基于有限元的金属基短纤维复合材料MMC的一种统计蠕变模型

在有限元分析的基础上建立了一个单向应力状态下金属基短纤维复合材料(MMC)的统计蠕变模型.首先建立细胞模型并进行有限元分析,得到了单向应力状态下材料细观尺寸及载荷方向对宏观蠕变响应的影响规律.通过在细胞模型中增加一界面层(考虑材料特性和厚度)来研究基体和纤维的界面对MMC宏观蠕变响应的影响.基于细胞模型的数值结果,提出了一适用于纤维平面随机分布的随机统计模型,该模型考虑了纤维的断裂.通过试验获得纤维的统计分布规律.分析结果表明随机统计模型可以满意地描述试验结果.进一步讨论了材料细观尺寸,纤维的断裂特性以及界面层的材料特性和厚度对MMC宏观蠕变响应的影响.     

泥质盐岩单轴蠕变寿命研究

岩石流变力学的研究中，蠕变寿命是一个重要问题.由于长期蠕变试验资料的缺乏，难以估计蠕变破坏时间.该文进行了泥质盐岩单轴全应力 应变压缩试验，并采用陈氏加载法进行了单轴蠕变试验.对蠕变曲线进行了处理，获得了不同应力水平下的蠕变曲线簇，进而得到了等时应力-应变曲线簇.通过拟合分析，建立了等时应力-应变曲线割线模量随时间变化关系模型和等时应力 应变曲线的数学模型.对等时应力 应变曲线与全应力-应变曲线之间的关系进行了分析，获得了蠕变破坏强度和破坏应变分别与蠕变寿命之间的数学表达式.该文研究成果可以估计泥质盐岩的蠕变寿命、长期强度、长期模量、蠕变破坏线和蠕变终止线，对相关岩石流变寿命的估计具有借鉴意义.     

蠕变损伤材料中缺口尖端稳定扩展的应力场

研究了在拉伸载荷和反平面载荷作用下蠕变损伤材料缺口尖端稳定扩展的应力场．假设材料的应力及位移势函数,得到了缺口尖端场的各参数分量,进而在小范围蠕变条件下,建立了缺口尖端稳定扩展的蠕变损伤控制方程,并考虑缺口尖端蠕变钝化作用和问题的边界条件,对控制方程进行了数值分析,得到了缺口尖端的应力场,并讨论了缺口尖端应力场随各影响参数的变化规律．结果表明,缺口尖端的应力具有r1/(1-n)的奇异性,应力率具有rn/(1-n)的奇异性,n是蠕变指数．     

聚氯乙烯接近玻璃化转变温度时的蠕变行为研究

利用动态力学分析仪,详细研究了小应力作用下聚氯乙烯（PVC）接近玻璃化转变温度（63℃）时的短期蠕变性能．根据Struik-Kohlrausch (SK)方程及Struik移位方法,分析了物理老化对PVC瞬时蠕变柔量的影响,并提出了基于多松弛机制的新模型定量描述初始蠕变柔量、松弛时间和形状因子．结果表明:SK方程虽可表征不同老化时间下PVC的蠕变性能,但通过Struik移位方法却无法获得理想的主曲线,且初始蠕变柔量、松弛时间、形状因子和蠕变率均受物理老化显著影响,并在双对数坐标下随老化时间呈明显的非线性变化;新模型不仅揭示了物理老化控制参量的演化规律,而且极好地预测了PVC在物理老化进程中的高温蠕变行为．     

基于有损伤体元件的广义Kelvin模型蠕变全过程探究

岩石的蠕变特性往往对隧道和地下工程的稳定性有着重要的控制作用.针对岩石蠕变的阶段性特征,可将岩石蠕变全过程分为四个阶段.广义Kelvin模型可较好地反映前三个阶段的岩石蠕变特性,不能理想地反映加速蠕变阶段特征.通过引入损伤体元件和Kachanov的损伤因子演化公式,构建了具有损伤体元件的广义Kelvin模型,从而建立了可以体现岩石蠕变全过程的蠕变模型,并提出较为简单的组合模型参数计算方法.该模型不仅能较好地反映岩石蠕变全过程,且模型参数易于确定.利用该模型对砂质泥岩单轴压缩蠕变实验曲线进行拟合分析,拟合效果良好,研究结果可为类似工程提供参考.     

Investigation on the 2D Contact of Multilayer Functionally Graded Piezoelectric Material Coating Under Conducting Indenters北大核心CSCD

考虑了材料参数可按照任意函数形式变化的功能梯度压电材料(FGPM)涂层在不同形状导电压头作用下的接触问题,研究了梯度系数对功能梯度压电涂层接触力学行为的影响.建立了多层功能梯度压电材料涂层模型,运用了Fourier积分变换和传递矩阵将多层功能梯度压电材料涂层的接触问题转化为奇异积分方程.利用GaussChebyshev数值计算方法,得到了多层功能梯度压电材料涂层-基底结构在刚性导电平压头和圆柱形压头作用下的表面应力分布和电荷分布.利用数值解,分析了材料参数按照不同变化形式的FGPM涂层对最大压痕和电势的影响,还分析了功能梯度压电涂层内部的应力和电位移分布.研究结果表明,功能梯度压电材料参数的不同变化形式对结构的接触性能具有重要的影响.     

