基于棒材拉伸试验确定金属材料真实应力应变关系的研究

根据颈缩段塑性变形前后体积不变的原理,并假设拉伸试棒颈缩后的轮廓为圆弧,推导出了颈缩段最小直径和颈缩段轮廓半径的计算公式。依据计算出的颈缩段最小直径和颈缩段轮廓半径,可以计算出材料的真实应力和真实应变。考虑到试样颈缩部位处于三向应力状态下,对计算得到的真实应力依据Bridgman的应力公式进行了修正。通过采用的真实应力应变的计算方法,对TC4材料单向拉伸的试验数据进行处理,获得了TC4材料的真实应力应变曲线。基于TC4真实应力应变曲线和拉伸试验条件,建立TC4材料单向拉伸试验的有限元模型,利用获得的力和位移曲线与单向拉伸试验获得的曲线进行比较,结果高度吻合,证明了这种计算真实应力应变方法的可行性和工程实用性。     

材料模型对弯管壁厚变化有限元仿真的影响

为研究材料模型对管材弯曲变形的影响,将管材拉伸试验所得真实应力应变曲线分别拟合成线性硬化和指数硬化材料模型,并用于有限元仿真。经对比分析认为,以真实应力应变曲线为加载曲线,对航空1Cr18Ni9Ti弯管壁厚变化的有限元计算与试验结果吻合程度最好;采用线性强化材料模型的仿真精度,高于指数硬化模型仿真精度。弯管壁厚变化主要集中在弯曲中段,弯、直管段交界处的变形缓和效应,使弯管两端壁厚变化逐渐减小,并且使未发生弯曲变形的直管部分也产生一定量的壁厚变化。     

逆法识别金属板材颈缩阶段应力应变曲线

金属板料成形过程的仿真需要定义材料在大应变处对应的流动应力曲线,即真应力-应变曲线。文章介绍了一种逆向识别方法,通过结合单向拉伸试验和有限元仿真分析来获得金属板料在大应变区间内的流动应力曲线。仿真分析过程中通过构建合适的有限元模型,正确模拟分散性失稳期间的各向异性塑性变形行为。在得出材料完整的直至颈缩阶段的流动应力曲线后,建立多个不同的硬化模型描述该曲线,并比较拟合结果,发现改进的Hockett-Sherby模型效果最好。     

2024铝合金高温损伤模型的建立及其应用

针对2024铝合金在其高温拉伸过程中颈缩现象显著,实验测得的应力-应变曲线并不能准确反映材料的真实应力应变关系的问题,通过分析高温拉伸曲线中的流变软化现象及微观组织演化规律,认为动态回复和损伤是导致材料软化的主要机制,提出了一种"去耦合"方法求解真实应力-应变曲线。采用由Avrami方程求解所得的动态回复曲线作为高温下"无损伤"2024铝合金的应力应变关系,并引入Johnson-Cook损伤模型描述2024铝合金高温损伤演化过程。利用ABAQUS有限元模拟软件,集成上述应力应变关系和损伤模型,预测了2024铝合金拉伸断裂和方盒件热拉深成形破裂过程。其预测结果和实验基本一致。     

核电用316LN奥氏体不锈钢延性断裂阈值研究

在Gleeble-1500D模拟机上对核电用316LN奥氏体不锈钢的热拉伸行为进行了研究,获得了钢在变形温度900~1200℃、应变速率0.01~1 s-1条件下拉伸变形的真应力-真应变曲线。通过Deform-3D有限元软件模拟了热拉伸过程,利用迭代法对试样颈缩后对应的真应力-真应变曲线进行修正。对试样颈缩前塑性变形部分对应的真应力-真应变数据拟合处理,得到了不同变形条件下316LN奥氏体不锈钢的硬化指数。将修正后的真应力-真应变曲线和计算得到的硬化指数导入Deform-3D软件中模拟热拉伸过程,计算得到了316LN奥氏体不锈钢热变形时的断裂阈值。     

