dependency, ANACAP also contains a heat transfer module that includes thermal effects due to fluid flow and moving material boundaries. This program has been applied to problems involving underground waste storage tanks and grout vaults at the DOE Hanford site in Richland, Washington.

Observation of the structural transition Pt(100) 1 × 1 → hex by LEED intensities

The structure transition Pt(100) 1 × 1 → hex is observed by means of LEED intensities. The clean metastable 1 × 1 phase is prepared by intermediate NO adsorption. The transition to the hexagonal surface structure, which is an activated process, is initiated by raising the temperature. Integral intensities for both integer order spots as well as for superstructure spots are taken as function of time for constant elevated temperatures and as function of temperature for continuous temperature increase. The development of spot profiles is recorded as well. It turns out that the spot half width of integer order spots remains constant during the transition. Intermediate stages during the structure change show considerable background which is concentrated to streaks and indicates structural disorder. The superstructure spots grow up from the streaks giving first a 1 × 5 pattern from which the final structure results by spot splitting in the very last stage of the transition. Their intensity increase is delayed with respect to the decrease of integer order spots. They occur already with a comparably narrow half width, which remains constant in a first part of the transition. In the second part, especially during the final splitting, a further reduction is observed. The results are qualitatively discussed to give a geometrical model of the reconstruction based on the shift of atomic rows. Quantitative evaluation of the intensity-time dependences results in the activation energy of the transition, ΔE≈1.1 eV. From the temperature dependent measurements the surface Debye temperatures of the 1 × 1 and reconstructed phases are deduced to be roughly equal.

An experimental and numerical study on the effect of the wall in a thermocline-type cylindrical enclosure—II Numerical model

Natural convection in cylindrical enclosures finds many applications in solor energy problems. An integral method to solve these internal flow problems is presented. The cases of a thermocline-type enclosure and a cylindrical enclosure subject to constant elevated wall temperatures are solved as examples. Correlations of the Nusselt number for various Rayleigh numbers and wall materials are presented for the laminar regime. The mixing rate of the fluid is estimated for various boundary conditions. Results computed with this method are in good agreement with the finite difference solution and the experimental data presented in Part I.

Critical time of compression of metal rods under creep due to the joint effect of constant loads and elevated temperatures

1. Dependences (3.8) and (4.8) have been obtained for calculation of the critical time of centrally and eccentrically compressed metal rods which are deformed under creep conditions due to the combined action of constant loads and elevated temperatures. 2. The critical time of a compressed rod, evaluated by the Bubnov-Galerkin method, is 5–20% greater than the critical time obtained in solving the problem by the collocation method for the middle section over the length of the rod. This difference is explained by taking account of the change in stiffness over the length of the rod when applying the Bubnov-Galerkin method.

Equipment for Conditioning Materials at Constant Humidities and Elevated Temperatures

Equipment is described for the study of the mois ture relations of materials in air of controlled relative humidities at elevated temperatures. The air is brought to the desired relative humidity and temperature by an arrangement including a saturator and automatically controlled heaters and then passed through a working chamber in which the materials to be studied can be weighed rapidly and accurately. Provision is made for measuring the rate of flow of air and for accurately de termining the relative humidity in the working chamber. The equipment has been used at temperatures from 96° F. to 300° F., relative humidities from 10% to 90% for temperatures up to 212° F. and to the maximum rela tive humidities obtainable at atmospheric pressure for temperatures between 212° F. and 300° F. The tem perature in the working chamber can be maintained con stant within ± 0.4° F. and the relative humidity within ± 1% relative humidity for experimental runs of several days' duration. Data are given illustrating the performance of the equipment.