A New Ductile Fracture Criterion and Its Application to the Prediction of Forming Limit in Deep Drawing

With considering the influence of equivalent plastic strain on void-damage and taking Lemaitre damage equivalent stress as plastic potential, based on continuous damage mechanics theory, a new criterion for ductile fracture is derived. The two key material constants in the criterion are determined by the combination of tension tests with FE (finite element) simulation. On the basis of the values of stress and strain calculated from commercial finite element software, the forming limit in cylindrical deep drawing of annealed aluminum alloy LY12(M) is predicted by means of the new ductile fracture criterion. Experiments verify that the predicted results are in agreement with the experimental ones. Hence, it is reliable to predict the forming limit in deep drawing by means of the new ductile fracture criterion.

New criterion in predicting glass forming ability of various glass-forming systems

It has been confirmed that glass-forming ability （GFA） of supercooled liquids is related to not only liquid phase stability but also the crystallization resistance. In this paper, it is found that the liquid region interval （T1 - Tg） characterized by the normalized parameter of Tg/T1 could reflect the stability of glass-forming liquids at the equilibrium state, whilst the normalization of supercooled liquid region △Tx=（Tx - Tg）, i.e. △Tx/Tx （wherein T1 is the liquidus temperature, Tg the glass transition temperature, and Tx the onset crystallization temperature） could indicate the crystallization resistance during glass formation. Thus, a new parameter, defined as ζ = Tg/T1＋△Tx/Tx is established to predict the GFA of supercooled liquids. In comparison with other commonly used criteria, this parameter demonstrates a better statistical correlation with the GFA for various glass-forming systems including metallic glasses, oxide glasses and cryoprotectants.

Theoretical prediction of forming limit diagram of AZ31 magnesium alloy sheet at warm temperatures

A theoretical prediction on forming limit diagram（FLD） of AZ31 magnesium alloy sheet was developed at warm temperatures based on the M-K theory. Two different yield criteria of von Mises and Hill＇48 were applied in this model. Mechanical properties of AZ31 magnesium alloy used in the prediction were obtained by uniaxial tensile tests and the Fields-Backofen equation was incorporated in the analysis. In addition, experimental FLDs of AZ31 were acquired by conducting rigid die swell test at different temperatures to verify the prediction. It is demonstrated from a comparison between the predicted and the experimental FLDs at 473 K and 523 K that the predicted results are influenced by the type of yield criterion used in the calculation, especially at lower temperatures. Furthermore, a better agreement between the predicted results and experimental data for AZ31 magnesium alloy sheet at warm temperatures was obtained when Hill＇48 yield criterion was applied.

Comparative study on forming limit prediction of AA7075-T6 sheet with M−K model and Lou−Huh criterion

In order to effectively predict the fracture of AA7075-T6 sheet, the forming limit curves of AA7075-T6 high-strength sheet were drawn according to Morciniak Kuczyski (M K) model and Lou Huh criterion, respectively. The errors between the predicted values of the two theoretical prediction models and experimental values were calculated by error analysis. The forming limit curves were verified by the punch stretch test to evaluate the prediction accuracy of M K model and Lou Huh criterion. The error analysis results show that the mean error of Lou Huh criterion with the optimal parameters for all tensile specimens is 25.04%, while the mean error of M K model for all tensile specimens is 74.24%. The prediction accuracy of Lou Huh criterion in predicting the fracture of AA7075-T6 sheet is higher. The punch stretch test results show that the forming limit curve drawn by Lou Huh criterion can effectively predict the fracture of AA7075-T6 sheet, but the prediction accuracy of M K model is relatively poor.

