A numerical study of crack tip constraint in ductile single crystals

In this work, the effect of crack tip constraint on near-tip stress and deformation fields in a ductile FCC single crystal is studied under mode I, plane strain conditions. To this end, modified boundary layer simulations within crystal plasticity framework are performed, neglecting elastic anisotropy. The first and second terms of the isotropic elastic crack tip field, which are governed by the stress intensity factor K and T-stress, are prescribed as remote boundary conditions and solutions pertaining to different levels of T-stress are generated. It is found that the near-tip deformation field, especially, the development of kink or slip shear bands, is sensitive to the constraint level. The stress distribution and the size and shape of the plastic zone near the crack tip are also strongly influenced by the level of T-stress, with progressive loss of crack tip constraint occurring as T-stress becomes more negative. A family of near-tip fields is obtained which are characterized by two terms (such as K and T or J and a constraint parameter Q) as in isotropic plastic solids.     

On the transformation toughening of a crack along an interface between a shape memory alloy and an isotropic medium

In this study, a bilinear cohesive zone model is employed to describe the transformation toughening behavior of a slowly propagating crack along an interface between a shape memory alloy and a linear elastic or elasto-plastic isotropic material. Small scale transformation zones and plane strain conditions are assumed. The crack growth is numerically simulated within a finite element scheme and its transformation toughening is obtained by means of resistance curves. It is found that the choice of the cohesive strength t₀ and the stress intensity factor phase angle φ greatly influence the toughening behavior of the bimaterial. The presented methodology is generalized for the case of an interface crack between a fiber reinforced shape memory alloy composite and a linear elastic, isotropic material. The effect of the cohesive strength t₀, as well as the fiber volume fraction are examined.     

基于改进遗传算法主动柔性结构压电元件位置优化

基于改进的遗传算法提出了一种解决多压电片布置在柔性结构上以实现振动主动控制的有效方法.本文采用D优化设计准则,即把Fisher信息矩阵行列式最大值作为目标函数的一种优化方法.通过对结构模态振型和动力特性的研究,使用一种简单的方法将所选择的低阶模态振型转化为归一化形式,最后通过结构模态振型可确定压电片最优位置.为了达到更好的振动控制效果,压电片布置在结构模态应变最大位置处.当在结构不同位置上布置压电片时,结构各阶模态振型也将随之发生不同程度的变化.本文建立了ANSYS软件和遗传算法的接口来实现对结构的模态重分析,从而提取各阶模态振型.     

Gradient-enhanced softening material models

Classical constitutive models exhibit strong mesh dependency during softening and the numerical responses tend towards perfectly brittle behavior upon mesh refinements. Such sensitivity can be avoided by adopting the gradient-enhanced formulation. The implicit approach incorporates the gradient contributions indirectly via an additional Helmholtz equation and requires only C⁰ continuity. The explicit approach computes the gradient terms directly from the local field variables. Assuming a weak satisfaction of the yield function, C¹ continuity or C⁰ continuity with additional degrees of freedoms in the penalty approach is required. This makes the explicit method less attractive computationally. However, the explicit approach is able to fully regularize some material models where the standard implicit method fails to perform. Drawing analogy to the over-nonlocal integral formulation, the over-implicit-gradient framework is proposed. In addition, an alternative framework for the explicit gradient method requiring only C⁰ continuity is proposed. The regularizing effects of the abovementioned two gradient frameworks show promising applications to strain-softening materials.     

Geometrically necessary dislocation structure organization in FCC bicrystal subjected to cyclic plasticity

Crystal plasticity finite element analysis of cyclic deformation of compatible type FCC bicrystals are performed. The model specimen used in the analysis is a virtual FCC bicrystal with an isotropic elastic property; therefore, the effect of constraint due to elastic incompatibility does not appear. The results of the analysis show the strain-amplitude-dependence of both the organization of the GND structure and the stress–strain behavior. The calculated stress–strain curve with the largest strain amplitude shows additional cyclic hardening. The microscopic mechanisms of the strain-amplitude-dependent organization of the GND structure and additional cyclic hardening behavior are discussed in terms of the activation of secondary slip system(s). Finally, the effects of the elastic anisotropy, the lattice friction stress and the interaction between dislocations are also argued.     

