用T积分研究硬聚氯乙烯粘塑性裂纹扩展

采用作者建立的硬聚氯乙烯的粘塑性本构模型,以CT试样为研究对象进行粘塑性有限元分析,并在此基础上编程计算T积分,然后再用T积分与实验所得的裂纹扩展速率关联.研究结果显示,描述蠕变延性材料和蠕变脆性材料粘塑性裂纹扩展速率的控制参量是不同的.T积分不适合作为硬聚氯乙烯粘塑性裂纹扩展的控制参量.     

循环稳定材料的棘轮行为：Ⅰ．实验和本构模型

在实验分析结果的基础上，对循环稳定材料的室温单轴和非比例多轴循环棘轮行为进行了本构描述，建立了一个简单而合理的、便于工程应用的粘塑性循环棘轮本构模型。给出了模型参数的确定方法，并根据实验获得的单拉应力．非弹性应变关系曲线确定了针对U71Mn轨道钢材料的参数值。在此基础上，通过对该材料棘轮行为的本构模拟检验了发展模型的预言能力。结果表明，模型具有很好的预测效果，模拟结果与对应的实验结果符合得很好。     

LY12超塑性成形有限元分析

基于LY12铝合金超塑性材料属性建立了弹-粘塑性本构模型.利用该本构模型并结合最大等效应变速率控制压力变化算法对LY12铝合金板超塑性圆杯成形进行数值模拟,得到圆杯变形过程中的应力应变分布、板料厚度变化及所需成形时间.根据模拟获得的优化压力时间曲线对圆杯进行超塑性气压胀形加载实验,制件厚度分布与模拟结果非常接近.     

循环稳定材料的棘轮行为:I.实验和本构模型

在实验分析结果的基础上,对循环稳定材料的室温单轴和非比例多轴循环棘轮行为进行了本构描述,建立了一个简单而合理的、便于工程应用的粘塑性循环棘轮本构模型。给出了模型参数的确定方法,并根据实验获得的单拉应力-非弹性应变关系曲线确定了针对U71Mn 轨道钢材料的参数值。在此基础上,通过对该材料棘轮行为的本构模拟检验了发展模型的预言能力。结果表明,模型具有很好的预测效果,模拟结果与对应的实验结果符合得很好。     

基于不可逆内变量热力学的岩石材料粘弹-粘塑性本构方程

岩石材料的粘弹性和粘塑性变形是与时间相关的能量耗散行为。在Rice不可逆内变量热力学框架下,引入两组内变量分别用来描述在粘弹性和粘塑性变形过程中材料的内部结构调整。通过给定比余能的具体形式和内变量的演化方程,推导出内变量粘弹-粘塑性本构方程。粘弹性本构方程具有普遍性,能涵盖Kelvin-Voigt和Poynting-Thomson在内的经典粘弹性模型的本构方程。并指出热力学力与应力呈线性关系是组合元件模型为线性模型的根本原因。粘塑性本构方程能较好地刻画岩石材料在粘塑性变形过程中的硬化现象。对模拟岩石的模型相似材料进行单轴加卸载蠕变试验,将蠕变过程中的粘弹性和粘塑性变形分离并根据试验数据对本构方程的材料参数进行辨识。试验数据和理论曲线对比结果表明该文提出的本构方程能很好地模拟材料的蠕变行为。该类型的本构方程能为岩石工程的长期稳定性的预测、评价以及加固分析提供基础。     

Drucker-Prager材料一致率型本构模型

根据一致粘塑性模型理论,结合所做的混凝土单轴动力特性试验,对常用的混凝土本构模型Drucker-Prager模型进行改进,引入应变率的影响,推导了Drucker-Prager材料的一致率型本构模型;并与试验的应力应变曲线进行比较,结果表明本文模型能够较好地反映混凝土的动力特性;最后,通过对一平面梁进行计算,并与线弹性及率无关塑性结果进行比较,结果表明考虑了应变率的影响后梁的动力特性发生了较大的变化。     

Bammann-Chiesa-Johnson粘塑性本构模型的参数识别方法与验证

Bammann-Chiesa-Johnson(BCJ)粘塑性本构模型对材料力学响应的再现和预测能力强烈依赖于其模型参数的确定,而模型参数的确定往往是通过反分析方法来进行。由于BCJ粘塑性模型包含了应变、应变率和温度耦合效应以及加载路径和温度历史,其常数多达18个,所以寻找最佳的模型参数识别值十分繁琐。针对BCJ本构模型参数复杂、识别困难的问题,本文基于参数的物理意义,在准静态、蠕变及动态加载试验基础上,通过模型参数解耦分离、粒子群智能优化的方法分6步对18个材料常数进行识别,并用识别结果对1060纯铝动态加载试验力学响应进行模拟,模拟结果与试验结果符合良好。通过定量化误差分析,证明了BCJ粘塑性模型对实验数据的预测具有较高精度,该模型参数识别方法科学可行。     

