一种基于Chaboche理论的混合硬化模型及其在回弹仿真中的应用

为提高相变诱发塑性钢板的回弹仿真精度,在Voce非线性各向同性硬化和Chaboche随动硬化理论的基础上建立混合硬化模型。采用LSDYNA软件建立实现正交应变路径的单向拉伸有限元模型,结合LS-OPT软件,以仿真获得的应力应变曲线与试验应力应变曲线的偏差最小为目标,对混合硬化模型中的待定系数进行优化识别。采用该混合硬化模型预测TRIP780钢U形件的回弹,并进行试验验证。对仿真和试验误差因素进行总结。结果表明,基于Voce各向同性硬化和2项背应力组成的Chaboche随动硬化模型的混合硬化模型具有比各向同性硬化模型更高的冲压件回弹预测精度,优化过程中近似模型和预应变水平的选取以及轮廓线获取均是本文误差的重要来源,采用优化软件与有限元软件相结合确定硬化模型参数的方法在实际应用中精度高且方便可行。     

各向异性非线性随动硬化本构模型的建立及应用

基于平面应力假设,采用HILL48各向异性屈服准则和A-F非线性随动硬化模型及塑性流动法则建立一种各向异性非线性随动硬化本构模型。运用向后欧拉回映算法,通过Fortran语言编写UMAT子程序,将该本构模型嵌入到ABAQUS软件中。以板料经过拉深筋的循环加载问题为研究对象,利用开发的本构模型、ABAQUS软件中的各向同性屈服及随动硬化模型对板料经过拉深筋的变形过程进行数值模拟分析,得到切向应力-应变循环变化曲线。对比试验结果,开发的弹塑性本构模型的计算精度更高,验证了所建本构模型的有效性,该模型可以应用于板料反向加载变形行为研究。     

基于循环拉-压试验的混合硬化模型参数确定及模型应用

材料弹塑性本构模型是影响有限元模拟精度的最重要因素,混合硬化本构模型能较准确表现材料塑性变形过程真实硬化特征,而本构模型中材料特性相关参数是否准确直接影响到有限元模拟的精度。基于Hill48各向异性屈服准则,结合Voce各向同性硬化模型和Armstrong-Frederic非线性随动硬化模型,建立一个考虑材料各向异性和Bauschinger效应的混合硬化弹塑性本构模型。通过循环拉伸-压缩试验,获得DC54D+ZF镀锌板的循环变形应力-应变曲线,并利用通用全局优化算法,根据单向应力状态混合硬化本构方程,准确地确定了混合硬化模型中的材料特性参数。最后,使用ABAQUS有限元软件对板材循环拉伸-压缩问题和板材过拉深筋问题进行该本构模型的适用性分析,验证了所建立的各向异性混合硬化材料本构模型的可靠性和精确性。循环拉伸-压缩试验是直接准确地获得本构模型材料参数的有效方法。     

列车车体SUS301L-HT不锈钢动态力学性能及其对结构吸能特性的影响

对SUS301L-HT不锈钢材料分别进行了准静态拉伸试验和动态冲击拉伸试验,以获得不同应变率下的材料本构关系。为准确地描述SUS301L-HT不锈钢材料的动态力学性能,采用了列表插值法,并通过对比有限元对标和试验数据来验证该方法的可靠性。以典型薄壁吸能结构为载体,采用2种材料参数,对比分析了SUS301L-HT不锈钢材料的动态力学性能对结构吸能特性的影响。研究结果表明：SUS301L-HT不锈钢具有明显的应变率强化效应,随着应变率的增大,材料的塑性硬化能力降低,表现出明显的温度软化效应;列表插值法相比动态本构模型能更好地描述SUS301L-HT不锈钢的动态力学性能,且采用列表插值法得到的结果与试验结果、有限元结果均有良好的一致性;列车碰撞的应变率属于中低应变率范围,对于SUS301L-HT不锈钢制成的车体吸能结构,考虑应变率效应的结构的实际吸能量要比不考虑应变率效应的相同结构的吸能量高,但初始峰值力相对较大。     

