功能梯度材料与构件的静动力识别

功能梯度材料具有复杂的细部结构,其内部构造远比匀质材料复杂.因此,以目前的实验条件,测量功能梯度材料的参数分布是十分困难的.建议一种新颖的功能梯度构件分析的细观元法.细观力学研究的目的在于建立材料的宏观性能同其组分材料性能及细观构造之间的定量关系,它可揭示不同的材料组合及其变异所具有不同的宏观性能的内在机制.利用细观元法探讨功能梯度材料与结构的识别问题,即在已知实验测量的位移或固有频率的情况下,对功能梯度材料的组分分布及组成材料名称进行识别.     

具有中等组分不同网状结构功能梯度构件的三维动力特性分析

功能梯度材料具有复杂的细部结构,其内部构造远比匀质材料复杂,因此其构件动力分析很难求得其解析解。该文建议一种新颖的功能梯度构件动力分析的细观元法。细观力学研究的目的在于建立材料的宏观性能同其组分材料性能及细观结构之间的定量关系,它可揭示不同的材料组合具有不同的宏观性能的内在机制。此法可实现材料细观结构到构件宏观响应的直接过渡分析,而计算单元与自由度又等同一般常规有限元,却使得组成功能梯度材料构件的各种材料细观构造得到反映。通过细观元技术,对具有中等组分不同网状结构功能梯度构件进行三维动力特性分析,并给出其三维固有频率及振型的三维分布,特别是给出了不同网格结构功能梯度板件应力振型的平面等值线图差异。结果表明:不同细观网格结构对功能梯度材料结构三维动力响应有较明显影响。     

非均质梯度功能材料三维本构关系及宏细观结构分析

对非均质耐热梯度功能材料的结构与性能进行了研究。通过对陶瓷-金属梯度功能结构的分析,建立了宏、细观力学模型,并在此基础上导出了反映其宏观力学性能的三维本构关系。      

沿板平面变异功能梯度板件的三维动力特性

功能梯度材料具有复杂的细部结构,其内部构造远比匀质材料复杂,因此其构件动力分析很难求得其解析解。文中建议一种新颖的功能梯度构件动力分析的细观元法。细观力学研究的目的在于建立材料的宏观性能同其组分材料性能及细观结构之间的定量关系,它可揭示不同的材料组合具有不同的宏观性能的内在机制。目前功能梯度板件分析只能处理材料特性沿厚度方向梯度变化,而细观元法则直接从制备时给定的材料组分分布出发计算构件宏观三维动力特性,并给出了沿板平面方向材料特性梯度变化的功能梯度板件三维固有频率及振型的三维分布。     

复杂形状功能梯度板件的三维动力特性分析

从细观力学角度出发根据材料细观组分分布对具有不同复杂形状功能梯度材料构件进行三维动力特性分析,并相应给出其三维固有频率及其基频对应的位移振型和应力振型沿厚度方向的三维分布。结果发现对同样材料细观组分分布的不同功能梯度结构,其固有频率和相应的振型分布均有很大差异。此结果为建立专门的功能梯度板壳理论提供定量的资料依据。     

空隙、杂质及组分突变对功能梯度构件动力特性的影响

功能梯度材料具有复杂的细部结构, 其内部构造远比匀质材料复杂, 因此其构件动力分析很难求得其解析解。本文中提出了一种新颖的功能梯度构件动力分析的细观元法, 其目的在于建立材料的宏观性能与其组分材料性能及细观构造之间的定量关系, 以便揭示不同的材料组合及其变异所具有不同的宏观性能的内在机制。利用细观元法对含有空隙、 杂质及组分突变等情况下的功能梯度构件进行动力分析, 求得其三维固有频率及振型的三维分布。从而可知空隙、 杂质及组分突变均对功能梯度材料构件的宏观动力特性有很大的影响。      

复杂边界条件功能梯度板三维分析的细观元法

发展一种细观分析和宏观计算相结合的计算方法—细观元法。细观元法是使功能梯度构件宏观响应和材料组分的几何、物理、构造参数直接发生关联的分析方法,实现材料细观结构到构件宏观响应的直接过度分析,为解决功能梯度构件宏、细观跨尺度分析提供了一种有力工具。细观元方法不增加结点自由度,却使得功能梯度板件的任意功能梯度变化、各种复杂边界条件得到反映。用细观元法得到了几种具有不同复杂边界条件的功能梯度板件的力学量三维分布形态。     