导电压头作用下的功能梯度压电涂层二维黏附接触问题研究

纳米压痕实验是研究材料的力学性能和表面形貌的重要手段,当接触区尺寸减小时,压头与试件接触表面间的黏附作用将无法忽视,因此,考虑黏附作用对压头作用下的接触问题具有重要的价值.功能梯度压电材料(FGPM)兼具梯度材料和压电材料的优点,用作涂层可有效地抑制接触损伤和破坏.该文将针对梯度压电材料在导电压头作用下的黏附接触问题开展研究,假设功能梯度压电涂层的材料参数按照指数形式变化,基于Maugis黏附模型,利用Fourier积分变换获得了功能梯度压电涂层在导电压头作用下的二维无摩擦黏附接触问题的控制奇异积分方程,并采用Erdogan-Gupta的数值方法求解,获得了黏附应力、梯度参数和压头所带电荷对力-电耦合响应的影响.研究结果为利用功能梯度压电材料涂层改善材料表面的接触行为提供了理论依据,同时可为压电结构及器件的设计提供帮助.     

皮肤层蠕变分析的混合解法

本文利用积分变换方法求解皮肤层在表面压力作用下的蠕变响应问题。使用数值方法分别求解了双积分逆变换和由表面应力边界条件所得的第一类Ｖｏｌｔｅｒｒａ积分方程。计算所得的加载瞬时和平衡状态时的位移结果分别与不可压缩和可压缩单相弹性体的应移值相同，证明了本文方法的正确性。文中还给出了皮肤层在压力作用下的蠕变响应曲线。     

低密度和高密度水化硅酸钙的纳米压痕数值模拟

水化硅酸钙(C-S-H)是决定以硅酸盐水泥为主要组分的水泥基材料性能的关键组分,水化硅酸钙(C-S-H)是存在于水化产物中连续的固体物质,占水化水泥浆体体积的50%～60%,是水泥浆体中决定硬化水泥浆体物理结构和性能的主要成分,同时水化硅酸钙(C-S-H)也是混凝土结构宏观尺度的凝聚性和耐久性的重要影响因素．模型的建立对于预测水泥的体积性能如收缩、蠕变、渗透率和裂缝等方面越来越重要．通过将水化硅酸钙(C-S-H)分为低密度和高密度两种数值模型,并且将水化硅酸钙(C-S-H)视为在纳米尺度上具有实际微观粒子性能的离散颗粒的集合,运用分子动力学方法,通过控制水化硅酸钙(C-S-H)的紧密堆积体积分数和保持颗粒级别的其他材料属性不变来进行纳米压痕实验模拟,进而验证了压痕模量和硬度符合纳米压痕实验数据的规律．     

幂律非线性粘弹性材料中的裂纹扩展*

对蠕变不可压幂律非线性粘弹性材料中裂纹的蠕变扩展进行了分析,为描述银纹带中的力学行为,假设在裂纹尖端邻域中断裂过程区中分布着阻抗裂纹张开的粘聚应力б_f,.通过对均匀应力参考状态平凡解的摄动,将非线性粘弹性问题化成线性问题处理.对于幂指数.n≌1的弱非线性情况得到了应力与位移表达式.提出断裂过程区局域能量判据,导出了裂纹孕育时间t*与蠕变扩展率a的预测公式.     

Analitical Description of the Creep of Expanded Polystyrene under Compressive Loading

Data obtained in testing the creep of expanded polystyrene plates in compression are discussed. Power-type and exponential regression equations are used for describing the compression creep curves. Within the limits of data spread, both these equations represent experimental results equally well. A correlation between the creep strain and density, the compressive stress at a 10% strain, and the elastic modulus in compression is revealed for the material investigated.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 4, pp. 525–534, July–August, 2005     

Creep of Poly(Vinyl Chloride)/ Chlorinated Polyethylene Blends

The results of experimental investigations of the creep behavior of blends of poly(vinyl chloride) (PVC) with chlorinated polyethylene (CPE) are presented. Eight types of specimens with the PVC/CPE weight ratios of 100/0, 90/10, 80/20, 60/40, 40/60, 20/80, 10/90, and 0/100 are examined. The creep tests were continued for 1000 h. It is discussed how the blend composition affects the elastic and inelastic behavior of the material. The elastic compliance of the blend can be determined from the properties of its components by using the Kerner and Budiansky equations for heterogeneous systems with a phase structure of statistic-dispersion type. The creep compliance (the total current compliance minus the elastic compliance) obeys the power law of creep with coefficients depending on the blend composition.     

