Transition Temperature Region Research Articles

Influence of Temperature on Charge Accumulation in Low-Density Polyethylene Based on Depolarization Current and Space Charge Decay.

Temperature is one of the key factors affecting space charge accumulation in high voltage direct current (HVDC) cable insulation material. The influence of temperature on charge accumulation in low density polyethylene (LDPE) has been investigated with a combined thermally stimulated depolarization current (TSDC) method and pulsed electro-acoustic (PEA) method. The experimental results indicate that there exists a transition temperature region of charge accumulation around 50 °C. The total accumulated charges all firstly increase and then decrease with the increasing polarization temperature under three typical polarization electric fields, and they have more accumulated charges in LDPE around 50 °C. The phenomenon has a close link with the dynamic processes of charge trapping and de-trapping, which were verified by TSDC results. At room temperature, the trapped charges are difficult to release from the traps, and these homocharges near the cathode can depress the further injection of the charges. More charges can be injected from the electrodes with the increase of temperature, while the charge migration is relatively lower before 50 °C, leading to more accumulated charges. When the temperature exceeds around 50 °C, the molecular movement is accelerated which can enhance the hopping probability of charges between the adjacent traps, resulting in few accumulated charges.

Experimental Characterization and Finite Element Prediction of Large Strain Spring-Back Behavior of Poly(Methyl Methacrylate)

Abstract Spring-back of poly(methyl methacrylate) (PMMA) at large strains, various embossing temperatures, and release temperatures below glass transition is quantified through modified unconfined recovery tests. Cooling, as well as large strains, is shown to reduce the amount of spring-back. Despite reducing the amount of spring-back, these experiments show that there is still a substantial amount present that needs to be accounted for in hot embossing processes. Spring-back is predicted using finite element simulations utilizing a constitutive model for the large strain stress relaxation behavior of PMMA. The model's temperature dependence is modified to account for cooling and focuses on the glass transition temperature region. Spring-back is predicted with this model, capturing the temperature and held strain dependence. Temperature assignment of the sample is found to have the largest effect on simulation accuracy. Interestingly, despite large thermal gradients in the PMMA, a uniform temperature approximation still yields reasonably accurate spring-back predictions. These experiments and simulations fill a substantial gap in knowledge of large strain recovery of PMMA under conditions normally found in hot embossing.

Failure of the ASTM E 1921 master curve to characterize the fracture toughness temperature dependence of ferritic steel and successful application of the stress distribution T-scaling method

The fracture toughness KJc of a material in the ductile-to-brittle transition temperature (DBTT) region shows a large temperature dependence, large scatter, and specimen thickness dependence. Thus, a large amount of data is necessary to satisfy the design requirements of equipment that experiences temperature changes. The ASTM E 1921 master curve (MC) method provides an engineering approach to solve these three issues and is intended to be applicable to an arbitrary ferritic material with a yield stress of 275–825 MPa. This study tested the CrMo steel JIS SCM440, which satisfies these conditions, but the ASTM E 1921 MC failed to characterize its fracture toughness temperature dependence. The material was tested at four temperatures: −55, 20, 60 and 100 °C. The obtained reference temperature T0 (i.e., the temperature at which a 25 mm thickness shows a fracture toughness of 100 MPa·m1/2) from these tests differed in the range of −9 to 89 °C. T0 from all of the test data was 16.6 °C. Thus, the MC using any of these T0 values failed to characterize the material’s fracture toughness temperature dependence. This material showed a T0 value higher than room temperature, which is a rare case for materials to which the MC has been successfully applied. The ASTM E 1921 MC is based on the Zerilli equation, which showed a poor fitting performance of the yield stress above room temperature for this material. When this yield stress temperature dependence was corrected (i.e., a temperature shift), the ASTM E 1921 MC better fitted the material’s fracture toughness test data. However, some discrepancy still remained. Because the Zerilli equation showed a small yield stress change and may differ from the experimental results above room temperature, the ASTM E 1921 MC may fail to characterize the fracture toughness temperature dependence close to or above room temperature. In contrast, the stress distribution T-scaling method was demonstrated to successfully predict the KJc temperature dependence of SCM440.

