Thermoelastic Stresses in SiC Single Crystals Grown by the Physical Vapor Transport Method

A finite element-based thermoelastic anisotropic stress model for hexagonal silicon carbide polytype is developed for the calculation of thermal stresses in SiC crystals grown by the physical vapor transport method. The composite structure of the growing SiC crystal and graphite lid is considered in the model. The thermal expansion match between the crucible lid and SiC crystal is studied for the first time. The influence of thermal stress on the dislocation density and crystal quality is discussed.

Thermal-mechanical modeling of nodular defect embedded within multilayer coatings

The initiation of laser damage within optical coatings can be better understood by thermal-mechanical modeling of coating defects. The result of this modeling shows that a high-temperature rise and thermal stress can be seen just inside the nodular defect compared to surrounding coating layers. The temperature rise and thermal stress tend to increase with seed diameter. Shallower seed tend to cause higher temperature rise and greater thermal stress. There is a critical seed depth at which thermal stress is largest. The composition of the seed resulting from different coating-material emission during evaporation can affect the temperature rise and thermal stress distribution.

Thermal stress analysis for GaInAsP multiple quantum well wafer chemically bonded to Si (100)

An n-InP-based InGaAsP multiple-quantum-well wafer was bonded with p-Si by chemical surface activated bonding at 70 degrees C, and then annealed at 450 degrees C. Different thermal expansion coefficients between InP and Si will induce thermal stresses in the bonded wafer. Planar and cross-sectional distributions of thermal stress in the bonded InP-Si pairs were analyzed by a two-dimensional finite element method. In addition, the normal, peeling, and shear stresses were calculated by an analytic method. Furthermore, x-ray double crystalline diffraction was applied to measure the thermal strain and the strain caused by the mismatching of the crystalline orientation between InP (100) and Si (100). The wavelength redshift of the photoluminescence (PL) spectrum due to thermal strain was investigated via the calculation of the band structure, which is in agreement with the measured PL spectra.

Critical Biot's number for Determination of the Sensitivity of Spherical Ceramics to Thermal Shock

A critical Biot number, which determines both the sensitivity of spherical ceramics to quenching and the durations of the temperature-wave propagation and the thermal stresses in the ceramics subjected to thermal shock, is theoretically obtained. The results prove that once the Biot number of a ceramic sphere is greater than the critical number, its thermal shock failure will be such a rapid process that the failure only occurs in the initial regime of heat conduction, whereas the thermal shock failure of the ceramic sphere is uncertain in the course of heat conduction. The presented results provide a guide to the selection of the ceramics applied in the thermostructural engineering with thermal shock.

Growth of composite sapphire/Ti : sapphire by the hydrothermal method

Composite sapphire/Ti:sapphire crystals for high-power laser application were grown by the hydrothermal method. The results of the X-ray rocking curve analysis indicate high crystalline quality of the surface Al2O3 material. The strong bonding between the overgrown Al2O3 and seed Ti:Al2O3 crystals is indispensable for withstanding high thermal stresses produced by intense optical pumping. The optical loss at the boundary of the composite crystal is considerably low, indicating the lack of scattering centers. (c) 2005 Elsevier B.V. All rights reserved.

水热法生长复合钛宝石激光晶体

水热法生长了复合钛宝石(Al2O3／Ti：Al2O3)激光晶体．对外层Al2O3X射线ω扫描表明其晶体质量很高．外层生长的Al2O3和内部的钛宝石激光棒结合牢固、紧密，因而能适应高强度泵浦所带来的激光介质内部的热应力．复合激光晶体界面层带来的光损耗很小．就我们所知，这是首次用水热法生长复合激光晶体．

不同基板预热温度对激光金属沉积成形过程热应力影响的研究

研究不同基板预热温度对激光金属沉积成形过程热应力的影响,对于降低成形过程的热应力,抑制成形过程裂缝的产生,减小成形过程试样和基板的翘曲变形具有非常重要的意义。根据有限元分析中的"单元生死"技术,编程建立了基板预热对激光金属沉积成形过程热应力影响的三维多道多层数值模拟模型,详细分析了基板未预热和分别预热到200℃、300℃、400℃、500℃、600℃时对沉积成形过程VonMise’s热应力、X方向、Y方向以及Z方向热应力的影响。在与模拟过程相同的参数下,利用镍基合金粉末分别在基板未预热和分别预热到300℃、400℃、500℃、600℃时进行了成形试验,试验的结果跟数值模拟结果吻合较好。

Thermal Residual Stresses in Particulate Composites and Its Toughening Effect

In this paper, an accurate formula for calculating the thermal residual stress field in a particle-reinforced composite are presented. Numerical examples are given to show r-variations of the thermal residual stresses. The increase in fracture toughness of matrix predicted by the thermal residual stress field is compared well with the experimentally measured increase.

