Reduction Of Thermal Stress Research Articles

Analysis of Cracking Risk in Early Age Mass Concrete with Different Aggregate Types

The main cause of early age cracks often observed in massive concrete structures is inhomogeneous volume changes associated with thermal and moisture gradients occurring in these structures due to the hydration process. Significant impact on a temperature rise which is the main reason of significant changes in concrete volume, and as a consequence, on the cracking risk in the massive foundation slab has the type of aggregate, which forms the thermal properties of concrete. In the paper, thermal properties of concretes made with different aggregate types and their impact on the cracking risk in a massive concrete slab are discussed. Thermal conductivity and heat capacity of concrete with different aggregate types have been experimentally measured and next used in the numerical study. The influence of gravel (consisting mainly of quartz), basalt, granite and limestone aggregate on the temperature development, stress level and the cracking risk has been studied in the numerical tests. The presented numerical analyses are conducted for a massive foundation slab with use of the original numerical model and computer programs TEMWIL and MAFEM. It has been shown that the use of the aggregate with the appropriate thermal properties may result in the reduction of thermal stresses and the cracking risk in early age concrete slabs.

Evaluation of functional reliability indices for DC-link capacitors in pulse-width modulation converters

The impact of kHz range harmonics on the power losses, thermal stress, and lifetime reduction of the dc-link capacitors in pulse-width modulation (PWM) converters was investigated. Expressions to evaluate the mean time to failure, survival probability, and unavailability of aging failure for dc-link capacitors are presented. The dc-link capacitors failures due to accelerated insulation aging were modeled using Weibull and normal distribution. A case study with the Siemens SINAMICS S120 frequency converter for driving of rolling mill leveler shows that the failures of motor modules due to breakdown of electrolytic dc-link capacitors registered for the time frame from May 2012 to October 2012 can be caused by the increased ambient temperature and additional heating due to high-frequency components of the ripple current. As a possible solution to improve reliability of motor modules, the four AVX FFVE4I0227K film capacitors instead of nine EPCOS B43564 electrolytic capacitors in dc-link were recommended.

IGBT Thermal Stress Reduction Using Advance Control Strategy

Next-generation advances in stress control strategy will enable renewable energies, such as solar energy, to become more reliable and available. Critical components, such as power electronics, present uncertainties to the system control in malfunctioning process, which reduces the target of more clean energy development and CO2 emission reduction. Thus, developing and harnessing sustainable energy requires mitigating the impact of the variability of the source of energy and the impact of the adaptive stress control deployed for the proportional, integral, derivative (PID) controller to minimize the thermal stress in the power switch insulated gate bipolar transistor (IGBT). In response to this challenge, a fuzzy linear matrix inequality (FLMI) PID controller proposes initiatives for customizing parameters of PID controller corresponding to the uncertainty of IGBTs. In this paper, the uncertainty of the boost converter has been evaluated in the dynamic of the LMI model and Takagi-Sugino (TS) has applied in closed loop control to overcome the instability of the Boost converter parameters.

Thermal stress measurement in steel plate strengthened by CFRP and aluminum alloy plates

In steel members strengthened by carbon-fiber reinforced polymer (CFRP) plates, the thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. As so far, the authors proposed a technique to reduce the thermal stress in steel members strengthened by CFRP plates, which involves bonding aluminum alloy plates with CFRP plates. In the proposed method, the thermal stress in steel member can be reduced so that there are negligible levels of stress in steel member when the cross sectional areas of CFRP and aluminum plates are designed to correspond to the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. In this study, to confirm the maintaining the thermal stress reduction in steel member by proposed method, thermal stress measurement in steel plate strengthened by CFRP and aluminum plates was carried out about 21 months. In this research, the thermal stress introduced in the steel plate strengthened by CFRP plates was also measured. Furthermore, to assume the thermal shear and normal (peel) stresses in adhesive layers, FE analysis with plane stress element was employed. As the result, it was shown the thermal stresses in steel plate with CFRP plate were able to calculate by using composite theory and measured temperature. Furthermore, in steel plate strengthened by CFRP and aluminum plates, the thermal stress introduced in steel plate was negligible-small through the all-season. It was found the thermal stresses in steel plate with CFRP plates as well as CFRP and aluminum plates were also estimated by using composite theory and measured temperature. In the steel plate strengthened by CFRP and aluminum plates, the thermal shear and normal stresses in adhesive layer glued to steel plate become smaller than that in the conventional CFRP bonded specimen. However, the shear stress in adhesive layers between CFRP and aluminum plates in proposed method was higher than the thermal stress in adhesive layers between CFRP plates in conventional method.

