19Mn5钢循环硬化特性与累积等效微观应变分布的有限元分析

本文用改进的空间轴对称大应变有限元程序,对19Mn5钢在低周疲劳下的应力-应变滞后回线以及试样纵截面、横截面内累积等效应变分布和水静应力在纵截面内的分布进行了计算.结果表明:19Mn5钢铁素体内的累积塑性应变和水静应力均高于珠光体,这就从微观上证明了为什么19Mn5钢在低周疲劳下呈现循环硬化,同时为19Mn5钢的微观断裂机制提供了一个清晰的物理图象.     

加载历史对循环硬化的影响

本文研究了45#钢和LY-12CZ铝合金在室温条件下拉扭复合比例及非比例加载时的循环硬化特性,其结果表明不同加载路径循环硬化程度不同,纯扭硬化程度最小,圆形路径硬化程度最大。着重研究了不同循环历史对后面循环稳定应力幅值的影响。     

加载路径和温度对NiTi合金相变特性的影响

利用MTS装置,对NiTi形状记忆合金在不同加载路径(拉、压、扭以及拉/压-扭比例加载)和不同温度(28~150℃)下的准静态相变行为进行了较系统的实验研究。结果表明,材料呈现明显的伪弹性效应和拉压不对称性。随着温度的升高,相变起始应力逐渐增大,伪弹性效应和拉压不对称性逐渐减弱。当温度达到一定阈值时,拉压不对称性消失,材料基本呈现弹塑性状态。通过实验曲线确定了NiTi合金的相变临界参数,在σ-τ应力空间中对三种常用宏观相变临界准则进行了讨论,比较了它们各自的适用性。     

铜双晶体的循环应力-应变响应

比较了铜单晶体和多晶体疲劳行为的异同,提出了研究双晶体疲劳行为的必要性．总结了具有不同晶体取向和晶界的铜双晶体的疲劳行为的最新进展．利用平行晶界铜双晶体的取向因子和晶界影响区,总结了在循环载荷作用下的晶界强化模型．分析了垂直晶界铜双晶体循环塑性变形行为的特点,讨论了组元晶体取向对垂直晶界铜双晶体循环应力－应变曲线的影响．提出了提高单晶体和双晶体疲劳强度的控制因素．     

循环硬化材料本构模型的隐式应力积分和有限元实现

针对新发展的、能够描述循环硬化行为应变幅值依赖性的粘塑性本构模型,讨论了它的数值实现方法。首先,为了能够对材料的循环棘轮行为（Ratcheting）和循环应力松弛现象进行描述,对已有的本构模型进行了改进;然后,在改进模型的基础上,建立了一个新的、全隐式应力积分算法,进而推导了相应的一致切线刚度（Consistent Tangent Modulus）矩阵的表达式;最后,通过ABAQUS用户材料子程序UMAT将上述本构模型进行了有限元实现,并通过一些算例对一些构件的循环变形行为进行了有限元数值模拟,讨论了该类本构模型有限元实现的必要性和合理性。     

PTFE材料在高应变率冲击下的力学性能

聚四氟乙烯(PTFE)在高速碰撞或者爆炸加载时的应变率可高达10⁶ s^-1,高应变率下PTFE材料的力学响应会对其材料性能产生较大影响。本文中采用压剪炮试验系统(PSPI)测试了PTFE材料在高应变率(10⁵~10⁶ s^-1)下的压缩力学性能,实验中碳化钨(WC)飞片板以一定速度撞击由前靶板、试件和后靶板组成的三明治结构,并采用激光干涉仪记录后靶板自由面的速度变化。对实验结果处理后得到该PTFE材料的应力应变数值,并拟合得到应力应变曲线。本研究对PTFE/金属复合材料制成的动能侵彻体强度及其冲击碎化机理的分析具有指导意义。

     

摩擦系数对钛合金等通道转角挤压影响的有限元分析

以Ti-6Al-4V钛合金高温变形行为研究为基础,建立了等通道转角挤压（ECAE）的三维模型,运用DEFORM-3D有限元分析软件模拟了600 ℃等温条件下不同摩擦系数对Ti-6Al-4V合金ECAE过程中的温度场,等效应力,等效应变以及等效应变率的影响。结果表明：核心高温区以及核心应力区主要集中于转角处;随着摩擦系数增大,核心高温区面积增大,转角入口区的应力也有所增加;点迹跟踪结果表明各点应变均在经过转角处达到最大值,内角点及外角点处的变形较不稳定。     

