Flow of a fluid-solid mixture: Normal stress differences and slip boundary condition

In this paper, using mixture theory we study the flow of a dense suspension, composed of solid particles and a fluid; the emphasis is on the influence of the slip boundary condition and the effect of normal stress differences. Very little work has been done considering both the slip at the walls and the normal stress effects in the frame of a two-component flow. In this paper, the stress tensor for the solid component is modeled as a nonlinear fluid which not only includes the viscous effects but also the normal stress effects; the fluid constituent is modeled as a viscous fluid. We look at the flow between two flat plates.     

向错偶极子对楔型裂纹尖端场

研究了材料中楔型向错偶极子与楔型裂纹的弹性干涉问题. 运用复变函数方法获得了复势函 数和应力场的封闭形式解答,导出了楔型裂纹尖端应力强度因子的解析表达式. 讨论了向错 偶极子的位置、方向和偶臂长度对楔型裂纹尖端应力强度因子的屏蔽和反屏蔽作用规律. 研 究结果表明,向错偶极子靠近裂纹尖端时,对应力强度因子的屏蔽或反屏蔽作用非常强烈. 在一定条件下,楔型向错偶极子能够延缓楔型裂纹的扩展;偶极子的方向也存在一个临界值 使其对应力强度因子的屏蔽或反屏蔽效应最大. 此外,楔型裂纹张开角以及偶极子臂长对应 力强度因子也有较大的影响.     

退火和调质42CrMo合金钢单轴循环塑性行为的实验研究

在室温下对退火和调质42CrMo合金钢进行了单轴应变控制和应力控制的系统循环实验,并对它们的应变循环和应力循环特性进行比较.揭示和分析了应变幅值、平均应变及其历史对材料应变循环特性的影响以及应力幅值、平均应力及其历史对棘轮行为的影响.讨论了应变循环和应力循环间的交互作用以及不同热处理工艺下材料循环变形行为间的区别.研究发现材料的热处理工艺、平均应力和应力幅值及其历史对材料的棘轮行为都有很大的影响.得到了一些有助于进行合理本构描述的结果.     

任意多孔多裂纹有限大板的应力强度因子分析

采用各向异性体平面弹性理论中的复势方法,以Faber级数为工具,应用保角映射技术和最小二乘边界配点法,导出内边界条件精确满足,外边界条件近似满足的含多椭圆孔及裂纹群有限大板在任意载荷作用下的应力场、位移场的级数解,建立了任意多椭圆孔及裂纹群有限大板应力强度因子的有效分析方法,讨论了各参数对裂尖应力强度因子及孔边应力集中的影响．数值结果表明,该方法具有计算精度高、收敛速度快、方便快捷等优点,有利于全面系统地研究各参数对结构断裂性能的影响．     

粘塑性薄壁管中复合应力波的传播特性研究

以本构关系一般理论为基础,导出了计及材料功硬化效应和应变率硬化效应的粘塑性薄壁管的本构关系及管中复合应力波的控制方程,应用有限差分方法研究了在压扭复合冲击载荷作用下粘塑性薄壁管中复合应力波的传播特性与演化规律,分析了复合应力波的耦合效应以及薄壁管中粘塑性参数和功硬化效应对复合应力波传播与演化规律的影响,并对有关现象进行了分析和解释。     

Effects of pressure-sensitivity and plastic dilatancy on void growth and interaction

Hydrostatic stress can affect the non-elastic deformation and flow stress of polymeric materials and certain metallic alloys. This sensitivity to hydrostatic stress can also influence the fracture toughness of ductile materials, which fail by void growth and coalescence. These materials typically contain a non-uniform distribution of voids of varying size-scales and void shapes. In this work, the effects of void shape and microvoid interaction in pressure-sensitive materials are examined via a two-prong approach: (i) an axisymmetric unit-cell containing a single ellipsoidal void and (ii) a plane-strain unit-cell consisting of a single large void and a population of discrete microvoids. The representative material volume in both cases is subjected to physical stress states similar to highly stressed regions ahead of a crack. Results show that oblate voids and microvoid cavitation can severely reduce the critical stress of the material. These effects can be compounded under high levels of pressure-sensitivity. In some cases, the critical stress responsible for rapid void growth is reduced to levels comparable to the yield strength of the material. The contribution of void shape and pressure-sensitivity to the thermal- and moisture-induced voiding phenomenon in IC packages is also discussed.     