人工种植黄芩根、茎、叶、花、种子中营养元素的光谱分析

使用火焰原子吸收光谱法对人工种植黄芩的根、茎、叶、花、种子5个部位中的Ca,Cu,Fe,Mn,Zn,K六种矿物元素的含量进行了测定分析。结果表明,黄芩各部位均含有丰富的矿物元素,根中营养元素由高到低的顺序是为Ca>Fe>Cu>Mn>Zn>K,茎中的为Fe>Mn>Zn=Cu>K>Ca,叶中的为Fe>Mn>Zn>Cu>K>Ca,花中的为Ca>Fe>Mn>Zn>Cu>K,种子中的为Ca>Fe>Zn>Mn>Cu>K。研究认为人工种植黄芩茎、叶、花、种子中Fe, Mn, Zn含量皆较高,花和种子中还富含Ca, 根中富含Ca和Fe元素。     

中药丹参及其近缘种中微量元素的主成分和聚类分析

采用原子吸收光谱法测定了不同产地的中药丹参及其近缘种植物白花丹参、雪山鼠尾草、云南鼠尾草、甘西鼠尾草、贵州鼠尾草、血盆草、粘毛鼠尾草、峨眉鼠尾草、黄鼠狼花、短唇鼠尾草和犬形鼠尾草中的Cu,Zn,Fe,Mg,Ca,Cr,Pb,Mo,Mn和Cd共10种微量元素的含量,应用主成分和聚类分析法对测定结果进行了研究。经过主成分分析得出3个主因子,其累计方差贡献率达79.3%。第一主因子的方差贡献率为49.6%,故所对应的Fe,Mn,Cu,Zn,Cd和Pb是丹参及其近缘种的特征元素。聚类分析将21个样品聚成2组,除浙江栽培丹参和白花丹参外,来自不同产地的9个丹参样品聚为一组;除贵州鼠尾草外,其他鼠尾草样品聚在一起,故此法可以将丹参和鼠尾草属其他植物区分开,总符合率达90%。因此利用原子吸收光谱法测定丹参及其近缘种中微量元素含量并对测定结果进行聚类分析是鉴别正品丹参的一种快速、准确的方法。     

五种郁金无机元素比较及样品元素间相关性分析

中药含有丰富的无机元素,特别是人体必需的无机元素含量较高。在样品中加入混酸,再用微波消解技术消化样品,采用火焰原子吸收光谱法对五种不同品种郁金（温郁金、姜黄、广西莪术、蓬莪术和川郁金）所含有的八种具有重要生理功能的无机元素——钾、钙、钠、镁、锌、铁、铜和锰的含量进行了测定。结果显示郁金类中药材含有相近的元素成分,不同来源的品种在个别元素含量上存在差异。基于各样品所含无机元素采用聚类分析进行相关性分析,药典所收载四种郁金（温郁金、姜黄、广西莪术和蓬莪术）聚为一类,与非正品川郁金有所区别,该聚类结果与依据形态特征的传统分类学结果一致。测定结果可为郁金中无机元素与其药效的相关性提供科学依据。     

Impact of woven fabric: Experiments and mesostructure-based continuum-level simulations

Woven fabric is an increasingly important component of many defense and commercial systems, including deployable structures, restraint systems, numerous forms of protective armor, and a variety of structural applications where it serves as the reinforcement phase of composite materials. With the prevalence of these systems and the desire to explore new applications, a comprehensive, computationally efficient model for the deformation of woven fabrics is needed. However, modeling woven fabrics is difficult due, in particular, to the need to simulate the response both at the scale of the entire fabric and at the meso-level, the scale of the yarns that compose the weave. Here, we present finite elements for the simulation of the three-dimensional, high-rate deformation of woven fabric. We employ a continuum-level modeling technique that, through the use of an appropriate unit cell, captures the evolution of the mesostructure of the fabric without explicitly modeling every yarn. Displacement degrees of freedom and degrees of freedom representing the change in crimp amplitude of each yarn family fully determine the deformed geometry of the mesostructure of the fabric, which in turn provides, through the constitutive relations, the internal nodal forces. In order to verify the accuracy of the elements, instrumented ballistic impact experiments with projectile velocities of 22-550 m/s were conducted on single layers of Kevlar^® fabric. Simulations of the experiments demonstrate that the finite elements are capable of efficiently simulating large, complex structures.     

Anisotropic finite elastoplasticity with nonlinear kinematic and isotropic hardening and application to sheet metal forming

The paper discusses the derivation and the numerical implementation of a finite strain material model for plastic anisotropy and nonlinear kinematic and isotropic hardening. The model is derived from a thermodynamic framework and is based on the multiplicative split of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong–Frederick kinematic hardening. Introducing the so-called structure tensors as additional tensor-valued arguments, plastic anisotropy can be modelled by representing the yield surface and the plastic flow rule as functions of the structure tensors. The evolution equations are integrated by a new form of the exponential map that preserves plastic incompressibility and uses the spectral decomposition to evaluate the exponential tensor functions in closed form. Finally, the applicability of the model is demonstrated by means of simulations of several deep drawing processes and comparisons with experiments.