Prediction and Verifcation of Forming Limit Diagrams Based on a Modifed Shear Failure Criterion

The forming limit diagram plays an important role in predicting the forming limit of sheet metals.Previous studies have shown that,the method to construct the forming limit diagram based on instability theory of the original shear failure criterion is efective and simple.The original shear instability criterion can accurately predict the left area of the forming limit diagram but not the right area.In this study,in order to improve the accuracy of the original shear failure criterion,a modifed shear failure criterion was proposed based on in-depth analysis of the original shear failure criterion.The detailed improvement strategies of the shear failure criterion and the complete calculation process are given.Based on the modifed shear failure criterion and diferent constitutive equations,the theoretical forming limit of TRIP780 steel and 5754O aluminum alloy sheet metals are calculated.By comparing the theoretical and experimental results,it is shown that proposed modifed shear failure criterion can predict the right area of forming limit more reasonably than the original shear failure criterion.The efect of the pre-strain and constitutive equation on the forming limits are also analyzed in depth.The modifed shear failure criterion proposed in this study provides an alternative and reliable method to predict forming limit of sheet metals.

Formability improvement and blank shape definition for deep drawing of cylindrical cup with complex curve profile from SPCC sheets using FEM

In the current work, to predict and improve the formability of deep drawing process for steel plate cold rolled commercial grade （SPCC） sheets, three parameters including the blanking force, the die and punch comer radius were considered. The experimental plan according to Taguchi＇s orthogonal array was coupled with the finite element method （FEM） simulations. Firstly, the data from the test of stress-strain and forming limit curves were used as input into ABAQUS/Explicit finite element code to predict the failure occurrence of deep drawing process. The three parameters were then validated to establish their effects on the press formability. The optimum case found via simulation was finally confirmed through an experiment. In order to obtain the complex curve profile of cup shape after deep drawing, the anisotropic behavior of earring phenomenon was modeled and implemented into FEM. After such phenomenon was correctly predicted, an error metric compared with design curve was then measured.

Stress based forming limit diagram for formability characterization of 6061 aluminum

Two numerical criteria of forming limit diagram（FLD） criterion and ductile fracture criterion（DFC） are presented for FLD prediction of 6061 aluminum. The numerical results are compared with the experimental FLD and also punch＇s load-displacement curve of experimental samples. Experimental FLD of this study is calculated using hemispherical punch test of Hecker. Experimental FLD is converted to FLSD and imported to the Abaqus software to predict necking of samples. Numerical results for FLSD prediction were compared with experimental FLSD. Results show that ductile fracture criterion has higher accuracy for FLD and FLSD prediction of 6061 aluminum. Comparison of numerical and experimental results for force-displacement curve of punch shows that numerical results have a good agreement with experiment.

Forming limits of nickel coating on right region

The forming limits of nickel coating on the right region were studied, so as to direct the preparation of the material and help the production of workpieces. The electrodeposited nickel coating was prepared on steel substrate to form advanced structures, and its plastic instability was investigated by the Swift Plastic Instability Theory. By using the compound law for laminated sheet metals, explicit equations for the calculation of the instable eigen values were deduced. The forming limit diagrams of the nickel coating on the right region were plotted. It is exhibited that the forming limit of the coating sheet is between the forming limits of the individual nickel coating and steel substrate. The forming limit of the nickel coating is not so good as that of the steel substrate, and the forming limit strain of the coating sheet tends to diminish with the increase of thickness of the coating. The greater the normal anisotropic coefficient of the materials is, the better the forming limit is.

Prediction of forming limit curve for pure titanium sheet

Commercially pure titanium（CP Ti） has been actively used in the plate heat exchanger due to its light weight, high specific strength, and excellent corrosion resistance. However, researches for the plastic deformation characteristics and press formability of the CP Ti sheet are not much in comparison with automotive steels and aluminum alloys. The mechanical properties and hardening behavior evaluated in stress-strain relation of the CP Ti sheet are clarified in relation with press formability. The flow curve denoting true stress-true strain relation for CP Ti sheet is fitted well by the Kim-Tuan hardening equation rather than Voce and Swift models. The forming limit curve（FLC） of CP Ti sheet as a criterion for press formability was experimentally evaluated by punch stretching test and analytically predicted via Hora＇s modified maximum force criterion. The predicted FLC by adopting Kim-Tuan hardening model and appropriate yield function shows good correlation with the experimental results of punch stretching test.