Influence of interlayer design on residual thermal stresses in trilayered and graded all-ceramic restorations

Residual thermal stresses are formed in dental restorations during cooling from high temperature processing. The aim of this study was to evaluate the influence of constructive design variables (composition and interlayer thickness) on residual stresses in alumina- and zirconia-graded restorations. Restorations' real-like cooling conditions were simulated using finite elements method and temperature-dependent material properties were used. Three different designs were evaluated: a bilayered restoration (sharp transition between materials); a trilayered restoration with a homogenous interlayer between core and veneer; and a trilayered restoration with a graded interlayer. The interlayer thickness and composition were varied. Zirconia restorations presented overall higher thermal stress values than alumina ones. Thermal stresses were significantly reduced by the presence of a homogeneous interlayer. The composition of the interlayer showed great influence on the thermal stresses, with the best results for homogeneous interlayers being observed for porcelain contents in the composite ranging between 30%–50% (vol.%), for both alumina and zirconia restorations. The interlayer's thickness showed a minor contribution in the thermal stress reduction. The graded interlayer showed an optimized reduction in restorations' thermal stresses. The use of graded interlayer, favoring enhanced thermal stress distributions and lower magnitude is expected to reduce the risk of catastrophic failure.

PCM for improving polyurethane-based cool roof membranes durability

Thermal Energy Storage systems are acknowledged solutions frequently used for reducing indoor thermal fluctuations by behaving as thermal buffer. In this view, phase change materials (PCMs) are included into building envelope components. At the same time, cool roofs represent passive cooling techniques aimed at maximizing the solar radiation reflected by the roof without contributing to the indoor overheating, in hot summer conditions, in particular. In this panorama, the authors developed a new composite material made by a polyurethane liquid waterproof cool membrane with non-capsulated PCMs acting as shape stabilized thermal buffer additive. In this work, the behaviour of such composite material when exposed to accelerated weathering long-term tests (QUV test) has been studied. Temperature, humidity and ultraviolet radiation are varied in order to test the membrane vulnerability to realistic environmental conditions. The purpose of such experimental analysis was to clarify if the PCM inclusion could help the membrane to better behave during the course of the time, since PCMs are supposed to be responsible for thermal stress reduction. Optic-energy, morphology, thermal and mechanical tests are carried out in order to analyse the membrane degradation imputable to the weathering stress. The inclusion of PCMs in different concentrations allowed identifying the best performing PCM-doping inclusion, i.e. 25wt%. Such composition, in fact, was able to register a better spectral reflectance reflectance in the near infrared region of the solar spectrum up to 10% with the fewest modification during the course of the QUV test, i.e. 7% of weight loss after 60 days of exposure, and to preserve the required flexibility of the membrane together with its superficial finishing characteristics.

