基于韧性断裂准则的铝合金板材成形极限预测

为了准确地预测铝合金板材成形极限，将韧性断裂准则引入到数值模拟中。在数值模拟获得的应力应变值基础上，采用简单拉伸试验和数值模拟相结合的方法确定了韧性断裂准则中的材料常数，并应用该韧性断裂准则预测了铝合金LYl2(M)的圆筒件拉深和半球形凸模胀形的成形极限。预测结果与实验值吻合较好，该韧性断裂准则能够预测铝合金板材成形极限。     

应用韧性断裂准则预测不同材料的胀形极限

为了准确地预测板料成形极限 ,将韧性断裂准则引入到有限元模拟中。在有限元模拟获得的应力应变场基础上 ,应用韧性断裂准则预测板料断裂的发生。本文应用作者提出的韧性断裂准则及材料常数的确定方法预测了铝合金板和钢板的半球形凸模胀形的成形极限。与实验结果比较表明 ,该方法能在较宽的材料范围内预测胀形成形极限。     

应用韧性断裂准则预测板料的成形极限图

将Oyan韧性断裂准则引入数值模拟预测板料的成形极限图(FLD).讨论了各向异性系数对不同应变状态下准则中各项的影响,通过测定单向拉伸和平面应变拉伸的断裂应变确定了准则中的材料常数.模拟凸模胀形实验得到每一时间步应力、应变值,代入韧性断裂准则预测板料的成形极限.应用Oyan韧性断裂准则预测了铝合金A5182-O和SPCC的成形极限图.模拟结果表明,用韧性断裂准则和数值模拟相结合能成功预测板料的成形极限图.     

基于MMC韧性断裂准则的高强双相钢成形极限研究北大核心CSCD

以高强双相钢CR450/780DP板材为研究对象,设计4种拉伸试样进行单向拉伸试验,获得了载荷-位移曲线与表面全场应变结果;采用有限元仿真分析手段,对4种试样的试验结果进行了对标分析,得到了材料硬化本构模型及应力三轴度、Lode角参数、等效塑性应变等历程数据;采用曲面拟合优化方法标定MMC韧性断裂准则的断裂参数。基于Keeler公式及简化的MMC韧性断裂准则分别绘制了高强双相钢CR450/780DP板材的理论和预测成形极限图,并通过半球形刚模胀形试验对预测结果进行验证。由对比结果可知,基于MMC韧性断裂准则预测的成形极限曲线与试验数据的吻合程度较高,验证了韧性断裂准则对高强双相钢CR450/780DP板材损伤与断裂预测的准确性与适用性。     

铝合金板材渐进成形韧性破裂过程模拟

针对铝合金板材渐进成形过程中材料受拉失稳产生韧性破裂的问题,建立考虑应变率效应的铝合金板材渐进成形韧性破裂准则。采用单向拉伸试验和有限元模拟相结合的方法,确定了铝合金板材渐进成形韧性破裂准则中的材料参数。以建立的破裂准则为破裂判断的依据,结合有限元软件MSC.Marc进行二次开发,通过嵌入韧性破裂子程序,实现了对1060铝合金板材渐进成形破裂过程的定量分析,并通过渐进成形试验进行了试验验证。结果表明,考虑应变率效应的韧性破裂准则适用于铝合金板材渐进成形破裂缺陷的预测,揭示了铝合金板材渐进成形过程中破裂产生的机制,有限元模拟结果与实验结果一致性较好。     

基于应力成形极限图的6061铝合金成形特征（英文）

采用成形极限图准则和韧性断裂准则两个数值准则对6061铝合金的成形特征进行预测。将数值模拟结果与实验所得成形极限图和冲头力-位移图进行对比。采用Hecker半圆形冲头计算实验成形极限图。将实验成形极限图转化为成形极限应力图并导入Abaqus软件中对样品的缩颈进行预测。将成形极限应力图预测结果与实验所得成形极限应力图进行比较。结果表明,韧性断裂准则对6061铝合金成形极限图和成形极限应力图的预测精度较高。冲头的力-位移图模拟结果和实验结果对比表明模拟结果和实验结果吻合。     

基于应力的断裂准则对AA7075-T6韧性断裂行为的预测研究

通过试验研究了AA7075-T6在剪切、单轴拉伸和平面应变拉伸应力状态下的塑性变形和韧性断裂行为。然后采用Swift-Voce硬化定律和pDrucker屈服函数表征了板材在不同加载状态下的塑性变形特性。最后，采用基于应力的pDrucker韧性断裂准则表征了不同应力状态下塑性变形过程中的韧性断裂行为。为了提高数值模拟预测精度，塑性本构模型和pDrucker韧性断裂准则参数均采用试验-模拟相结合的逆向工程方法进行优化标定。将有限元预测的韧性断裂位移与不同试件载荷-行程曲线进行了比较。结果表明，通过逆向工程优化的pDrucker韧性断裂准则能够准确描述AA7075-T6不同应力状态下的韧性断裂行为。     