Haynes230镍基合金拉伸DSA效应和循环硬化特性研究

基于Haynes230镍基合金760℃下的拉伸和疲劳实验结果，讨论了拉伸过程中的动态应变时效(DSA)效应以及不同应变幅值下的循环硬化特性，采用ABAQUS有限元软件对拉伸和疲劳特性进行了模拟。实验结果表明，Haynes230镍基合金在760℃拉伸过程中表现出明显的DSA效应，最大应力跌幅为3.8 MPa。在单轴疲劳过程中，材料表现出明显的循环硬化现象。采用ABAQUS有限元软件和基于热激活位错运动的指数流动准则对拉伸过程中的DSA效应进行了模拟。模拟结果与实验结果吻合很好，整个拉伸过程最大应力误差仅为3.9%,最大应力跌幅为9.3 MPa。选择Chaboche混合模型进行了应变范围0.6%和0.8%下的循环硬化模拟计算，模拟得到的前100周次内峰值应力的平均误差分别为4.3%和3.4%。     

冷轧钢单拉本构关系与韧性断裂参数研究

以SPCC钢为研究对象并采用拉伸试验与有限元模拟相结合的方法来获得准确表征颈缩后材料变形行为的本构关系与韧性断裂参数值。通过试验得到了工程应力-应变曲线,使用SEM观察断口形貌并分析了断裂机理。利用Abaqus软件进行三维拉伸模拟,对颈缩后工程应力-应变曲线进行拟合,获得了最优拟合的本构关系。假定等效断裂应变与应力三轴度间关系为反比例函数,利用反求法使得实际断裂应变与具体变形路径下的累积名义断裂应变相等,从而确定了断裂参数值。模拟结果表明,材料模型能够准确描述整个变形过程,合理预测韧性断裂的发生。此外,模拟中颈缩发生时等效应变值与理论计算值吻合度很高,证实了模型的可靠性。     

汽车仪表盘PP材本构关系计算

利用PP材料拉伸试验的位移载荷曲线,结合弹塑性力学中的相关假设,分阶段计算该材料的真应力-真应变曲线,并利用LS-DYNA显示分析算法,输入该真应力-真应变曲线,模拟计算该材料的单轴拉伸试验,通过模拟结果与试验结果的比较,验证了真应力-真应变曲线的有效性。同时提出了一种塑性材料颈缩阶段的本构简化模型。     

基于单轴拉伸试验、模拟及优化方法的材料本构识别

基于材料单轴拉伸试验及与其有限元模拟的结果的对比,采用优化方法对材料本构模型参数进行识别。假定材料的应力应变关系可以用Ludwick型本构模型来拟合,以材料单轴拉伸试验拉伸载荷-位移关系曲线与有限元数值模拟得到的拉伸载荷-位移曲线的差值的绝对值作为目标函数,以材料本构参数为设计变量,建立了材料本构参数识别优化模型。采用多目标粒子群算法,通过使目标函数极小化来识别材料本构参数。以Q235钢为例,获得了拉伸试验试件颈缩后的真应力与真应变的本构关系,验证了该方法的有效性。     

钢制汽车轮轮缘缩口成形有限元分析

为了保证轮缘的成形质量，提高生产效率，通过分析轮缘的成形工艺，采用有限元数值模拟与试验相结合的方法，建立了缩口成形和终成形的有限元模型，分析了模具结构、成形速度和摩擦因数对缩I：2件周向应力、应变分布情况的影响。模拟结果表明，采用内外支撑缩口模时，缩口件的周向应力应变值较大；随着成形速度的增加，周向应力应变先增加后减小；随着摩擦因数的增加，周向应变和应变的波动性随之增大。试验结果表明，有限元数值模拟发挥了较好的指导作用。     

DC05同材同厚拼焊板焊缝组织与力学性能

焊缝对同材同厚拼焊板成形的影响远大于母材强度或厚度差异的拼焊板.焊缝的力学性能是评价同材同厚拼焊板成形性及有限元仿真的关键.文中在"混合法"的基础上,通过微观组织观测与微硬度测试,结合数字影像相关技术,获得了拼焊板单向拉伸实时的全场应变场,解析计算出了焊缝的真实应力-应变关系曲线.将此曲线与小尺寸试件单向拉伸试验曲线对比分析后发现,曲线在前期稳定变形阶段是吻合的,在屈服阶段和颈缩阶段的差异是由于标准试件中母材对焊缝变形的限制作用,阻止焊缝过早颈缩.结果表明,焊缝的应变路径(应变比为-0.63)已经偏离单向拉伸状态(应变比为-0.50),纯焊缝试件则无此限制作用.计算所得曲线才是包含母材焊缝的真实应力应变曲线.     