The role of the stress regime on microseismicity induced by overpressure and cooling in geologic carbon storage

AbstractFluid injection in deep geological formations usually induces microseismicity. In particular, industrial‐scale injection of CO2 may induce a large number of microseismic events. Since CO2 is likely to reach the storage formation at a lower temperature than that corresponding to the geothermal gradient, both overpressure and cooling decrease the effective stresses and may induce microseismicity. Here, we investigate the effect of the stress regime on the effective stress evolution and fracture stability when injecting cold CO2 through a horizontal well in a deep saline formation. Simulation results show that when only overpressure occurs, the vertical total stress remains practically constant, but the horizontal total stresses increase proportionally to overpressure. These hydro‐mechanical stress changes result in a slight improvement in fracture stability in normal faulting stress regimes because the decrease in deviatoric stress offsets the decrease in effective stresses produced by overpressure. However, fracture stability significantly decreases in reverse faulting stress regimes because the size of the Mohr circle increases in addition to being displaced towards failure conditions. Fracture stability also decreases in strike slip stress regimes because the Mohr circle maintains its size and is shifted towards the yield surface a magnitude equal to overpressure minus the increase in the horizontal total stresses. Additionally, cooling induces a thermal stress reduction in all directions, but larger in the out‐of‐plane direction. This stress anisotropy causes, apart from a displacement of the Mohr circle towards the yield surface, an increase in the size of the Mohr circle. These two effects decrease fracture stability, resulting in the strike slip being the least stable stress regime when cooling occurs, followed by the reverse faulting and the normal faulting stress regimes. Thus, characterizing the stress state is crucial to determine the maximum sustainable injection pressure and maximum temperature drop to safely inject CO2.

Reduction of Thermal Stress during Soec Unsteady Operation

In recent years, natural energy sources such as solar and wind power have been introduced into the electric grid. However, the wind and solar power suffer instability with respect to changes in the weather. As the proportion of these energies increases, a technique to store a surplus electric power is required. The hydrogen generation systems using solid oxide electrolysis cells (SOECs) are expected as large-capacity power storage facilities with the ability to stabilize the electrical power supply. For the use of the SOEC, it is important to grasp the unsteady characteristics, especially temperature distribution which is the one of the biggest destruction factor of the cell. In this study, the unsteady, two dimensional, tubular numerical model of a cathode supported micro-tubular SOEC was developed to clarify the temperature distributions and its responses .The temperature of anode surface were measured directly by K-type thermocouples during the simultaneous electrochemical performance measurements. The operating condition of SOEC was controlled to reduce the unsteady temperature distribution in SOEC. Rough description of the numerical model is shown in Fig.1. The model calculates heat-mass-charge transports and electrochemical reactions, simultaneously. The construction materials of SOEC cathode electrode was mixed materials of nickel (Ni) and yttria-stabilized zirconia (YSZ). The electrolyte was YSZ, and the anode electrode was mixture of lantern strontium manganite (LSM) and YSZ. The thicknesses of cathode electrode, electrolyte and anode electrode were 200 mm, 20 mm and 20 mm, respectively. The cell length was about 50 mm, with an outer diameter of about 2 mm. The flow rate of hydrogen and steam was set to be same. The operating temperature was 1123K. Experiments of SOEC were conducted in same conditions. The calculated temperature distributions at steady states are shown in Fig.2. The temperature decreased at 1.1 V, 1.2 V and increased at 1.3 V, 1.4 V. The maximum temperature gradient was observed around z=0.042 at 1.4 V. This position was consistent with the point where a clack was observed during a measurement. The maximum temperature gradients change during potential sweep with various intermediate voltage are shown in Fig.3. This shows that the maximum temperature gradient could be reduced by controlling the sweeping conditions. Figure 1

Fabrication and stress analysis of annular-trench-isolated TSV

The large mismatches among the coefficients of thermal expansion (CTE) of the metal via, insulator liner, and Si substrate of the through-silicon via (TSV) induce thermal stresses within and around the TSV during thermal-cycled fabrication processes. Reduction of thermal stress in the Si substrate is important for minimizing the deviations in the device characteristics. An annular-trench-isolated (ATI) structure was proposed for the TSV to solve the thermal issues, which occur during the three-dimensional (3D) integrated circuit (IC) integration, by stress redistribution. The concept of ATI TSV is based on retaining a Si-ring between the metal core and insulator layer during the fabrication process. We realized the ATI TSV using a via-last fabrication approach, with two deep silicon etching processes (Bosch processes) for the insulator layer and the metal core. Parylene-HT was utilized as the insulator to achieve high uniformity. With a vacuum-assisted filling system, the vias were filled with a solder material. ATI TSVs with diameters of 10μm and 2-μm-thick Parylene-HT insulation layers were demonstrated. Studies on the thermal stress levels of the ATI TSV were carried out by finite-element method (FEM) simulation, along with comparisons with regular and annular TSVs. We revealed that the ATI TSV shows lower thermal stresses in the Si substrate than the regular and annular TSVs. The ATI TSV is a possible candidate for 3D IC integration with stress-sensitive devices.