板料双向加压液压胀形数值模拟

应用商用有限元软件ANSYS/LS-DYNA,对板料的双向加压液压胀形过程进行了数值模拟.并利用韧性断裂准则作为胀形高度极限的判断依据,将模拟得到的必要数据代入到准则中.对两种材料、不同初始液体压力条件下的模拟结果表明,随着初始液体压力的升高,胀形的极限高度也相应的增加,当初始压力与板料的强度相当时,极限高度提高的更明显.     

一种改进的Lemaitre断裂准则及其在弯曲成形裂纹预测中的应用

针对弯曲成形中的裂纹缺陷,应用Lemaitre韧性断裂准则,同时考虑应力三轴度,最大主应力比,以及塑性应变对损伤的影响,对Lemaitre准则进行改进,有效预测了弯曲成形中金属板料的成形极限。以7075-T6铝合金为研究对象,模拟得到该合金板材的裂纹产生条件,获取改进的Lemaitre准则材料参数,确定破裂阈值。对6 mm厚7075-T6铝合金板材进行三点弯曲实验,并观察其金相组织。对其产生裂纹时的压下量与断裂准则所得的理论压下量进行比较,验证了改进后断裂准则对裂纹预测的准确性。通过对比产生裂纹时的压下量,结果表明,改进后的Lemaitre断裂准则所得理论压下量为9.7 mm,与模拟和实验结果一致,证明改进后的Lemaitre准则对弯曲成形裂纹预测具有一定的准确性。     

6016铝合金板材室温成形性及其数值模拟

通过实验和数值模拟方法研究了6016-T4P铝合金板材在室温下的成形性能。通过刚模胀形实验获得了成形极限图(FLD),通过单向拉伸实验建立了6016-T4P铝合金的室温Johnson-Cook材料模型,并利用ABAQUS有限元软件对刚模胀形实验进行了数值模拟。模拟过程中采用最大凸模力准则和应变失稳准则获得了计算的成形极限结果,并讨论了凸模速度、摩擦系数等对成形极限模拟结果的影响。结果表明:当摩擦系数为0.1,凸模速度为1000 mm·s~(-1)时,模拟和实验结果吻合较好。随摩擦系数的增大,平面应变值增大,成形极限曲线(FLC)垂直向上移动,且摩擦系数的变化对FLC的右侧部分影响较显著;随凸模速度增加,FLC略垂直向下移动,平面应变值减小。进而应用合理的成形参数成功预测了6016铝合金汽车B柱零件的冲压成形效果。     

Evaluation of forming limit in viscous pressure forming of automotive aluminum alloy 6k21-T4 sheet

A ductile fracture criterion is introduced into numerical simulation to predict viscous pressure forming limit of the automotive body aluminum alloy 6k2 l-T4. The material constant in the ductile fracture criterion is determined by the combination of the viscous pressure bulging （VPB） test with numerical simulation. VPB tests of the aluminum alloy sheet are carried out by using various elliptical dies with different ratios of major axis to minor axis（β）, and the bugling processes are simulated by the aid of the finite element method software LS-DYNA3D. On the basis of the stress and strain calculated from numerical simulations, the forming limits of bulging specimens obtained are predicted by the ductile fracture criterion, and compared with experimental results. The fracture initiation site and the minimal thickness predicted by the ductile fracture criterion are in good agreement with the experimental results.     

Application of artificial neural network and Taguchi method to preform design in metal forming considering workability

This study describes a new method of perform design in muti-stage metal forming processes considering workability limited by ductile fracture. The finite element simulation combined with ductile fracture criterion has been performed in order to predict ductile fracture. The artificial neural network using the Taguchi method has been implemented for minimizing objective functions relevant to the forming process. The combinations of design parameters used in finite element simulation are selected by orthogonal array in statistical design of experiments. The orthogonal array and the result of simulation are used as train data for artificial neural networks. The cold heading process is taken as an example of designing preforms which do not form any fracture in the finished product. The results of analysis to validate the proposed design method are presented.     

