面心立方铝剪切变形的准连续介质模拟

采用准连续介质多尺度方法模拟了纳米尺度单晶铝的剪切变形.分别采用三种不同晶体取向(分别为x[1-10],y[001],z[-1-10]; x[-1-12],y[111],z[-110]; x[-110],y[-1-12],z[111])和三种不同长宽比的几何模型(分别为1:1,2:1,4:1)进行模拟,得出各模型应力一应变响应曲线.加载过程中,对晶体内部变形比较剧烈的部分画出原子图,并从微观角度分析了产生剧烈变形的原因;讨论了三种晶体取向下的尺寸效应:单晶铝的剪切屈服应力随模型几何尺寸的增加而降低,并且随晶体取向的不同变化很大.最后,使用指数法则来描述模型的尺寸效应,提出了针对不同取向的指数法则参数,表明了晶体不同取向对剪切屈服应力的大小有影响.     

基于分子动力学与耗散粒子动力学串行耦合的面心立方金属粗粒化模型

通过全原子分子动力学(MD)与等温耗散粒子动力学(DPD)的串行耦合,提出了面心立方金属粗粒化模型的建立方法。该方法将一定数量的原子粗粒化为单个介观 DPD 粒子,假设 DPD 粒子间作用势的表达式为Sutton-Chen势函数形式,利用遗传算法,以 MD和DPD计算的单晶金属常温(298 K)等温线相一致为目标,确定了DPD粒子间作用势函数的参数。对单晶铜纳米棒的轴向拉伸开展 MD 和 DPD 对比模拟,发现在纳米棒弹性响应阶段,两者计算结果吻合较好,而屈服应力和屈服应变存在一定差距。建议在优化 DPD势函数参数时,引入更多的材料力学响应信息,进一步提高介观DPD模型的准确性。     

基于分子动力学与耗散粒子动力学串行耦合的面心立方金属粗粒化模型（英文）

通过全原子分子动力学(MD)与等温耗散粒子动力学(DPD)的串行耦合,提出了面心立方金属粗粒化模型的建立方法。该方法将一定数量的原子粗粒化为单个介观DPD粒子,假设DPD粒子间作用势的表达式为Sutton-Chen势函数形式,利用遗传算法,以MD和DPD计算的单晶金属常温(298K)等温线相一致为目标,确定了DPD粒子间作用势函数的参数。对单晶铜纳米棒的轴向拉伸开展MD和DPD对比模拟,发现在纳米棒弹性响应阶段,两者计算结果吻合较好,而屈服应力和屈服应变存在一定差距。建议在优化DPD势函数参数时,引入更多的材料力学响应信息,进一步提高介观DPD模型的准确性。     

纳米双晶铜单向拉伸弹性性能的应变率效应和尺寸效应

利用分子动力学方法研究了双晶铜在单向拉伸载荷作用下弹性性能的应变率效应和尺寸效应.结果表明:随着加载应变率的改变,纳米双晶铜的初始弹性模量在低应变率区和高应变率区表现出不同的规律.在低应变率区,弹性模量基本不随应变率的变化而变化;但在高应变率区,弹性模量随应变率的增大而增大,弹性模量和加载应变率之间满足近似的对数线性关系.因此,存在一个控制弹性模量应变率效应的应变率阈值.进一步的研究发现,双晶纳米铜的应变率阈值随着截面尺寸的增大而减小,具有明显的尺寸依赖性.     

微纳米晶金属的应变率敏感性及应变硬化行为分析

基于亚微米、纳米晶粒组织塑性变形过程中多种变形机制(位错机制、扩散机制及晶界滑动机制)共存,建立了理论模型,用于定量研究亚微米、纳米晶粒组织的塑性变形行为.以铜为模型材料,计算分析了晶粒尺度、应变率以及温度对亚微米、纳米晶粒组织塑性变形行为的影响.结果表明:相比粗晶铜,亚微米晶铜表现出明显的应变率敏感性,并且应变率敏感系数随晶粒尺度及变形速率的减小而增大;同时,增大变形速率或降低变形温度都能提高材料的应变硬化能力,延缓颈缩发生,进而提高材料的延性.计算分析结果与实验报道吻合.     

