考虑应变梯度及刚度劣化的剪切带局部变形分析

基于梯度塑性理论,研究了应变软化阶段的刚度劣化对剪切带内部的局部应变及相对剪切位移的影响。剪切带被看作一维剪切问题,本构关系为线弹性及线性应变软化。考虑刚度劣化后,剪切带的弹性应变由弹性剪切模量、损伤变量及残余剪切模量确定。剪切带的非局部总应变由双线性的本构关系确定。将非局部总应变减去弹性应变,可得剪切带的非局部塑性应变。剪切带非局部塑性应变与流动应力及损伤变量等参数有关,此关系即为在经典弹塑性理论框架之内的考虑刚度劣化的屈服函数。将二阶应变梯度项引入该函数,可得剪切带内部的局部塑性剪切应变及局部总剪切应变的分布规律。对局部塑性剪切应变积分,得到了局部塑性剪切位移。结果表明:考虑了刚度劣化后,剪切带内部的弹性剪切应变及位移增加,而局部塑性剪切应变及位移降低。若不考虑刚度劣化,理论结果可蜕化为以前的结果。理论结果与岩石局部变形的观测结果在定性是一致的。     

较大塑性变形下纳晶镍剪切带的演化模型

较大塑性变形下,采用TEM原位拉伸实验揭示了纳晶镍内剪切带演化的微观机制,优化了微观力学模型。结果表明:在纳米尺度范围内,随着晶粒尺寸的增加,应变软化的趋势越来越明显,剪切带宽度也随着增加;晶粒尺寸降低将导致纳晶材料中剪切带的产生提前;内部特征长度的值随着晶粒尺寸的增加呈现先增加后下降的特点;剪切带内的塑性应变分布为:在剪切带两个边界处应变为零,在剪切带中央剪切塑性应变达到最大值。     

较大塑性变形下纳晶镍剪切带的演化模型EI北大核心CSCD

较大塑性变形下,采用TEM原位拉伸实验揭示了纳晶镍内剪切带演化的微观机制,优化了微观力学模型。结果表明:在纳米尺度范围内,随着晶粒尺寸的增加,应变软化的趋势越来越明显,剪切带宽度也随着增加;晶粒尺寸降低将导致纳晶材料中剪切带的产生提前;内部特征长度的值随着晶粒尺寸的增加呈现先增加后下降的特点;剪切带内的塑性应变分布为:在剪切带两个边界处应变为零,在剪切带中央剪切塑性应变达到最大值。     

剪切带内部应变(率)分析及基于能量准则的失稳判据

应用应变梯度塑性理论对局部化剪切带内部(塑性)剪应变(率)规律进行了理论分析.研究了剪切降模量及岩石材料内部长度等参数对剪切带内部应变(率)的影响.推导了剪应力(率)与剪切带相对错距(速度)的本构关系.研究了剪切降模量和岩石材料内部长度对剪切带稳定性的影响.将岩石试件直剪试验试验机简化为钢块,采用能量准则对岩石试件(剪切带)及钢块系统的稳定性进行了理论研究,提出了系统失稳判据.研究表明;岩石材料的剪切降模量越大,岩石材料的内部长度越小,试验机的剪切刚度越小及试验机的等效高度越大剪切带--钢块系统越容易失稳.     

非晶合金塑性理论研究进展

非晶合金是一类兼具玻璃和金属双重特性的新型材料,具有一系列优异的力学、物理和化学性能,已经在国防、空天等领域显示出广阔的应用前景。非晶合金内部原子排列长程无序、短程有序,没有位错、晶界等传统意义上的晶体缺陷。因此,基于位错、孪生等微观机制的经典塑性理论在描述这类材料的塑性行为时遇到了极大的挑战。目前普遍认为,非晶合金宏观塑性流动是微观动态"流动事件"时空演化的结果。但是,对于"流动事件"的认知还很不清楚。主要介绍了目前几种代表性的非晶塑性流动理论:自由体积理论、剪切转变理论、剪切转变区理论以及协同剪切模型,并对非晶塑性流动的结构起源以及局部化剪切带机理进行了评述,最后简要展望了非晶塑性机理发展的几个问题。     

