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。     

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。     

基于连续介质损伤力学的7075铝合金高温成形极限理论预测与数值仿真

开展7075铝合金高温单向拉伸试验和成形极限试验,获得了不同温度和应变率的应力-应变曲线和成形极限曲线。结果表明,在较高的温度和应变率时7075铝合金的强度减小、成形性提高。为描述7075铝合金高温损伤演化过程,提出一种改进的连续介质损伤模型,并建立了耦合损伤的多轴统一黏塑性本构模型。基于试验结果,运用NSGAII遗传算法标定了模型中的参数,标定后的本构模型可以很好地预测7075铝合金的高温热力行为和极限应变。通过有限元软件Abaqus的用户材料子程序VUMAT,该本构模型被编入到Abaqus软件中进行数值仿真计算。结果表明,仿真获得的成形极限曲线和应变场分布与试验和理论结果吻合度好,进一步证明了所建立的耦合损伤的多轴本构模型的正确性及其在高温成形极限有限元仿真中的适用性。     

基于多尺度晶体塑性有限元的GCr15轴承钢拉-压不对称性研究

将应力三轴度引入晶体塑性本构模型,从宏、细观尺度阐明了造成GCr15轴承钢拉-压不对称性的原因.首先,拓展晶体塑性本构模型,在滑移系中引入应力三轴度对屈服准则和流变准则的影响.其次,采用了基于代表性体元(RVE)的多尺度晶体塑性有限元仿真模拟,对GCr15轴承钢的准静态单向拉伸和压缩试验进行了数值实现.研究结果表明,由...     

机织复合材料各向异性超弹性本构模型

基于纤维增强连续介质力学理论,提出一种超弹性本构模型来描述机织复合材料织物在成形过程中由于大变形所引起的非线性各向异性力学行为。在这一模型中,应变能函数被分解成两部分:一部分代表由于经纱和纬纱各自拉伸所产生的拉伸应变能;另一部分代表由于经纱和纬纱之间角度变化而产生的剪切应变能。本构模型中所需的材料参数通过对机织复合材料织物的单向拉伸以及偏拉试验数据的分析与拟合来求得。利用对一平纹机织复合材料的单曲率半球形冲压成形试验进行有限元模拟来验证模型。模拟结果与试验结果对比表明所提出的超弹性本构模型能够很好地表征机织复合材料在大变形下的非线性各向异性力学行为。这一本构模型具有简单实用、材料参数容易确定的优点。该模型对于机织复合材料成形的数值模拟与成形工艺优化设计有着重要的意义。     

球压痕试验法评价金属材料的强度

通过单次压痕试验与有限元模拟相结合的方法,结合反向分析方法与模拟退火粒子群算法,从获得的载荷-深度曲线加载部分提取材料的塑性参数,基于Ludwig硬化模型预测了不同金属材料的强度,并与单轴拉伸试验结果进行对比。结果表明:模拟得到的载荷-深度曲线与试验得到的几乎重合,二者的相对误差小于0.5%,说明模拟退火粒子群算法可有效地从压痕载荷-深度曲线中提取出金属材料的塑性参数;基于Ludwig硬化模型,利用反向分析方法从压痕载荷-深度曲线中提取的真应力-真塑性应变曲线不是唯一的,但从真应力-真塑性应变曲线计算得到的强度具有明显的收敛趋势;采用压痕试验得到不同金属材料的强度均接近于由拉伸试验得到的,屈服强度与抗拉强度的最大相对误差分别为5.9%,4.3%,说明采用压痕试验法可以准确地评价金属材料的强度。     

管材激光成形的数值模拟研究

建立了管材激光弯曲成形的有限元模型,对成形过程进行了数值模拟分析。模拟结果表明对应于加热和冷却阶段,管材的激光弯曲成形分别经历反向弯曲和正向弯曲两个变形阶段。管材加热区与未加热区域之间形成的较大的塑性应变差使得管材整体上产生了朝向激光束的正向弯曲。成形的机理主要表现为增厚机理。未加热区域的管材壁厚基本不变化,不会出现机械弯曲中管材的破裂问题。     

不同硬化模型对铝合金板冲压成形模拟结果的影响

研究不同塑性变形硬化模型对汽车5182-O铝合金板材冲压成形模拟结果的影响。采用材料单向拉伸试验得到应力应变关系曲线,基于Hollomom、Krupskowsky与Power方程对曲线进行拟合,建立材料室温下塑性变形硬化模型,对厚度为1.5 mm和0.85 mm的5182板材进行冲压试验和有限元模拟分析,对比分析冲压试验与模拟结果。试验与模拟结果显示,当板料厚度为1.5 mm时,板料冲压试验的成形力最大为42.95 kN,板料拉深深度为30.58 mm,基于Power方程计算得到的最大成形力为41.5kN与试验结果比较接近,Hollomom方程计算得到的拉深深度为30.546 mm,板材成形厚度分布与试验结果比较接近;当板料厚度为0.85 mm时,板料冲压试验的成形力最大为34.47kN,板料拉深深度为33.792 mm,基于Power方程计算得到的最大成形力为34.27 kN与试验结果比较接近,Hollomom方程计算得到的拉深深度为33.636 mm,板材成形厚度分布与试验结果比较接近。基于三种硬化模型铝合金冲压成形过程的计算模拟分析结果,并通过与试验对比得到不同硬化模型对铝合金板材冲压成形计算模拟的影响,进一步为汽车铝合金覆盖件在成形工艺的研究分析提供理论指导。     