连云港淤泥流变特性研究

研究了体积容重为Ｃ＿ｖ＝０．０５７～０．４１连云港西墅地区淤泥的流变特性和弹性模量。在剪切率的变化范围２×１０￣（－２）ｓ￣（－１），淤泥具有三种不同的流变特性，在很低剪切率情况下，淤泥剪切力基本保持常值；在低剪切率范围，表现为宾汉流体性质，其屈服应力和宾汉动性系数以Ｃ＿ｖ指数增长；在高剪切率，淤泥流动体现出宾汉伪塑性特征，划分三个特征区的临界剪切率值是随淤泥含水量变化，最后指出，连云港淤泥弹性模量与高岭土结果很相近，对淤泥含水量的变化极其敏感。     

连云港淤泥流变特性研究

研究了体积容重为Ｃ＿ｖ＝０．０５７～０．４１连云港西墅地区淤泥的流变特性和弹性模量。在剪切率的变化范围２×１０￣（－２）ｓ￣（－１），淤泥具有三种不同的流变特性，在很低剪切率情况下，淤泥剪切力基本保持常值；在低剪切率范围，表现为宾汉流体性质，其屈服应力和宾汉动性系数以Ｃ＿ｖ指数增长；在高剪切率，淤泥流动体现出宾汉伪塑性特征，划分三个特征区的临界剪切率值是随淤泥含水量变化，最后指出，连云港淤泥弹性模量与高岭土结果很相近，对淤泥含水量的变化极其敏感。     

基于界面层模型的双周期纳米夹杂复合材料反平面问题研究

利用复变函数方法并结合双准周期Riemann边值问题理论，获得了含双周期分布非均匀相（夹杂/界面层）的复合材料在远场均匀反平面应力下弹性场的全场解答．该解答可用于对纳米夹杂复合材料的应力进行分析，结合平均场理论也用于预测纳米夹杂复合材料的有效性能．计算结果表明：当夹杂尺度在纳米量级时，应力和有效反平面剪切模量具有明显的尺度依赖性，并且随着夹杂尺寸的增加，趋近于不考虑界面效应时的结果；界面层厚度和性能对应力和有效反平面剪切模量明显变化时所对应的夹杂尺度范围和趋近于无界面效应结果的快慢有显著影响；当界面厚度足够薄时，界面层模型可用于模拟零厚度界面情况．     

确定材料在高温高应变率下动态性能的 Hopkinson杆系统

描述了一种利用Hopkinson杆装置确定在高温（温度可高达1 173 K）、高应变率下材料动态性能的试验方法。在试样加温过程中,试样不与入射杆及透射杆接触。当试样加热到预定温度时,气压驱动同步组装系统,推动透射杆及试样,使得应力波到达入射杆与试样接触面时,入射杆、试样及透射杆紧密接触。利用以上系统,完成了连铸单晶铜及上引法连铸多晶铜从室温到1 085 K范围内的应力应变曲线。测试结果表明,不论是上引法连铸多晶铜还是连铸单晶铜,流动应力随温度的升高而下降,在温度低于585 K时,材料的应变硬化率明显大于在温度高于585 K时的应变硬化率。     

晶体位错尺寸效应的多尺度分析

本文采用准连续介质多尺度方法，分析了面心立方（fcc）晶体铝阶梯孪晶界在不同尺寸情况下（试件尺寸长高比从1：1到8：1）受剪切作用的晶界变形。了解在不同尺寸下，晶界结构位错的成核过程，得到了大试件比值与小试件比值下作用力与应变的关系曲线及不同试件尺寸下应变能的变化曲线。其中随着试件比值的增加，作用力在应变比较小的时候变化情况相似，但当应变达到3％以后，呈现出明显的不同；应变能随试件长高比的增大而减小，各个试件在各自不同的加载阶段，应变能变化趋势同作用力变化趋势相一致。本计算揭示了不同尺寸下阶梯孪晶界在剪切作用下的微观机理，证实其尺寸效应性质。     

An investigation of the combined size and rate effects on the mechanical responses of FCC metals

The molecular dynamics (MD) simulations are performed with single-crystal copper blocks under simple shear to investigate the size and strain rate effects on the mechanical responses of face-centered cubic (fcc) metals. It is shown that the yield stress decreases with the specimen size and increases with the strain rate. Based on the theory of dislocation nucleation, a modified power law is proposed to predict the scaled behavior of fcc metals, which agrees well with the numerical and experimental data ranging from nanoscale to macro-scale. In the MD simulations with different strain rates, a critical strain rate exists for each single-crystal copper block of given size, below which the yield stress is nearly insensitive to the strain rate. A hyper-surface is therefore formulated to describe the combined size and strain rate effects on the plastic yield strength of fcc metals. The preliminary results presented in this paper demonstrate the potential of the proposed simple procedure for engineering design at various spatial and temporal scales.     