由材料组分直接计算功能梯度复合材料开孔板宏观响应的跨尺度分析

建议一种新颖的功能梯度构件分析的细观元法, 给出了方法模型、基本算式及特点与功能。细观元法对构件的常规有限单元内部设置密集细观单元以反映材料组分梯度变化, 又通过协调条件将各细观元结点自由度转换为同一常规有限元自由度, 再上机计算。此法可实现材料细观结构到构件宏观响应的直接过渡分析, 而计算单元与自由度又等同于常规有限元, 为解决功能梯度构件宏观、细观跨尺度分析提供了一种有效工具。本文中直接从制备时给定材料组分分布出发计算构件宏观响应, 给出了不同开孔形状与数量功能梯度板的力学量三维分布形态。      

考虑单向复合材料复杂微观结构的细化单胞模型

通用单胞模型常被应用在复合材料细观力学分析上。但原始的通用单胞模型存在求解量大、计算效率低的问题。该文对其改进,建立了以子胞界面细观应力为未知量的细化单胞模型。利用该模型研究复杂的微观结构包括纤维截面形状/排列方式,界面相材料属性/几何厚度,夹杂/空隙对单向纤维复合材料宏观弹性常数的影响。通过与其他研究方法和试验数据对比证实了该预测模型具有更高的计算效率,计算精度和更广泛的普适性。该文模型子胞划分更细致,克服了原始通用单胞模型无法分析复杂微观结构的不足。有望将损伤力学引入该模型中建立一个有力的分析工具,来进行复合材料结构宏/细观多尺度损伤力学分析。     

孔隙对陶瓷基复合材料强度影响的研究进展

陶瓷基复合材料具有较强的结构特性,是一种多相体材料.其力学性能及损伤破坏规律不仅取决于各组分材料性能,同时也取决于细观结构特征.对于复合材料的强化,通常是通过掺入夹杂物以提升其界面强度.但是由于夹杂掺入的不均匀性,以及制备工艺无法排净的空气和其他杂质,使得基体尤其是界面附近的基体,不可避免的存在孔隙,这些孔隙在受到外部载荷时产生裂纹继而扩展,造成材料失效.因此通过研究材料在遭受外部载荷冲击时,其在细观层面的损伤演化与宏观失效表现的关系,从细观层面分析材料损伤演化的规律和断裂机理,不仅可以为复合材料的制备提供理论上的指导,而且还可以通过对材料细观结构的进一步优化达到设计材料的目的.本文介绍了陶瓷基复合材料在制备过程中产生的缺陷、缺陷致使材料发生损伤失效以及缺陷对复合材料有效强度影响的研究现状,在总结复合材料相关研究成果及不足的基础上,结合仿真建模分析手段对下一步复合材料有效强度问题研究进行了展望.     

Dynamic stress from a subsurface cylindrical inclusion in a functionally graded material layer under anti-plane shear waves

The multiple scattering of shear waves and dynamic stress resulting from a subsurface cylindrical inclusion in a functionally graded material (FGM) layer bonded to homogeneous materials are investigated, and the analytical solution of this problem is derived. Image method is used to satisfy the traction free boundary condition of the FGM layer. The analytical solutions of wave fields around the actual and image inclusions are expressed by employing wave functions expansion method, and the expanded mode coefficients are determined by satisfying the continuous boundary conditions around the inclusions. Through the numerical solutions of dynamic stress concentration factors (DSCFs) around the inclusion, the effects of the position of the inclusion in the material layer, the properties of the inclusion, and the properties of the two phases of composites on the DSCFs are analyzed. Analyses show that when the cylindrical inclusion is stiffer than the two phases of FGMs, the dynamic stress around the inclusion increases greatly. When the distance between the surface of the structure and the inclusion is smaller, the effect of the properties of the inclusion becomes greater. When the cylindrical inclusion is softer than the two phases of FGMs, the maximum dynamic stress shows little difference; however, the variation of the distribution of the dynamic stress around the inclusion is greater.     