Instability of common beans during storage causes hardening: The role of glass transition phenomena

Long-term storage of common beans leads to loss of cooking quality and an ill-defined solution, appropriate storage, is recommended. Therefore, the polymer science theory of glasses that hypothesizes stability of a system below its glass transition temperature (Tg) was applied to determine bean stability during storage in relation to cooking behavior. Since composition influences Tg, powders of cotyledons and seed coats in addition to whole beans were equilibrated above different saturated salt solutions in order to generate materials with different moisture contents. A thermal mechanical compression test which measures compressibility changes in a system upon reaching its glass-rubber transition temperature region was conducted to obtain the Tg. A Tg-moisture relation was established, whose relevance was confirmed by storage and cooking experiments which showed development of hard-to-cook in beans stored above Tg but not below it. Therefore, this relation constitutes a stability map for storage of common beans.

Nature of Microcracks in Ferritic Steels Occurred during Fracture under Conditions of Ductile-Brittle Transition Temperature Region

It has been shown by means of EBSD techique that fracture of ferritic steel in ductile-brittle transition temperature region, along with the formation of previously discribed cleavage microcracks, results in the formation of ductile microcracks. It has also been shown that microstructure of plastic zones under brittle and ductile fracture components produced by the main crack propagation differ significantly. Better developed plastic zone under ductile fracture component protects steel from overstress. The plastic zone under brittle fracture surface, apparently, has a reduced local plasticity. Consequently, the cleavage microcracks formation precedes the fracture process. During the main crack formation such microcracks occur in steel microvolumes located both in front of its tip and in adjacent to its edges microvolumes. Further propagation of the main crack is realized in steel which already contains scattered cavities and reduces to ductile fracture of the connections between them.

Correlation of local parameters of brittle crack initiation and propagation with the fracture toughness of VVER-1000 pressure vessel steels

Base metal of the VVER-1000 reactor pressure vessel steel in the unirradiated state was studied. The purpose of the work was to find a correlation between local fracture microparameters and macroparameters of samples under quasi-static loads. Fracture toughness tests of steel were performed in the transition temperature region. Structural parameters and brittle fracture origin types were defined by means of fractographic analysis of the tested samples. The stress-strain state of the studied samples at brittle fracture initiation moment was calculated using the complex experimental-calculative method.

Heat capacities of potential organic phase change materials

Abstract The heat capacity of a material is an important factor for many applications, including phase change material (PCM) selection for thermal energy storage. Aside from storing energy via the latent heat of a phase transition, significant sensible heat can be stored using the material’s heat capacity outside the transition temperature region. Furthermore, accurate heat capacity values are required to model and thereby optimize PCM heat storage systems. Various group additivity models can be used to estimate the heat capacity of organic liquids. However, these methods do not distinguish isomers built of the same base units. In the present study the liquid phase heat capacities of five isomeric, 12-carbon, linear, saturated fatty esters (potential PCMs) have been measured experimentally, and the results are compared to calculations from several group additivity methods. It was found that the heat capacities of the liquid esters are well described by the group additivity models, and that each of the esters had approximately the same heat capacity at a given temperature, despite their structural differences. The heat capacity of another isomer, dodecanoic acid, was also compared to the ester isomers, and found to be significantly different. The group additivity methods also were able to accurately represent the heat capacity of the acid isomer.

Fracture Toughness in the Transition Temperature Region

The steels fracture toughness was measured at two different temperature T= - 60°C and T = -90°C, at v=0.02 mm/min and v=0.5mm/min. The following steels: CrMoV, 20MnMoNi55, A533B and A508 were tested with three different sizes of CT specimens 50 and 100 and 200. Those steels are weldable, although the authors investigated the fracture behaviour of base material. In order to satisfy statistical analyses, a large number of specimens were tested. Fracture behaviour has turned out to be typical, S-shaped curve for transitional fracture at low temperatures. Apart from the other variables, specimen’s width significantly affects measured toughness. Smaller specimens, CT50, might be considered of upper bound reliability while the CT200 specimens were shown to be the most conservative. In this way it has been shown that wider specimens are more reliable in a fracture assessment of the examined steels. In this paper, the fracture probabilities of specimens in function of fracture toughness were determined as well, and it could be concluded that the widest specimens are the most likely to be broken for the same values of the fracture toughness.

