Application of response surface methodology to drive GTN model parameters and determine the FLD of tailor welded blank

Tailor welded blanks (TWBs) have found great usage in auto industries; however there are some drawbacks, low formability and weld line movement, which hinder their applications. Prediction of their formability through forming limit diagrams (FLDs) enables one to consider the effects of forming parameters on forming behavior of TWBs. Different numerical approaches have been applied to predict the experimental FLD of these blanks. In the present research Gurson-Tvergaard-Needleman (GTN) model was applied as failure criterion. The parameters of GTN model were firstly derived for both thin and thick monolithic blanks of studied TWB by application of response surface methodology. Then experimental method of obtaining FLD was simulated and FLD of transversely welded TWB was predicted using Abaqus software. The results indicate that the FLD of studied TWB is lower than those related to the parent sheets and there is an appropriate agreement between the predicted and experimental FLDs of the studied TWB.     

Local bifurcation and instability theory applied to formability analysis

The design and optimization of both sheet metal formed parts and processes are nowadays carried out virtually making use of numerical tools by finite element analysis. Such virtual try-out approach contributes with significant savings in terms of money, time and effort in the design, production and process set-up of deep drawn parts. The analysis of either forming success operation or surface defects, in each of the development phases, is generally performed by means of the material’s forming limit diagram (FLD), since it allows to define a safe region that reduces the probability of: (i) necking; (ii) wrinkling and (iii) large deformation occurrence. However, the FLD represented in the strain space is known to present some disadvantages. To overcome this problem, Ito and Goya proposed a local bifurcation criterion that defines the critical state for a local bifurcation to set in as a function of the stress level to work-hardening rate ratio, leading to a FLD represented in the stress space. This suggests that the FLD obtained is completely objective in the sense that it is completely independent of the strain or stress history paths (Ito et al. 2000). In this work the Ito and Goya model is used to evaluate formability, as well as fracture mode and direction on the deep drawing of a square cup. Since the analysis is performed based on the stress state, it is also possible to determine an instability factor that “measures” the degree of acceleration by current stress for the local bifurcation mode towards fracture. The selected example highlights the potential use of the criterion which, once combined with the finite element analysis, can undeniably improve the mechanical design of forming processes.     

Experimental and theoretical formability analysis using strain and stress based forming limit diagram for advanced high strength steels

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) and Forming Limit Stress Diagram (FLSD) for two Advanced High Strength Steel (AHSS) sheets grade DP780 and TRIP780 were performed. Initially, the forming limit curves were experimentally determined by means of the Nakazima forming test. Subsequently, analytical calculations of both FLD and FLSD were carried out based on the Marciniak–Kuczinsky (M–K) model. Additionally, the FLSDs were calculated using the experimental FLD data for both investigated steels. Different yield criteria, namely, von Mises, Hill’s 48, and Barlat2000 (Yld2000-2d) were applied for describing plastic flow behavior of the AHS steels. Both Swift and modified Voce strain hardening laws were taken into account. Hereby, influences of the constitutive yield models on the numerically determined FLDs and FLSDs were studied regarding to those resulted from the experimental data. The obtained stress based forming limits were significantly affected by the yield criterion and hardening model. It was found that the forming limit curves calculated by the combination of the Yld2000-2d yield criterion and Swift hardening law were in better agreement with the experimental curves. Finally, hole expansion tests were conducted in order to verify the different failure criteria. It was shown that the stress based forming limit curves could more precisely describe the formability behavior of both high strength steel sheets than the strain based forming limit curves.     

