刚塑性广义变分不等原理及其在平面应变分析中的应用

首先利用Lagrangian乘子法，从势能角度出发构造了考虑摩擦效应这一能导致变分不等形式的广义能量泛函，把一般的有条件的变分原理化为无条件的变分原理唯一确定，得出了各Lagrangian乘子所代表的物理意义。建立了刚塑性理论中的Coulomb摩擦约束的广义变分不等原理。而后基于退化的摩擦约束广义变分等式原理，对长矩形板镦粗进行了塑性加工工步分析，所得结果与经典上限法结果相吻合。     

Modeling the evolution of crystallographic dislocation density in crystal plasticity

Dislocations are the most important material defects in crystal plasticity, and although dislocation mechanics has long been understood as the underlying physical basis for continuum crystal plasticity formulations, explicit consideration of crystallographic dislocation mechanics has been largely absent in working constitutive models. Here, dislocation density state variables evolve from initial conditions according to equations based on fundamental concepts in dislocation mechanics such as the conservation of Burgers vector in multiplication and annihilation processes. The model is implemented to investigate the polyslip behavior of single-crystal aluminum. The results not only capture the mechanical stress/strain response, but also detail the development of underlying dislocation structure responsible for the plastic behavior.     

Homogenization of inelastic solid materials at finite strains based on incremental minimization principles. Application to the texture analysis of polycrystals

The paper presents new continuous and discrete variational formulations for the homogenization analysis of inelastic solid materials undergoing finite strains. The point of departure is a general internal variable formulation that determines the inelastic response of the constituents of a typical micro-structure as a generalized standard medium in terms of an energy storage and a dissipation function. Consistent with this type of finite inelasticity we develop a new incremental variational formulation of the local constitutive response, where a quasi-hyperelastic micro-stress potential is obtained from a local minimization problem with respect to the internal variables. It is shown that this local minimization problem determines the internal state of the material for finite increments of time. We specify the local variational formulation for a distinct setting of multi-surface inelasticity and develop a numerical solution technique based on a time discretization of the internal variables. The existence of the quasi-hyperelastic stress potential allows the extension of homogenization approaches of finite elasticity to the incremental setting of finite inelasticity. Focussing on macro-deformation-driven micro-structures, we develop a new incremental variational formulation of the global homogenization problem for generalized standard materials at finite strains, where a quasi-hyperelastic macro-stress potential is obtained from a global minimization problem with respect to the fine-scale displacement fluctuation field. It is shown that this global minimization problem determines the state of the micro-structure for finite increments of time. We consider three different settings of the global variational problem for prescribed displacements, non-trivial periodic displacements and prescribed stresses on the boundary of the micro-structure and develop numerical solution methods based on a spatial discretization of the fine-scale displacement fluctuation field. Representative applications of the proposed minimization principles are demonstrated for a constitutive model of crystal plasticity and the homogenization problem of texture analysis in polycrystalline aggregates.     

Plasticity criterion for porous medium with cylindrical voidUn critère de plasticité pour un matériau poreux à vide cylindrique

A simple Gurson-based yield criterion for porous materials with cylindrical voids in plane stress is proposed. With no adjustable parameters, it compares quite satisfactorily with recent numerical data by Francescato et al. for different porosities. It is non-analytic with respect to the porosity, and displays an angulous point. To cite this article: Y.-P. Pellegrini, C. R. Mecanique 330 (2002) 763–768.     

螺旋藻C-藻蓝蛋白亚基分离及抗肿瘤活性研究

为解决C-藻蓝蛋白分子较大,难以直接与细胞发生作用的问题,对藻蓝蛋白亚基的分离纯化方法进行研究,并初步探讨了其对肿瘤细胞的抑制作用.对通过热变性、盐析和自制的羟基磷灰石吸附层析,从钝顶螺旋藻中获得高纯度的藻蓝蛋白(C-PC,A620/A2804.5).C-PC用8 mol.L-1的尿素溶液变性,过CM-Sepharose F.F.阳离子交换层析、透析、复性,获得具活性的C-PC亚基.用CCK-8试剂盒分别研究了C-PC、α和β亚基对一个肺腺癌细胞株SPC-A-1的生长的影响,结果表明藻蓝蛋白及其亚基对该细胞株的生长具有抑制作用,β亚基效果最好,预示着它们具有一定的抗肿瘤潜力.     