Crack initiation, arrest and tearing assessments of a RPV subjected to PTS events

Although the resistance of reactor pressure vessel (RPV) against fast fracture has to be proven by comprehensive analyses, there are few published literatures systematically discussing the crack initiation, arrest and tearing instability assessment in the pressurized thermal shock (PTS) events. In this paper, three types of assessment are discussed and studied. For the rapid cool-down of the typical PTS transients, the temperature of the deepest point along the crack tip is often high enough to ensure that both crack arrest assessment and crack tearing instability assessment can be used to prevent the RPV failure after the crack initiation occurs. When the assessment is changed from the crack initiation assessment to the crack arrest assessment, the transition temperature region of the material toughness curve moves towards to the right which leads to an increase of the area in which the brittle cleavage fracture may occur. It is not possible to determine the most dangerous moment before the detailed analysis of fracture mechanism. So the method of assessment the most dangerous moment which is given in the initial input data may be non-conservative. As the conservative crack shape parameter and method to consider the plastic correction are used in the French RCC-M code, the size of critical crack is very small in the case study. While according to the new study results, the tearing process of the RPV is still stable even for a big initial crack which is larger than the maximum hypothetical crack size in the code.

Application of T-scaling method to account for the continuity of pre-cracked and notched specimen fracture toughness in the ductile-to-brittle transition temperature region

Application of T-scaling method to account for the continuity of pre-cracked and notched specimen fracture toughness in the ductile-to-brittle transition temperature region

Molecular dynamics simulation of melting of 2D glassy monatomic system

The melting of two-dimensional (2D) glassy monatomic systems is studied using the molecular dynamics simulation with Lennard–Jones–Gauss interaction potential. The temperature dependence of various structural and dynamical properties of the systems during heating is analyzed and discussed via the radial distribution functions, the coordination number distributions, the ring statistics, the mobility of atoms and their clustering. Atomic mechanism of melting is also analyzed via tendency to increase mobility and breaking clusters of atoms upon heating. We found that melting of a 2D glass does not follow any theory of the melting of 2D crystals proposed in the past. The melting exhibits a homogeneous nature, i.e. liquid-like atoms occur homogeneously throughout the system and melting proceeds further leading to the formation of an entire liquid phase. In addition, we found a defined transition temperature region in which structural and dynamical properties of systems strongly change with increasing temperature.

Fracture Toughness Prediction under Compressive Residual Stress by Using a Stress-Distribution T-Scaling Method

The improvement in the fracture toughness Jc of a material in the ductile-to-brittle transition temperature region due to compressive residual stress (CRS) was considered in this study. A straightforward fracture prediction was performed for a specimen with mechanical CRS by using the T-scaling method, which was originally proposed to scale the fracture stress distributions between different temperatures. The method was validated for a 780-MPa-class high-strength steel and 0.45% carbon steel. The results showed that the scaled stress distributions at fracture loads without and with CRS are the same, and that Jc improvement was caused by the loss in the one-to-one correspondence between J and the crack-tip stress distribution. The proposed method is advantageous in possibly predicting fracture loads for specimens with CRS by using only the stress–strain relationship, and by performing elastic-plastic finite element analysis, i.e., without performing fracture toughness testing on specimens without CRS.

Precise estimation of glass transition and crystallization temperatures of Zr55Cu30Ni5Al10 metallic glass using step-scan modulated temperature differential scanning calorimeter

The glass transition temperature (Tgonset) and super-cooled liquid region (ΔTx) of Zr55Cu30Ni5Al10 metallic glass ribbon and bulk glassy plate have been measured using conventional and step-scan modulated temperature differential scanning calorimeter (MTDSC). A wide variation of Tgonset(±14.8 K) and ΔTx (±18.5 K) have been estimated using conventional DSC at heating rates of 5–400 K/min in the present study as well the data reported in the literature. We report that Tgonset can be precisely (±1.3 K) measured using step-scan MTDSC from the reversible heat flow curve, which excludes the structural relaxation near to glass transition.