Thermal Creep Flow for the Boltzmann Equation

It is known that the Boltzmann equation has close relation to the classical systems in fluid dynamics. However, it provides more information on the microscopic level so that some phenomena, like the thermal creep flow, can not be modeled by the classical systems of fluid dynamics, such as the Euler equations. The author gives an example to show this phenomenon rigorously in a special setting. This paper is completely based on the author's recent work, jointly with Wang and Yang.     

Investigation of the relation between the depth of penetration and indentation diameter in polymer hardness measurements

It has been experimentally demonstrated that for polymeric materials the area of the indentation should not be calculated from the depth of penetration of the spherical indenter, since this gives hardness values that are too low. It is shown that when the surface of a polymeric specimen is indented by a rigid ball, the effect of the spherical stress tensor on the yield point is important; the average stress on the contact area at the beginning for forced high-elastic deformation is close to the compressive yield stress of the polymeric material and considerably higher than its tensile yield stress.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 995–998, November–December, 1971.     

Analysis of nanoindentation experiments by means of atomic force microscopy

Nanoindentation is quite a common method for local material characterization. Values for hardness and Young's modulus can be determined directly from the recorded data. Essential for the correct determination of the material parameters is the precise measurement of the actual indentation depth of the indenter. The indenter measures the current depth by means of a Wheatstone bridge which correlates the indentation depth to a change in voltage. A possible tool for the verification of the recorded indentation depths is Atomic Force Microscopy (AFM). AFM is able to scan an area of indents for almost any surface. The deflection of the tip is measured by a laser spot reflected from the surface of the cantilever. The difference in height between the surface and the indent can directly be read off from the plotted image. However, using an AFM only allows us to measure the depth of the permanent indentation depth after unloading the indenter. Nevertheless, correlation between the remaining indentation depths measured by the explained methods allows for a first assessment of the correctness of the online recorded depth-data by the nanoindenter. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-Temperature Creep of Fibrous Metal-Matrix Composites under Varying Stresses

The present paper is aimed at testing the hypothesis about the failure of the relatively weak fiber/matrix interface under cyclic loading, which causes an increase in the steady-state creep rate. The hypothesis is tested qualitatively by comparing the creep behavior of composite specimens with various interface strengths under the conditions mentioned (loading-unloading-loading to the original level). The hypothesis is tested semi-quantitatively by estimating the interface strength in relation to the action decreasing the strength. The latter requires the use of a microstructural calculation model. Both the approaches are used in the paper, and the results found support this hypothesis. The experimental data obtained are an additional argument for the necessity of developing metal-matrix composites with a strong interface, which can be a basis for real creep-resistant high-temperature composites.     

Long-Term Prediction of Creep Strain of Expanded Polystyrene

Data obtained in investigating the creep of expanded polystyrene in compression are discussed. An analysis of the results of creep tests 65–608 days long showed that the creep can be predicted for 50 years based on experiments of less duration than the 605 days indicated in EN 13163. For this purpose, it is suggested to employ a 95-fold extrapolation in time if the creep curves are described by a power function (as recommended by EN 13163) or a 50-fold extrapolation if the exponential equation proposed in the present paper is used. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 795–802, November–December, 2005.     

Long-Term Creep of Hybrid Aramid/Glass-Fiber-Reinforced Plastics

The results of experimental investigation of the long-term creep of SVM aramid fibers, EDT-10 epoxy resin, aramid-epoxy FRP (fiber-reinforced plastics), glass-epoxy FRP, and aramid/glass-epoxy hybrid FRP with different volume fractions of aramid and glass fibers are presented. The long-term tests were continued for 50,000 h (5.7 years). A structural approach for predicting the long-term creep from the properties and content of the components is considered. The effect of hybridization (partial replacement of the aramid fibers by glass fibers) on the inelastic deformation of hybrid FRP is discussed. The redistribution of stresses in the components during long-term creep of the hybrid composites is analyzed.     

Creep in unidirectionally reinforced polymer composites

Conclusions We have explored here the possibilities of predicting the permanent creep in unidirectionally reinforced polymer composite materials from the results of accelerated testing of their components. The mutually independent components of the viscoelastic compliance tensor under conditions of creep have been calculated with the aid of the Laplace transformation and an earlier verified variant of determining the mean elastic characteristics of a composite material, whereupon the originals of the sought functions have been obtained by a numerical inverse Laplace transformation. Experiments were performed with unidirectionally reinforced materials, a glass-plastic, an organoplastic, a carbon-plastic, and a boroplastic, all tested in tension in the direction of fiber reinforcement and in the transverse direction, also in longitudinal shear. The possibilities of predicting the creep in composite materials with the aid of functions characterizing the viscoelastic compliance of the binder and obtained by the method of temperature-time analogy have been confirmed twofold: by long-duration (up to 3 yr) control tests performed on the given materials and by control calculation of the creep in these composite materials from results of direct long-duration (up to 5 yr) tests performed on specimens of the binder.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 215–223, March–April, 1984.