Computational visco‐plasticity for kinematically constrained crystals

Crystals possessing less than five independent slip systems cannot undergo any arbitrary isochoric deformation. For these kinematically deficient crystal structures, besides incompressibility, extra constraints need to be taken into account. A visco‐plastic constitutive model, based on crystallographic slip, is formulated for these materials, incorporating the kinematic deficiency. A finite element formulation is presented that enforces these kinematic restrictions by an approximate weighting method. The procedure is applicable for large deformations. The orientation of the crystal lattice is incrementally updated. The formulation is applied to a fictitious two‐dimensional analogon of a kinematically constrained crystal. Copyright © 2000 John Wiley & Sons, Ltd.     

地质缺陷对拱坝及坝基力学行为的影响

  通过对一个坝址地质结构较为发育的拱坝作三维有限元非线性分析,分析了温度、渗流,地形地貌,地基上的地质缺陷等对拱坝及大坝运行时地基的力学行为的影响。并给出了复杂地基的拱坝的位移场、应力场的分布规律和传力特性。对大坝及基岩上塑性区产生的机理,形式和发展趋势作了分析。     

Recent advances in the hole drilling method for residual stress measurement

The main activities in the hole drilling residual stress measurement technique recently developed at the University of Pisa are reviewed and presented. Particular attention was paid to developing tools for increasing the limits indicated by the presently applied standard procedures for residual stress evaluation. For residual stresses that were assumed to be uniform through-thickness, the effect of plasticity was numerically analyzed and results formed the basis for a procedure that allows an increase in the maximum measurable residual stress up to 0.9 of the material yield strength. For nonuniform through-thickness residual stress, accurate analytical influence functions are proposed by which arbitrary interpolation of the influence coefficients is avoided and all the experimentally obtained strains, with no regard to their number, can be used as input for residual stress evaluation.     

热机械循环载荷下短纤维增强金属基复合材料性能分析

从复合材料内部组分的细观力学关系人手,选取代表体积元,基于Eshelby椭圆夹杂理论和瞬时体积平均的概念,通过集中张量描述纤维与基体以及纤维与纤维间的相互作用,并把在弹性范围内得到的各集中张量推广到弹塑性范围内,建立能够在弹塑性范围内分析热机械循环载荷作用下短纤维增强金属基复合材料的性质的模型。为了接近工程实际,假设纤维始终保持线弹性,对基体材料采用能反映bauschinger效应的混合硬化模型,依据基体的弹塑性状态决定复合材料整体的弹塑性状态。在塑性范围内,从各向异性的角度出发,采用增量法迭代得出每个加载步结束时复合材料整体以及各相的应力应变增量。编写控制应变和温度加载条件下,计算复合材料应力应变响应的程序,着重讨论纤维的外形、空间分布、体积百分比以及温度载荷对复合材料宏观性质的影响,并与相关的实验结果和数值结果进行比较。     

Digging Deeper: Advancements in Visualization of Inhibitory Synapses in Neurodegenerative Disorders

Recent research has provided strong evidence that neurodegeneration may develop from an imbalance between synaptic structural components in the brain. Lately, inhibitory synapses communicating via the neurotransmitters GABA or glycine have come to the center of attention. Increasing evidence suggests that imbalance in the structural composition of inhibitory synapses affect deeply the ability of neurons to communicate effectively over synaptic connections. Progressive failure of synaptic plasticity and memory are thus hallmarks of neurodegenerative diseases. In order to prove that structural changes at synapses contribute to neurodegeneration, we need to visualize single-molecule interactions at synaptic sites in an exact spatial and time frame. This visualization has been restricted in terms of spatial and temporal resolution. New developments in electron microscopy and super-resolution microscopy have improved spatial and time resolution tremendously, opening up numerous possibilities. Here we critically review current and recently developed methods for high-resolution visualization of inhibitory synapses in the context of neurodegenerative diseases. We present advantages, strengths, weaknesses, and current limitations for selected methods in research, as well as present a future perspective. A range of new options has become available that will soon help understand the involvement of inhibitory synapses in neurodegenerative disorders.     