Effects of stress dependent electrochemical reaction on voltage hysteresis of lithium ion batteries

Intercalation of lithium ions into the electrodes of lithium ion batteries is affected by the stress of active materials, leading to energy dissipation and stress dependent voltage hysteresis. A reaction-diffusion-stress coupling model is established to investigate the stress effects under galvanostatic and potentiostatic operations. It is found from simulations that the stress hysteresis contributes to the voltage hysteresis and leads to the energy dissipation. In addition, the stress induced voltage hysteresis is small in low rate galvanostatic operations but extraordinarily significant in high rate cases. In potentiostatic operations, the stresses and stress induced overpotentials increase to a peak value very soon after the operation commences and decays all the left time. Therefore, a combined charge-discharge operation is suggested, i.e., first the galvanostatic one and then the potentiostatic one. This combined operation can not only avoid the extreme stress during operations so as to prevent electrodes from failure but also reduce the voltage hysteresis and energy dissipation due to stress effects.     

任意多椭圆孔多裂纹无限大各向异性板应力强度因子求解的一种新方法

采用各向异性体平面弹性理论中的复势方法,应用保角变换技术,以F aber级数为工具,导出含任意多椭圆孔和裂纹群无限大各向异性板在远场载荷作用下其应力场和位移场的级数解,并在此基础上利用断裂力学方法确定裂纹尖端的应力强度因子,通过算例讨论了材料参数及裂纹、孔的尺寸等对应力强度因子的影响规律,得出了一些有益的结论。数值结果表明本文方法具有计算精度高、收敛速度快、方便快捷等优点,有利于全面系统地研究各参数对结构断裂性能的影响。     

Analytical solutions for thermal vibration of nanobeams with elastic boundary conditions

A nonlocal Euler beam model with second-order gradient of stress taken into consideration is used to study the thermal vibration of nanobeams with elastic boundary.An analytical solution is proposed to investigate the free vibration of nonlocal Euler beams subjected to axial thermal stress.The effects of the nonlocal parameter,thermal stress and stiffness of boundary constraint on the vibration behaviors of nanobeams are revealed.The results show that natural frequencies including the thermal stress are lower than those without the thermal stress when temperature rises.The boundary-constrained springs have significant effects on the vibration of nanobeams.In addition,numerical simulations also indicate the importance of small-scale effect on the vibration of nanobeams.     

On a principle of virtual work for thermo-elastic bodies with couple stresses

A principle of virtual work is proposed for thermo-elastic bodies with couple stresses. From it are derived the equations of motion, the stress principles of Cauchy and the Cosserats, and the stress, couple stress and entropy relations. The principle is also used to analyze the effects of an internal constraint on the response of the body.     

An Experimental Study on the Effects of Water Content on Coalbed Gas Permeability in Ground Stress Fields

Experimental coal cores were collected from a coalbed of Sihe colliery and Zhaozhuang colliery, Qinshui Basin, China. Their gas effective permeability was studied under effects of water content and effective stress. The experiments were mainly carried out on a self-made ??Triaxial Stress Thermal?Chydrological?Cmechanical Coal Gas Permeameter.?? The results showed that when the temperatures of gas and coal were constant, a negative effect of either water content or effective stress was reported on the gas transportation, i.e., the gas effective permeability decreased with the increasing of water content under constant effective stress and it also decreased as the effective stress increased when the water content was constant. Under experimental conditions as in this study, the effects of water content and effective stress on the gas effective permeability was described by a linear?Cexponential equation, which presented that the gas effective permeability had a linear relationship with the water content and an exponential relationship with the effective stress. The permeation pores were defined as the primary places of transporting the coalbed gas. They were affected by water content and effective stress in different ways. The water content occupied the space of permeation pores, while the effective stress changed the shape of permeation pores. Consequently, the gas effective permeability was also affected by the two aspects.     

