超塑拉伸时缩颈的粘塑性力学分析及变速拉伸

本文基于“粘塑性流动理论”,求解了金属超塑性拉伸变形时粘塑性流动的力学平衡方程.得出了缩颈部位金属流动速度的解析式;从而应用应变速率敏感时塑性变形的稳定流动判据,对超塑性拉伸过程中的缩颈游动进行了力学分析.并用此理论解释了近几年发现的超塑性变速拉伸时延伸率显著增加的现象,试图为超塑性的进一步开发和应用提供新的启示,从能量的观点对超塑性变形行为进行新的解释.     

多晶体变形、应力的不均匀性及宏观响应

从单晶滑移变形分析的角度探讨多晶体塑性变形和应力的不均匀性及宏观力学响应：建议了 一种当前构形下以应力为基本变量的单晶黏塑性增量迭代计算方法；用Voronoi晶粒集合体 模型研究多晶体由于晶粒几何及取向的随机性造成的变形和应力的不均匀性, 进行了多晶集 合体的宏观响应和晶粒位向演化数值分析. 结果表明：(1)多晶体内等效塑性应变和应力分量在统计上呈现高斯分布，在应变硬化过程中, 随着塑性变形增加多晶体微观应力的统计变异系数会越来越大；(2)用Voronoi模型计算可得到沿最大剪应力方向的滑移变形带；(3)多晶体内最高三轴拉应力一般出现在晶界特别是三晶交界处；(4)Voronoi模型能用于织构分析.     

位错动力学方法在辐照损伤力学中的应用

核能是人类最理想的清洁能源之一，在世界能源结构中发挥着巨大作用。核裂变或核聚变导致的辐照环境会引起材料的辐照损伤，进而显著影响材料的力学性能，造成辐照硬化、脆化、蠕变、肿胀等现象。无论是预测辐照材料的服役寿命，还是设计新型的抗辐照材料，都迫切需要建立强辐照环境下的塑性力学和损伤力学理论。分子动力学方法为理解辐照材料中的原子级相互作用机理提供了诸多有价值的信息，然而受限于时空尺度难以直接用于力学理论模型的建立。晶体塑性有限元方法可用于预测辐照材料的力学响应，但是往往需要基于已知的物理模型，并且拟合实验数据。位错动力学方法是联系纳米力学与连续介质力学的桥梁，是揭示大量微结构的累积相互作用机理，建立基于物理机制的塑性力学和损伤力学理论的强有力手段。位错动力学方法起源于上个世纪八十年代，起初主要用于研究位错间的短程和长程相互作用、计算位错运动引起的塑性变形、硬化、软化、变形局部化等。本文将展示三种耦合位错动力学和辐照损伤场的方法，并系统地综述研究者近年来使用该方法在理解辐照硬化、塑性变形局部化、晶界效应、温度效应、和发展多尺度耦合方法等方面取得的进展。     

有限弹塑性变形的几何模型1)

以连续介质力学内变量理论为基础,建立了一个以材料内部微结构变量为底流形,材料外部变形状态为对应纤维的材料状态纤维丛模型,使材料的力学特性与模型的几何性质自然对应起来.在模型上讨论和分析了有限弹塑性变形中变形梯度的Lee和Clifton的分解和联系,并证明了塑性变形为沿内变量演化在纤维丛的水平空间的运动.由此获得了塑性变形随内变量演化的变化方程和塑性速率梯度与内变演化的协调关系.     

不同细料含量土石混合料塑性行为离散元模拟

土石混合料是指由大粒径的块石和作为填充成分的细粒土组成的二元混合料, 其塑性行为与细料含量密切相关. 目前对细粒含量如何影响土石混合料塑性行为及其细观机制的研究尚不充分, 为此本文开展了不同细料含量土石混合料的二维离散元数值模拟, 基于二阶功失稳准则与细观力学理论, 探究了细料含量对石料骨架土石混合料失稳特性与非关联流动特性的影响, 并揭示了细料含量影响土石混合料塑性力学行为的细观机制. 研究结果表明, 细颗粒可通过限制集合体塑性变形从而起到促进集合体整体稳定的作用; 细颗粒控制颗粒集合体塑性变形的方向(即塑性势面法方向), 随着细料含量增大, 土石混合料的塑性势面法方向和屈服面法方向之间的夹角减小, 非关联流动性减弱, 材料分岔失稳区域变窄; 尽管加入到石颗粒中的部分细颗粒与石颗粒共同承担骨架作用, 但是细颗粒的加入不影响颗粒集合体的力学状态, 不改变材料屈服面法方向. 相关研究结果可为建立考虑细料含量的土石混合料弹塑性本构模型提供理论依据.      