先进高强双相钢DP780温度相关应力-应变关系

为了研究先进高强度双相钢DP780的温热成形力学行为,通过不同温度(300℃、400℃、500℃、600℃)及不同应变率(10^-4～10^-3 s^-1)下的单向温拉伸试验获得了先进高强度双相钢DP780在相应条件下的基本力学性能指标和应力-应变曲线,并对试验结果进行了分析;为描述温热成形DP780板料应力-应变曲线,求解、修正了Grosman本构模型的相关参数;以单向拉伸试样为例,基于Fortran语言通过编写Abaqus-Vumat子程序进行了数值模拟验证。结果表明,所用Grosman模型预测值与试验数值拟合较好,验证了所求模型的有效性。     

大弯曲半径管材推弯回弹有限元分析

大弯曲半径管材是在推弯机的推弯过程中成形的,因在推弯过程中,模具与管坯间各参数的相互影响导致管坯在推弯成形后,不可避免的产生回弹角,因而影响了管坯在后期应用中的精度和准确率。因此采用动态有限元平台ABAQUS/Explicit软件对大弯曲半径管材的推弯成形建立符合实际推弯参数的有限元模型,并通过有限元数值模拟对推弯过程中影响回弹角的主要因素:相对弯曲半径R/D、相对壁厚D/t、摩擦系数、以及材料的硬化指数和强度系数进行有限元分析,并得出这些参数对回弹角影响的规律曲线。     

Finite element simulation of flexible roll forming with supplemented material data and the experimental verification

Flexible roll forming is a promising manufacturing method for the production of variable cross section products. Considering the large plastic strain in this forming process which is much larger than that of uniform deformation phase of uniaxial tensile test, the widely adopted method of simulating the forming processes with non-supplemented material data from uniaxial tensile test will certainly lead to large error. To reduce this error, the material data is supplemented based on three constitutive models. Then a finite element model of a six passes flexible roll forming process is established based on the supplemented material data and the original material data from the uniaxial tensile test. The flexible roll forming experiment of a B pillar reinforcing plate is carried out to verify the proposed method. Final cross section shapes of the experimental and the simulated results are compared. It is shown that the simulation calculated with supplemented material data based on Swift model agrees well with the experimental results, while the simulation based on original material data could not predict the actual deformation accurately. The results indicate that this material supplement method is reliable and indispensible, and the simulation model can well reflect the real metal forming process. Detailed analysis of the distribution and history of plastic strain at different positions are performed. A new material data supplement method is proposed to tackle the problem which is ignored in other roll forming simulations, and thus the forming process simulation accuracy can be greatly improved.     

Reproducing hoop stress–strain behavior for tubular material using lateral compression test

Identification of material properties in the hoop direction, such as stress–strain behavior, is essential in tube hydroforming processes. Conventional tests such as uniaxial tension and compression tests have some drawbacks and limitations. In the current investigations a simple technique to identify the stress–strain behavior in the hoop direction for tubular material is introduced, based on the experimental data obtained from tube lateral compression test. In the proposed technique, an assumed stress–strain curve is used in finite element simulation to predict the load deflection curve of the tube lateral compression. An iterative algorithm is used to compare the calculated and experimental load deflection curves until a good agreement with a percentage deviation less than 4% is obtained. The suggested technique was used to obtain the material properties of Cu–40%Zn brass tube. The predicted stress–strain curve was compared with that obtained from uniaxial compression test. Comparison between the experimental and predicted stress–strain curve showed that the proposed technique is effective in the prediction of the material properties from the tube lateral compression test with percentage deviation less than 1%.     

金属薄膜弹簧揿压特性仿真研究

随着移动通讯和家电的普及，轻触式按键（Light touch switches)得到广泛的运用。其中关键部件金属薄膜弹簧（Metal diaphragms)是由高强度极薄金属板冲压制成，对其成形特性和工作特性进行仿真研究，可以为优化模具设计，预测产品的疲劳寿命提供依据，本文首先介绍金属薄膜弹簧成形仿真建模的原则，其次，以大变形、小应变弹塑性有限元理论为基础，在单向拉试验的基础上，给出极薄板各向异性指数的计算公式，并采用Hill屈服准则和静力隐式法对金属薄膜弹簧成形进行建模，对成形过程和揿压特性进行仿真研究，预测了冲压成形后的几何形状和揿压过程中的力感特性曲线，最后，比较了仿真结果和试验结果，二者较为符合，验证了建模的正确性。     