Identification of the plastic deformation characteristics of AL5052-O sheet based on the non-associated flow rule

This study aims to determine the plastic deformation characteristics of aluminum 5052-O based on non-associated flow rule. To achieve this goal, a new strain hardening model named as Kim-Tuan hardening model is proposed to perfectly describe the stress-strain relation of the studied material in terms of the uniaxial tensile test and to predict the material’s post-necking behavior. Additionally, the plastic behaviors of AL5052-O sheet are described by two approaches: the associated flow rule with YLD2000-2d yield function and the non-associated flow rule with Hill's quadratic function (NAFR-Hill48). The parameters of these functions were derived from the material properties that were obtained from uniaxial tensile tests and bulge test. The flow curve based on Kim-Tuan model and plastic behaviors obtained from two above-mentioned approaches were imported into a finite element analysis code to simulate the hydraulic bulge test for this material to confirm the precision of material characteristics achieved before. The simulation results based on the NAFR-Hill48 match well with the experiment results of bulge test while the YLD2000-2d provides highly accurate predictions for anisotropy of this material.     

The finite-element deformation modeling of superplastic Al-8090

The deformation behaviors of an Al-Li-Cu alloy (Aluminum 8090) during uniaxial and biaxial stress states were modeled using the nonlinear finite element analysis package ABAQUS 5.8. Two different material models were used for the purpose of comparison and to propose a valid and accurate superplastic material deformation behavior. Using a power law strain hardening model, the material was modeled with either constant or variable properties of strain-rate sensitivity and strain-hardening exponent. Results from the numerical models were compared to experimental results by Chen and Huang, who investigated the uniaxial and biaxial states of stress. Experimental data were primarily used to acquire the needed uniaxial material parameters for the model. The strain-rate sensitivity and strain-hardening exponent were dependent on the strain and strain rates. Comparing the two material models for both stress states indicated that the material model with variable properties better represented the true behavior of the material when compared to the experimental results. For more information, contact Noha M. Hassan, the American University in Cairo, 113 Kasr El Aini St., P.O. Box 2511, Cairo 11511, Egypt; +202-797-5336; fax +202-797-7565; e-mail nhassan@vt.edu.     

TC4钛合金GTN损伤模型反向标定法研究

通过拉伸试验得到TC4钛合金载荷-位移曲线,研究反向标定法确定GTN孔洞损伤模型参数。通过有限元模拟,研究GTN模型6个待定参数对模拟结果的影响。结果表明,参数f0和fN需要通过反向标定法确定,εn可由拉伸试验确定,Sn可通过计算确定,fc和fF可由模拟结果直接给出;TC4钛合金的孔洞损伤随颈缩过程的产生迅速发展,直至断裂。     

温度对焊接热模拟X80管线钢断裂韧性的影响

通过试验和3D有限元模拟相结合的方法分析了焊接热模拟X80管线钢在不同温度（-90，-60，-30和0℃）下的断裂韧度.文中选取标准三点弯曲试样在不同温度下进行断裂韧度测试，同时对测试件进行三维有限元实体建模与分析计算.结果表明，X80管线钢的断裂韧性随温度减低显著减小，并使其倾向于脆性断裂.材料在不同温度下的真实应力-应变曲线行为从光滑拉伸到断裂力学试样具有良好的可传递性，温度对材料的硬化行为没有明显影响，有限元计算的结果表明，应用该方法可以准确地计算X80钢焊接热影响区不同温度下的CTOD值.     