Preparation of phase change material emulsions with good stability and little supercooling by using a mixed polymeric emulsifier for thermal energy storage

A mixed polymeric emulsifier consisting of polyvinyl alcohol (PVA) and polyethylene glycol-600 (PEG-600) was explored for preparing high-performance phase change material emulsions (PCMEs) containing the paraffin with a melting point of 62–64°C. After the effects of the mass ratio of PVA to PEG-600, the mass ratio of the mixed emulsifier to the paraffin and the homogenization rate on the size distribution, viscosity and dispersion stability of the PCMEs containing 20wt% paraffin were systematically investigated, the optimal emulsifying process parameters have been determined to be the mass ratio of PVA to PEG-600 of 50:50, the mass ratio of the mixed emulsifier to paraffin of 1:5 and the homogenization rate of 10,000rpm. Accordingly, the PCMEs with the paraffin mass fractions varying from 10wt% to 30wt% were prepared. It is found that the paraffin has been well dispersed in water in the form of sphere-like droplets with average diameters ranging from 3µm to 11µm in the obtained PCMEs. Significantly, the PCMEs with different mass fractions of the paraffin exhibit no supercooling, owing to the function of the mixed polymeric emulsifier as a nucleating agent. The apparent specific heat of PCMEs are 1.51–2.18 times as high as that of water at the phase transition temperature region, due to the existent of the paraffin in them. Their apparent thermal conductivity gradually decreases with an increase in the mass fraction of the paraffin, while the viscosity increases significantly with mass fraction of the paraffin at the same temperatures. Furthermore, the pumping power consumption of the PCMEs shows a drastic reduction as compared to that of water at the same heat storage capacity. It is revealed that the PCMEs show great potential for use as a novel heat transfer fluid (HTF) in thermal energy storage (TES) systems.

Thermal and electrical behaviors of acid functionalized MWCNT/ABC-type triblock copolymer grafted-MWCNT nanocomposites

ABSTRACTIn this work, ABC-type triblock copolymer grafted onto the surface of the MWCNT/acid functionalized MWCNT (MWCNT-COOH) composites were prepared and the properties of nanocomposites were characterized extensively using differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), thermogravimetric analysis (TGA), ac electrical conductivity and dielectrical measurements.DSC study showed that the glass transition temperatures of the nanocomposites are a some higher than that of the matrix polymer. The increase in oxidized MWCNT in the nanocomposite improved the thermal stability of the composite, according to initial decomposition temperatures. The ac electrical conductivity has increased moderately with increasing frequency, but has increased slowly with increase in the oxidized MWCNT content in the nanocomposites. The electrical conductivity increases slowly with increasing temperature to about the glass transition temperature, then it increases faster. The dielectric constants for the matrix polymer and all the composites decreases slightly with increasing frequency from 0.1 kHz to 2.0 kHz. The dielectric constant increases slightly with increasing temperature up to about the glass transition temperature region and then the increase in temperature is accelerated the increase in the dielectric constant.

Bioinspired Design of a Robust Elastomer with Adaptive Recovery via Triazolinedione Click Chemistry.

It is a significant but challenging task to simultaneously reinforce and functionalize diene rubbers. Inspired by "sacrificial bonds", the authors engineer sacrificial hydrogen bonds formed by pendent urazole groups in crosslinked solution-polymerized styrene butadiene rubber (SSBR) via triazolinedione click chemistry. This post-crosslinking modification reveals the effects of the sacrificial bonds based on a consistent covalent network. The "cage effect" of the pre-crosslinked network facilitates the heterogeneous distribution of urazole groups, leading to the formation of hydrogen-bonded multiplets. These multiplets further aggregate into clusters with vicinal trapped polymer segments that form microphase separation from the SSBR matrix with a low content of urazole groups. The clusters based on hydrogen bonds, serving as sacrificial bonds, promote energy dissipation, significantly improving the mechanical properties of the modified SSBR, and enable an additional wide transition temperature region above room temperature, which endows the modified SSBR with promising triple-shape memory behavior.