基于耦合扩展多尺度有限元方法的功能梯度材料热应力分析

以高效模拟功能梯度材料(FGM)微观非均质性对整体热力学性能的影响为研究目的,通过随机形态描述函数(RMDF)法和体积分数的指数分布建立FGM二维微结构,在此基础上,发展了FGM热应力分析的耦合扩展多尺度有限元方法(CEMsFEM)。该方法基于扩展多尺度有限元方法(EMsFEM)的基本思想,对温度场和位移场构造数值基函数,以把微观非均质材料性质带到宏观响应中。同时为了考虑泊松效应导致的不同方向间的耦合作用,在位移场数值基函数中增加了耦合附加项。通过数值基函数建立宏微观单元信息的映射关系,在宏观尺度求解有效方程,节约计算量。为了更好地考虑微观载荷的影响,把结构的真实响应分解为宏观响应和微观扰动,进一步推导出修正的宏观载荷向量。通过不同体积分数分布的FGM在不同载荷工况下的热应力分析算例验证了本文中方法的正确性和有效性,最后讨论了微结构的尺寸效应对结构热力学响应的影响。     

梯度功能涂层的剥落现象分析与梯度分布的优化

梯度功能材料的优化设计, 可以转化为梯度分布函数的设计。采用幂函数来描述梯度分布, 使材料优化问题简化为该幂指数(称为梯度指数) 的优化。对梯度功能涂层-基底结构的剥落现象进行了分析, 推广了文献[ 9 ]的结果, 给出了在梯度功能涂层的基底中平行于界面稳态扩展的裂纹的能量释放率和应力强度因子的解析表达式, 并由此计算了最优的梯度分布指数。同时, 还讨论了梯度功能涂层耐高温抗氧化性能与热应力缓和之间的关系。      

XFEM simulation of cracks, holes and inclusions in functionally graded materials

The present work aims at the numerical simulation of inhomogeneities/discontinuities (cracks, holes and inclusions) in functionally graded materials (FGMs) using extended finite element method (XFEM). A FGM with unidirectional gradation in material properties is modeled under plane strain condition. The domain contains a major crack either at the center or at the edge of the domain along with multiple minor discontinuities/flaws such as minor cracks and/or voids/inclusions distributed all over the domain. The effect of the variation in stress intensity factor (SIF) of the major crack due to the presence of the minor cracks and voids/inclusions is studied in detail. The simulations show that the presence of minor discontinuities significantly affects the values of SIFs.     

Ti/Al2O3 Functionally Gradient Material Prepared by the Explosive Compaction/SHS Process

1.IntroductionDistinctinterfacesoccurincompositesorasare-sultofthefabricationprocessasincoatingandbond-ing.Thepresenceofadistinctillterfacecanleadtoproblemsduringfabricationorserviceuse.Forexam-ple,delaminationorcrackingalongabondinglayerformetal-ceramiccompositemayoccurasaresultofmechanicalstressesorthermalstressesthataredevelopedfromthedifferenceinthermalexpansionbetweenmaterialsifthebondinglayerissubjectedtotemperaturevariation[1].Therecentdevelopmentintheprocessingoffunctionallygradientmat…     

Response of spatially tailored structures to thermal loading

The paper presents the formulation and analysis of composite plates serving as STATs, i.e., spatially tailored advanced thermal structures where the distribution of the constituent phases varies throughout the surface as well as through the thickness. This is an extension of the well-known concept of functionally graded materials (FGM) and structures with the constituent phases varying only in the latter direction. As a result of two- or three-dimensional grading it is possible to optimize the response and properties of the structure providing multitask and multi-scale optimization. The response of plates with two- or three-dimensional grading to an arbitrary thermal loading is elucidated, including the conditions that result in thermal bending versus thermal instability.     