A mesoscale investigation of strain rate effect on dynamic deformation of single-crystal copper

A combined finite element (FE) simulation and discrete dislocation dynamics (DD) approach has been developed in this paper to investigate the dynamic deformation of single-crystal copper at mesoscale. The DD code yields the plastic strain based on the slip of dislocations and serves as a substitute for the 3D constitutive form used in the usual FE computation, which is implemented into ABAQUS/Standard with a user-defined material subroutine. On the other hand, the FE code computes the displacement and stress field during the dynamic deformation. The loading rate effects on the yield stress and the deformation patterning of single-crystal copper are investigated. With the increasing of strain rate, the yield stress of single-crystal copper increases rapidly. A critical strain rate exists in each single-crystal copper block for the given size and dislocation sources, below which the yield stress is relatively insensitive to the strain rate. The dislocation patterning changes from non-uniform to uniform under high-strain-rate. The shear stresses in the bands are higher than that in the neighboring regions, which are formed shear bands in the crystal. The band width increases with the strain rate, which often take places where the damage occurs.     

Mechanics of very fine-grained nanocrystalline materials with contributions from grain interior, GB zone, and grain-boundary sliding

In this paper, we formulated an atomically-equivalent continuum model to study the viscoplastic behavior of nanocrystalline materials with special reference to the low end of grain size that is typically examined by molecular dynamic (MD) simulations. Based on the morphology disclosed in MD simulations, a two-phase composite model is construed, in which three distinct inelastic deformation mechanisms disclosed from MD simulations are incorporated to build a general micromechanics-based homogenization scheme. These three mechanisms include the dislocation-related plastic flow inside the grain interior, the uncorrelated atomic motions inside the grain-boundary region (the GB zone), and the grain-boundary sliding at the interface between the grain and GB zone. The viscoplastic behavior of the grain interior is modeled by a grain-size dependent unified constitutive equation whereas the GB zone is modeled by a size-independent unified law. The GB sliding at the interface is represented by the Newtonian flow. The development of the rate-dependent, work-hardening homogenization scheme is based on a unified approach starting from elasticity to viscoelasticity through the correspondence principle, and then from viscoelasticity to viscoplasticity through replacement of the Maxwell viscosity of the constituent phases by their respective secant viscosity. The developed theory is then applied to examine the grain size- and strain rate-dependent behavior of nanocrystalline Cu over a wide range of grain size. Within the grain-size range from 5.21 to 3.28 nm, and the strain rate range from 2.5 × 10⁸ to 1.0 × 10⁹/s, the calculated results show significant grain-size softening as well as strain-rate hardening that are in quantitative accord with MD simulations [Schiotz, J., Vegge, T., Di Tolla, F.D., Jacobsen, K.W., 1999. Atomic-scale simulations of the mechanical deformation of nanocrystalline metals. Phys. Rev. B 60, 11971–11983]. We have also applied the theory to investigate the flow stress, strain-rate sensitivity, and activation volume over the wider grain size range from 40 nm to as low as 2 nm under these high strain rate loading, and found that the flow stress initially displays a positive slope and then a negative one in the Hall–Petch plot, that the strain-rate sensitivity first increases and then decreases, and that the activation volume first decreases and then increases. This suggests that the maximum strain rate sensitivity and the lowest activation volume do not occur at the smallest grain size but, like the maximum yield strength (or hardness), they occur at a finite grain size. These calculated results also confirm the theoretical prediction of Rodriguez and Armstrong [Rodriguez, P., Armstrong, R.W., 2006. Strength and strain rate sensitivity for hcp and fcc nanopolycrystal metals. Bull. Mater. Sci. 29, 717–720] on the basis of grain boundary weakening and the report of Trelewicz and Schuh [Trelewicz, J.R., Schuh, C.A., 2007. The Hall–Petch breakdown in nanocrystalline metals: a crossover to glass-like deformation. Acta Mater. 55, 5948–5958] on the basis of hardness tests. In general the higher yield strength, higher strain rate sensitivity, and lower activation volume on the positive side of the Hall–Petch plot are associated with the improved yield strength of the grain interior, but the opposite trends displayed on the negative side of the plot are associated with the characteristics of the GB zone which is close to the amorphous state.     