Effect of continuum damage mechanics on springback prediction in metal forming processes

The influence of considering the variations in material properties was investigated through continuum damage mechanics according to the Lemaitre isotropic unified damage law to predict the bending force and springback in V-bending sheet metal forming processes, with emphasis on Finite element (FE) simulation considerations. The material constants of the damage model were calibrated through a uniaxial tensile test with an appropriate and convenient repeating strategy. Holloman’s isotropic and Ziegler’s linear kinematic hardening laws were employed to describe the behavior of a hardening material. To specify the ideal FE conditions for simulating springback, the effect of the various numerical considerations during FE simulation was investigated and compared with the experimental outcome. Results indicate that considering continuum damage mechanics decreased the predicted bending force and improved the accuracy of springback prediction.     

Earing predictions for strongly textured aluminum sheets

Metallic alloy sheets develop crystallographic texture and plastic anisotropy during rolling. Deep drawing of a cylindrical cup from a rolled sheet is one of the typical forming operations where the effect of this anisotropy is most evident. Generally, in the finite element analyses of this process, the evolution of anisotropy during forming is neglected. In this paper, results of an experimental program carried out to quantify the anisotropy of aluminum alloy AA5042-H2 are reported. In addition to tensile tests along seven directions in the plane of the sheet, cup-drawing tests were conducted. It was observed that the material displays eight ears. The effects of the evolution in anisotropy and the directionality in hardening on the predictions of the earing profile for this material are investigated using a new methodology that incorporates multiple hardening curves corresponding to uniaxial tension along several orientations with respect to the rolling direction, and to biaxial tension. Yielding is described using the anisotropic yield function Yld2000-2D [1] and a form of CPB06ex2 yield function [2], which is tailored for metals with no tension–compression asymmetry. It is shown that even if distortional hardening is neglected, the latter yield function predicts a cup with eight ears as was observed experimentally. Consideration of distortional only leads to improved accuracy in prediction of the non-uniformity of the cup height profile.     

方矩形管连续辊弯成形过程的变形和应力场

应用三维大变形弹塑性有限元法,对方矩形管连续辊弯成形过程中金属流动规律以及应力分布进行了模拟分析.考虑了各道次之间的数据信息传递,保证了下一道次变形历史的延续,从而真实模拟了方矩形连续辊弯成形过程.获得的结果对连续辊弯成形中的产品质量的控制和孔型的优化有着重要意义.     

不同强化模型下的板料成形极限

介绍Hill48屈服准则下基于不同强化模型的屈服方程。推导出能够用来确定随动强化模型和混合强化模型中参数的方程。采用单向拉伸曲线上所取得的数据,对所得方程进行拟合,得到参数值,并使用所得参数值得出三种强化模型下的单向拉伸曲线。结果表明采用上述方法能够准确地确定强化模型中的参数。给出随动强化模型和混合强化模型下成形极限的计算方法。基于三种强化模型,针对分散性失稳准则、Hill集中性失稳准则、凹槽失稳准则和平面应变漂移失稳准则,得到简单加载路径下的成形极限图和成形极限应力图。从这些图中可以看出,强化模型对成形极限图和成形极限应力图影响明显。因此应当确定板料在成形过程中的强化规律,选择合适的强化模型进行成形极限预测。     

Evaluating the flow stress of aerospace alloys for tube hydroforming process by free expansion testing

In order to obtain accurate tube hydroforming (THF) simulation results, one of the important inputs in the finite element model (FEM) of the process is the mechanical response of the material during THF. Generally, the mechanical response is defined by the stress–strain behavior that can be determined from tensile testing of the specimens extracted either from the sheet used for roll forming of the tubes or directly from the tubes. More recently, free expansion testing has been used to characterize the mechanical response of the material for hydroforming applications. The free expansion test can emulate process conditions similar to those found during THF, and as such, can be used to obtain reliable and accurate information on the mechanical response/properties of the tubular material. The aim of this research is to present an approach for evaluating the stress–strain behavior of different materials using a 3D deformation measurement system in conjunction with an analytical model. Here, to characterize the mechanical response of the materials, free expansion and tensile testing were used for austenitic stainless steel types 321 (SS 321) and 304L (SS 304L), INCONEL® alloy 718 (IN 718), and aluminum alloy 6061 in the annealed “0” temper condition (AA 6061-0). The mechanical response of each material, measured through free expansion testing of tubular forms, was compared to the respective stress–strain behavior determined from the uniaxial tensile test using ASTM E8 geometry specimens extracted from the tubes. For each material studied in this work, the two flow stress behaviors were distinct, indicating that the test method can have a noticeable effect on the mechanical response. Finite element analysis (FEA) of the free expansion of each material was also utilized to simulate the THF process with the flow stress curves obtained experimentally; the predicted expansion and burst pressure results were close to the experimental data indicating that the approach developed and described in this work has merit for characterizing the mechanical response of aerospace alloys for hydroforming applications.