Impacts of Thermally Induced Stresses on Fracture Stability During Geological Storage of CO2

Geomechanical stability issues may arise due to induced thermal stresses because CO2 will generally reach the storage formation at a temperature lower than that of the reservoir. Cold injection will form a cold region around the injection well, which will induce thermal stress reduction. We simulate cold CO2 injection in deep saline formations in a normal faulting stress regime and investigate under what conditions thermal stresses may jeopardize the caprock sealing capacity by studying the effect of the heterogeneity of the thermal expansion coefficient between the reservoir and caprock. Furthermore, we use an elastoplastic constitutive model to account for inelastic deformation related to fracture instability. We find that the temperature difference should be limited in the presence of very stiff reservoirs, because the thermal stress reduction is proportional to the product of the rock stiffness, the temperature difference and the thermal expansion coefficient. Simulation results show that inelastic strain occurs in the cooled region within the reservoir, but fracture instability does not propagate into the caprock in the considered normal faulting stress regime. However, the cooled region of the lower portion of the caprock may experience yielding if the thermal expansion coefficient of the caprock is larger than that of the reservoir, because the thermal stress reduction in the caprock becomes larger than in the reservoir, which increases the deviatoric stress. Nevertheless, irreversible strain caused by cooling within the caprock are limited to a small region of the lower portion of the caprock and thus, the overall sealing capacity of the caprock is not compromised, so CO2 leakage is unlikely to occur because of cooling.

Residual Stresses in Intrinsic UD-CFRP-Steel-Laminates - Experimental Determination, Identification of Sources, Effects and Modification Approaches

This paper focuses on the reduction of process-related thermal residual stress in fiber metal laminates and its impact on the mechanical properties. Different modifications during fabrication of co-cure bonded steel/carbon epoxy composite hybrid structures were investigated. Specific examinations are conducted on UD-CFRP-Steel specimens, modifying temperature, pressure or using a thermal expansion clamp during manufacturing. The impact of these parameters is then measured on the deflection of asymmetrical specimens or due yield-strength measurements of symmetrical specimens. The tensile strength is recorded to investigate the effect of thermal residual stress on the mechanical properties. Impact tests are performed to determine the influence on resulting damage areas at specific impact energies. The experiments revealed that the investigated modifications during processing of UD-CFRP-Steel specimens can significantly lower the thermal residual stress and thereby improve the tensile strength.

Strain-related phenomena in TiO2 nanostructures spin-coated on porous silicon substrate

In this study, anatase TiO2 nanostructures were synthesized on silicon and porous silicon (PS) substrates by sol–gel spin coating. The PS template was formed by electrochemical anodization on p-type silicon wafer. The field emission electron microscopy (FESEM) images showed a uniform morphology with average diameter of 15nm and 20nm for PS and TiO2 nanostructures, respectively. The X-ray diffraction studies of residual stress and detailed discussion about intrinsic and extrinsic stresses as its origins are presented. Measurements reveal the presence of compressive strain in the TiO2 layer with negative dilatation about 1.83%. A reduction of thermal stress of TiO2 layer deposited on PS substrate compared to silicon substrate was shown. We also used Williamson–Hall (W–H) method to study the individual contributions of crystallite sizes and microstrain on the peak broadening of the TiO2 nanostructures. Our obtained results showed that the mean crystallite size of the TiO2 nanostructures estimated from the FESEM and W–H analysis were highly in agreement.

