Maximum Tensile Thermal Stress Research Articles

Influence of pore structure on thermal stress distribution inside coal particles during primary fragmentation

Thermal stress is an important reason of coal particle primary fragmentation, during which the role of pore structure is ambiguous. Thermal stress induced fragmentation experiments were conducted with low volatile coal/char particles, and the results show that the fragmentation severity enhances with increasing porosity. Various porous thermal stress models were developed with finite element method, and the influences of the pore shape, size, position and porosity on the thermal stress were discussed. The maximum thermal stress inside particle increases with pore curvature, the pore position affects the thermal stress more significantly at the particle center and surface. The expectation of the maximum tensile thermal stress linearly increases with porosity, making the particles with higher porosity easier to fragment, contrary to the conclusion deduced from the devolatilization theory. The obtained results are valuable for the analysis of different thermal processes concerning the thermal stresses of the solid feedstocks.

Nanometer-Scale Strain Measurements in AlGaN/GaN High-Electron Mobility Transistors During Pulsed Operation

Electric, thermal, and mechanical strain fields drive the degradation of AlGaN/GaN high-electron mobility transistors (HEMTs). The resulting mechanical strains within the devices are particularly important. However, a lack of high-resolution measurements of device deformation has limited progress in understanding the related phenomena. This paper presents the atomic force microscope measurements of thermomechanical deformation of AlGaN/GaN HEMT devices during pulsed operation. We investigate the devices with various operating conditions: drain–source voltage, $V_{\mathrm {\mathrm {DS}}}$ , of 0–50 V; drain–source power of 0–6 W/mm; and operating frequency of 55–400 kHz. As $V_{\mathrm {\mathrm {DS}}}$ increases, thermomechanical deformation decreases, especially in the region above the gate. An electrothermomechanical model closely matches with and helps to explain the measurements. According to the model, the maximum periodic tensile thermal stress, which occurs at the drain-side edge of the gate footprint, is 55% larger for $V_{\mathrm {\mathrm {DS}}} = 10$ V than for $V_{\mathrm {\mathrm {DS}}} = 48$ V for the same device power. The maximum tensile thermal stress in the device depends on the gate temperature and not the maximum device temperature. As $V_{\mathrm {\mathrm {DS}}}$ increases, the hotspot moves away from the gate, leading to lower gate temperature rise and lower tensile thermal stress.

Solid State Bonding of Carbon Material to Copper with Nickel Insert

Carbon material/nickel/copper system was heated under a compressive stress of 13 MPa in a vacuum at 1073 K for different keeping times, and the bending strength of the joint, the microstructure, hardness and carbon concentration near the joining interface were examined. Thermal stress induced in the joint was estimated by a finite element method. In carbon material/copper system solid state bonding is difficult. However, good solid state bonding becomes feasible when nickel is used as an insert metal. Axisymmetric thermoelastic finite element analysis reveals that the maximum tensile thermal stress is induced on the surface of carbon material near the joining interface, and it decreases with increasing nickel thickness. The maximum residual tensile thermal stress in practical joints is less than the maximum tensile thermal stress calculated by finite element method because the thermal stress is released. [doi:10.2320/matertrans.H-MRA2008819]

Solid State Bonding of Graphite to Nickel

Graphite was bonded to nickel under joining compressive stress of 3 to 33 MPa in a vacuum at temperatures within the range of 973 to 1273 K using an RF-induction furnace. The influence of joining conditions on the bending strength of the graphite/nickel joint, and changes in the microstructure and hardness of nickel near the joining interface, were investigated. Thermal stress induced in the joint was estimated using the finite element method. On the basis of these results, the influence of thermal stress on the bending strength of the joint was examined Completion of the graphite/nickel joint depends on both joining compressive stress and joining temperature. At high joining temperature, good solid-state bonding under relatively low joining compressive stress becomes feasible. Axisymmetric thermoelastic finite element analysis suggests that the maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of graphite anal is increased with increasing joining temperature. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. Part of the thermal stress in practical joints is relaxed and less than that calculated using the finite element method. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of the maximum tensile thermal stress depends on the amount of carbon which diffuse into nickel. This may be related to changes in plasticity of both nickel and graphite, that is, changes in the amount of carbon that dissolves in nickel to supersaturation and the point defects introduced in graphite.

Solid State Bonding of Graphite to SUS304 Steel

Graphite was bonded to SUS304 steel under a compressive stress of 25 MPa in a vacuum over a temperature range from 873 to 1023 K using a RF-induction furnace. The bending strength of the graphite/SUS304 steel joint, and changes of microstructure and hardness near the joining interface of SUS304 steel, were investigated. Thermal stress induced in the joint was estimated with a finite element method. On the basis of these results the influence of thermal stress on the bending strength of the joint was examined. Good solid state bonding becomes feasible when the joining temperature exceeds 923 K. Axisymmetric thermoelastic finite element analysis suggests that maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of the graphite and is increased with increasing joining temperature. The position of fracture in a bending test specimen corresponds approximately to that of the maximum tensile thermal stress. A part of the thermal stress is released with the deformation of SUS304 steel and graphite. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of thermal stress depends on the amount of carbon atoms which diffuse into SUS304 steel. This may be related to the changes in plasticity of SUS304 steel and graphite, that is, the changes in the amounts of Cr 23 C 6 precipitated in SUS304 steel and point defect introduced in graphite.