高强钢变模量随动强化本构模型匹配与解耦标定策略研究

高强钢通过微观组织调控获得高强度,但不同牌号高强钢微观的不均匀变形和微观诱导塑性机理不同,使得高强钢卸载及反向加载行为更加复杂,并且牌号间差异增大,为此给出模型自适应匹配及参数解耦匹配的系统化策略,实现了高强钢回弹精确预测。首先建立幂函数和指数函数混合硬化模型,基于混合模型给出自由弯曲加载的弯矩平衡方程和曲率约束方程,基于变模量模型构建截面弹性弯矩的积分方程,基于加载和卸载解析模型建立逆向识别卸载参数的子优化模型。确定变模量线性随动强化、变模量非线性随动强化模型和含边界面的变模量随动强化模型的匹配策略。基于自由弯曲、单向拉伸和拉压试验数据,确定相应本构的子优化模型参数的优化次序,最终形成本构匹配及其参数解耦标定的系统化策略,并基于Fortran语言开发标定程序库。建立U形弯曲件和弧形弯曲件预测模型,分别对DP980和DH980两种高强钢不同应变水平下的识别结果及回弹预测结果进行对比分析,验证了解耦标定策略不仅提高了不同牌号数据的相关度,而且大幅度提升了同一牌号下的模型精度和稳定性,为基于数据的材料性能统一自辨识方法研究奠定了基础。     

316L不锈钢循环特性的有限元分析及疲劳寿命预测

针对316L奥氏体不锈钢进行了低周疲劳试验,采用有限元方法针对材料的循环特性进行了模拟计算。试验结果表明,该材料在不同应变范围下均表现出了明显的循环附加硬化,且硬化程度随着应变范围的增加而更为明显。在Abaqus模拟计算中,使用非线性随动强化与各向同性强化的混合模型描述材料弹塑性行为。不同应变范围下,模拟得到的第二周次下的应力应变曲线与实验结果吻合较好,模拟得到的前20周的最大应力与实验结果的误差与应变范围有关,最大平均误差为3.20%。基于实验结果和模拟结果,采用能量方法进行疲劳寿命预测,预测结果均位于两倍分散带内。     

TC4-DT钛合金材料动态力学性能及其本构模型

利用同步组装的高温分离式Hopkinson压杆试验装置,对TC4-DT钛合金材料分别进行了常温下不同应变率（930~9700s-1）和应变率为5000s-1时不同温度下（20~800℃）的动态力学性能测试,获得了各种冲击载荷下的应力-应变曲线。试验数据表明,TC4-DT材料具有应变率增塑效应且存在着临界应变率值,当应变率高于此值时应变率敏感性增强明显,此外随着材料加热温度的升高,软化效应减弱。利用试验所得的数据拟合了基于Power-Law和Johnson-Cook两种热-黏塑性本构方程且获得这两种动态本构模型参数,并将所得的两种拟合曲线与试验所得数据进行对比分析,结果表明两曲线吻合度都较好,此外还对这两种曲线的拟合精度进行对比,对比结果表明两种模型的拟合误差相差不大,但是Power-Law模型拟合精度要略优于Johnson-Cook模型的拟合精度。     

冲击载荷下铜基粉末冶金材料性能及本构关系研究

在hopkinson压杆上做了铜基粉末冶金摩擦材料的冲击压缩试验。结果表明:在1000/s应变率以下时,材料表现为应变率强化效应;在1000/s应变率以上时,材料表现为应变率弱化效应。得出结论:应变率1000/s为材料的临界应变率。根据实验得出的应力应变曲线,在朱王唐模型的基础上加入表征材料粘性特征的塑性项,对试验曲线进行数值模拟,取得了较好效果,填补铜基粉末冶金摩擦材料在冲击动态参数上的空白,为工程应用提供了理论依据。     

15CrMoR焊缝区及母材区低周疲劳寿命预测

制作了焦炭塔用材15Cr MoR的对接试件,对其母材区及焊缝区金属进行了不同温度的单轴拉伸试验与全寿命单轴棘轮效应试验。利用OW-II随动强化模型对材料的棘轮效应进行预测,模型能较好地预测材料稳定段的棘轮应变率。通过单轴拉伸试验数据得出预测低周疲劳寿命的基本参数,选用考虑棘轮效应的MSRS模型对母材区焊缝区材料疲劳寿命进行预测,预测结果均在2倍误差带以内,而且预测结果均匀分布在45°精确线两侧。利用OW-II随动强化模型在相同工况下预测出的母材区焊缝区稳定段棘轮应变率,应变幅值,带入MSRS模型,预测结果表明母材区疲劳寿命明显低于焊缝区,稳定段棘轮应变率母材区明显高于焊缝区。     

利用双线性应力-应变模型求解管道爆裂压力

针对硬化材料模型求解圆柱容器破裂压力的非线性微分方程的解,基于容器塑性应变过程的体积不变定律,采用应力-应变强度的双线性模型进行分析。通过冲击压力实验对管道模型进行实验测量以及使用有限元方法进行的数值建模进行对比,验证解析方程。实验结果与有限元分析结果基本吻合。     