Yield stress strengthening in ultrafine-grained metals: A two-dimensional simulation of dislocation dynamics

The effect of grain size on the tensile plastic deformation of ultrafine-grained copper polycrystals is investigated using a two-dimensional simulation of dislocation dynamics. Emphasis is put on the elementary mechanisms governing the yield stress in multislip conditions. Whatever the grain size, the yield stress is found to follow a Hall-Petch law. However, the elementary mechanism controlling slip transmission through the grain boundaries at yield is observed to change with the grain size. For the larger grain sizes, the stress concentrations due to dislocations piled-up at grain boundaries are responsible for the activation of plastic activity in the poorly stressed grains. For the smaller grain sizes, the pile-ups contain less dislocations and are less numerous, but the strain incompatibilities between grains become significant. They induce high internal stresses and favor multislip conditions in all grains. Based on these results, simple interpretations are proposed for the strengthening of the yield stress in ultrafine grained metals.     

冲击下混凝土试样应变率效应和惯性效应探讨

结合混凝土试件的真三轴静载冲击实验结果,分别运用考虑应变率效应的Holmquist-Johnson-Cook (HJC)模型和考虑静水压效应的Drucker-Prager (DP)模型进行数值分析,以探讨研究混凝土试样应变率效应和惯性效应的方法。在探究混凝土的应变率效应和横向惯性效应的关系时,使用HJC模型的数值模拟结果来拟合DP准则的各个参数。结果表明:随着应变率的升高,混凝土的强度会提高,并且这种强度的提高,也有一部分原因是第一应力不变量I₁的增大所导致的。因此,混凝土试件的应变率效应和横向惯性约束具有较强的耦合作用。理论和数值分析了冲击下试样内部的横向应力分布特征与应变率、静水压和试样尺寸的关系,结果发现:试样内部横向应力的幅值随着应变率、静水压的升高而增大,但随着试样尺寸的增大而减小。为了探讨横向惯性带来的强度提升效果,提出了一个有关冲击方向最大应力σ_x和等效应力σ_e的参数ξ,且ξ=(σ_x?σ_e)/σ_x。此参数具有尺寸效应、应变率效应和静水压效应,但是此参数与应力三轴度的关系表现出应变率无关特性,可为应变率效应的研究提供新的思路。     

A self-consistent plasticity theory for modeling the thermo-mechanical properties of irradiated FCC metallic polycrystals

A self-consistent theoretical framework is developed to model the thermo-mechanical behaviors of irradiated face-centered cubic (FCC) polycrystalline metals at low to intermediate homologous temperatures. In this model, both irradiation and temperature effects are considered at the grain level with the assist of a tensorial plasticity crystal model, and the elastic-visocoplastic self-consistent method is applied for the scale transition from individual grains to macroscopic polycrystals. The proposed theory is applied to analyze the mechanical behaviors of irradiated FCC copper. It is found that: (1) the numerical results match well with experimental data, which includes the comparison of results for single crystals under the load in different directions, and for polycrystals with the influences of irradiation and temperature. Therefore, the feasibility and accuracy of the present model are well demonstrated. (2) The main irradiation effects including irradiation hardening, post-yield softening, strain-hardening coefficient (SHC) dropping and the non-zero stress offset are all captured by the proposed model. (3) The increase of temperature results in the decrease of yield strength and SHC. The former is attributed to the weakened dislocation–defect interaction, while the latter is due to the temperature-strengthened dynamic recovery of dislocations through the thermally activated mechanism. The present model may provide a theoretical guide to predict the thermo-mechanical behaviors of irradiated FCC metals for the selection of structural materials in nuclear equipment.     