高强钢TRB高温下热流变特性

为了准确仿真高强钢板热冲压成形过程,获得高强钢高温下的材料本构关系模型,利用Gleeble3500热模拟试验机在不同温度和应变速率下对不同厚度的高强钢B1500HS钢板进行了单向拉伸试验,获得各种工艺条件下的应力-应变曲线,并基于变形抗力数学模型,引入板材厚度参数,通过最小二乘法进行数据拟合获得高强钢TRB高温下的材料本构关系.利用试验结果对本构关系模型进行的拟合验证表明,拟合程度较好,说明建立的材料本构关系能很好地描述高强钢TRB在高温下的应力-应变关系.     

一种新的多轴高周疲劳模型

将疲劳强度以上加载等效为塑性应变,建立了塑性应变与加载应力呈线性关系的表达式,由此得到循环加载的塑性应变能。该塑性应变能使材料微观组织结构发生不可逆变化而引起等效宏观应力。假定该应力符合一种特定的分布函数,导出其最大应力与外加应力叠加达到材料本征断裂应力时的裂纹成核寿命,从而并由微裂纹引起上述两部分应力变化,得到继续加载直至宏观裂纹出现的疲劳寿命。所建立的多轴疲劳寿命公式由3个材料参数表达,并通过单轴疲劳试验数据确定。初步研究表明：该模型对所引用的多轴疲劳试验数据有很好的预测能力。     

混凝土材料的粘塑性损伤统一本构模型

发展了只适用于金属类材料的粘塑性统一本构理论,借助经典塑性理论的基本法则,建立了无屈服面和无破坏面的混凝土材料的粘塑性损伤统一本构模型。放弃了传统统一本构模型的静水压不影响非弹性变形和无非弹性体积膨胀的基本假设;发展了间断的经典塑性乘子,使其为连续函数,并提出了相应的构造方法,拓展定义了其物理意义。数值模拟显示,此本构模型能够模拟混凝土材料的率相关性质、在压缩载荷作用下的体积膨胀现象和由损伤引起的应力软化和刚度退化现象。     

Assessment of Viscoplastic Properties of Titanium Alloys

The paper presents findings of the study of viscoplastic properties of several titanium alloys subjected to cyclic loading under complex stress state conditions. The applicability of elastic viscoplastic models to structural materials has been experimentally verified.__________Translated from Problemy Prochnosti, No. 3, pp. 37 – 44, May – June, 2005.     

Viscoplastic response and modeling of asphalt-aggregate mixes

The strain response of asphalt-aggregate mixes to applied stresses is decomposed additively into a viscoelastic part and a viscoplastic part. The paper focuses on the response and modeling of the viscoplastic component; it includes the development of a multiaxial constitutive formulation that is capable of generating: (i) strain hardening when the loading is applied in one direction; (ii) strain softening immediately after stress reversals; (iii) volumetric changes under uniaxial conditions or isotropic conditions, or both; and (iv) directional non-symmetry. In order to investigate the model’s capabilities, four tests were performed sequentially on one asphalt sample. The tests were limited to pre-peak conditions and one temperature and consisted of creep and recovery sequences in uniaxial tension, uniaxial compression, isotropic compression and uniaxial tension-compression. Analysis of the results showed that the new theory, once calibrated, was able to adequately reproduce the viscoplastic strain component; its forecastability, however, was found limited.     

Viscoplastic regularization of local damage models: revisited

Local damage models are known to produce pathological mesh dependent results. Regularization techniques are therefore mandatory if local damage models are used for academic research or industrial applications. The viscoplastic framework can be used for regularization of local damage models. Despite of the easy implementation of viscoplasticity, this method of regularization did not gain much popularity in comparison to the non-local or gradient damage models. This work is an effort to further explore viscoplastic regularization for quasi-static problems. The focus of this work is on ductile materials. Two different types of strain rate hardening models i.e. the Power law (with a multiplicative strain rate part) and the simplified Bergström van Liempt (with an additive strain rate part) models are used in this study. The modified Lemaitre’s anisotropic damage model with a strain rate dependency was used in this study. It was found that the primary viscoplastic length scale is a function of the hardening and softening (damage) parameters and does not depend upon the prescribed strain rate whereas the secondary length scale is a function of the strain rate. As damage grows, the effective regularization length gradually decreases. When the effective regularization length gets shorter than the element length numerical results become mesh dependent again. This loss of objectivity can not be solved but the effect can be minimized by selecting a very fine mesh or by prescribing high deformation velocities.     