黒鉛とニッケルの固相接合

Graphite was bonded to Ni under a compressive stress range from 3 to 33 MPa in a vacuum over a temperature range from 973 to 1273 K using a RF-induction furnace. The influence of joining conditions on the bending strength of the graphite/Ni joint, and changes of microstructure and hardness near the joining interface of Ni, were investigated. Thermal stress induced in the joint was estimated with a finite element method. On the basis of these results the influence of thermal stress on the bending strength of the joint as examined.Completion of the graphite/Ni joint depends on both the compressive stress and the joining temperature. Good solid state bonding becomes feasible under low compressive stress in case of higher joining temperatures. Axisymmetric thermoelastic finite element analysis suggests that maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of the graphite and is increased with increasing joining temperatures. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. A part of the thermal stress in practical joints is relaxed and less than that calculated by finite element method. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of maximum tensile thermal stress depends on the amount of carbon atoms which diffuse into Ni. This may be related to the changes in plasticity of Ni and graphite, that is, the changes in the amounts of C super-saturately dissolved in Ni and the point defects introduces in graphite.

Estimation of Thermal Shock Strength of Cr3C2 Ceramics by Laser Irradiation.

Nuclear power plants have the greatest potential for use in space ships, underwater vehicles, and so on. Because the space and the weight for those plants have limitation, it is very important to develop new radiation shielding materials which have excellent mechanical strength and heat-resistant characteristics. Chromium carbide ceramics (Cr3C2) is the neutron absorber which is more excellent than titanium boride ceramics and has been investigated as a radiation shielding material. This report presents an evaluation of thermal shock strength by the laser irradiation technique for chromium carbide ceramics. Their temperature and thermal stress distributions are analyzed by the finite element method (FEM), and critical fracture curves, which can specify a critical power density for a given laser beam spot diameter, are obtained from the relationships between the spot diameter of the laser beam and the maximum tensile thermal stress. And furthermore, size effects to the fracture criterion of chromium carbide ceramics are examined, and the laser irradiation experiments are performed on chromium carbide ceramics using a CO2 laser. Finally, theoretical results are compared with experimental ones, and the thermal shock strength PL of Cr3C2 ceramics with laser irradiation is ∼3.4 W/mm2 for tension by the FEM analyses.

黒鉛とオーステナイト系ステンレス鋼SUS304の固相接合

Graphite was bonded to SUS304 steel under a compressive stress of 25 MPa in a vacuum over a temperature range from 873 to 1023 K using a RF-induction furnace. The bending strength of the graphite/SUS304 steel joint, and changes of microstructure and hardness near the joining interface of SUS304 steel, were investigated. Thermal stress induced in the joint was estimated with a finite element method. On the basis of these results the influence of thermal stress on the bending strength of the joint was examined.Good solid state bonding becomes feasible when the joining temperature exceeds 923 K. Axisymmetric thermoelastic finite element analysis suggests that maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of the graphite, increasing with increasing joining temperature. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. A part of the thermal stress is released with the deformation of SUS304 steel and graphite. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of thermal stress depends on the amount of carbon atoms which diffuse into SUS304 steel. This may be related to the changes in plasticity of SUS304 steel and graphite, that is, the changes in the amounts of Cr23C6 precipitated in SUS304 steel and point defect introduced in graphite.

STRESS INTENSITY RELAXATION AT THE TIP OF AN EDGE CRACK IN A FUNCTIONALLY GRADED MATERIAL SUBJECTED TO A THERMAL SHOCK

We analyze thermal stresses and the stress intensity factor in an edge-cracked strip of a functionally graded material (FGM) subjected to sudden cooling at the cracked surface. It is assumed that the shear modulus of the material decreases hyperbolically with the higher value at the surface exposed to the thermal shock and that the thermal conductivity varies exponentially. Volume fractions of the constituents in a ceramic-metal FGM are then determined with the assumed shear modulus gradient using a three-phase model of conventional composites. The differences between the other assumed material properties and those predicted by the three-phase model are delineated and the applicability of the assumed FGM is discussed. It is shown that the maximum tensile thermal stress in the strip without cracks is substantially reduced by the assumed thermal conductivity gradient and that the magnitude of the compressive stress is increased. A strong compressive zone just away from the thermally shocked surface is devel...

Estimation of Thermal Shock Resistance of Titanium Boride Ceramics with Different Porosities by Laser Irradiation.

Nuclear power plants have the greatest potential for use in space ships, underwater vehicles, and so on. It is very important to develop new radiation shielding materials which have excellent mechanical strength and heat-resistant characteristics. Titanium boride ceramic (TiB2) is an excellent neutron absorber and has been investigated as a radiation shielding material. This report presents an evaluation of thermal shock resistance by the laser irradiation technique for three kinds of titanium boride ceramics with 3, 8, and 15% porosity. Their temperature and thermal stress distributions are analyzed by the finite-element method, and the relationship between the spot diameter of the laser beam and the maximum tensile thermal stress is derived for various power densities of the laser beams. Critical fracture curves are obtained from these relationships, which can specify a critical power density for a given laser beam spot diameter. The irradiation experiments are performed on titanium boride ceramics using a C02 laser. Finally, theoretical results are compared with experimental ones.