基于中间构形的大变形弹塑性模型

在大变形弹塑性本构理论中,一个基本的问题是弹性变形和塑性变形的分解.通常采用两种分解方式,一是将变形率(或应变率)加法分解为弹性和塑性两部分,其中,弹性变形率与Kirchhoff应力的客观率通过弹性张量联系起来构成所谓的次弹性模型,而塑性变形率与Kirchhoff应力使用流动法则建立联系;另一种是基于中间构形将变形梯度进行乘法分解,它假定通过虚拟的卸载过程得到一个无应力的中间构形,建立所谓超弹性-塑性模型.研究了基于变形梯度乘法分解并且基于中间构形的大变形弹塑性模型所具有的若干性质,包括:在不同的构形上,塑性旋率的存在性、背应力的对称性、塑性变形率与屈服面的正交性以及它们之间的关系.首先,使用张量函数表示理论,建立了各向同性函数的若干特殊性质,并导出了张量的张量值函数在中间构形到当前构形之间进行前推后拉的简单关系式.然后,基于这些特殊性质和关系式,从热力学定律出发,建立模型在不同构形上的数学表达,包括客观率表示的率形式和连续切向刚度等,从而获得模型所具有的若干性质.最后,将模型与4种其他模型进行了比较分析.      

位错动力学方法在辐照损伤力学中的应用

核能是人类最理想的清洁能源之一,在世界能源结构中发挥着巨大作用。核裂变或核聚变导致的辐照环境会引起材料的辐照损伤,进而显著影响材料的力学性能,造成辐照硬化、脆化、蠕变、肿胀等现象。无论是预测辐照材料的服役寿命,还是设计新型的抗辐照材料,都迫切需要建立强辐照环境下的塑性力学和损伤力学理论。分子动力学方法为理解辐照材料中的原子级相互作用机理提供了诸多有价值的信息,然而受限于时空尺度难以直接用于力学理论模型的建立。晶体塑性有限元方法可用于预测辐照材料的力学响应,但是往往需要基于已知的物理模型,并且拟合实验数据。位错动力学方法是联系纳米力学与连续介质力学的桥梁,是揭示大量微结构的累积相互作用机理,建立基于物理机制的塑性力学和损伤力学理论的强有力手段。位错动力学方法起源于上个世纪八十年代,起初主要用于研究位错间的短程和长程相互作用、计算位错运动引起的塑性变形、硬化、软化、变形局部化等。本文将展示三种耦合位错动力学和辐照损伤场的方法,并系统地综述研究者近年来使用该方法在理解辐照硬化、塑性变形局部化、晶界效应、温度效应、和发展多尺度耦合方法等方面取得的进展。     

金属非晶的强度和变形特性

金属非晶发展至今已有多种体系并可实现厘米量级的块体制备,其各种性能也都有了广泛的研究。本文主要介绍金属非晶的单轴拉伸、单轴压缩、微柱压缩、薄板弯曲、拉伸-扭转等物理力学特性及关于其变形的理论分析。文章涵盖了金属非晶的以下一些力学特性：金属非晶的弹性模量和其溶剂金属的相近性―金属非晶通常具有2% 左右的弹性应变极限,对应着GPa量级的高失效强度;金属非晶单轴拉伸、压缩时的宏观塑性特征及塑性变形的典型机制;金属非晶微观上的短程与中程原子团簇结构特点及其与非晶塑性的关联;金属非晶塑性屈服与静水压力的相关性,拉扭组合时呈现的螺旋断口特征,以及Mohr-Coulomb本构模型对这些屈服特征的适用性。最后,作者也介绍了金属非晶塑性变形的微观物理模型及连续介质力学本构,以及金属非晶的断裂与疲劳特性。     