Forming limit of electrodeposited nickel coating in the left region

A uniform nickel (Ni) coating was bilaterally electrodeposited on the low-carbon steel substrate for the application of advanced battery shells. Its forming limit was investigated by Hill localized necking theory coupled with finite element simulation and scanning electron microscopy. The effective stress and effective strain in the Ni coating and steel substrate are deduced using Hill’s anisotropic yield function. The localized necking condition is derived by sandwich sheet analysis, and the forming limit strains are obtained by solving the nonlinear equation of the localized necking condition. Extensive calculations are carried out using the proposed model. This study exhibits the nickel coating thickness and the normal anisotropic coefficients of the coating and substrate have little influence on the forming limit curve (FLC) in the left region of the coated sheet, but the strain hardening exponents of the coating and substrate have much effect on it. The calculated result matches well with the measured data in uniaxial tension. This investigation is useful for the preparation of the electrodeposited Ni coating and helpful for the forming operation of the battery shells.     

Springback After the Lateral Bending of T-Section Rails of Work-Hardening Materials

This paper studies the springback after the lateral bending of T-section rails, considering the work-hardening materials. A linear-hardening model and an elastic-plastic power-exponent hardening model of the material are adopted and compared with the real experimental stress-strain curve obtained from the uniaxial tension tests. The analytical formulas for the springback and residual curvatures are given. The numerical results indicate that the material hardening directly affects the accuracy of springback prediction compared with the experimental results. Besides, springback prediction is not sensitive to hardening parameters in the beginning of elastic-plastic bending deformation. Although there is an apparent yield stage in the true stress-strain curve, the adopted hardening models can achieve an allowable relative error, if hardening parameters are properly selected.     

基于应变补偿和修正项的新型铝锂合金温热变形本构模型

采用UTM5000电子万能拉伸试验机,在变形温度573~648K和应变速率0.001~0.1s-1条件下对2060-T8铝锂合金进行等温恒应变速率拉伸试验,得到其在变形过程中的真应力-真应变曲线,建立了基于应变补偿和修正项的温热变形本构方程。通过扫描电子显微镜(SEM)分析拉伸断口,对2060-T8铝锂合金的温热变形行为进行研究。结果表明：2060-T8铝锂合金对变形温度和应变速率具有较高的敏感性,流变应力曲线呈现出应变硬化和流变软化的特征,随着变形温度的升高和应变速率的降低,稳态流变特征逐渐消失,其在温热变形条件下的断裂形式为韧性断裂。修正的本构模型与实验值吻合度较高,可以为2060-T8铝锂合金温热变形的有限元模拟提供前提条件。     

INFLUENCE OF FERRITE-MARTENSITE MICROSTRUCTURE ON STRENGTH AND STRAIN HARDENING BEHAVIOR OF 12CrMo STEEL ANALYSED WITH FINITE ELEMENT METHOD

本文用有限元网格模拟12CrMo钢铁素体(F)+马氏体(M)双相组织的显微结构,利用轴对称弹塑性大应变有限元程序,计算了几种不同马氏体含量12CrMo钢的应力应变曲线,和相应的实验结果吻合较好.结果表明:马氏体含量是影响双相组织强度的主要因素。在低应变时,双相组织的流变应力和马氏体含量成线性关系,在高应变时呈非线性关系.强度随马氏体岛的直径减小而略增大.当马氏体含量为50％时,以马氏体为基体的双相组织强度高于以铁素体为基体的.计算的应力应变曲线在颈缩之前可用两段幂硬化规律近似,第一阶段的硬化指数n_1≈0.30—0.45,n_1对基体相比较敏感,而对第二相的尺寸、形状变化不敏感.硬化的第二阶段代表了12CrMo钢双相组织的应力应变曲线上绝大部分硬化规律。和n_1相反,n_2对第二相尺寸、形状变化比较敏感.对基体相不敏感.n_2在数值上约等于(1/2)n_1,和Mileiko理论值接近,但n_2大于实际测量的最大均匀应变ε_u.