STABILITY OF A PROPAGATING INTERPHASE BOUNDARY IN A THERMOPLASTIC MATERIAL

We study the morphological stability of a propagating planar interphase boundary in a thermoplastic material deformed in antiplane shear. The plane interphase boundary is found to be stable if σ₀/(₄ρcrpar;V₀ ²where c is the specific heat per unit uolume; p is the mass density; V₀is a proagation speed; and σ₀is a function of the material moduli and the state of deformation of the body.     

温度波作用下建筑内墙定型相变材料板的热特性

通过数值模拟研究了正弦温度波作用下建筑内墙定型相变材料板的热特性并与传统的墙体材料——砖和泡沫混凝土板相比较.利用焓法建立的一维瞬态模型采用全隐差分格式迭代求解.计算结果表明,与砖和泡沫混凝土板相比,定型相变材料板表面温度变化不仅波幅有较大的衰减,时间延迟也较明显;相变温度是影响波幅衰减程度的重要因素;对于一定的温度波,相变材料潜热、导热系数和表面换热系数均存在极限值,超过此值,相变材料板表面温度受室内温度波动的影响很弱;相变材料板厚度对表面温度波动几乎无影响;相变温度区间的扩大使表面温度有轻微的波动.所得结果为太阳能建筑及相关领域中相变材料的选择与应用提供参考.     

THERMOELASTIC PROBLEM FOR A GRIFFITH CRACK IN A PLATE WITH TEMPERATURE-DEPENDENT PROPERTIES UNDER A LINEAR TEMPERATURE DISTRIBUTION

This paper is concerned with a method for calculating the thermal-stress distribution in a plate exhibiting temperature-dependent properties. Using the perturbation method, general equations for the displacements are found. The equations can be solved by using four displacement functions. Following this method, a solution is derived for calculating the stress intensity factor for a Griffith crack in a temperature-sensitive plate under a linear temperature distribution, where we assume that the temperature field is not disturbed by the crack. (Under these conditions, if a plate is temperature-insensitive, the stress intensity factor for the crack is zero.) In this case, whether the stress intensity factor is zero or not depends only on the temperature variation of the coefficient of thermal expansion. A numerical example is given for a plate made of steel.     

MULTIPLE SURFACE CRACKING IN GLASS-FIBER-REINFORCED PLASTICS DUE TO A THERMAL SHOCK AT A LOW TEMPERATURE

We consider the transient thermal singular stress problem of multiple surface cracking in glass-fiber-reinforced plastics due to a thermal shock at a low temperature. The layered composite is made of a layer bonded between two layers of different physical properties, and it is suddenly cooled on the surfaces. The surface layers contain parallel arrays of the embedded or edge cracks perpendicular to the boundaries. The thermal and elastic properties of the material are dependent on the temperature. For the case of the crack that ends at the interface between orthotropic elastic materials, the order of stress singularity around the tip of the crack is obtained. Finite element calculations are carried out, and the transient thermal stress intensity factors are shown graphically.     

FATIGUE CRACK PROPAGATION UWDER OSCILLATING TEMPERATURE DISTRIBUTIONS

A method has been presented for the estimation of fatigue crack growth in structures with harmonically varying temperuture distributions. The number of cycles required to extend a fatigue crack of a given depth to a certain amount has been calculated as a function of the temperature amplitude and frequency of the oscillating temperature distribution. The concepts from linear elastic fracture mechanics have been used in these evaluations.     

EDGE CRACK PROBLEMS IN HOMOGENOUS AND FUNCTIONALLY GRADED MATERIAL THERMAL BARRIER COATINGS UNDER UNIFORM THERMAL LOADING

In this study the axisymmetric crack problem for thermal barrier coatings under a uniform temperature change is considered. Modes I and II stress intensity factors and the strain energy release rate are calculated for various sizes and locations of the crack. The main variables in the problem are the material inhomogeneity parameter of the functionally graded material coating, the size and the location of the crack, and the relative dimensions of the specimen. The effect of the temperature dependence of the material properties on the stress intensity factors and the strain energy release rate is also investigated. The finite element method is used to solve the problem. The material property grading is accounted for by developing special inhomogeneous elements and the stress intensity factors are calculated by using enriched crack tip elements.     

金属材料常温高压氢脆研究进展

安全经济的氢气储运技术是氢能利用推向实用化、产业化的关键,35～75MPa高压储氢是现阶段可以商用的一种储氢方式.金属材料长期在常温高压氢环境中使用,可能出现氢脆现象,进而引发脆性破坏事故.该文以国内外的研究成果为基础,介绍常温高压氢脆机理,着重分析几种金属的抗氢脆性能,并提出若干关于我国压力容器常用金属抗氢脆性能研究的建议.     