基于中间构形的大变形弹塑性模型

在大变形弹塑性本构理论中,一个基本的问题是弹性变形和塑性变形的分解.通常采用两种分解方式,一是将变形率(或应变率)加法分解为弹性和塑性两部分,其中,弹性变形率与Kirchhoff应力的客观率通过弹性张量联系起来构成所谓的次弹性模型,而塑性变形率与Kirchhoff应力使用流动法则建立联系;另一种是基于中间构形将变形梯度进行乘法分解,它假定通过虚拟的卸载过程得到一个无应力的中间构形,建立所谓超弹性–塑性模型.研究了基于变形梯度乘法分解并且基于中间构形的大变形弹塑性模型所具有的若干性质,包括:在不同的构形上,塑性旋率的存在性、背应力的对称性、塑性变形率与屈服面的正交性以及它们之间的关系.首先,使用张量函数表示理论,建立了各向同性函数的若干特殊性质,并导出了张量的张量值函数在中间构形到当前构形之间进行前推后拉的简单关系式.然后,基于这些特殊性质和关系式,从热力学定律出发,建立模型在不同构形上的数学表达,包括客观率表示的率形式和连续切向刚度等,从而获得模型所具有的若干性质.最后,将模型与4种其他模型进行了比较分析.     

近代塑性力学发展概况

经典塑性理论研究在50年代已经成熟,主要结果已总结在Hill的名著“塑性数学理论和Prager & Hodge的名著“理想塑性的固体理论”中。近15年来,非经典塑性理论突破了经典理论的先验限制,在下述六个方面有独特的发展:①由微小变形向有限变形发展,确立了有限塑性变形理论;②由宏观唯象理论向细观深度发展,确立了塑性细观力      

均匀等轴细晶金属超塑性本构关系

本文根据超塑变形机理的金属学研究结果,应用细观力学方法建立了包括三种变形机制的均匀等轴细晶超塑性本构关系。该本构关系能解释超塑拉伸实验中观察到的基本实验现象:晶粒度、应变率对σ～、m～曲线形态的影响。     

A two-dimensional finite element method for simulating the constitutive response and microstructure of polycrystals during high temperature plastic deformation

We describe a finite element method designed to model the mechanisms that cause superplastic deformation. Our computations account for grain boundary sliding, grain boundary diffusion, grain boundary migration, and surface diffusion, as well as thermally activated dislocation creep within the grains themselves. Front tracking and adaptive mesh generation are used to follow changes in the grain structure. The method is used to solve representative boundary value problems to illustrate its capabilities.     

Constitutive behavior of superplastic materials

Superplasticity is an intriguing inelastic process in solid materials with deformation upto several thousand percent. Forming sheet and bulk materials using superplastic forming has become an established manufacturing method in aerospace and lately in other industries. Developing the right constitutive behavior is important not only for modeling the process for manufacturing by engineering mechanicians but for choosing the right composition and processing for material scientists. Such an ideal constitutive equation has been eluding the analysts so far. This paper examines some of the fundamental misgivings about the origin of inelastic process in superplasticity compared to other well known deformation processes. Also an attempt is made to understand the basic characteristics of superplastic inelastic deformation at macroscopic, mesoscopic and atomic levels.     

Identification of Phenomenological Superplasticity Models Based on the Results of Technological Experiments

A general scheme for identifying the constitutive equations of superplasticity, according to which the values of material constants are calculated by the results of technological experiments using interpretation techniques based on the adoption of additional hypotheses of the stress-strain state nature in the deformation region, is proposed. The verification of the result is carried out by comparing the experimental data with the corresponding solutions of the boundary problems for superplasticity mechanics obtained using software complexes such as ANSYS and ABAQUS. The obtained result is used as input data for a program, and the solution of the boundary value problem is found without additional hypotheses of the stress-strain state nature. The practical application of the proposed approach for the standard power model of superplasticity shows that the accuracy of the forming duration simulation is no less than 4%.     

