三点弯曲试样应力强度的动态响应

采用振动理论分析了三点弯曲试样的动态响应,得到了一个计及冲击速度影响的动态应力强度因子计算公式。当不考虑冲击速度影响时,本文给出的计算模型可退化成经典的K.Kishimoto模型。数值计算的结果表明,无论是在阶跃载荷作用下,还是在周期载荷作用下,冲击速度对三点弯曲试样应力强度因子的动态响应都有明显的影响。     

In-Situ SEM High Strain Rate Testing of Large Diameter Micropillars Followed by TEM and EBSD Postmortem Analysis

Experimental Mechanics - Dislocation dynamic simulations are intended as a tool to understand and predict the mechanical behavior of metallic materials, but its prediction has never been directly...     

泡沫铝合金填充圆管三点弯曲实验研究

用实验方法研究了三种不同管壁厚度、两种跨径的泡沫铝合金填充圆管的三点弯曲力学性能,得到了泡沫铝合金填充管结构承载过程中的三种变形模式,即压入、压入弯曲和管壁下缘拉裂破坏。给出了空管和泡沫铝合金填充管的载荷位移曲线,并进行了比较。实验发现泡沫铝合金填充管结构的承载能力随泡沫铝合金密度的增大而增大,但破坏应变则随之减小。结构承载力的相对提高量随着管壁厚度的减小和跨径的增大而增大。此外,分析了泡沫铝合金提高填充管结构承载能力的机理。泡沫铝合金填充使管壁压入量和管截面抗弯刚度的损失显著减小,从而提高了结构的抗弯能力。     

冲击载荷下含空孔三点弯曲梁的动态断裂行为

为得到圆孔缺陷对运动裂纹扩展过程的影响规律,采用动态焦散线实验方法进行模型实验,研究了冲击载荷下含空孔三点弯曲梁的动态断裂行为。研究结果表明:空孔对裂纹扩展有极大的阻碍作用,在一定范围内,空孔直径越大,阻碍作用越明显。裂纹扩展到空孔附近时,扩展速度会下降。裂纹在空孔上部再次起裂后,最大扩展速度远远大于裂纹与空孔贯通前的最大扩展速度。裂纹扩展至空孔附近时,裂纹尖端动态应力强度因子KdⅠ和KdⅡ均会下降。裂纹在空孔上部再次起裂后,裂尖的应力强度因子KdⅠ和KdⅡ均大于裂纹与空孔贯通前裂尖的KdⅠ和KdⅡ。在整个扩展过程中,裂纹尖端的动态应力强度因子KdⅡ远小于KdⅠ,说明KdⅠ在裂纹扩展过程中起主要作用。     

薄膜材料微观力学行为的有限元分析

采用有限元法对材料表面改性层和薄膜材料在显微压入过程中的力学行为进行了计算机模拟。从所得的载荷与压入深度的关系曲线、压痕周围应力、应变场的大小和形状分布曲线等为依据，对在超显微硬度测试中基底材料及界面层的影响进行了详细的讨论，并得出：为排除基底材料的影响，通常规定压入深度（Ｄ）不得超过膜厚（ｔ）的１０～２０％的规则并不适用于所有薄膜系统。测试时，允许的Ｄ／ｔ的临界比值将随薄膜系统不同而异。对软膜硬基底系统而言，由于压头下的塑性应变区更多的是沿膜层的横向扩展，故Ｄｃ／ｔ允许大于上述规定值，而对硬膜软基底系统而言，则由于压头下的塑性应变区很容易扩展到基底材料中去，其Ｄｃ／ｔ值将小于上述规定值。     

第四次国际散体介质细观力学会议简介

１会议概况第四次国际散体介质细观力学会议（简称Ｐｏｗｄｅｒｓ　＆　Ｇｒａｉｎｓ　２００１）于２００１年５月２１日到５月２５日在日本仙台召开．来自法、日、美、英、德、中等１７个国家的１４０多人参加了会议,共收到论文１４０篇,其中７１篇在大会报告交流．口头报告和墙展均已收入会议论文集《Ｐｏｗｄｅｒｓ　ａｎｄ　Ｇｒａｉｎｓ　２００１》,论文数量是历次会议中最多的．前３次会议分别在法国（１９８９）,英国（１９９３）和美国（１９９７）召开,下次会议已决定２００５年在德国召开．这是大陆（３人）和香港（１人）…     

A Quantitative In Situ SEM Bending Method for Stress Relaxation of Microscale Materials at Room Temperature

Experimental Mechanics - Although the time-dependent deformation behaviors of microscale materials have been investigated through experiments with uniaxial loading conditions, the influence of the...     