Evaluation of glass forming ability of alloys

Key step of exploiting a new type BMG (Bulk Metallic Glass) is quickly judging GFA (Glass Forming Ability) of the alloys. The mole melting heats of BMGs are calculated using the weighted averages principle. The reliability and limitation of Trg criterion for GFA are discussed. The reason why Trg of BMGs is larger than 0.5 is discussed. Two new criteria for GFA, ΔHmg and ΔGg, are proposed. GFA sequence of BMGs is calculated using the ΔHmg criterion, the result agrees with that of A. Inoue and the Rc criterion. Furthermore, as an example, the Rc of the alloys developed by Chuang DONG et al is calculated using the ΔHmg and ΔGg. The ascending sequence of these alloys calculated with the ΔHmg criterion agrees with that of Chuang DONG et al. On the contrary, the result by the ΔGg criterion is in contrary with Chuang DONG et al, indicating that the ΔHmg criterion is better and more convenient than the ΔGg criterion. Calculation showed that the optimum ΔHmg is -15.16 kJ/mol.

FORMABILITY OF TERNARY INTERMEDIATE COMPOUNDS IN SOME MOLTEN SALT PHASE DIAGRAMS

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A Linearized and Unified Yield Criterion of Metals and Its Application

In this paper a linearized and unified yield crierion of metals is presented, which is in a form of a set of linear functions with two pararneters. The parameters are ex- pressed in terms of tension yield stress and so-called “shear-stretch ratio” and can bereadily determined from experimental data. It is shown that in stress space the set of yield functions is a set of polygons with twelve edges located between the Tresca’s hexagon and twin-shear-stress hexagon￣[1]. In this paper the present yield function isused to analyse the prestressiap loose running fit cylinders.

Theoretical and Numerical Comparison of Limit Point Bifurcation and Maximum Force Criteria. Application to the Prediction of Diffuse Necking

A large number of criteria has been developed to predict material instabilities, but their choice is limited by the lack of existing comparison of their theoretical basis and application domains. To overcome this limitation, a theoretical and numerical comparison of two major models used to predict diffuse necking is present in this paper. Limit Point Bifurcation criterion is first introduced. An original formulation of the Maximum Force Criterion (MFC), taking into account the effects of damage and isotropic and kinematic hardenings, is then proposed. Strong connections are shown between them by comparing their theoretical basis. Numerical Forming Limit Diagrams at diffuse necking obtained with these criteria for different metallic materials are given. They illustrate the theoretical link and similar predictions are shown for both models.

RELIABILITY ANALYSIS OF FRP LAMINATED PLATES WITH CONSIDERATION OF BOTH INITIAL IMPERFECTION AND FAILURE SEQUENCE

A probabilistic progressive failure analyzing method is applied to estimating the reliability of a simply supported laminated composite plate with an initial imperfection under bi-axial compression load. The initial imperfection and the strength parameters are considered as random variables. Ply-level failure probability is evaluated by the first order reliability method (FORM) together with the Tsai-Wu strength criterion and Tan criterion. Current stresses in the laminated structure are calculated by the classical lamination theory with the stiffness modified based on the last step ply failure. Probabilistically dominant ply-level failure sequences leading to overall system failure are identified, based on which the system failure probability is estimated. A numerical example is presented to demonstrate the methodology proposed. Through parameter studies it is shown that the deviation of the initial imperfection and some of the strength parameters largely influence the system reliability.

THE UNIFIED FORM FOR YIELD CRITERION OF METALLIC MATERIALS

Ⅰ. INTRODUCTIONThe yield criteria are important foundations in the theory of plasticity and the strength of metallic materials. As the shear stress is the major cause for yielding, the yield criteria based on shear stress as a physical interpretation are: maximum shear stress (Tresca)criteri-

基于DEFORM-3D的微型螺钉冷成形过程有限元分析

典型的螺钉头部成形过程包括镦粗(预成形)和冲槽(最终成形)两个工步,是个比较复杂成形过程。特别是冲槽阶段,镦粗和反挤压同时进行的,情况更为复杂。利用非线形有限元分析软件DEFORM-3D对微型螺钉冷塑性成形过程进行仿真,可以获得成形过程中的应变、应力、材料流动情况,同时还可以对成形过程中的缺陷如折叠、裂纹,进行预测,从而为工艺的设计和改进提供信息。