Computation of collapse states with von Mises type yield condition

A new computational approach to the problem of limit analysis with quadratic yield condition is developed and tested. The problem is solved using the exact convex yield condition and the general case of unbounded yield set, corresponding to unrestricted hydrostatic pressure, is treated. The discretization by the finite element method is based on an analysis of the duality between the static principle and the kinematic principle of limit analysis. Also the solution method for the discrete optimization problem is new and exploits this duality. The method simultaneously computes approximations to the fields of stresses and flow in the collapse state. The software used for the optimization problem is independent of continuum mechanics, but has been developed with applications in limit analysis as a primary objective. The efficiency and accuracy of the method for large problems is demonstrated by solving a classical problem in the plane strain model: approximately 90 000 finite element nodes with 3 stress components and 2 velocity components at each node. In two space dimensions this may be overkill, but it shows that we are able to solve problems in three space dimensions. Copyright © 1999 John Wiley & Sons, Ltd.     

Toughening of glassy polymers by prepackaged deformation‐activated diluents

The use of low molecular weight liquid rubber diluents in concentrations of ∼ 1%, dispersed finely in the form of submicron‐sized pools, had been demonstrated in the past by Gebizlioglu and colleagues (Macromolecules 1990, 23, 3968) to be an effective source of toughening of brittle glassy polystyrene through deformation‐accentuated plasticization effects of craze matter during the development of craze plasticity. Herein, we present results of a study extending the range of this plasticization effect through the development of a new “precipitation‐molding” process that can routinely incorporate diluent pools of up to 4–5% by volume and can increase the tensile toughness by about a factor of 3 over what was achieved before. The experiments demonstrate that the toughening mechanism operates primarily through a substantial reduction of the craze flow stress that promotes an increase in craze plasticity that renders inactive adventitious inclusions that normally initiate fracture. Experiments with all blends at different temperatures and strain rates indicate that the fracture response can be represented by a universal behavior pattern of a single thermally assisted process of fracture of craze matter where the diluent merely modulates the plastic resistance. However, the experimental results show that the sorption of the diluent that controls the plasticization effect during straining competes against the time‐dependent fracture process and that the toughening effect decreases monotonically with increasing strain rate and decreasing temperature that limits the effectiveness of the process to a range considerably below what is required for impact response. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1469–1490, 1999     

Toughening of glassy polystyrene through ternary blending that combines low molecular weight polybutadiene diluents and ABS or HIPS‐type composite particles

The effectiveness of toughening brittle glassy polymers such as polystyrene (PS) through deformation‐induced plasticization by low molecular weight diluents of polybutadiene (PB) was amply demonstrated in earlier studies. In those applications, surface‐initiated crazes of unusual growth kinetics and stability could produce effective toughening in sheet samples of millimeter thicknesses, but would have been ineffective in more massive parts where crazes could not be initiated in the interiors to promote a plastic response of the entire volume. This shortcoming has now been rectified through the development of ternary blends incorporating into the previous PS/PB blends a critical small volume fraction of ABS‐ or HIPS‐type composite particles that serve to initiate crazes throughout the volume. Thus, we demonstrated in the present study that incorporation of 10% commerical ABS or 20% commercial HIPS into the most effective PS/PB‐3K blend results in tensile toughnesses equal to or exceeding those of commercial ABS or HIPS in full concentration. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2319–2328, 1999     

An approach to limit analysis with cone-shaped yield surfaces

The paper presents an algorithm for limit analysis with mixed approach by finite elements with application to geotechnical problems. The emphasis is on a special technique to deal with the singularity at the apex of a cone-shaped yield surface. The approach is based on the direct application of the sub-differential concept to the flow law and is essentially different from the Lagrange multipliers technique. The proposed algorithm does not require the regularization usually adopted for the singularities at the boundary of the plastic admissible stress region that occur in soil models. The case of a Drucker–Prager model is particularly considered, as well as the Mohr–Coulomb model in plane strain conditions. The algorithm is applied to a footing and a shallow tunnel. © 1997 John Wiley & Sons, Ltd.