A different approach for considering the effect of non-proportional loading path on the forming limit diagram of AA5083

In this paper, the effects of strain path change on the forming limit diagram (FLD) of AA5083 sheet were investigated. The aim is to predict the forming limit curve (FLC) with non-proportional loading path by ductile fracture criteria. For this purpose, some square blanks were pre-strained by uniaxial tension in rolling direction (RD) and transverse direction (TD), and some others were pre-strained by biaxial stretching over a hemispherical punch. Then, the FLD test specimens were prepared by trimming the pre-strained blanks with the longitudinal axis in the RD and TD directions. The out-of-plane formability test was used for obtaining the FLD. The commercial finite element software ABAQUSE 6.9 was used for simulation in accordance with the experimental procedure. For trimming in the simulation environment, a program was written in MATLAB 7.6 that could determine the elements and introduce their properties to the new simulation model. Ductile fracture criteria were used for predicting the failure, and the Hill’79 criterion was used for applying the anisotropic coefficients. The results show that pre-straining in biaxial tension generally reduces the FLC and shifts it to the right-hand side of the FLD, whereas pre-straining in uniaxial tension raises the FLC and shifts it to the left-hand side. The numerical results were compared with the experimental findings, and relatively good agreement was achieved.     

模拟参数对橡皮成形TB5钛合金凸弯边起皱模拟的影响

目的 针对高强钛合金成形时遇到的起皱问题,引入有限元模拟技术。方法 利用MATLAB对模拟结果的峰高进行量化。以橡皮成形TB5钛合金的凸弯边为考察对象,对板料初始网格、加载时间、加载方式及与橡皮接触的摩擦等模拟参数建立正交设计表L9(34),应用ABAQUS/Explicit模拟起皱,全面研究模拟参数对皱纹的影响。结果 研究全面评估了模拟参数对起皱模拟结果的影响,板料网格是影响起皱的最重要因素。结论 试验证明了TB5钛合金拥有良好的冷成形性能,但在成形过程中有起皱问题。     

Prediction of forming limit diagrams using the modified M-K method in hydroforming of aluminum tubes

The purpose of this study is to predict forming limit diagrams (FLDs) for AA6063 and AA6065 aluminum seamless extruded tubes. The Modified Marciniak and Kuczynski (M-K) method is used with Barlat’s 1989 anisotropic yield function and Voce equation. Furthermore, a new calibration method for FLD determination based on the modified M-K method is also applied. The predicted forming limits correlate well with experimental data. It is shown that in comparison to other methods, this method is able to predict necking in tube hydroforming with more accuracy. Therefore, this method can be used to predict bursting in a wide range of practical tube hydroforming of aluminum alloys.     

利用最小厚度准则预测高强热轧钢板的FLC曲线

为更便捷地获得成形极限图(FLD)中的成形极限曲线(FLC),用最小厚度准则,通过少量成形极限试验结合数值模拟来预测FLC.采用Barlat1989屈服准则对QStE340TM、SAPH370、ZStE260P三种高强度热轧钢板进行成形极限模拟,并以最小厚度准则作为极限判据,根据数值模拟结果绘制FLC图.计算结果表明,采用平面应变路径下的成形极限实验数据作为最小厚度准则的已知参数时,数值预测结果与实验结果能较好吻合.故采用平面应变路径下的成形极限实验数据,结合最小厚度准则和数值模拟,即可得到材料完整的FLC曲线.     

润滑条件对圆管镦挤法兰成形的影响

对圆管镦挤法兰成形工艺进行了详细的研究 ,采用有限元数值模拟方法分析了润滑条件对成形过程中金属流动的影响及缺陷产生的原因 .给出了两种内径、三种壁厚情况下成形法兰宽度 -摩擦系数关系曲线 ,并进行了圆管镦挤法兰成形试验 ,数值模拟结果与试验结果具有较好的一致性 .     

非轴对称件拉深的破裂临界压边力控制原理

变压边力控制技术在拉深工艺中是一种简单有效的工艺手段,可以有效地改善成形质量.本文以锥壁方盒形件为例,以板材成形极限图(FLD)为理论依据,通过数值模拟的方法获得了破裂的压边力-行程临界曲线,得出了压边力沿着该曲线加载有利于改善成形质量的结论,进而提出了破裂临界压边力控制原理,给出了实际应用中获得变压边力加载曲线的方法,为实现非轴对称件拉深智能化控制的压边力变化规律预测奠定了基础.     