湿热环境对挖补修理后复合材料层合板振动特性的影响

通过对湿热环境下阶梯挖补胶接修理后碳纤维/双马树脂(T300/QY8911)层合板振动特性的分析,探究了挖补修理后碳纤维/双马树脂层合板在不同湿热效应下的振动特性。基于Mindlin-Reissner板理论与最小势能变分原理,在考虑湿热效应前提下,推导了正交各向异性复合材料层合板的本构关系及四边简支条件下的自由振动控制方程;其次,利用ABAQUS软件建立了三维黏弹性有限元挖补修理模型,应用Fortran语言将湿热耦合下的应力-应变本构关系编入UMAT二次开发文件中,采用三维八节点五自由度等参单元进行了离散。通过与参考文献对照,验证了胶接修理后T300/QY8911层合板有限元模型和编译的湿热环境下UMAT子程序的合理性与有效性。算例结果表明:湿热环境对挖补修理后层合板固有频率的影响比对完整层合板严重;温度对材料性能的影响大于湿度;相比于材料性能变化对振动特性的影响,湿热残余应力对胶接修理后层合板振动特性的影响占主导;参考环境下,胶接修理后层合板的固有频率增大;湿应力对层合板固有频率的影响大于热应力。     

用CE/SE法对弯曲与分叉河道的溃坝洪水波的数值研究

对作者的二维溃坝洪水波的数学模型进一步推广,得到了一般形式的基于任意四边形网格的时空守恒元和解元方法(简称CE/SE法)的新的格式.CE/SE法从守恒积分型浅水方程出发,设立守恒元和解元,严格保证其物理意义上的守恒律,并且构造思想简单,格式通用性好.首先采用CE/SE法计算等宽矩形河道的溃坝洪水波,并与Stoker解析解进行比较,在此基础上,数值模拟了180度强弯曲河道、45度三支分叉河道的二维溃坝洪水波的演进过程,揭示了溃坝洪水波在弯曲河道中内外两岸速度与水位的变化,在分叉河道中自动进行流量与动量的再分配,在分叉点处形成旋涡,水位变化剧烈等复杂的运动特征,算例结果表明基于任意四边形网格的CE/SE法精度高,稳定性好,该格式对各种不规则几何区域内的溃坝问题具有较强的适应性,对溃坝洪水波的间断具有较高的分辨率.     

Constitutive modeling of cyclic plasticity deformation of a pure polycrystalline copper

Cyclic plasticity experiments were conducted on a pure polycrystalline copper and the material was found to display significant cyclic hardening and nonproportional hardening. An effort was made to describe the cyclic plasticity behavior of the material. The model is based on the framework using a yield surface together with the Armstrong–Frederick type kinematic hardening rule. No isotropic hardening is considered and the yield stress is assumed to be a constant. The backstress is decomposed into additive parts with each part following the Armstrong–Frederick type hardening rule. A memory surface in the plastic strain space is used to account for the strain range effect. The Tanaka fourth order tensor is used to characterize nonproportional loading. A set of material parameters in the hardening rules are related to the strain memory surface size and they are used to capture the strain range effect and the dependence of cyclic hardening and nonproportional hardening on the loading magnitude. The constitutive model can describe well the transient behavior during cyclic hardening and nonproportional hardening of the polycrystalline copper. Modeling of long-term ratcheting deformation is a difficult task and further investigations are required.     

Gradient plasticity constitutive model reflecting the ultrafine micro-structure scale: the case of severely deformed copper

A further development of the mechanism-based strain gradient plasticity model well established in literature is reported. The major new element is the inclusion of the cell size effect in dislocation cell forming materials. It is based on a ‘phase mixture’ approach in which the dislocation cell interiors and dislocation cell walls are treated as separate ‘phases’. The model was applied to indentation testing of copper severely pre-strained by equal channel angular pressing. The deformation behaviour and the intrinsic length scale parameter of the gradient plasticity model were related to the micro-structural characteristics, notably the dislocation cell size, resulting from the deformation history of the material.     