Potential fracture propagation into the caprock induced by cold [formula omitted] injection in normal faulting stress regimes

Thermal effects are an important component in the analysis of geologic carbon storage because the injected CO2 reaches the storage formation at a lower temperature than that of the reservoir rock. The main fear is related to the possibility that the shear slip that may occur within the reservoir due to cooling could propagate into the caprock, which could result in CO2 leakage. We model a baserock–reservoir–caprock system in a normal faulting stress regime using an axisymmetric model in which we inject cold CO2 and use an elasto-plastic constitutive model to simulate inelastic deformation. CO2 forms a cold region around the injection well of a significantly smaller extension than the CO2 plume. Within this cold region, inelastic strain is yielded due to the thermal stress reduction caused by the thermal contraction of the rock. This inelastic strain occurs only within the reservoir and does not propagate into the caprock. Actually, the stability of the lower portion of the caprock improves in the cooled region as a result of a stress redistribution that occurs to satisfy stress equilibrium and displacement compatibility caused by the thermal stress reduction of the reservoir. This stress redistribution tightens the caprock and prevents CO2 leakage. Thus, thermally induced stresses are not likely to generate fracture propagation into the caprock in normal faulting stress regimes.

Configuration Development of Autothermal Solid Oxide Fuel Cell: A Review

Solid Oxide Fuel Cell (SOFC) is typically operated at high temperature. Both electricity and heat are generated during operation. Therefore, SOFC can be efficiently designed to integrate the endothermic reformer with the cell for optimizing heated utilization, as called autothermal operation. However, the mismatch between rate of exothermic electrochemical reaction and endothermic reforming reaction is easily resulted in material cracking of those integrated structure. In order to overcome the thermal mismatch limitation, various approaches for autothermal SOFC configurations have been widely developed; these configurations can be classified into 2 main groups including direct internal reforming (as called DIR-SOFC) and indirect internal reforming (as called IIR-SOFC) operations. This review focuses on the technological progress of these various configurations. In detail, computer simulation technique has been applied to study the thermal behavior inside the autothermal SOFC configurations using various primary hydrocarbon fuels. In addition, the advantage and disadvantage on thermal stress reduction of each configuration are also discussed.

Flexible one diode-one phase change memory array enabled by block copolymer self-assembly.

Flexible memory is the fundamental component for data processing, storage, and radio frequency communication in flexible electronic systems. Among several emerging memory technologies, phase-change random-access memory (PRAM) is one of the strongest candidate for next-generation nonvolatile memories due to its remarkable merits of large cycling endurance, high speed, and excellent scalability. Although there are a few approaches for flexible phase-change memory (PCM), high reset current is the biggest obstacle for the practical operation of flexible PCM devices. In this paper, we report a flexible PCM realized by incorporating nanoinsulators derived from a Si-containing block copolymer (BCP) to significantly lower the operating current of the flexible memory formed on plastic substrate. The reduction of thermal stress by BCP nanostructures enables the reliable operation of flexible PCM devices integrated with ultrathin flexible diodes during more than 100 switching cycles and 1000 bending cycles.

The role of carbon nanofibers on thermo-mechanical properties of polymer matrix composites and their effect on reduction of residual stresses

In this research, the effects of carbon nanofibers (CNFs) on thermo-elastic properties of carbon fiber (CF)/epoxy composite for the reduction of thermal residual stresses (TRS) using micromechanical relations were studied. In the first step, micromechanical models to calculate the coefficient of thermal expansion (CTE) and Young's modulus of CNF/epoxy and CNF/CF/epoxy nanocomposites were developed and compared with experimental results of the other researchers. The obtained results of the CTE and Young's modulus of modified Schapery and Halpin-Tsai theories have good agreement with the experimental results. In the second step, the classical lamination theory (CLT) was employed to determine the TRS for CNF/CF/epoxy laminated nanocomposites. Also, the theoretical results of the CLT were compared with experimental results. Finally, reduction of the TRS using the CLT for different lay-ups such as cross ply, angle ply, and quasi-isotropic laminates were obtained. The results demonstrated that the addition of 1% weight fraction of CNF can reduce the TRS that the most reduction occurred in the unsymmetric cross-ply laminate by up to 27%.