基于补偿因子的直弯边下陷零件回弹补偿研究

橡皮成形是飞机钣金零件制造的一种重要成形工艺,回弹是橡皮成形过程中主要缺陷之一.为提高钣金零件橡皮成形效率,基于补偿因子对下陷区回弹补偿进行研究,实现钣金零件一步法成形.首先对弯曲回弹公式进行推导,结合弯曲过程中刚度概念引出补偿因子作为直弯边下陷零件回弹补偿公式;然后利用CATIA二次开发功能将补偿公式写入程序并与数据库进行连接,并对直弯边下陷零件进行回弹补偿;采用Pamstamp 2G软件对橡皮成形过程进行有限元模拟,通过实际成形试验对带补偿因子的回弹公式进行验证,最后将模拟值和试验值进行比较.结果表明:有限元模拟过程能够很好的预测回弹,补偿结果达到精准成形要求且误差在0.5°范围内,成形压力对回弹影响很小.对于下陷成形,需要较大的成形压力才能达到一次成形的目的,验证了补偿因子回弹公式的可靠性.     

6A02铝合金异形截面小弯曲半径薄壁管液压成形工艺研究

目的 研究6A02铝合金异形截面薄壁管的液压成形过程，改进管件的成形质量。方法 使用Abaqus软件进行数值模拟，通过考察管件壁厚分布情况、管件轴线的最小弯曲半径及管壁与模具贴合情况，研究了内压、轴向进给量、加载路径及合模过程中的内压与进给对成形质量的影响，并提出了合模力–轴向进给–内压三者同时配合的加工方法。结果 通过数值模拟确定了无轴向进给情况下管件薄弱处发生破裂时的内压为7.5 MPa，发生起皱前的最大轴向进给量为2 mm，最低整形内压为80 MPa。确定了在合模过程中进给0.75 mm、合模后继续按照特定加载路径进行内压提升和轴向进给、最后施加80 MPa的整形压力的情况下成形效果最好。通过此路径加工出的管件最小壁厚为0.42 mm，最大减薄率为16%，轴线最小弯曲半径为1.258 mm，与模具间隙面积为0.065 mm²。结论 适当的内压–轴向进给可以实现较好的成形质量。在合模过程中，施加与合模力相配合的轴向进给和内压能增加管件弯曲处薄弱部分的补料量，改善管件在合模后的壁厚分布情况，进一步提升管件最终成形质量。     

Modelling macroscopic imperfections for the prediction of flow localization and fracture

Ductile materials subjected to plastic deformation experience the different stages of void nucleation, growth and coalescence that eventually lead to ductile fracture. Several models have been proposed to assess the influence of this damage on flow localization and fracture. In general, the plastic behaviour is represented by a constitutive model for porous or damaged materials. It is typical to introduce a material imperfection, with porosity higher than average, which evolves up to localization and fracture. However, the void volume fraction in the imperfection is chosen more or less arbitrarily. In the present work, a model that evaluates this void volume fraction more rigorously is developed. The forming limit diagram (FLD) for a dual phase‐steel is calculated using the damage‐based imperfection calculation and validated with experimental results. The effect of void shape on the imperfection porosity level and limit strains in sheet forming is also assessed with the present method.     

Experimental and numerical evaluation of forming limit diagram for Ti6Al4V titanium and Al6061-T6 aluminum alloys sheets

The forming limit diagram (FLD) is a useful concept for characterizing the formability of sheet metal. In this work, the formability, fracture mode and strain distribution during forming of Ti6Al4V titanium alloy and Al6061-T6 aluminum alloy sheets has been investigated experimentally using a special process of hydroforming deep drawing assisted by floating disc. The selected sheet material has been photo-girded for strain measurements. The effects of process parameters on FLD have been evaluated and simulated using ABAQUS/Standard. Hill-swift and NADDRG theoretical forming limit diagram models are used to specify fracture initiation in the finite element model (FEM) and it is shown that the Hill-swift model gives a better prediction. The simulated results are in good agreement with the experiment.     