基于分子动力学模拟的铜锆晶体/非晶双相纳米复合材料力学行为

金属玻璃因其较差的室温塑性限制了其广泛应用,因此提升金属玻璃的力学性能、探明金属玻璃的变形机制已经成为当前材料领域的研究热点。采用分子动力学方法研究了晶粒尺寸和分布对晶体/非晶B2-CuZr/CuZr双相复合材料力学行为的影响。研究结果表明,随着纳米晶粒的尺寸增大,复合材料变形模式发生了从相对均匀变形到单一剪切带的局部变形的转变。研究指出,增大纳米晶粒尺寸/体积分数能有效提高复合材料的峰值应力,但除了较小尺寸纳米晶粒模型外,双相复合材料的塑性没有明显增强。此外,相对于交叉排列,纳米晶粒的对齐排列导致了更严重的塑性应变局部化。本文的研究结果对于设计和制备高性能的金属玻璃材料具有重要的参考价值和指导意义。      

AISI D2钢力学性能尺寸效应实验研究

为研究AISI D2钢力学性能尺寸效应现象,在常温下采用电子万能试验机和分离式霍普金森压杆(SHPB)实验装置对3种不同剪切带宽度(分别为800,400,50μm)的帽形试样进行了准静态和动态加载实验.实验结果表明,流动应力和失效应变随着剪切带宽度的减小而增大,但产生流动应力和失效应变尺寸效应现象的剪切带宽度不同.基于应变率强化项修正的Johnson-Cook本构模型,通过实验数据拟合得到材料的本构关系.研究表明,修正的Johnson-Cook本构模型与实验结果吻合较好.     

不同温度和应变率下Wf/BMG的力学性能

本文对纤维体积百分含量(Vf)为60%的钨丝增强Zr41.25Ti13.75Ni10Cu12.5Be22.5大块非晶基体复合材料(Wt/BMG)的力学性能进行了试验研究,测得该材料在77K到473K温度变化范围内的动静态应力应变曲线.发现其流动应力具有明显的正应变率敏感性;随着试验环境温度的升高,材料的弹性模量、屈服应力不断降低,材料的最终塑性应变值则呈升高趋势;Wf/BMG的断裂模式主要为剪切和纵向劈裂及二者的复合,具体形式受钨丝和基体界面的结合强度影响;钨丝的加入限制剪切带的传播,在界面处形成多条剪切带是复合材料塑性提高的主要原因;Wf/BMG的屈服与基体中由于绝热加热降低粘性而形成的剪切带有关,环境温度和非晶基体玻璃转变温度之差则直接影响剪切带的生成,使得Wf/BMG的屈服极限具有一定的温度敏感性.     

超塑性材料单向拉伸时孔洞长大对延伸率的影响

本文在描述单向拉伸的应变集中现象的局部化塑性理论中结合了孔洞长大模型式和孔洞长大速率的变化模型式,首次提出一个能同时描述应变集中和孔洞长大的模型,此模型所予测的断裂应变与实验结果吻合较好,本文还对影响孔洞长大的有关参数进行了讨论。     

复合成形轧制铜极薄带变形局部化的晶体塑性有限元模拟

随极薄带厚度的进一步减薄，轧制极薄带变形由于轧件厚度/晶粒尺寸比值小的尺寸效应和变形程度导致各向异性与局部化已完全不同于轧制厚件时的变形特性。采用具有拉拔-压缩-剪切复合成形功能的微型异步轧机开展系列厚度铜极薄带的箔轧实验，结果表明复合成形轧制工艺和极薄带尺寸显著影响轧制力能参数与箔材质量。宏观有限元理论已不再适用出现这些新现象的极薄带轧制变形的建模。将嵌入初始晶粒形貌和取向等微观组织结构信息的介观晶体塑性有限元模型(CPFE)用于复合成形条件下铜极薄带轧制变形局部化的模拟与分析，指导箔轧工艺优化和提高箔材质量。晶粒层次的晶体塑性有限元模型，准确预测了单层晶铜极薄带轧制变形局部化的现象和趋势，模拟与实验的轧制力吻合较好，尤其是各向异性。随上下工作辊异速比的增大，箔材厚度方向剪切变形增强，变形带、滑移带形成且局部化趋势显著。晶粒变形局部化的差异，对轧制制备极薄带材的控形控性造成困难。     