TRANSIENT THERMO-VISCOELASTIC RESPONSE OF A CRACK IN A LAYERED STRUCTURE

This article examines the application of a numerical method of transient stress analysis for the study of a crack located in a viscoelatic layer bonded to an elastic substrate. The layered structure containing the defect is subjected to heat conduction and associated thermo-viscoelastic effects. The finite-element technique is used to examine the time-dependent variation in the stress intensity factor at the crack tip due to a sudden reduction in the temperature at the surface of the layered structure. Numerical results presented in the article illustrates the influence of the thermo-viscoelastic coupling on the crack behavior.     

ACCURATE SURFACE TEMPERATURE PREDICTION AT HIGH SPEEDS

Computational tools of turbulent combustion have practical applications for various fields including liquid rocket engines, but some numerical issues are still presented for solving supercritical combustion. In the present study, several of these numerical issues are studied and discussed. Turbulent flow and thermal fields of gaseous hydrogen/cryogenic liquid oxygen flame at supercritical pressure are simulated by a turbulence model. To realize real-fluid combustions, the modified Soave-Redlich-Kwong (SRK) equations of state (EOS) are implemented into the flamelet model with a look-up table as functions of mean and variance of mixture fraction, scalar dissipation rate, enthalpy, and pressure. For supercritical combustion flows, modified forms of the pressure implicit with splitting of operator (PISO) algorithm for solving the pressure-velocity linked equation are introduced. From a comparison of instantaneous temperature distributions for gaseous hydrogen/cryogenic liquid oxygen flame at supercritical pressure, the capability of each method based on the different solution sequence is examined and the effective sequence is explored. The results show that the updated mixture fraction reflected in the pressure correction loop is a critical factor for numerical stability. Also, the relative performance of six convection schemes for supercritical combustion is discussed.     

TRANSIENT THERMOELASTIC PROBLEM FOR AN INFINITE PLATE CONTAINING A PENNY-SHAPED CRACK AT AN ARBITRARY POSITION

This article deals with the transient thermoelastic problem for an infinite plate containing a penny-shaped crack that is parallel to the surfaces of the plate but at an arbitrary position of the plate. The transient thermal stresses are set up by the heat generation on the surfaces and the sudden heat exchange on the surfaces. By using the finite difference method for the time variable, the analytical solution for spatial variables can be obtained. The numerical results for the temperature and stress intensity factor are obtained, and results are shown in graphs.     

EXACT THERMOELECTROELASTICITY SOLUTION FOR A PENNY-SHAPED CRACK IN PIEZOELECTRIC MATERIALS

This paper investigates the electromechanical fracture behavior of a penny-shaped crack in piezoelectric materials subjected to a uniform heat flow far away from the crack region. The crack is treated as isothermal. First, the temperature field is obtained by using the Hankel transform technique. The thermoelectromechanical field is then investigated by solving the themopiezoelectoelasticity governing equations. The explicit solutions for the stress intensity factors near the crack tip are presented. The exact solution for the whole-field stresses and electric displacements are obtained.     

EDGE CRACK IN A NONHOMOGENEOUS HALF PLANE UNDER THERMAL LOADING

This paper deals with the problem of an edge crack in a semi-infinite nonhomogeneous plate under steady heat flux loading conditions. The objective of the study is to assess the effect of material nonhomogeneity on the thermal stress intensity factor. All material properties are supposed to be exponentially dependent on the distance from the boundary of the plate. By using the Fourier transform, the problem is reduced to a singular integral equation that is solved numerically. The thermal stress intensity factors for various material constants are calculated. The results show that by selecting the material constants appropriately the stress intensity factor can be reduced.     

CRACK ARREST ANALYSIS UNDER CYCLIC THERMAL SHOCK FOR AN INNER-SURFACE CIRCUMFERENTIAL CRACK IN A FINITE-LENGTH THICK-WALLED CYLINDER

This article presents a crack arrest depth analysis under cyclic thermal shock for an inner-surface circumferential crack in a finite-length thick-walled cylinder with rotation-restrained edges. The inside of the cylinder is cooled from a uniform temperature distribution. The effects of heat transfer conditions on the maximum transient stress intensity factor for the problem were investigated with systematical evaluation methods formerly developed. Then, under an assumption of a tentative threshold stress intensity range j K th together with the Paris law, the crack arrest depth under cyclic thermal stress was evaluated. The results suggested the existence of an upper limit for the normalized crack arrest depth, independent of the cylinder material in an engineering sense. Finally, the validity of applying j K max h j K th as a crack arrest criterion under cyclic thermal shock was confirmed by fatigue tests under mechanical loads equivalent to those induced by cyclic thermal shock.     