剪应力对培养内皮细胞表面ATP浓度的调节

剪应力信号转导是目前细胞生物力学的研究热点之一。Ca^2＋作为一种第二信使在剪应力信号转导中扮演了重要角色，钙信号转导是通过胞内自由Ca^2＋浓度变化来实现的。胞内Ca^2＋浓度的大小不仅依赖于细胞承受剪应力的大小，同时依赖于细胞外ATP的浓度。本文采用剪应力诱发内皮细胞分泌ATP的时变模型，通过对平行平板流动腔中的ATP浓度场进行数值模拟，分析了剪应力对培养内皮细胞表面ATP浓度的影响。结果表明，剪应力诱发的ATP分泌明显影响细胞表面附近ATP的浓度；剪应力、对流和扩散效应、ATP的水解作用共同决定了细胞外表面附近ATP的浓度。这一结果提示，剪应力大小并非剪应力信号转导过程中的唯一决定因素；剪应力的作用、激动剂的浓度改变共同参与了剪应力信号转导。     

Effects of characteristic material lengths on mode III crack propagation in couple stress elastic–plastic materials

The asymptotic fields near the tip of a crack steadily propagating in a ductile material under Mode III loading conditions are investigated by adopting an incremental version of the indeterminate theory of couple stress plasticity displaying linear and isotropic strain hardening. The adopted constitutive model is able to account for the microstructure of the material by incorporating two distinct material characteristic lengths. It can also capture the strong size effects arising at small scales, which results from the underlying microstructures. According to the asymptotic crack tip fields for a stationary crack provided by the indeterminate theory of couple stress elasticity, the effects of microstructure mainly consist in a switch in the sign of tractions and displacement and in a substantial increase in the singularity of tractions ahead of the crack-tip, with respect to the classical solution of LEFM and EPFM. The increase in the stress singularity also occurs for small values of the strain hardening coefficient and is essentially due to the skew-symmetric stress field, since the symmetric stress field turns out to be non-singular. Moreover, the obtained results show that the ratio η introduced by Koiter has a limited effect on the strength of the stress singularity. However, it displays a strong influence on the angular distribution of the asymptotic crack tip fields.     

Love–Bishop rod solution based on strain gradient elasticity theory

In the present work, the propagation of longitudinal stress waves is investigated with a strain gradient elasticity theory given by Lam et al. In principle, the analysis of wave motion is based on the Love rod model including the lateral deformation effects, but in the same time is also taken into account the shear strain effects with Bishop?s correction. By applying Hamilton?s principle, a general explicit strain gradient elasticity solution is developed for the longitudinal stress waves, and it is compared with the special solutions based on the modified couple stress and classical theories. This work gives useful information with regard to the meaning of the three scale parameters in the strain gradient elasticity theory used here.     

On the anti-plane shear of an elliptic nano inhomogeneity

The elastic field of an elliptic nano inhomogeneity embedded in an infinite matrix under anti-plane shear is studied with the complex variable method. The interface stress effects of the nano inhomogeneity are accounted for with the Gurtin–Murdoch model. The conformal mapping method is then applied to solve the formulated boundary value problem. The obtained numerical results are compared with the existing closed form solutions for a circular nano inhomogeneity and a traditional elliptic inhomogeneity under anti-plane. It shows that the proposed semi-analytic method is effective and accurate. The stress fields inside the inhomogeneity and matrix are then systematically studied for different interfacial and geometrical parameters. It is found that the stress field inside the elliptic nano inhomogeneity is no longer uniform due to the interface effects. The shear stress distributions inside the inhomogeneity and matrix are size dependent when the size of the inhomogeneity is on the order of nanometers. The numerical results also show that the interface effects are highly influenced by the local curvature of the interface. The elastic field around an elliptic nano hole is also investigated in this paper. It is found that the traction free boundary condition breaks down at the elliptic nano hole surface. As the aspect ratio of the elliptic hole increases, it can be seen as a Mode-III blunt crack. Even for long blunt cracks, the surface effects can still be significant around the blunt crack tip. Finally, the equivalence between the uniform eigenstrain inside the inhomogeneity and the remote loading is discussed.     

Propagation behavior of Love waves in a functionally graded half-space with initial stress

The propagation behavior of Love waves in a functionally graded material layered non-piezoelectric half-space with initial stress is taken into account. The Wentzel–Kramers–Brillouin (WKB) technique is adopted for the theoretical derivations. The analytical solutions are obtained for the dispersion relations and the distributions of the mechanical displacement and stress along the thickness direction in the layered structure. First, these solutions are used to study the effects of the initial stress on the dispersion relations and the group and phase velocities, then the influences of the initial stress on the distributions of the mechanical displacement and shear stresses along the thickness direction are discussed in detail. Numerical results obtained indicate that the phase velocity of the Love waves increases with the increase in the magnitude of the initial tensile stress, while decreases with the increase in the magnitude of the initial compression stress. The effects on the dispersion relations of the Love wave propagation are negligible as the magnitudes of the initial stress are less than 100 MPa. Some other results are obtained for the distributions of field quantities along thickness direction. The results obtained are not only meaningful for the design of functionally graded structures with high performance but also effective for the evaluation of residual stress distribution in the layered structures.     