A new hardening rule for metallic foam plasticity

Metallic foams are a class of porous materials widely used in the industry because of their advantages. In recent years, extensive studies on the behavior of these materials have been conducted. Several constitutive equations have also been presented and applied. This study proposes a new constitutive equation that predicts metallic foam behavior using the stress–strain curve in uniaxial compression. The proposed model offers a new functionality for work hardening and is evaluated for both isotropic and combined hardening. The constitutive equations are implemented in MATLAB and integrated using return mapping algorithm. The material parameters are identified using genetic algorithm and through a comparison of the experimental and numerical results. The aluminum foams discussed in this paper are the commercially available types, Foaminal and Alporas. The comparison of numerical and experimental results indicate that this new constitutive equation predicts foam behavior in a reasonable manner. Moreover, a good agreement is observed between the experimental and computational curves.     

Finite element simulation of springback for a channel draw process with drawbead using different hardening models

The objective of this work is to predict the springback of Numisheet’05 Benchmark#3 with different material models using the commercial finite element code ABAQUS. This Benchmark consisted of drawing straight channel sections using different sheet materials and four different drawbead penetrations. Numerical simulations were performed using Hill's 1948 anisotropic yield function and two types of hardening models: isotropic hardening (IH) and combined isotropic-nonlinear kinematic hardening (NKH). A user-defined material subroutine was developed based on Hill's quadratic yield function and mixed isotropic-nonlinear kinematic hardening models for both ABAQUS-Explicit (VUMAT) and ABAQUS-Standard (UMAT). The work hardening behavior of the AA6022-T43 aluminum alloy was described with the Voce model and that of the DP600, HSLA and AKDQ steels with Hollomon's power law. Kinematic hardening was modeled using the Armstrong-Frederick nonlinear kinematic hardening model with the purpose of accounting for cyclic deformation phenomena such as the Bauschinger effect and yield stress saturation which are important for springback prediction. The effect of drawbead penetration or restraining force on the springback has also been studied. Experimental cyclic shear tests were carried out in order to determine the cyclic stress-strain behavior. Comparisons between simulation results and experimental data showed that the IH model generally overestimated the predicted amount of springback due to higher stresses derived by this model. On the other hand, the NKH model was able to predict the springback significantly more accurately than the IH model.     

A single cone-cap plasticity with an isotropic hardening rule for powder materials

In this paper, a new single cone-cap plasticity with an isotropic hardening rule is presented for powder materials. A general form is developed for the cap plasticity, which can be compared with some common double-surface plasticity models proposed for powders in literature. The constitutive elasto-plastic matrix and its components are derived based on the definition of yield surface, hardening parameter and nonlinear elastic behavior, as a function of relative density of powder. Different aspects of the model are illustrated and the procedure for determination of powder parameters is described. Finally, the applicability of the proposed model is demonstrated in numerical simulation of triaxial and confining pressure tests.     

板料屈服行为及强化规律的研究进展

研究板料塑性成形的理论基础是屈服准则、强化规律以及本构模型。随着新材料、新工艺的不断出现,温度和应变速率对塑性成形过程中的影响也不容忽视,原有的塑性理论已无法满足研究和工程应用的需求。从板料屈服准则研究、包辛格效应与强化模型研究、屈服强化规律试验方法研究以及涉及应变速率和温度的板料屈服强化研究4个方面阐述板料屈服行为及强化规律的研究进展,指出常用屈服准则的特点和不足,说明各种强化模型中组合强化模型仍然是研究重点。试验方法主要从研究屈服轨迹的双向拉伸试验及确定强化模型参数试验的2个方面进行介绍。此外,指出针对板料在复杂应力状态下应力张量与应变张量之间的涉及应变率和温度的屈服准则和相应的流动准则的本构关系还有待研究。提出随着新材料、新工艺的不断出现,涉及应变速率和温度的屈服准则和强化规律、试验方法以及在有限元模拟中的应用等研究将是未来的研究热点。     