Micromechanics and macromechanics of carbon nanotube-enhanced elastomers

The effects of carbon nanotubes on the mechanical behavior of elastomeric materials is investigated. The large deformation uniaxial tension and uniaxial compression stress-strain behaviors of a representative elastomer are first presented. This elastomer is then reinforced with multi-wall carbon nanotubes (MWNTs) and the influence of weight fraction of MWNTs on the large deformation behavior of the resulting composite is quantified. The initial stiffness and subsequent strain-induced stiffening at large strains are both found to increase with MWNT content. The MWNTs are also found to increase both the tensile strength and the tensile stretch at break. A systematic approach for reducing the experimental data to isolate the MWNT contribution to the strain energy of the composite is presented. A constitutive model for the large strain deformation behavior of MWNT-elastomer composites is then developed. The effects of carbon nanotubes are modeled via a constitutive element which tracks the stretching and rotation of a distribution of wavy carbon nanotubes. The MWNT strain energy contribution is due to the bending/unbending of the initial waviness and provides the increase in initial stiffness as well as the retention and further enhancement of the increase in stiffness with large strains. The model is shown to track the stretching and rotation of the CNTs with macroscopic strain as well as predict the dependence of the macroscopic stress-strain behavior on the MWNT content for both uniaxial tension and uniaxial compression.     

Micromechanics of cleavage fracture initiation in ferritic steels by carbide cracking

Cleavage fracture in ferritic steels is often initiated in brittle carbides randomly distributed in the material. The carbides break as a result of a fibre loading mechanism in which the stress levels in the carbides are raised, as the surrounding ferrite undergoes plastic deformation. The conditions in the vicinity of the nucleated micro-crack will then determine whether the crack will penetrate or be arrested by the ferrite. The ferrite is able to arrest nucleated cracks through the presence of mobile dislocations, which blunt and shield the microcrack and thus lowers the stresses at the crack tip. Hence, the macroscopic toughness of the material directly depends on the ability of the ferrite to arrest nucleated micro-cracks and in turn on the plastic rate sensitivity of the ferrite. The initiation of cleavage fracture is here modelled explicitly in the form of a micro-crack, which nucleates in a brittle carbide and propagates into the surrounding ferrite. The carbide is modelled as an elastic cylinder or in a few cases an elastic sphere and the ferrite as an elastic viscoplastic material. The crack growth is modelled using a cohesive surface, where the tractions are governed by a modified exponential cohesive law. It is shown that the critical stress, required to propagate a microcrack from a broken carbide, increases with decreasing plastic rate sensitivity of the ferrite. The results also show that a low stress triaxiality and a high aspect ratio of the carbide promote the initiation of cleavage fracture from a broken carbide.     

Bending of layers weakened by cuts

In the present paper, a method proposed by one of the authors is extended to a class of skew-symmetric elastic problems for the stress analysis of a layer supported by sliding fixed supports and weakened by several stress raisers. The corresponding boundary value problem is reduced to an infinite system of one-dimensional singular integral equations of the second kind. The expressions for the stress components in an elastic layer weakened by stress raisers are presented. Based on the developed analytical algorithm, extensive numerical investigations have been conducted. The results of these investigations are illustrated graphically exposing some novel qualitative and quantitative knowledge about stress concentration in the layer depending on some geometric parameters of stress raisers and Poisson’s ratio of a layer material.     

Measurement of Residual Stresses in Nuclear-grade Zircaloy-4(R) Tubes—Effect of Heat Treatment

Nuclear-grade Zircaloy-4(R) tubes are produced by a unique manufacturing process known as pilgering, which leaves the material in a work-hardened state containing a pattern of residual stresses. Moreover, such tubes exhibit elastic anisotropy as a result of the pilgering process. Therefore, standard equations originally proposed by Sachs (Z Met Kd, 19: 352–357, 1927; Sachs, Espey, Iron Age, 148: 63–71, 1941). for isotropic materials do not apply in this situation. Voyiadjis et al. (Exp Mech, 25: 145–147, 1985) proposed a set of equations for treating elastically anisotropic materials, but we have determined that there are discrepancies in their equations. In this paper, we present the derivation for a set of new equations for treating elastically anisotropic materials, and the application of these equations to residual stress measurements in Zr-4(R) tubes. To this end, through thickness distribution of residual stress components in as-received and heat treated (500°C) Zr-4(R) tubes was measured employing the Sachs’ boring-out technique in conjunction with electrochemical machining as the means of material removal, and our new equations. For both as-received and the heat treated materials, the axial and tangential residual stresses were significantly higher than the radial and shear residual stresses. The largest residual stress was the tangential stress component in the as-received material, showing a tensile value at the outer surface and a compressive value at the inner surface. At high values of von Mises equivalent stress, the principal directions of residual stress coincided with the principal axes of the tube for the as-received material, as well as for the material heat treated at 500°C.     