An analytical method for thermal stresses of a functionally graded material cylindrical shell under a thermal shock

In this paper, the thermal stresses of a thin functionally graded material (FGM) cylindrical shell subjected to a thermal shock are studied. An analytical method is developed. The studied problem for an FGM cylindrical shell is reduced to a plane problem. A perturbation method is used to solve the thermal diffusion equation for FGMs with general thermal properties. Then, the transient thermal stresses are obtained. The results show that the thermal shock is much easier to result in failure than the steady thermal loading. The present method can also be used to solve the crack problem of an FGM cylindrical shell with general thermal properties.     

A multi-scale model for coupled heat conduction and deformations of viscoelastic functionally graded materials

An integrated micromechanical-structural framework is presented to analyze coupled heat conduction and deformations of functionally graded materials (FGM) having temperature and stress dependent viscoelastic constituents. A through-thickness continuous variation of the thermal and mechanical properties of the FGM is approximated as an assembly of homogeneous layers. Average thermo-mechanical properties in each homogeneous medium are computed using a simplified micromechanical model for particle reinforced composites. This micromechanical model consists of two isotropic constituents. The mechanical properties of each constituent are time–stress–temperature dependent. The thermal properties (coefficient of thermal expansion and thermal conductivity) of each constituent are allowed to vary with temperature. Sequentially coupled heat transfer and displacement analyses are performed, which allow analyzing stress/strain behaviors of FGM having time and temperature dependent material properties. The thermo-mechanical responses of the homogenized FGM obtained from micromechanical model are compared with experimental data and the results obtained from finite element (FE) analysis of FGMs having microstructural details. The present micromechanical-modeling approach is computationally efficient and shows good agreement with experiments in predicting time-dependent responses of FGMs. Our analysis forecasts a better design for creep resistant materials using particulate FGM composites.     

Geometry effects of substrate and coating layers on the thermal stress response of FGM structure

Summary Due to transient temperature change, the plane strain elastic-plastic problem for a functionally graded material (FGM) bonded to a homogeneous coating layer and a metal substrate is considered by the use of the finite element method (FEM). The substrate and the coating are assumed to be aluminum and partially stabilized zirconia, respectively. The FGM layer is a particulate composite of aluminum and partially stabilized zirconia with volume fractions continuously varying through the thickness. Generally in high temperature applications, the FGM system is sandwiched between a substrate layer and a coating layer. The coating layer increases the protection from heat but decreases the thermal shock resistance while the substrate layer increases the rigidity of the structure and decreases strength-related properties at high temperature. In order to compromise the thickness of both the coating and substrate layers, different values of the substrate and coating thickness are studied in order to evaluate their effects on the thermal stress response of the FGM structure. Since the main objective of the FGMs is using them in different applications with severe thermal loading conditions, the thermal stresses may be so high that some reinforcements may be fractured and/or debonded from the matrix giving a weakening effect instead of a reinforcing one. Hence, the behaviors of the reinforcements and the matrix are essential to be studied. In this regard, microscopic constitutive equations along with the temperature-dependent properties of the constituent materials are considered to enable us obtaining more realistic results of thermal stresses. Since the FGM structures are fabricated at high temperatures, thermal residual stresses are produced. In order to find out the importance of the consideration of the residual stresses arising from the fabrication process, the FGM structure with stress-free conditions is heated to the operating temperature, and its thermal stress response is compared with that one where the residual stresses are taken into account. Also, several functional forms of gradation of the constituents in the FGM layer are examined to reach the optimum profile giving the minimum stress level for the FGM structure under thermo-elasto-plastic behavior.     

Exact solutions for characterization of electro-elastically graded materials

The present paper deals with a class of functionally graded materials (FGM), called active FGM that has electro-elastically graded material phases. An active FGM system leads to minimization of stress concentration that arises due to mismatch in the electrical and elastic properties of the constituent phases. This work focuses on the characterization of the through thickness stresses of an active FGM subjected to electrical excitation. The structure is comprised of a substrate, an electro-elastically graded layer and an active layer. A formulation for exact solutions of the system based on Euler–Bernoulli theory is presented. Power-law variation of the composition of the two phases in the graded layer is considered. Performance of linearly gradient FGM for a range of stiffness and electrical property ratios of the active and substrate materials have been studied. It is observed that the electrical strain component and the compositional gradation significantly influence the stress characteristics of the active FGM.