異種材料熱溶着接合部の熱応力低減と強度の測定

異種材料間の接合部では，被着体間の熱膨張率差に起因する熱応力や熱変形が発生し，その強度に影響を与える。したがって，接合強度の測定に使用される試験片自体に熱応力が存在する場合は，その影響を適切に評価し，真の接合強度を求める必要がある。本研究では接合部の強度試験として，限界エネルギー解放率の測定を目的とした双片持ちはり（DCB）試験を取り上げた。熱応力を有するDCB試験片では，その影響によりはく離しやすい状態となる。そこで，予変形を試験片に与え熱応力の影響を低減した試験片を作製することにより，接合強度を正確に評価する新たな手法を提案する。具体的には，被着体に曲率一定の予変形を与えつつ接合することにより，DCB試験片の熱変形および熱応力を低減した。また，本手法により作成した試験片を用いて，熱応力の影響を排除した接合部固有の限界エネルギー解放率を測定した。

Research on reduction of thermal stress induced in steel member by bonding carbon fiber

Research on reduction of thermal stress induced in steel member by bonding carbon fiber

Reduction of Thermal Stress Induced in a Steel Plate Strengthened by Bonded Aluminum-CFRP Composite

In steel members strengthened with carbon-fiber reinforced polymer (CFRP) plate, thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. Therefore, the thermal stress caused by temperature change has to be considered when designing the repair or strengthening of a steel member with CFRP plates. With this in mind, the authors proposed a technique to reduce thermal stress in steel members strengthened by CFRP plate on both side, which involves bonding aluminum alloy plates with CFRP plates. In this proposed method, the thermal stresses in steel member can be reduced so that there are negligible levels of stress when the cross sectional areas of CFRP and aluminum plates are designed to correspond the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. So far, a thermal bending moment was not considered in the proposed method, because the steel members strengthened by CFRP-aluminum laminated plate on top and bottom sides were assumed. However, if the proposed method is applied on one side of steel member, the thermal stress in the steel member might not be reduced completely by generated thermal bending moment. Therefore, to confirm the effectiveness of the proposed method for one-sided bonding, heat tests were conducted on a steel plate with a laminated plate bonded on one side. Additionally, to verify test results and calculate the shear and peel stresses in the adhesive layers, a numerical model that used Eigenvalue analysis was proposed and applied. The tests revealed that using the proposed method to create a two-layered laminated plate consisting of CFRP and aluminum plates could not reduce the thermal stress completely in a thin steel plate. However, it was found that the thermal stress in steel plate can be completely reduced, even in thin steel plate, when the proposed method is used to create a three-layered laminated plate consisting of one CFRP plate between two aluminum plates, which when composited has a thermal bending moment equal to zero.

Plasma Sprayed 4-Phase Ceramic Abradable Seal Coatings and their Thermal Shock Resistance on Nickel-Based Superalloy

Novel 4-phase ceramic abradable seal coatings have been prepared by plasma spray. The as-prepared coatings were characterized by SEM, EDS and XRD. The results showed that the 4-phase ceramic abradable coating exhibited porous structures and had good adhesion with the bond coat. The ZrO2 and Al2O3 in the coating existed in the form of tetragonal-ZrO2 and α-Al2O3, respectively. The result of thermal shock test revealed that compared with the 3-phase abradable coating, the thermal shock life of the 4-phase abradable coating had increased tenfold to 1900~2000 cycles from 100~200 cycles. The reason is owing to the reduction of oxygen partial pressure and thermal stresses at the top/bond coat interface due to the addition of the fourth nanoAl2O3 phase.