The effects of rate sensitivity and plastic potential surface curvature on plastic flow localization in porous solids

Plastic flow localization in porous elastic-viscoplastic solids is analyzed with an emphasis on the effects of material rate sensitivity and plastic potential surface curvature. The effect of rate sensitivity is included in a material model that accounts for a change of yield surface curvature in a rate-insensitive porous ductile solid. Shear band formation under plane strain and axisymmetric tension, and localized necking in biaxially stretched sheets are analyzed by using the present material model. The results illustrate the interactions of the effects of void nucleation and growth, material rate sensitivity and plastic potential surface curvature on plastic flow localization. The effects of nonproportional straining paths on localized necking in thin sheets are also demonstrated.     

Localization of deformation in rate sensitive porous plastic solids

The effect of material rate sensitivity on the localization of deformation in a porous visco-plastic solid is examined under plane strain tension and axisymmetric tension conditions. The plastic flow rule proposed by Gurson [3], modified to account for material rate sensitivity, is adopted to model the plastic softening behavior that arises due to void nucleation and growth. An initial imperfection in the form of a planar band is assumed and a material instability is sought as the deformation proceeds. Comparisons are made with the results of a rate-independent analysis [10]. The present rate-dependent results show that the retardation effect on flow localization is larger when the material is more rate-sensitive, and that, with a given rate sensitivity, the retardation effect on flow localization is greater in plane strain tension than in axisymmetric tension. Results are also obtained by employing parameter values representative of spheroidized carbon steels studied by Fisher [21], and the predictions of the model are in good agreement with experimental observations.     

Modelling macroscopic imperfections for the prediction of flow localization and fracture

Ductile materials subjected to plastic deformation experience the different stages of void nucleation, growth and coalescence that eventually lead to ductile fracture. Several models have been proposed to assess the influence of this damage on flow localization and fracture. In general, the plastic behaviour is represented by a constitutive model for porous or damaged materials. It is typical to introduce a material imperfection, with porosity higher than average, which evolves up to localization and fracture. However, the void volume fraction in the imperfection is chosen more or less arbitrarily. In the present work, a model that evaluates this void volume fraction more rigorously is developed. The forming limit diagram (FLD) for a dual phase‐steel is calculated using the damage‐based imperfection calculation and validated with experimental results. The effect of void shape on the imperfection porosity level and limit strains in sheet forming is also assessed with the present method.     

MODELLING FOR TRANSCRYSTALLINE AND INTERCRYSTALLINE FRACTURE BY VOID NUCLEATION AND GROWTH

A criterion has been formulated for transcrystalline and intercrystalline fracture caused by the evolution of voids located both in a grain and on grain boundaries. The criterion is based on the idea of plastic collapse for a unit cell that is a regular structural mezovolume of polycrystalline material. The criterion does not require the introduction of any empirical parameters, such as critical void size, critical size of ligament between voids and critical void volume fraction, which are used in most models.
Modelling has been performed for void nucleation and growth in a grain and on grain boundaries for elastic–plastic deformation and under creep conditions. A scheme is proposed to describe the transition from transcrystalline to intercrystalline cavitation fracture as a function of strain rate and temperature.
The effect of stress triaxiality on the critical strain and the lifetime for both transcrystalline and intercrystalline fracture has been investigated. A comparison of the results predicted by the suggested criterion with available empirical data has been performed.     

