Reduction Of Thermal Stress Research Articles

Residual Thermal Stresses in Filamentary Polymer-Matrix Composites Containing an Elastomeric Interphase

A three-phase micromechanical model based on the method of cells is for mulated to characterize residual thermal stresses in filamentary composites containing an in terphase between the fiber and the matrix. This is the first such study to incorporate a true three-phase version of the method of cells. The model's performance is critically evaluated using data generated from other micromechanical models. Subsequently, a parametric study is performed to quantify the residual stresses in two hypothetical graphite fiber/epoxy matrix composites: one containing an elastomeric interphase whose Young's modulus is less than that of the fiber and the matrix and one incorporating an interphase whose Young's modulus is intermediate with respect to the fiber and the matrix. The data correlate the residual ther mal stresses in the fiber, interphase and matrix as a function of the interphase thickness and fiber volume fraction within each model composite. The study makes a broad assessment of the stress-attenuating characteristics that each interphase imparts to the graphite/epoxy com posites. Over the range of variables considered, properly dimensioning the elastomer inter phase leads to a more favorable reduction of residual thermal stress.

Design of thermoelastic materials using direct sensitivity and optimization methods. Reduction of thermal stresses in functionally gradient materials

A methodology is presented for thermoelastic material design in functionally gradient materials (FGMs) to reduce the thermoelastic thermal stresses induced under the would-be operational thermal boundary conditions. Direct sensitivity analysis and optimization techniques are employed to reach an optimal distribution of the volume fraction of the phases in the FGM. A numerical example is given to show the potential applicability of the proposed formulation.

Active Cooling and Thermal Stress Reduction by Use of Porous Materials : Hollow Cylinder Model

Thermal stresses induced in a fluid saturated porous hollow cylinder whose inner surface is heated by burning gas, and the wall of which is cooled by the fluid injection from its outer surface, are analyzed. Two situations of loading conditions are considered: The cylinder is subjected to a sudden rise in temperature of the gas and is simultaneously pressurized at its outer surface to cool the wall, and steady state cooling is abruptly disturbed by a sudden loss of pressurization, and after a while it is recovered

An Analytical Solution for Transient Thermal Stress in a Functionally Gradient Plate with Arbitrary Nonhomogeneities and Thermal Boundary Conditions. Material Properties Determined by Fuzzy Inference.

Functionally gradient material (FGM) is one of the typical nonhomogeneous materials. In this paper, an analytical solution based on a kind of integral transform developed by Vodicka for the composite regions is presented for the transient heat conduction problem in a nonhomogeneous plate with arbitrary nonhomogeneous thermophysical properties. The material properties of SiC/Al FGM with the uncertainty of distinction between the matrix phase and the filler phase in an intermediate composite and the uncertain change in the microstructure of the filler phase are estimated based on Mamdani's method of fuzzy inference. The associated transient thermal stresses under the various thermal boundary conditions are analysed with the use of the closed- form solution derived by one of the authors for a nonhomogeneous plate with arbitrary variations in mechanical properties through the thickness, and the composition suitable for the reduction of thermal stress is discussed.

Reduction of Thermal Stresses in Continuous Fiber Reinforced Metal Matrix Composites with Interface Layers

The potential of using interface layer to reduce thermal stresses in the matrix of composites with a mismatch in coefficients of thermal expansion of fiber and matrix has been investigated. It was found that compliant layers, with properties of readily available materials, do not have the potential to reduce thermal stresses significantly. How ever, interface layers with high coefficient of thermal expansion can compensate for the mismatch and reduce thermal stresses in the matrix significantly.

Joining of silicon nitride to SUS304 stainless steel using a sputtering method

Silicon nitride with thin sputter-deposited titanium and nickel films was joined to SUS304 stainless steel (18% Cr-8% Ni) using metallic buffers in a series of silicon nitride/nickel/ molybdenum/nickel/SUS304, and the joining strength and microstructures were investigated. Four-point bending tests showed fracture strength of the joints up to 169 MPa. Cracks were formed at the interface between the silicon nitride and its adjacent nickel buffer, and frequently extended into the silicon nitride. Microstructural analyses revealed that the silicon nitride reacted with the sputter-deposited titanium producing titanium nitride and isolated silicon atoms, and that silicon and titanium diffused into the nickel buffer. Calculations using a finite-element method indicated a marked reduction in thermal stress induced in the joined silicon nitride with increasing thickness of the molybdenum buffer. The strong interfacial bond inducing the fracture of the joined silicon nitride was interpreted in terms of a good interfacial reaction, the interdiffusions and the reduction of thermal stress being due to the insertion of the molybdenum buffer.

Active Cooling and Thermal Stress Reduction by Use of Porous Materials. Hollow Cylinder Model.