半固态材料触变成形通用本构方程及其优化

采用半固态成形机理分析和试验研究相结合的方法,建立半固态触变成形的粘塑性本构方程,并提出本构方程的优化新方法。通过半固态Al-4Cu-Mg合金的等温压缩试验研究,分析试验数据,得到本构方程中的4个待定系数,并以此作为优化设计的初试值。结合本构方程的形式,对其进行特性分析和优化。将含优化变量的本构方程作为子程序引入到有限元数值模拟中,可以得到对照热模拟试验结果的若干工艺条件下半固态Al-4Cu-Mg合金的应力应变曲线。通过比较有限元数值模拟结果和热模拟试验结果可知,利用提出的本构方程优化新方法,不仅可以剔除热模拟试验数据中几何效应的影响,而且还能准确地描述半固态材料的触变成形规律,从而可以提高数值模拟的精度与可靠性。     

基于包辛格效应的回弹仿真分析

金属材料在一个方向上的应变硬化降低了反方向的屈服强度,材料包辛格效应的存在对车身成形仿真精度产生了重要影响,尤其现今高强钢和铝合金的大量应用,使车身成形件的回弹问题日益突出,车身模具制造对有限元回弹预测的准确性提出更高的要求。为了提高回弹的仿真精度有必要对材料的包辛格效应进行研究,利用一套夹具对DC06和DP600两种材料的薄板进行拉伸压缩试验,获得不同预应变下的位移加载曲线,通过拉伸压缩试验结果与仿真结果的对比分析,得到能反映材料包辛格效应的非线性混合硬化模型材料参数。开展U形件成形试验,并建立试验的仿真模型,计算DC06和DP600薄板的U形件成形回弹量,分析等向强化、混合强化和随动强化本构模型对回弹预测精度的影响,针对回弹仿真结果和试验结果的差别,对影响仿真精度的材料模型因素进行分析。结果表明,DC06和DP600的包辛格效应大小存在差别,考虑包辛格效应有助于回弹仿真精度的提高,但小曲率弯曲成形回弹计算对材料本构模型的敏感性,限制了回弹仿真精度的提高。     

Prediction of sheet forming limits with Marciniak and Kuczynski analysis using combined isotropic-nonlinear kinematic hardening

The forming limit curve (FLC), a plot of the limiting principal surface strains that can be sustained by sheet metals prior to the onset of localized necking, is useful for characterizing the formability of sheet metal and assessing the forming severity of a drawing or stamping process. Both experimental and theoretical work reported in the literature has shown that the FLC is significantly strain-path dependent. In this paper, a modified Marciniak and Kuczynski (MK) approach was used to compute the FLC in conjunction with two different work-hardening models: an isotropic hardening model and a mixed isotropic-nonlinear kinematic hardening model, which is capable of describing the Bauschinger effect. Predictions of the FLC using the MK analysis have been shown to be dependent on the shape of the initial yield locus and on its evolution during work hardening; therefore the hardening model has an influence on the predicted FLC. In this investigation, published experimental FLCs of AISI-1012 low carbon steel and 2008-T4 aluminum alloy sheets that were subjected to various nonlinear loading paths were compared to predictions using both hardening models. The predicted FLCs were found to correlate quite well with experimental data and the effects of strain path changes and of the hardening model on predicted FLCs are discussed.     

Prediction of history-dependent forming limits by applying different hardening models

The loading history-dependent forming limits have been computed for sheet metals undergoing various combinations of plane-stress loading conditions. The analysis method is essentially an extension of Marciniak and Kucźynski's inhomogeneous model, except that the roles of isotropic and Prager-Ziegler kinematic hardening have been examined in detail while the flow theory of plasticity is applied. A suitable modification of the constitutive equations for the kinematic hardening model converts the rate form of the constitutive equations into the finite-increment form which satisfies the yield criterion precisely. Representative combinations of strain history consisted of an initial proportional straining to either a fixed strain state or different levels of strain state followed by continued loading under different conditions of strain ratios. Comparison of computed forming limits with available experimental data shows that the ultimate choice of either an isotropic or a kinematic hardening model is dependent on a specific combination of strain history and the material properties.     

Shakedown analysis of beams using nonlinear kinematic hardening materials coupled with continuum damage mechanics

The nonlinear kinematic hardening theory of plasticity based on the Armstrong-Fredrick model and isotropic damage was used to evaluate the cyclic loading behavior of a beam under the axial, bending, and thermal loads. Damage and inelastic deformation were incorporated and they were used for the beam shakedown and ratcheting analysis. The beam material was assumed to follow the nonlinear strain hardening property coupled with isotropic damage. The effect of the damage phenomenon coupled with the elastoplastic nonlinear kinematic hardening was studied for deformation and load control loadings. The Bree's diagram was obtained for two different types of loading, and all numerical results confirmed the reduction of the safe loading domain due to material damage.