Whisker alignment of Ti–6Al–4V/TiB composites during deformation by transformation superplasticity

We examine the effect of 10 vol.% TiB whisker reinforcement on transformation superplasticity of Ti–6Al–4V, induced by thermal cycling about the α/β phase transformation range of the matrix under a uniaxial tensile stress. During superplastic deformation, the whiskers gradually align along the external loading axis, as measured by electron back-scattering diffraction of deformed specimens. The composites exhibit a gradual decrease in effective deformation rate during superplastic elongation. This effect is attributed to increased load transfer from matrix to whisker upon whisker alignment, and is explained with elastic calculations of stress partitioning.     

Numerical simulation for deformation of nano-grained metals

Electro-deposition technique is capable of producing nano-grained bulk copper specimens that exhibit superplastic extensibility at room temperature. Metals of such small grain sizes deform by grains sliding, with little distortion occurring in the grain cores. Accommodation mechanisms such as grain boundary diffusion, sliding and grain rotation control the kinetics of the process. Actual deformation minimizes the plastic dissipation and stored strain energy for representative steps of grain neighbor switching. Numerical simulations based on these principles are discussed in this paper. The project supported by the National Natural Science Foundation of China (19972031)     

Dislocation loop nucleation and plastic deformation of nanocrystalline materials

We propose a three-dimensional model of plastic deformation of a mechanically loaded nanocrystalline material by means of heterogeneous nucleation of loops of complete and partial lattice dislocations and by means of grain boundary dislocations on the already formed dislocation loops. We calculate and compare the energy variations characterizing various versions of dislocation loop nucleation. We discover three basic regions of variation of nanocrystalline material grain dimensions which are characterized by their own types of loop nucleation. We also study the role played by loop nucleation in plastic and superplastic deformations of nanocrystalline materials.     

Grain- and macro-scale kinematics for granular micromechanics based small deformation micromorphic continuum model

Macro-scale deformation of granular solids comprising large number of grains (>10⁶) are most efficiently described within the framework of continuum mechanics. It is notable, however that the micro-scale deformations in these materials are concentrated at the grain-boundaries or grain-contacts. Thus, the deformation energies in these systems must be modeled by considering the deformations concentrated in the neighborhood of the grain-boundaries or grain-contacts. To address this issue, grain-interactions has been widely described in the Hertzian sense by considering the relative movement of points on either side of a grain boundary or contact treated as an imperfect interface. This communication introduces the relevant kinematic variables given in the terms of the grain displacements, spins and size that can be used to estimate the relative movement of a grain boundary or contact. The macro-scale kinematic variables useful for continuum modeling are then identified with the grain-scale kinematic variables. The deformation energy density of the granular solid can thus be expressed both in terms of the grain-scale as well as the macro-scale kinematic variables providing the necessary pathway for micro-macro identification which can lead to non-classical micromorphic continuum models that incorporate grain-scale representation.     

An investigation of the origin of the superplasticity of cast TiAl alloys

Two high-Cr TiAl alloys, with different compositions of Ti–46Al–8Cr–2Nb–0.15B and Ti–46Al–8Cr–2Nb–0.5W–0.15B (at.%), were studied. Superplastic deformation was observed in these alloys in their cast conditions. The superplastic behavior was attributed to the development of a fine-grained microstructure containing a metastable β phase with heavy alloying of Cr. The fine-grained microstructure is favorable for the occurrence of grain boundary sliding, and the presence of the β phase and its transformation to γ phase during deformation are beneficial to stress relaxation and suppression of cavitation.     

Superplastic characteristics of high purity yttria-stabilized zirconia polycrystals

The mechanical characteristics of superplastic yttria-stabilized zirconia polycrystals have been analyzed as a function of stress, temperature and grain size. The evolution of the stress exponent n with stress found in high purity materials is similar to that observed in superplastic metals. True creep parameters can be ascribed to the deformation mechanism at high stresses. By contrast, the creep parameters exhibit a continuous evolution with stress, temperature and grain size at low stresses. The threshold stress formalism used in conventional and high strain rate superplastic metals accounts for the mechanical characteristics observed in fine-grained zirconia polycrystals.