By using the thermo-poro-elastic theory proposed previously by the author, thermal stresses are analyzed which are induced in a fluid-saturated porous hollow cylinder whose inner surface is heated by burning gas, the wall of which is cooled by the fluid injection from its ourter surface. Two stories of loading conditions are considered : (A) the cylinder is subjected to a sudden rise in temperature of the gas and simultaneously pressurized at its outer surface to cool the wall, and (B) steady state cooling is abruptly disturbed by a sudden loss of pressurization, and after a while it is recovered. Main attention is placed on the effect of heat advection due to active fluid injection on the reduction of temperature and thermal stresses. Since the formulated problem is axisymmetric, the displacement field is decoupled from the temperature and pore-pressure fields, which are still coupled to each other. The coupled nonlinear diffusion equations are solved by the Crank-Nicolson implicit method.

Computer modelling of temperature and stress distributions in LEC-grown GaAs crystals

Temperature and thermal stress distributions have been modelled in a LEC GaAs crystal at various stages of growth using boundary conditions which allow for radiation into the argon atmosphere and thermal conduction into the boric oxide encapsulant. Results have been obtained as a function of oxide conductivity, oxide depth, ambient temperature above the oxide, and shape of the GaAs solid-melt interface shape. Heat losses into the encapsulant are important, both in reducing thermal stresses in the early stages of growth and in increasing them once the conical region has fully formed. It is shown that in both situations, a reduction in thermal stress can be effected by increasing the depth of oxide and the temperature of the environment it loses heat to. The convex crystal-melt interface shapes that occur in practice are not optimum for reducing stress: flatter interfaces should be aimed for. These results are discussed in the context of other studies.

Thermal stress and defect reduction in undercut GaAs on Si substrate

Results of a TCA (thermal cycle annealing) experiment on UCGAS (undercut GaAs on Si) in which the GaAs/Si interface is partially etched to separate the GaAs layer from the substrate are reported. The structure is developed to reduce the thermal stress in GaAs grown on Si. A drastic EPD (etch pit density) reduction by TCA is observed by eliminating the GaAs/Si interface which acts as a dislocation source, and a dislocation-free region about 6 μm wide is obtained at the edge of the structure. Therefore, it is possible to eliminate both the thermal stress and the dislocations at the edge of the UCGAS.

Structures for Thermal Stress Reduction in GaAs Layers Grown on Si Substrate

The structures to reduce the thermal stress in GaAs grown on Si substrates produced by the thermal expansion coefficient mismatch are proposed and analyzed by the finite-element method. It was shown that the stress can be below 1/10 of the stress in the planar GaAs on Si by partially separating the GaAs layer from the substrate. The preliminary experimental results show that this technique reduces the stress to 1/10 of the normal stress in the area of as wide as 200×250 µm2.

A novel pressure sensor structure for integrated sensors

A novel pressure sensor ‘silicon box’ structure is proposed, fabricated by one-sided processing technology. Tests and analyses show that, while the excellent performance of single crystal silicon diaphragm sensors is maintained, the new technique has advantages for both discrete and integrated pressure sensors of high yield, good uniformity, low cost and feasibility of mass production. This structure also provides some reduction of thermal stress caused by the mismatch between the chip and the substrate occurring in a silicon cup structure.

傾斜機能を有する粒子が分散した複合体中の熱応力の解析

This paper discusses the effects of a film at the particle/matrix interface in a particle-dispersed ceramic composite on the stresses due to thermal contraction mismatch upon cooling by using the formal elastic theory. The dispersed particles are assumed to be spherical and uniformly covered with films of FGMs or of other materials having various properties.Analysis is conducted for the particle/matrix systems, in particular for SiC/Al2O3 and Al2O3/MgO composites. Reduction of thermal stresses is shown to occur when the interfacial film is a sufficiently thick FGM film or has a lower Young's modulus.Also, when the particle has a lower thermal expansion coefficient than the matrix, the interfacial stresses decrease as the thermal expansion coefficient of the film increases.

円盤状焼結傾斜機能材料の設計と作製

The fabrication of disk shaped functionally gradient material with an optimum distribution of composition for thermal stress reduction in a specific material combination of PSZ/Stainless Steel is described. The thermal stress generated during cooling from sintering temperature was analized by finite element method as a function of conpositional distribution and sample size.According to the optimum compositional gradient, the sintered compact of the PSZ /SUS304 functionally gradient material was successfully fabricated.

傾斜機能材料の開発

This paper discribes the development of Functionally Gradient Material (FGM) which is promoted by Science and Technology Agency with special coordination funds for promoting science and. technology from 1987. FGM is an advanced metal/ceramic compositional gradient from a metal side to a ceramic side to obtain the function of the thermal stress reduction In the high temperatur circumstance such as aerodynamic heating or combustion chamber of space plane. The aim of the development Is completion of FGM with the function to withstand 2000 K In the ceramic side and the difference of 1000 K between both sides. which are generated by the high temperature gas and the active cooling system. The small-sized FGM with a diameter of 30 mm and a thickness of 1-10 mm is scheduled to be completed by March, 1990 and the large-sized FGM of 300 mm by 300 mm with 1-10 mm in thickness by March. 1992. The development is shared by three research groups such as material designing, fabrication and evaluation, which consist of 30 research Institutes including national institues, universities and private companies. Material design group developed expert system. Fabrication group produced small FGM test pieces by physical-chemical combination vapor deposition process, thin sheet lamination method, low pressure plasma spray method and self-propagating high temperature synthesis process. Evaluation group developed various techniques for thermal barrier performance, thermal fatigue properties, thermal shock resistance and etc.