A MODE-I CRACK PROBLEM FOR AN INFINITE SPACE IN GENERALIZED THERMOELASTICITY

ABSTRACT

In this work, we solve a dynamical problem of an infinite space with a finite linear crack inside the medium. The Fourier and Laplace transform techniques are used. The problem is reduced to the solution of a system of four dual integral equations. The solution of these equations is shown to be equivalent to the solution of a Fredholm integral equation of the first kind. This integral equation is solved numerically using the method of regularization. The inverse Laplace transforms are obtained numerically using a method based on Fourier expansion techniques. Numerical values for the temperature, stress, displacement, and the stress intensity factor are obtained and represented graphically.     

TWO CRACK GROWTHS IN A FUNCTIONALLY GRADED PLATE UNDER THERMAL SHOCK

This article examines the problem of two thermal cracks under a transient temperature field in a ceramic/metal functionally graded plate. When the functionally graded plate is subjected to thermal shock, multiple cracks often occur on the ceramic surface. It is shown that the crack paths are influenced by interaction between multiple cracks and a compositional profile of the functionally graded plate. Transient thermal stresses are treated as a linear quasi-static thermoelastic problem for a plane strain state. The crack paths of two cracks are obtained using the finite element method with mode I and mode II stress intensity factors.     

低温热电器件的结构尺寸优化

建立了热电器件模型,在考虑最大输出功率的条件下对热电器件的长度进行了优化.结果表明:随着热电器件p/n结长度的减小,热电器件的功率呈上升趋势并达到一个极值,但随着p/n长度的进一步减小,热电器件的功率开始下降.热电器件的最大功率随着端板热阻、接触电阻和接触热阻的增大而减小,最大功率优化下的p/n结长度随着端板热阻、接触电阻和接触热阻的增大逐渐增大.     

STEADY THERMAL STRESSES IN AN INFINITE NONHOMOGENEOUS ELASTIC SOLID CONTAINING A CRACK

This article considers the crack problem for an infinite nonhomogeneous elastic solid subjected to steady heat flux over the crack surfaces. The aim is to understand the effect of nonhomogeneities of materials on stress intensity factors. By using the Fourier transforms, the problem is reduced to a Fredholm integral equation of the second kind which is solved numerically. Results are presented illustrating the influence of the nonhomogeneity of the material on the stress intensity factors. For some groups of the material constants, there exist minimum stress intensity factors, which is very interesting for the understanding of compositions of advanced functionally gradient materials.     

低温沉积氧化锌薄膜电学特性

利用金属有机物化学气相沉积系统在玻璃衬底上沉积氧化锌薄膜,在约160℃的低温生长条件下,通过改变n型掺杂气体硼烷的流量来调制薄膜电阻率和光学透过率.基于Hall效应测试分析,研究了掺杂剂硼烷流速对氧化锌薄膜电阻率,载流子浓度,Hall迁移率以及光学透过率的影响,此外还研究了薄膜电学特征参量随薄膜厚度的变化规律.经过一系列优化实验,在玻璃衬底上能够获得低温生长的氧化锌薄膜电阻率约2×10~(-3)Ω·cm,光学透过率的截止波长发生蓝移,从380nm延伸到近340nm.     

稻草低温燃烧过程中床料聚团机理的实验研究

在小型燃煤鼓泡流化床上对稻秆低温燃烧的聚团现象进行了试验研究并对其机理进行了探讨.对所取床料样品的分析结果表明,床料中碱金属主要以钾并以KCI和K2SO4水溶性钾方式存在,且随着燃烧时间的增长碱金属在床料中呈增加趋势,同时床料聚团颗粒增多.对稻秆在650℃和700℃成灰的XRD检测也表明,稻秆在该温度范围内燃烧后灰的物相主要是KCI、K2SO4和SiO2,而KCI和K2SO4的两相低温共熔温度约为684℃,说明KCI和K2SO4低温共溶对稻秆低温燃烧床料聚团具有重要作用.     

THERMAL STRESSES IN A LONG ELASTIC CYLINDER CONTAINING A PENNY-SHAPED CRACK AND EMBEDDED IN A THERMALLY CONDUCTIVE ELASTIC INFINITE MEDIUM

This paper is concerned with- the state of stress in a long circular elastic cylinder with a concentric penny-shaped crack whose surfaces are subjected to a prescribed temperature. The plane of the crack is assumed to be perpendicular to the axis of the cylinder. The cylinder is bonded to a thermally conductive elastic infinite medium. The thermal and elastic constants of the cylinder and infinite medium are assumed to be different. By assuming suitable representations for the temperature function, the heat conduction problem is reduced to the solution of a Fredholm integral equation of the second kind. Similarly, the thermoelastic problem is also reduced to the solution of a Fredholm integral equation of the second kind. A closed-form expression is obtained for the stress-intensity factor. The integral equations are solved numerically, and the results are used to obtain numerical values for the stress intensity factor. These values are presented graphically.