Prediction of forming limit in hydro-mechanical deep drawing of steel sheets using ductile fracture criterion

It has been observed that the forming limit curve at fracture (FLCF) of steel sheets, with a relatively higher ductility limit have linear shapes, similar to those of a bulk forming process. In contrast, the FLCF of sheets with a relatively lower ductility limit have rather complex shapes approaching the forming limit curve at neck (FLCN) towards the equi-biaxial strain paths. In this study, the FLCFs of steel sheets were measured and compared with the fracture strains predicted from specific ductile fracture criteria, including a criterion suggested by the authors, which can accurately describe FLCFs with both linear and complex shapes. To predict the forming limit for hydro-mechanical deep drawing of steel sheets, the ductile fracture criteria were integrated into a finite element simulation. The simulation, results based on the criterion suggested by authors accurately predicted the experimetal, fracture limits of steel sheets for the hydro-mechanical deep drawing process.     

Prediction of ductile fracture for advanced high strength steel with a new criterion: Experiments and simulation

This paper is concerned with prediction of the onset of ductile fracture by a newly proposed micro-mechanism-motivated macroscopic ductile fracture criterion in various stress states from shear to plane strain tension where most ductile fracture takes place in sheet metal forming processes. The new ductile fracture criterion (Lou et al., 2012) is calibrated by the equivalent plastic strain to fracture measured by the hybrid experimental–numerical method from four types of specimens manufactured from DP980 sheet whose fracture locus is eventually constructed. The calibrated criterion is utilized to construct the fracture locus of DP980. The constructed fracture locus is then implemented into the ABAQUS/Explicit code to predict the onset of ductile fracture for these three types of specimens. Three types of notched specimens are further designed for the validation of the ductile fracture criterion from uniaxial tension to plane strain tension by comparison of experimental results to those numerically predicted by the ductile fracture criterion. Three types of shear specimens are then utilized to validate predictability of the ductile fracture criterion between shear and uniaxial tension. The validation demonstrates that the ductile fracture criterion can accurately predict the onset of ductile fracture for these specimens. The comparison result with high accuracy reveals that the criterion can correctly describe ductile fracture behaviors of metals in various stress states from shear to the plane strain tension.     

Coupled thermomechanical finite element analysis to improve press formability for camera shape using AZ31B magnesium alloy sheet

In this study, a finite element analysis aimed at predicting and improving the press formability for a camera casing made of AZ31B magnesium alloy sheet was conducted. First, stress-strain curves and forming limit curves (FLDs) for warm temperatures were obtained. The data from these curves and the ductile fracture criterion of FLDs were then input into ABAQUS/Explicit finite element code to predict the failure occurrence of the camera casing. In the finite element method (FEM) simulation, for investigating the effect of reduced temperature during the punch cooling process on the formability of the camera casing, coupled thermomechanical computational modeling was considered and verified by comparison with experimental results. Based on the good agreement between the simulation and the experimental results, three parameters-blank holding force, elevated temperature, and friction coefficient-were selected to improve the press formability of the camera casing in the coupling of the FEM simulations and Taguchi??s orthogonal array experiment. The optimum simulation case was confirmed through an experiment.     

基于Lemaitre韧性断裂准则的1060铝板成形极限研究

针对成形极限曲线的测定需建立在大量实验基础上,耗时耗材,且需要特定的成形实验机才能完成的不足,采用韧性断裂准则与数值模拟相结合来预测。建立测定成形极限曲线的有限元分析模型,对不同尺寸1060铝板试样成形过程进行数值模拟与分析,并采用Lemaitre韧性断裂准则作为板料破裂与否的判据,找出破裂极限应变值,拟合成形极限曲线;为验证提出方法的正确性,采用实验方法制作1060铝板成形极限曲线,并将其与模拟得到的曲线进行对比,两曲线走向基本一致,有较好的吻合度,表明该方法能够应用于成形极限曲线的预测。     

AISI-1035钢精冲成形与断裂的数值模拟

使用DEFORM2D软件对AISI-1035钢的落料、冲孔精冲工艺进行了弹塑性大变形有限元数值模拟,将McClintock断裂准则应用于精冲韧性断裂的预测中,预测了材料变形过程中静水应力、等效应力和等效应变的分布以及发展趋势、精冲最后阶段微裂纹产生发展和最终断裂。     

法向应力对板料成形极限影响的研究进展

在板料成形过程中应力状态对板料(板材和管材)的成形极限有很大影响,通过对板料施加法向应力可以提高板料的成形极限。文章综述并分析了法向应力对板料成形极限影响的理论模型、有限元模型以及实验研究方面的进展。理论模型方面的主要进展,是根据经典塑性失稳理论和M-K理论,建立了考虑法向应力影响的成形极限理论模型,可以准确地预测法向应力对板料成形极限的影响;有限元分析的主要进展,是在韧性断裂准则的基础上,利用体单元建立了考虑法向应力影响的数值模型;有关实验研究方面只是初步的探索,还有待进行深入的研究;对影响法向应力提高板料成形极限的因素进行了总结分析。