连续变通道挤压–剪切工艺对镁合金薄壁管材组织性能的影响

目的 针对高性能镁合金薄壁管材的加工和制备,提出一种新型复合挤压制备工艺,并验证新工艺对于管材性能的影响。方法 通过数值模拟结合实验,建立镁合金管材连续变通道挤压-剪切工艺(CVCES)成形三维有限元模型,分析CVCES成形过程中坯料的等效应力和累积应变;对坯料纵截面沿挤压方向上的不同位置进行微观组织观察,对比分析CVCES成形管材与普通成形管材的金相结果;对成形管材进行强度测试对比。结果 与普通挤压镁合金管材相比,CVCES成形管材晶粒细化效果更明显,抗拉强度约为320 MPa,硬度高出13%左右。结论 CVCES工艺可以有效地细化晶粒,成形管材的平均硬度和抗拉强度明显增大。     

A method for modelling of fatigue life for rubbers and rubber isolators

A method for modelling fatigue life of rubbers and rubber isolators is presented in this paper. Firstly, a fatigue experiment is carried out for a rubber dumbbell cylindrical specimen and a rubber isolator. Based on the finite element analysis, the damage parameters including the strain energy density, the maximum principal Green–Lagrange strain and the effective stress are calculated and discussed. Secondly, three fatigue life prediction models are established by using the three damage parameters and using the relation between the measured fatigue life of a dumbbell cylindrical specimen and the computed value of the damage parameters. Thirdly, three proposed prediction models are used to investigate which one can be best used to predicting fatigue life of rubber isolators, taking a typical powertrain rubber isolator as studying example. The fatigue lives of the rubber isolator predicted by the three models are compared with the experimental life. The results demonstrate that the predicted fatigue lives of the rubber isolator using the three fatigue models agree well with the experimental fatigue life within a factor of four, and the model using the effective stress as the damage parameter can predict the fatigue life within a factor of two, which has the best accuracy among the three models.     

Dynamics of Plug Formation in a Circular Cylinder Under Low Bond Number Conditions: Experiment and Simulation

The goal of the current study is to investigate the dynamics of two phase interface under a low Bond number condition. Silicone oil is injected into a cylinder under a Bond number of about 0.47 via a side tube forming a T-junction with the former. The time evolution of the interface of silicon oil in a cylinder is captured using a high speed camera. The volume at which the plug is formed is then determined using an image processing tool to analyze the captured images. A numerical simulation is carried out where fluid is injected into a cylinder, under a less than unity Bond number condition, via a side tube. Numerical and experimental results are then compared.     

板材软模成形数值模拟研究现状

分析了板材软模成形数值模拟的特点,对目前主要采用的软模成形数值模拟方法进行了阐述和评价,并对三类典型软模成形(液压成形,橡皮成形及粘性介质压力成形)过程数值模拟应用进行了综述,最后对板材软模成形数值模拟发展趋势进行了展望.     

非线性路径下板料拉深成形过程中破裂预测

在优选模型参数和简化孔洞形核规律的基础上,采用Gurson-Tvergaard (GT) 多孔材料本构模型分析圆筒件拉深过程;根据金属成形工艺特点,综合考虑拉伸型和剪切型2种不同韧性断裂机制,提出一个统一的韧性断裂准则形式.对于未经过预变形和经过预变形的圆筒件拉深试验和数值模拟进行了比较,结果表明:相对于成形极限图,新的韧性断裂准则可以更加准确地预测非线性路径下圆筒件的拉深破裂.     

丁腈橡胶和氢氧化铝对氯化丁基橡胶阻尼性能的影响

通过对各阶复合损耗因子的测定 ,考察了加入耐油配合胶丁腈橡胶和阻燃剂氢氧化铝对氯化丁基橡胶体系阻尼性能的影响。试验结果表明 ,加入丁腈橡胶 ,阻尼体系在 70Hz、2 0 0Hz和 40 0Hz下的复合损耗因子均下降。加入氢氧化铝可改善体系的阻尼性能 ,随着氢氧化铝用量的增加 ,体系在上述频率下的复合损耗因子有一定程度的提高 ,但在与橡胶的质量比达 4∶10的用量时 ,体系的阻尼性能转而下降     

坐标网分析技术与成形极限图的应用

用网格自动应变测试分析系统ASAME测试了11炉批冷轧薄板的成形极限，对实际测量的成形极限与模拟计算的成形极限进行了比较。对冲压的摩托车油箱进行应变分析，根据成形极限图（FLD）确定冲压零件的安全裕度，评价材料的适用性。