Micromechanics Contribution to Coupled Transport and Mechanical Properties of Fractured Geomaterials

This article is devoted to the modelling of interdependent mechanical and hydraulic behaviours of geomaterials in presence of a single through-wall fracture by means of micromechanics arguments. Experimental results of fractured concrete samples show non-linear evolutions for both mechanical and hydraulic behaviours with respect to confinement intensity. These non-linear responses are interpreted by the progressive closure of crack-like pores defining the pore volume of the fracture interfacial domain. Disregarding tortuosity effects, we adopt a 2D representation for these cracks. The key role of the fracture initial porosity is also emphasized. It allows to discuss the shape of the distribution of the local apertures distribution function classically used, intercepted here in terms of the distribution of initial crack–aspect ratio within the fracture domain. Application on fractured concrete samples shows the capability of the theoretical model to accurately reproduce the experimental results.     

Bending of rectangular plates carried by vacuum cups

In order to carry thin plates, vacuum cups are frequently used. When the over-hang is large, the deflections and stresses of the plate have considerably large values. In this paper, the rectangular plate hung by circular vacuum cups is treated. The analysis is carried out for the plate, which is subjected to line loads and radial bending moments at the inner circular boundary and free at the outer rectangular boundary. In addition to these boundary conditions, the plate is subjected to different distributed loads on inner and outer domains. First, the general solutions for the deflections on each domain are obtained by using infinite series, which are expressed by the polar coordinate system. The several undetermined constants in these equations are decreased by the conditions of continuity at the inner boundaries. Satisfying the boundary conditions at the finite points on the outer edges of the plate, the deflections and stresses of the plate and the contact pressures between the plate and the vacuum cup are calculated. Typical results are presented in dimensionless graphical form for different parameters and vacuum cup edge conditions.     

Micromechanics of breakage in sharp-edge particles using combined DEM and FEM

By combining DEM （Discrete Element Method） and FEM （Finite Element Method）, a model is established to simulate the breakage of twodimensional sharp-edge particles, in which the simulated particles are assumed to have no cracks. Particles can, however, crush during different stages of the numerical analysis, if stress-based breakage criteria are fulfilled inside the particles. With this model, it is possible to study the influence of particle breakage on macro- and micro-mechanical behavior of simulated angular materials. Two series of tests, with and without breakable particles, are simulated under different confining pressures based on conditions of biaxial tests. The results, presented in terms of micromechanical behavior for different confining pressures, are compared with macroparameters. The influence of particle breakage on microstructure of sharp-edge materials is discussed and the related confining pressure effects are investigated. Breakage of particles in rockfill materials are shown to reduce the anisotropy coefficients of the samples and therefore their strength and dilation behaviors.     

Micromechanics of breakage in sharp-edge particles using combined DEM and FEM

By combining DEM (Discrete Element Method) and FEM (Finite Element Method),a model is established to simulate the breakage of two-dimensional sharp-edge particles,in which the simulated particles are assumed to have no cracks.Particles can,however,crush during different stages of the numerical analysis,if stress-based breakage criteria are fulfilled inside the particles.With this model,it is possible to study the influence of particle breakage on macro- and micro-mechanical behavior of simulated angular materials.Two series of tests,with and without breakable particles,are simulated under different confining pressures based on conditions of biaxial tests.The results,presented in terms of micromechanical behavior for different confining pressures,are compared with macroparameters.The influence of particle breakage on microstructure of sharp-edge materials is discussed and the related confining pressure effects are investigated.Breakage of particles in rockfill materials are shown to reduce the anisotropy coefficients of the samples and therefore their strength and dilation behaviors.     

Micromechanics and structural response of functionally graded,particulate-matrix,fiber-reinforced composites

Reinforcement of fibrous composites by stiff particles embedded in the matrix offers the potential for simple, economical functional grading, enhanced response to mechanical loads, and improved functioning at high temperatures. Here, we consider laminated plates made of such a material, with spherical reinforcement tailored by layer. The moduli for this material lie within relatively narrow bounds. Two separate moduli estimates are considered: a “two-step” approach in which fibers are embedded in a homogenized particulate matrix, and the Kanaun–Jeulin (Kanaun, S.K., Jeulin, D., 2001. Elastic properties of hybrid composites by the effective field approach. Journal of the Mechanics and Physics of Solids 49, 2339–2367) approach, which we re-derive in a simple way using the Benveniste (1988) method. Optimal tailoring of a plate is explored, and functional grading is shown to improve the performance of the structures considered. In the example of a square, simply supported, cross-ply laminated panel subjected to uniform transverse pressure, a modest functional grading offers significant improvement in performance. A second example suggests superior blast resistance of the panel achieved at the expense of only a small increase in weight.     

Bending of rectangular flat slabs supported by four columns

In this paper an analytical solution is proposed for the bending of uniformly loaded rectangular plates supported only by four intermediate columns, the edges and corners of which are all free. For several particular cases, the numerical results, which contain the column reaction and the values for the deflection and the bending moments at several points of the plate, are given.Calculations indicate that the method proposed in this paper is valid.