非线性加载路径下金属薄板成形极限研究进展

成形极限图是评判金属薄板局部成形能力的重要工具,研究非线性加载路径下的成形极限图对提高实际生产过程中工艺设计的合理性和板料的成形性具有重要意义。总结分析了国内外学者对非线性加载路径下金属薄板成形极限的研究进展,主要包括试验方法、理论预测模型及路径无关成形极限图3个方面。重点研究了不同试验方法的适用范围及优缺点,阐述了多种预测非线性加载路径下成形极限图的理论模型及应用实例,并归纳了当前在应变路径无关的成形极限图方面取得的研究成果。最后,从上述3个方面总结了当前更具潜力的研究方向,并对未来的发展趋势以及面临的困难和挑战进行了展望。

基于GISSMO失效准则的1500 MPa热成形钢断裂失效模型分析

以CR1000/1500HS为研究对象,基于GISSMO断裂失效准则,设计表征不同失效形式的5种测试试样;基于万能拉伸试验机和DIC测试系统,获取不同试样拉伸过程的力-位移曲线和断裂时刻的极限塑性应变;基于LS-DYNA搭建各失效试样的仿真分析模型,提取各试样断裂区域的应力三轴度;拟合获得断裂失效曲线和塑性失稳曲线,迭代获取其他模型参数;采用车门防撞梁动态三点弯曲冲击试验和仿真模型对承载特性进行分析;对比GISSMO失效模型、传统应变失效模型与试验测试的力-位移曲线差异,以验证失效模型的准确性。结果表明:Swift和Hockett-Sherby混合硬化模型可准确描述材料的塑性变形行为,关键参数误差小于5%;GISSMO失效模型获得的车门防撞梁承载特性关键参数和承载力-位移曲线与试验测试结果保持一致,误差低于5%;而采用传统应变失效准则获得的结果误差较大,最大位移的误差达到30%;表明基于GISSMO失效准则的材料模型可以准确表征CR1000/1500HS的塑性变形行为及断裂失效行为。

考虑非线性阻尼效应的孤岛微电网暂态稳定性分析及提升策略研究

随着可再生能源发电及并网技术的快速发展,以电力电子变流设备为主导的微电网受到了越来越多的关注。由于电力电子变流器的低惯量和强非线性特征,孤岛微电网在大扰动下易发生暂态失稳。考虑微电网中构网逆变器与跟网逆变器的交互作用,提出1种基于等面积法则的暂态稳定性判据和暂态稳定提升控制策略。首先,建立了孤岛微电网的简化二阶动态模型,包含依赖于功角的非线性阻尼项。然后,从能量角度分析了非线性阻尼对加减速面积的影响规律。考虑非线性阻尼的分布特性,推导出正阻尼区域内的暂态稳定性判据。根据稳定边界条件,提出1种基于电压前馈的暂态稳定性提升控制策略。最后,利用MATLAB/Simulink仿真对所提稳定性判据和改进控制策略的有效性进行了验证。结果表明,所提微电网暂态稳定性判据和改进控制策略可为电力电子变流器的参数优化设计和提高微电网稳定运行能力提供理论依据。

基于韧性断裂准则的AZ31B镁合金板材成形极限预测

结合损伤起始判据和损伤演化准则,建立了完整的韧性断裂准则,基于ABAQUS中韧性损伤材料模型对AZ31B镁合金板材成形极限进行了预测。通过拟合单向拉伸应力应变曲线得到材料本构模型及损伤演化参数,建立了板材的Nakazima半球形凸模胀形有限元仿真模型,再基于韧性断裂准则预测了AZ31B镁合金板材室温下的成形极限,并分析了不同板材断裂失效判据对成形极限的影响。研究结果表明,基于所建立的韧性断裂准则,并以损伤演化过程中应变路径转变作为断裂失效判据,可以较准确地预测镁合金板材成形极限,得到的成形极限图与实验结果吻合较好。