Recent Developments in Laser Melt/Particle Injection Processing

Investigations into crack formation in particle injected surfaces and into microstructural evolution during cladding were some of the recent developments in laser melt/particle injection processing. Both surface modification techniques involved blowing powder particles into a melt pool formed by a laser beam. When the particulate materials were metal carbides composite surface layers were formed and the thermal stresses that developed during cooling of the carbide injected layer resulted in the formation of microcracks. It was found that modest levels of preheating were sufficient to prevent cracking and that cracking was eliminated mainly by the reduction in thermal stresses and not by modifications in microstructure. With lower melting alloy powders clad overlays were formed. It was found that the limited mixing between the clad material and the substrate resulted in complex macrostructures containing fluid-flow driven segregation bands. The degree of microstructural in-homogeneity depended upon the type of substrate material and had a direct influence on the soundness of the cladding.

Reduction of Thermal Stress in Mbe Grown GaAs/Si by Patterning

AbstractThe biaxial tensile stress of 2.65 kbar in as-grown GaAs/Si for T < 100K is reduced by post-growth patterning of the GaAs and the reduction in stress, as determined by photoluminescence and cathodoluminescence, is dependent on the pattern size and shape. For stripe patterns less than 15 gm wide the stress is largely uniaxial with stress relief normal to the stripe direction. Rectangular patterns exhibited stress relief in orthogonal directions, and a 9 x 12 µm2 rectangle exhibited an average stress of 0.5 kbar. For as-grown GaAs/Si layers 0.9 to 3.25 µm thick, the stress is weakly dependent on layer thickness. For T > looK the stress in as-grown GaAs/Si is reduced and at 295K a value of 1.51 ± 0.21 kbar is obtained. With patterned growth, using a native SiO2 mask, no reduction in stress was observed irrespective of the pattern size, indicating the importance of free GaAs edges in obtaining stress relief.

繰り返し応力下で接合したAl<SUB>2</SUB>O<SUB>3</SUB>-Cu接合体の接合強度

The effect of alternating stress, which was superimposed on the bonding pressure, on the bonding strength of alumina-copper joint was investigated. Alumina-copper joint with a thin aluminum interlayer deposited in the vacuum was performed at temperatures above 773 K in argon gas at a pressure of 0.4 MPa under the alternating stress.The main results obtained are as follows:(1) At each bonding temperature, the strength of joint increased with increasing time and reached a peak strength and then decreased with increasing time.(2) The addition of alternating stress on the bonding pressure has the effect to increase the peak strength and to promote the bonding process.(3) Apparrent activation energy for the bonding process is calculated to be about 49 kJ/mol.(4) A reasonable explanation for improving the bonding strength due to the alternating stressing can be made on basis of the reduction in thermal stress due to local fatigue deformation.(5) Bonding reaction proceeding during heating up has no significant effect on the experimental results.

The development of personal conditioning in military aviation.

Abstract This paper describes the development of personal conditioning systems in UK military aviation from the air-ventilated suit systems in the Second World War to the modern liquid-cooled garments supplied by thermoelectric cooling devices. It describes the decreasing need for personal cooling brought about by the reduction in environmental operating temperatures as the RAF became based almost entirely in Europe. It goes on to describe how this reduction in thermal stress from environmental sources has been more than offset by increasing stresses attributable to aircraft performance, avionic equipment and complicated air-crew clothing assemblies.

Edge stabilized ribbon (ESR) growth of silicon for low cost photovoltaics

The edge stabilized ribbon (ESR) growth method is a shaped crystal growth technique wherein a ribbon is grown from the surface of the melt and the edge positions are determined by capillary attachment to wetted strings which pass up through the melt and are frozen into the ribbon as it grows. The method is compared to other ribbon growth techniques and found to have significant advantages, notably stability of growth and ease of production scale-up. A continuous growth melt replenished furnace has been constructed and run around the clock on a three-shift basis. This single ribbon machine was operated over a run period of 100 h with a duty cycle of 96% resulting in the production of 62,000 cm 2 of ribbon. A panel of 36 series-connected 50 cm 2 ESR solar cells was found to have an AM1 encapsulated cell efficiency of 12.7%. Future work should include reduction of thermal stress and the growth of multiple ribbons.

CONSIDERATION ON AMBIENT THERMAL STRESS OF CIRCULAR PRESTRESSED CONCRETE WATER TANKS

The temperature and strain distributions are observed in a circular prestressed concrete water tank of 10000t capacity. The observed strains are compared with calculated strains obtained from finite element models. Close agreement was obtained for the both strains not only in elastic component but also in creep component. It confirm the propriety of used model and assumption.25°C difference in the temperature of each wall face is proposed for design from the observed data for 3 years and the meterological data for 15 years. It induced the tensile stress of 5.2MPa at the inside surface of the wall concrete. It is also shown that the reduction of thermal stress by creep effect can not be expected.