等径角挤压后Ti5553钛合金的冲蚀磨损机理演变

本文中系统研究了经等径角挤压(ECAE)处理后的Ti5553钛合金在海砂环境中的冲蚀磨损机理演变历程和失效原因.发现Ti5553钛合金在高角度冲蚀工况下,其冲蚀磨损机理从初期的微切削转变为冲击挤压变形,并保持稳定;合金显微组织中弥散的α相以及ECAE工艺影响了冲蚀机理演变过程及冲蚀磨损程度;提高合金的强度、韧塑性对于抵抗冲蚀磨损起到了促进作用.     

Effect of microstructure on the hardening and softening behavIors of polycrystalline shape memory alloys Part I: Micromechanics constitutive modeling

The effects of microstructure and its evolution on the macroscopic superelastic stress-strain response of polycrystalline Shape Memory Alloy (SMA) are studied by a microstructure-based constitutive model developed in this paper. The model is established on the following basis: (1) the transformation conditions of the unconstrained single crystal SMA microdomain (to be distinguished from the bulk single crystal), which serve as the local criterion for the derivation of overall transformation yield conditions of the polycrystal; (2) the micro- to macro-transition scheme by which the connection between the polycrystal aggregates and the single crystal microdomain is established and the macroscopic transformation conditions of the polycrystal SMA are derived; (3) the quantitative incorporation of three microstructure factors (i.e., nucleation, growth and orientation distribution of martensite) into the modeling. These microstructural factors are intrinsic of specific polycrystal SMA systems and the role of each factor in the macroscopic constitutive response is quantitatively modeled. It is demonstrated that the interplay of these factors will result in different macroscopic transformation kinematics and kinetics which are responsible for the observed macroscopic stress-strain hardening or softening response, the latter will lead to the localization and propagation of transformation bands in TiNi SMA. The project supported by the Research Grant Committee (RGC) of Hong Kong SAR, the National Natural Science Foundation of China and the Provincial Natural Foundation of Jiangxi Province of China     

Effect of microstructure on the hardening and softening behaviors of polycrystalline shape memory alloys Part II: Numerical simulation under axisymmetrical loading

Based on the microstructure-based constitutive model established in Part I, a detailed numerical investigation on the role of each microstructure parameter in the kinematical and kinetic evolution of polycrystalline SMA under axisymmetrical tension loading is performed. Some macroscopic constitutive features of stress-induced martensite transformation are discussed. The subject supported by the Research Grant Committee (RGC) of Hong Kong SAR, the National Natural Science Foundation of China and the Provincial Natural Science Foundation of Jiangxi Province of China     

Cyclic multiaxial and shear finite deformation responses of OFHC Cu. Part II: An extension to the KHL model and simulations

In this part, the Khan–Huang–Liang (KHL) constitutive model was extended to account for kinematic hardening characteristic behavior of materials. The extended model is then generalized and used to simulate experimental response of oxygen free high conductivity (OFHC) copper under cyclic shear straining and biaxial tension–torsion (multiaxial ratchetting) experiments presented in Part I (Khan et al., 2007). In addition, a new modification for the non-linear kinematic hardening rule of Karim–Ohno (Abdel-Karim and Ohno, 2000) is proposed to simulate multiaxial ratchetting behaviors. Although, the kinematic hardening contributes the most to the response, it is shown that, the loading rate effect, and a coupled isotropic and kinematic hardening effect should also be considered while simulating the multiaxial ratchetting behavior of OFHC copper. Furthermore, the newly modified kinematic hardening rules is able to fairly well simulate the multiaxial ratchetting experiments under different loading conditions, irrespective of the value of applied axial tensile stress, shear strain amplitude, pre-cyclic hardening and/or loading sequence.     

两种再生粗骨料混凝土单轴受压性能研究

为研究影响再生混凝土力学性能的因素,将建筑结构中使用最为广泛的C30与C40两种强度等级的废混凝土破碎成再生粗骨料RCA(recycled coarse aggregate),根据实测RCA吸水率调整了配合比。以RCA来源和RCA取代率为变量,设计了9组再生混凝土试件,进行立方体抗压强度及棱柱体单轴受压试验。基于试验数据,得到了两种不同来源RCA的再生混凝土弹性模量、峰值应变等重要力学性能参数,绘制了再生混凝土应力-应变曲线。结果表明,随着RCA取代率的增加,两种不同来源RCA再生混凝土弹性模量和立方体抗压强度均表现为下降,RCA强度对再生混凝土各项力学性能均有影响。