Fracture toughness of hydrogen charged as-quenched ultra-high-strength steels at low temperatures

The effect of hydrogen on the fracture and impact toughness of ultra-high-strength steels at sub-zero temperatures in the transition temperature region has been investigated with arctic applications in mind. Two types of as-quenched microstructure were studied, i.e. autotempered martensite and a mixture of martensite and bainite, both having yield strengths close to 1000MPa. These were charged with hydrogen using passive cathodic protection and then tested in both the charged and uncharged condition at sub-zero temperatures. Hydrogen contents were measured with melt-extraction. Fractography, kernel average misorientation measurements and cohesive zone modelling were used to analyse the results considering the degree and the active mechanisms of hydrogen embrittlement. It is shown that hydrogen embrittlement is present at sub-zero temperatures, causing an increase in fracture toughness reference temperature T0 and a small decrease in deformation capability. The relationship between the T0 and the impact toughness transition temperature T28J, which, in the case of ultra-high-strength steel, deviates from that observed for lower strength steels, is proposed to be affected by the hydrogen content.

応力分布Tスケーリング法適用による圧縮残留応力付与時破壊靭性値の予測

The fracture toughness Jc of the material in the ductile to brittle transition temperature (DBTT) region shows test specimen thickness (TST) effect and temperature dependence, and apparently increases when compressive residual stress is applied. In this paper, as TST effect and temperature dependence, the fact that Jc apparently increases due to compressive residual stress is attributable to the loss in the one-to-one correspondence between J and the crack-tip stress distribution, and then, the fracture prediction of the specimen with compressive residual stress was performed using the T-scaling method proposed by the authors, and its validity was confirmed for high strength steel of 780 MPa class and 0.45 % carbon steel JIS S45C. In addition, it was suggested that the minimum of Jc with compressive residual stress can be predicted by requiring only the tensile test.

Electrical Switching in Semiconductor-Metal Self-Assembled VO2 Disordered Metamaterial Coatings.

As a strongly correlated metal oxide, VO2 inspires several highly technological applications. The challenging reliable wafer-scale synthesis of high quality polycrystalline VO2 coatings is demonstrated on 4” Si taking advantage of the oxidative sintering of chemically vapor deposited VO2 films. This approach results in films with a semiconductor-metal transition (SMT) quality approaching that of the epitaxial counterpart. SMT occurs with an abrupt electrical resistivity change exceeding three orders of magnitude with a narrow hysteresis width. Spatially resolved infrared and Raman analyses evidence the self-assembly of VO2 disordered metamaterial, compresing monoclinic (M1 and M2) and rutile (R) domains, at the transition temperature region. The M2 mediation of the M1-R transition is spatially confined and related to the localized strain-stabilization of the M2 phase. The presence of the M2 phase is supposed to play a role as a minor semiconducting phase far above the SMT temperature. In terms of application, we show that the VO2 disordered self-assembly of M and R phases is highly stable and can be thermally triggered with high precision using short heating or cooling pulses with adjusted strengths. Such a control enables an accurate and tunable thermal control of the electrical switching.

Investigations on the Fracture Toughness of Electron Beam Welded Steels

Beam welded joints made from fine grain steels show martensitic micro structure, high weld hardness and thus low toughness in the weld seams. At the same time there is a risk for these welds to have crack-like defects that cannot be detected during the production and which grow due to cyclic loading. If such structures are used in areas with low ambient temperatures, it may come to brittle failure of the component. To secure components against such failure, fracture mechanic tests were carried out on electron beam welded SE(B)-samples made from S690QL and S960QL and the J-integral was determined. In order to describe the scattering of the results in the temperature transition region the results were evaluated by means of the Master Curve concept in accordance with ASTM E 1921. Afterwards the reference temperature in the transition range, T0, was determined and correlated with the T27J- temperature of Charpy V-notch tests.