圆柱外表面受热冲击问题的广义热弹性分析

基于 L-S 广义热弹性理论, 针对实心圆柱体在外表面受均匀热冲击作用下的一维广义热弹性问题进行研究分析. 利用热冲击的瞬时特征, 借助于 Laplace 正、反变换技术及柱函数的渐近性质, 推导了热冲击作用周期内温度场、位移场和应力场的渐近表达式. 通过计算, 得到了热冲击条件下各物理场的分布规律以及延迟效应和耦合效应对热弹性响应的影响规律. 结果表明: 当考虑延迟效应和耦合效应时, 热扰动将以两组速度不同的波的形式向前传播, 延迟效应和耦合效应对各物理场的建立时间, 阶跃间隔和阶跃峰值均产生影响, 且延迟效应和耦合效应均在一定程度上削弱了热冲击的作用效果.      

Material effects during monotonic-cyclic loading

The effects commonly related with deformation caused by proportional and non-proportional loading types were identified experimentally. In the case of non-proportional cyclic loading along circular strain path the second order effects such as: phase shift between stress and strain signals was observed. An analysis of experimental data from tests under non-proportional cyclic loading along square strain path exhibited a significant reduction of stress independently on direction of deformation.The paper also presents experimental results concerning evaluation of an influence of cyclic loading on stress variations during monotonic deformation carried out on the pure copper and X10CrMoVNb9-1 steel. All strain controlled tests were performed at room temperature using thin-walled tubular specimens. The experimental programme contained selected combinations of monotonic and cyclic loadings, i.e. the torsion-reverse-torsion cycles were superimposed on the monotonic tension. It is shown that such cycles associated with monotonic tension caused essential variations of tensile stress. For both materials, a significant decrease of the axial stress was visible. The effects observed during monotonic and cyclic loading combinations were theoretically described using the Mróz and Maciejewski model.     

Local effects in plates - theoretical and practical consequences

The term “local effect” denotes local, pronounced, three-dimensional stress states which occur primarily at the boundaries of flat beams and stretched plates made of homogeneous or composite materials, particularly in the regions of notches, cracks, local mechanical and thermal loads, etc., but which may also occur in any other region. These effects are still ignored in common procedures of analytical mechanics and in traditional procedures of experimental stress analysis, despite the fact that they often increase drastically the maximal stress values occurring in critical regions of structures made of homogeneous and composite materials.The most consistent and rapid response to the needs of modern engineering design with respect to local effects has developed within the field of analytical mechanics. The emphasis has been placed on mathematically and physically admissible analytical solutions. The problem is that the developed analytical models and solutions must be tested experimentally and that this task cannot be reliably accomplished by using experimental procedures based on the concept of plane stress state.This paper presents three new analytical/experimental groups of methods developed by the author and his co-workers: isodyne methods, strain gradient methods and thermoelastic methods. The first two methods have been developed to analyze local effects: static and dynamic. The third method was developed to study dynamic strain/stress fields. The presented empirical evidence shows that the isodyne techniques and strain gradient techniques can be used to evaluate major stress components of three-dimensional stress states in plates and beams caused by local effects. This evidence can be used either directly in engineering design, or as a measure to test the predictive power of analytical models and solutions, and in particular, to quantify errors caused by the simplifying assumptions accepted in analytical solutions.     

复杂应力状态下岩土材料非线性本构模型研究

基于统一强度理论,将洛德参数引入统一强度理论,推导出材料的统一强度参数,进而分析中间主应力以及主剪应力系数对材料统一强度参数的影响。在此基础上,对邓肯张双曲线模型进行改进,使得该模型能够反映复杂应力状态下的应力应变关系。采用粘土的平面应变试验进行验证,结果表明:在平面应变情况下,当b=0.4时,模型能够较好地反映粘土的应力应变关系,及其强度参数。分析了洛德参数以及中间主应力系数对模型的影响,进一步说明岩土材料存在主应力效应。该模型能够反映不同材料在复杂应力状态下的应力应变关系,有其更为广泛的适用性。