Finite multiplicative plasticity for small elastic strains with linear balance equations and grain boundary relaxation

This paper is concerned with the formulation of a phenomenological model of finite elasto-plasticity valid for small elastic strains for initially isotropic polycrystalline material. As a basic we assume the multiplicative split of the deformation gradient into elastic and plastic part. A key feature of the model is the introduction of an independent field of 'elastic' rotations which eliminate the remaining geometrical nonlinearities coming from finite elasticity in the presence of small elastic strains. In contrast to micro-polar theories an evolution equation for is presented which relates to making use of a new device found by the author to perform the polar decomposition asymptotically. The model is shown to be invariant under both change of frame and rotation of the so called intermediate configuration. The corresponding equilibrium equations at frozen plastic and viscoelastic configuration constitute then a linear, elliptic system with nonconstant coefficients which makes this model amenable to a rigorous mathematical analysis. The introduced hysteresis effects within the elastic region are related to viscous elastic rotations of the grains of the polycrystal due to internal friction at the grain boundaries and constitute as such a rate dependent transient texture effect. The inclusion of work hardening will be addressed in future work. Received March 07, 2002 / Published online February 17, 2003 RID="*" ID="*"Communicated by Kolumban Hutter, Darmstadt     

A comparison of nonlocal continuum and discrete dislocation plasticity predictions

Discrete dislocation simulations of two boundary value problems are used as numerical experiments to explore the extent to which the nonlocal crystal plasticity theory of Gurtin (J. Mech. Phys. Solids 50 (2002) 5) can reproduce their predictions. In one problem simple shear of a constrained strip is analyzed, while the other problem concerns a two-dimensional model composite with elastic reinforcements in a crystalline matrix subject to macroscopic shear. In the constrained layer problem, boundary layers develop that give rise to size effects. In the composite problem, the discrete dislocation solutions exhibit composite hardening that depends on the reinforcement morphology, a size dependence of the overall stress-strain response for some morphologies, and a strong Bauschinger effect on unloading. In neither problem are the qualitative features of the discrete dislocation results represented by conventional continuum crystal plasticity. The nonlocal plasticity calculations here reproduce the behavior seen in the discrete dislocation simulations in remarkable detail.     

A crystal plasticity model of a formation of a deformation band structure

The formation of deformation bands with the typically alternating sign of the misorientation across their boundaries is interpreted as spontaneous deformation instability caused by anisotropy of hardening. To analyse the nature of the fragmentation, a model of a rigid-plastic crystal domain deformed by symmetric double slip in a plane-strain compression is considered. The basic reason for the deformation band existence is that a local decrease in number of active slip systems in the bands is energetically less costly than a homogeneous deformation by multislip. However, such model of the bands predicts their extreme orientation and their width tends to zero. This trend is modified by hardening caused by a build up of the band boundaries and by a dislocation bowing (Orowan) stress. The model provides an explanation of observed orientation of the bands, their width and the significant change in the structural morphology seen as the band reorientation occurs at large strains. The predictions are in a favourable agreement with the available observations.     

Effects of ultrasound pretreatment with different frequencies and working modes on the enzymolysis and the structure characterization of rice protein

The effects of ultrasound pretreatment with different frequencies and working modes, including mono-frequency ultrasound (MFU), dual-frequency ultrasound (DFU) and tri-frequency ultrasound (TFU), on the degree of hydrolysis (DH) of rice protein (RP) and angiotensin-I-converting enzyme (ACE) inhibitory activity of RP hydrolysate were investigated. Ultraviolet–visible (UV) spectroscopy, fourier transform infrared (FTIR) spectroscopy, surface hydrophobicity and scanning electron microscopy (SEM) of RP pretreated with ultrasound were measured. The results showed that ultrasound pretreatment did not increase DH of RP significantly (p > 0.05). However, all the ultrasound pretreatment increased the ACE inhibitory activity of RP hydrolysate significantly (p < 0.05). The MFU of 20 kHz showed higher ACE inhibitory activity compared to that of other MFU. The ACE inhibitory activity of sequential DFU was higher than that of simultaneous with the same frequency combination. Sequential TFU of 20/35/50 kHz produced the highest increase in ACE inhibitory activity in contrast with other ultrasound frequencies and working modes. All the results under ultrasound pretreatment showed that ultrasound frequencies and working modes were of great effect on the ACE inhibitory activity of RP. The changes in UV–Vis spectra and surface hydrophobicity indicated the unfolding of protein and exposure of hydrophobic groups by ultrasound. The FTIR analysis showed that all the ultrasound pretreatment with different frequencies and working modes decreased α-helix, β-turn content and increased β-sheet, random coil content of RP. The SEM results indicated that ultrasound pretreatment resulted in the deformation of RP. In conclusion, the frequency selection of ultrasound pretreatment of RP is essential for the preparation of ACE inhibitory peptide.     

MgO单晶冲击变形产生点缺陷吸收谱的实时测量

为研究冲击塑性变形产生的缺陷对MgO单晶透明性的影响,采用40通道(波长)纳秒时间分辨高温计和冲击波原位光源技术,对在一维应变冲击加载下MgO单晶的透射谱进行了实验测量。冲击波加载方向垂直于样品(100)晶面,测谱范围为400~800nm,得到了2个压力点(约50GPa和约70GPa)的吸收系数随波长的变化曲线,从实测曲线发现了6个明显的特征吸收峰,其中心波长分别为410、460、490、520、580和660nm。通过对比分析,确定出410、460、490和580nm处的吸收峰为F聚心吸收,520nm处的吸收峰为V-心吸收,而660nm处的吸收峰则可能为与填隙原子相关的吸收。这是在冲击压缩条件下,首次实时观测到的MgO单晶样品冲击塑性变形产生的点缺陷色心吸收现象。