Non-Steady State Temperature Distribution and Thermal Stress History in Ceramic Disks Heated under Constant Heat Flux Conditions

Temperature distribution of ceramic disks subjected to suddenly heating from outside under constant heat flux conditions was measured to estimate the thermal stress history and evaluate the first kind thermal shock resistance parameter of ceramic materials. Also, numerical calculations were conducted to compare with experimental data for each heating condition. Temperature dependence of thermal conductivity and thermal diffusivity was introduced into the calculation to realize the practical heat conductive condition. Two types of ceramics were used for the experiment; alumina ceramics having high and varying thermal conductivies and mica ceramics having low and constant thermal conductivities. As a result, it was found that the thermal stress history of heated disks is easily estimated from measurement of temperature distribution of the specimen. Temperature dependence of thermal properties was substantially affected on temperature distribution and thermal stress history, and it is shown that varying thermal properties in alumina ceramics can lead to a significant increase in the maximum tensile thermal stress generated in the center of the disks. As demonstrated by the numerical calculation for many kinds of other ceramics, maximum thermal stress of heated disks under constant heat flux condition shows non-steady-state value.

溶融亜鉛めっき時における溶接継手の液体ぜい化 ＩＩＩ 液体金属ぜい化割れの再現試験およびその防止方法

In the second report, thermal stress during dipping the simple plate in the molten zinc was analyzed. And it was indicated that maximum tensile thermal stress, (σt) max occurred at the lower side of plate, and caused LME-cracking at this position.In this report, because of confirming the above crack position, the simulation test specimens for LME-cracking were devised, and were dipped in the molten zinc.The results are as follows.(1) LME-cracking occurred at the lower side of plate, and the results consisted with the results by analysis of thermal stress.(2) The addition of the thermal stress and residual stress affected the initiation of the cracking.(3) To make dipping velocity high, to give dipping direction careful consideration, or to make shot peening is a good example of preventing the crack.

溶融亜鉛めっき時における溶接継手の液体ぜい化 ＩＩ めっき浴浸せき時に生じる平板およびパイプの熱応力

In previous report, the effect of welding factors on the critical stress for LME-cracking of steel was examined. In addition, because of preventing the crack, it is necessary to study the behavior of thermal stress and residual stress during dipping in the molten zinc that cause the crack in steel structure.In this report, the distribution of temperature and thermal stress during dipping simple plate and pipe in the molten zinc were analyzed. The results are as follows.(1) Maximum tensile thermal stress, (σt)max occurred at the lower side of plate and pipe, and causedLME-cracking at this position.(2) (σt)max increased as plate width is large and plate thickness is small. And (σt)max increased as pipe diameter is large.(3) For the plate and pipe, (σt)max increased as dipping velocity is small.

Effect of Spatially Varying Thermal Conductivity on the Magnitude of Thermal Stress in Brittle Ceramics Subjected to Convective Heating

The effect of a spatial variation of the thermal conductivity on the magnitude of the maximum tensile thermal stress in a solid circular cylinder subjected to sudden convective heating was calculated by finite element methods. The general results show that by lowering the thermal conductivity in the surface region of the cylinder, the magnitude of the maximum tensile thermal stress at the center of the cylinder is reduced significantly. The negative temperature dependence of the thermal conductivity in dielectric materials, which indirectly creates a spatial variation in thermal conductivity, also causes a significant decrease in the magnitude of the tensile thermal stresses, as demonstrated by a numerical example for aluminum oxide. It is also shown that a spatial variation in thermal conductivity can significantly affect the time to maximum tensile stress as well.

ChemInform Abstract: ENHANCED THERMAL STRESS RESISTANCE OF STRUCTURAL CERAMICS WITH THERMAL CONDUCTIVITY GRADIENT

AbstractEs wird die Hypothese aufgestellt, daß die maximalen thermischen Zugspannungen in spröden Keramiken durch eine Veränderung der Temp.‐ Profile mit Hilfe modifizierter räumlicher Wärmeleitfähigkeitsverteilungen signifikant herabgesetzt werden können, und am Beispiel eines Hohlzylinders mit radial auswärts oder einwärts gerichtetem stationärem Wärmefluß durchgerechnet.

Enhanced Thermal Stress Resistance of Structural Ceramics with Thermal Conductivity Gradient

The hypothesis is presented that the maximum tensile thermal stresses in brittle ceramics can be reduced significantly by a redistribution of the temperature profile using a spatially varying thermal conductivity. On the basis of a hollow circular cylinder subjected to radially inward or outward steady‐state heat flow, it is shown that such a spatially varying thermal conductivity can lead to significant decreases in magnitude of the maximum tensile thermal stress. Possible techniques of creating such thermal conductivity variations are discussed briefly.