A New Analytical Model for Residual Stresses due to Thermal Surface Coating

发展了一种新的分析涂层结构（平板、梁）热残余应力的模型,可以研究骤冷过程(Quenching)和冷却过程(Cooling)在涂层结构内引发的残余应力分布。与以往模型相比,其优势在于：它可以考虑源于喷涂过程的涂层孔隙率、温度梯度等因素对于涂层和基底内残余应力的影响。其中孔隙率和温度分布由计算机模拟涂层沉积过程得到。另外,当基底的材料和几何参数被固定时,我们分析了诸如涂层的理想模量、厚度、热膨胀系数等参数,对于涂层结构中最终残余应力分布的改变机理。

Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations. (C) 1998 Elsevier Science Ltd.

Thermal Residual Stresses in SiCw/Al Composite

<正> The SiCw/6061Al composites were fabricated by squeeze casting method. Varia-tions of thermal residual stresses with quenching temperature, cooling manner, aging time and thethermal-cold cycle process in thin specimens,and the distributions of thermal residual stresses alongthe distances from the surface and changes with heating temperatnres in thick specimens were stud-ied by means of X-ray diffraction (XRD). The effects of residual stresses on the mierostructure, di-mensional stability and age-hardening behavior were studied by SEM, TEM observations, and tensiletest. The results showed that there existed macrostress, microstress and thermal mismatch stress inSiCw/Al compo-site,and the presence of microstress and thermal mismatch stress has no influenceon the measurement of macrostress, but the macrostress can affect the measured value of thermalmismatch stress. Thermal res dual stress induced during the composite fabrication process, will be further in-creased when the composite were subjected to the e

FEM Simulations and Optimization about Residual Stresses in Coating Structures with Functionally Graded Materials Layer

An elasto-plastic finite element method is developed to predict the residual stresses of thermal spraying coatings with functionally graded material layer. In numerical simulations, temperature sensitivity of various material constants is included and mix

Thermal fracture characteristics induced by laser beam

The damage mechanism of a cracked material due to laser beam thermal shock is an important problem when the interactions of high power laser beam with materials are studied. The transient thermal stress intensity factors (TSIFs) for a center crack in an infinite plate subjected to laser beam thermal shock are investigated. When the crack is in the heat affected zone, the compressive thermal stress causes the crack surface to come into contact with each other over a certain contact length, but the high tensile stresses around the crack tip tend to open the crack. In this case, the problem may be treated as a pair of embedded cracks problem with a smooth closure condition of the center crack. The TSIFs and the crack contact lengths are calculated with different laser beam spatial shapes. The TSIFs induced by thermal shock are in marked different from those induced by general mechanical loading.

Characterising Of Internal Stresses In Duplex Coating By Fem

The internal stresses in a duplex coating involving a prequenched layer are believed to change if it is exposed to thermal loading. To characterise the internal stresses in such a duplex coating, a gradient model of finite element method is set up. The initial stress within the substrate developed in as quenching and the internal stresses due to the tempering of the prequenched layer ( TPQL) in such a duplex coating are calculated. The synthetical internal stresses in coating can be estimated by superposing uniform initial stresses developed during plating. The results indicate that the residual tensile stresses due to fabrication in coating will be decreased greatly, or even synthetical compressive internal stresses may arise in the coating.

Study of thermal behaviors in CO2 laser irradiated glass

To understand mechanisms underlying laser-induced damage of BK7 and fused silica, we calculate the temperature field of the substrates with CO2 laser irradiating at a given laser power and beam radius. We find that the two glasses show different thermal behaviors. A model is developed for estimating the time t to heat the surface of the substrates up to a particular temperature T with cw CO2 laser irradiation. We calculate theoretically the duration t that the samples are irradiated, from the beginning to visual catastrophic damage, with the assumption of damage threshold determined by the critical temperature. The duration t that the samples are irradiated, from the beginning to visual catastrophic damage, is investigated experimentally as well. Here we take the melting point or softening point as the critical temperature, given the thermomechanical coupling properties, which is enough to cause damage for BK7. Damage features are characterized by the sound of visual cracks. Finally, we calculate stresses induced by laser heating. The analysis of stress indicates that the damage of BK7 is due to the stresses induced by laser heating. (c) 2005 Society of Photo-Optical Instrumentation Engineers.