A Viscoplastic Flow Modeling of Ceramic Tape Casting

The flow behavior in a ceramic tape casting process is analytically investigated using a viscoplastic fluid constitutive model. The model accommodates both yield stress and shear-thinning characteristic of tape casting materials. An analytical solution for the flow field in a two dimensional channel is obtained at a variety of pressure gradients and moving velocities of the channel wall, and the thickness of wet tape is subsequently characterized. Two critical pressure gradients are identified which mark patterns of the flow velocity profiles in the channel. The differences between the results from existing publications and the present work are clearly indicated.     

Nonlinear Viscoelastic and Viscoplastic Constitutive Equations Based on Thermodynamics

An approach to modeling the mechanical behavior of fiber reinforced and unreinforced plastics with an evolving internal state is described. Intrinsic nonlinear viscoelastic and viscoplastic behavior of the resin matrix is taken into account along with growth of damage. The thermodynamic framework of the method is discussed first. The Gibbs free energy is expressed in terms of stresses, internal state variables (ISVs), temperatureand moisture content. Simplifications are introduced based on physical models for evolution of the ISVs and on experimental observations of thedependence of strain state on stress state and its history. These simplifications include use of master creep functions that account for multiaxial stresses, environmental factors and aging in a reduced time and other scalars. An explicit representation of the strains follows, which isthen specialized to provide three-dimensional homogenized constitutiveequations for transversely isotropic, fiber composites. Experimentalsupport for these equations is briefly reviewed. Finally, physicalinterpretation of some of the constitutive functions is discussed usingresults from a microcracking model as well as molecular rate process andfree volume theories. It is shown that the present thermodynamicformulation leads to a generalized rate process theory that accounts for abroad distribution of thermally activated transformations in polymers.     

磁流变液在圆筒间的粘塑性流动

磁流变液具有粘性和粘塑性流体特性.基于Navier-Stokes方程,分析了外加磁场对在两圆筒间隙间的磁流变液粘塑性流动的影响,得到了速度和流量表达式,为圆筒式磁流变器件的设计提供了理论依据.研究结果表明:速度沿磁场方向呈抛物线和直线分布;流量可由外加磁场连续调控.     

Viscoplastic microstructural processes in solids under impulsive thermal loading

The stress–strain state of a disk caused by a thermal pulse is investigated within the framework of a dynamic coupled thermomechanics problem accounting for microstructural phase transformations caused by the heating and subsequent cooling of the material. The solution of the axisymmetric problem is obtained numerically from a thermodynamically consistent theory for the inelastic behavior of the material which takes into account the thermal dependencies of the material properties using the finite element method. The influence of the microstructural transformations on the dynamic and quasistatic response of the material as well as the residual stress–strain state within the irradiated zone are also studied in detail.     

Flow of a Viscoplastic Arsenic Selenide Melt in Circular-Cylindrical Channels

The flow rate of molten arsenic selenide in circular-cylindrical channels was measured as a function of temperature and excess gas pressure over the melt with application to the fabrication of optical fibers by crucible methods. The channel diameter was 3.0, 4.2, and 4.5 mm, and the channel length was 120 mm. The temperature was varied from 285 to 320°C, and the excess pressure of the inert gas was up to 1.5 × 10⁵ Pa. The results demonstrate that there is a threshold pressure for melt flow, suggesting that the melt is a viscoplastic fluid. The experimental data are interpreted in terms of the Shvedov-Bingham rheological model.     

Viscoplastic modeling of ABS material under high-strain-rate uniaxial elongational deformation

Uniaxial tensile tests were performed on the newest type of Meissner rheometer with an ABS (acrylonitrile-butadiene-styrene) material. Tests were conducted for constant strain rates varying from 0.01 to 1 (1/s), at the temperatures ranging from 150 to 200 °C. Based on the experimental data, a new model was proposed, in which, strain hardening, strain rate sensitivity, temperature changes and the variation in the hardening index could be taken into consideration. In this new model, a new parameter, w, was introduced, which represents the variation in the hardening index. The proposed model can approximate the experimental data of the uniaxial tensile test quite well. Two existing models were also employed to approximate the material behavior, however, both of them exhibited the poor accuracy. Finally, a simple stretch deformation was simulated employing the three different models, and the differences in the final thickness and shape were confirmed.