Molecular dynamics study of solute strengthening in Al/Mg alloys

The strengthening of Al by Mg solute atoms is investigated using molecular dynamics (MD) studies of single dislocations moving through a field of randomly placed solutes. The MD method permits explicit treatment of “core” effects, dislocation pinning and deceleration, and dislocation unpinning by thermal activation, all under an applied load. Choice of an appropriate MD simulation cell size is assessed using analytic concepts developed by Labusch. The interaction energy of a single Mg atom with straight edge and screw dislocations is computed and compared with continuum models. Using the single Mg energies, a one-dimensional energy landscape for the motion of a straight edge dislocation through a random field of Mg solutes is computed. The minima in this landscape match well with those found in the MD simulations at zero temperature. The stress to unpin a straight edge dislocation trapped in a local energy minimum generated by the solutes is then predicted semi-analytically using the energy landscape, and good agreement is obtained with the MD results. At temperatures of 300 and 500 K, the thermally activated rate of unpinning vs. stress and temperature is calculated semi-analytically, and agreement with the full MD results is again obtained with the fitting of a single attempt frequency in a transition state model. The agreement of the semi-analytical models provides a basis for calculating yield stress vs. strain rate and temperature, resulting from statistical pinning, for the case of non-interacting dislocations on a single slip system, and for extending the analysis to study dynamic strain aging effects resulting from diffusion of Mg atoms around a pinned dislocation.     

单晶铜中的类层错四面体稳定性

层错四面体作为受核辐照作用金属材料中一类常见的三维缺陷，会极大的改变材料塑性变形行为。本文借助分子静力学和分子动力学方法，针对不同构型、不同尺寸的类层错四面体，考察了不同形状空位团簇演化为类层错四面体的位错反应机理和形成能变化趋势，研究了类层错四面体附近空位形成能分布特征和最小空位形成能随类层错四面体尺寸增大的变化规律，分析了含不同构型、不同尺寸的类层错四面体铜单晶体的微观变形机理和单晶体屈服应力随类层错四面体尺寸增大的变化规律。研究发现：类层错四面体通过Silcox-Hirsch机制形成，且经历了空位团簇坍塌、Frank位错环分解和Shockley位错交汇形成层错四面体棱边三个过程；类层错四面体附近最小空位形成能随着类层错四面体尺寸变化而变化，且变化趋势与类层错四面体构型在稳态、亚稳态和非稳态之间过渡相关，稳态无尖端类层错四面体的最小空位形成能变化趋势表现出明显的尺寸效应；剪切会导致含类层错四面体铜单晶体产生两类层错四面体位错开动模式，即：斜面Shockley偏位错滑移和层错四面体底面压杆位错分解，且含类层错四面体的单晶体屈服应力基本上随着类层错四面体尺寸增大而逐渐减小。     

单晶铜中的类层错四面体稳定性

层错四面体作为受核辐照作用金属材料中一类常见的三维缺陷,会极大的改变材料塑性变形行为。本文借助分子静力学和分子动力学方法,针对不同构型、不同尺寸的类层错四面体,考察了不同形状空位团簇演化为类层错四面体的位错反应机理和形成能变化趋势,研究了类层错四面体附近空位形成能分布特征和最小空位形成能随类层错四面体尺寸增大的变化规律,分析了含不同构型、不同尺寸的类层错四面体铜单晶体的微观变形机理和单晶体屈服应力随类层错四面体尺寸增大的变化规律。研究发现：类层错四面体通过Silcox-Hirsch机制形成,且经历了空位团簇坍塌、Frank位错环分解和Shockley位错交汇形成层错四面体棱边三个过程;类层错四面体附近最小空位形成能随着类层错四面体尺寸变化而变化,且变化趋势与类层错四面体构型在稳态、亚稳态和非稳态之间过渡相关,稳态无尖端类层错四面体的最小空位形成能变化趋势表现出明显的尺寸效应;剪切会导致含类层错四面体铜单晶体产生两类层错四面体位错开动模式,即：斜面Shockley偏位错滑移和层错四面体底面压杆位错分解,且含类层错四面体的单晶体屈服应力基本上随着类层错四面体尺寸增大而逐渐减小。