Localized plastic flow and spatiotemporal distribution of acoustic emission in steel

A relationship between the macroscopic localization of plastic flow and the spatiotemporal distribution of acoustic emission has been experimentally established in low-carbon steel deformed via the development of the Chernov-Lüders (CL) band followed by the parabolic strain hardening. It is shown that, as the CL band front propagates at a constant velocity in the sample, the pattern of acoustic emission is different at various stages of motion of this strain localization focus. Immobile foci of the localized flow, which appear at the stage of parabolic strain hardening, are spatially related to the inhomogeneity of strain developed during the CL band propagation.     

Analysis of shear localization in porous materials based on a lower bound approach

The conditions for shear localization in porous materials are examined based on the lower bound approach proposed by the present authors. The influence of void nucleation and material inhomogeneity on the critical strain to localization is investigated and an improved plastic strain controlled nucleation criterion is proposed which makes it possible to include the influence of hydrostatic stress and avoid the ambiguity caused by the non-normality flow rule. The constitutive behavior of porous materials (including the yield loci, the void growth rate and the stress-strain curve) is also examined and comparison is made between the theoretical result and the experiment. Finally the instability and fracture of sintered CP Ti alloy and AISI4340 steels is analyzed and results are compared with the experiment.     

Void growth and coalescence in metals deformed at elevated temperature

For metals deformed at elevated temperatures the growth of voids to coalescence is studied numerically. The voids are assumed to be present from the beginning of deformation, and the rate of deformation considered is so high that void growth is dominated by power law creep of the material, without any noticeable effect of surface diffusion. Axisymmetric unit cell model computations are used to study void growth in a material containing a periodic array of voids, and the onset of the coalescence process is defined as the stage where plastic flow localizes in the ligaments between neighbouring voids. The focus of the study is on various relatively high stress triaxialities. In order to represent the results in terms of a porous ductile material model a set of constitutive relations are used, which have been proposed for void growth in a material undergoing power law creep.     

Elastic–plastic void expansion in near-self-similar shapes

For void growth in an elastic–plastic strain hardening material the preferred shape of the void is calculated, dependent on the macroscopic stress state. Axisymmetric cell model analyses are carried out with a very small initial void size relative to the cell dimensions. Large deformations of the material around the void are modeled until the void volume is four orders of magnitude larger than the initial volume. An iterative procedure is used until the final void shape and the initial void shape are identical. Even when this convergence has been obtained, the void shape does not stay constant during the growth. Thus, the shapes found give only approximately self-similar growth. The results are compared with self-similar shapes determined previously for nonlinear viscous solids, subject to power law creep. For the time independent elastic–plastic material considered here the effect of the strain hardening level and of the initial yield strain are studied.     

Plastic deformation behavior of a nickel-based superalloy on the mesoscopic scale

Nickel-based superalloys have become the key materials of micro-parts depending on excellent mechanical properties at high temperatures. The plastic deformation behavior is difficult to predict due to the occurrence of size effect on the mesoscopic scale. In this paper, the effect of specimen diameter to grain size ratio(D/d) on the flow stress and inhomogeneous plastic deformation behavior in compression of nickel-based superalloy cylindrical specimens was investigated on the mesoscopic scale. The results showed that when D/d is less than 9.7, the flow stress increases with the grain size. Aiming at this phenomenon, a flow stress size effect model considering compressive strain partitioning was established.The calculated flow stress values agree well with the experimental values, thus revealing the effect of D/d on the flow stress in compression of nickel-base superalloy on the mesoscopic scale. The inhomogeneous plastic deformation during compression deformation increases with the grain size. The end surface profiles evolve from a regular circular shape to an irregular shape with the grain size. The surface folding phenomenon occurs only in partially compressed specimen with a few grains across the diameter.Crystal plasticity finite-element(CPFE) simulation of compression deformation on the mesoscopic scale real-time displayed the evolution of microstructure. The study of this paper has important guiding significance for understanding the influence of D/d on the compression deformation behavior of nickel-based superalloy on the mesoscopic scale.