汽车用先进高强钢韧性断裂模型的研究与应用进展

韧性断裂预测对汽车轻量化产品设计与成形工艺优化有着重要的意义.全面综述了强耦合型与弱(非)耦合型韧性断裂模型的发展与研究现状;重点围绕考虑加载路径与应力状态的断裂失效与成形极限曲线预测、各向异性耦合的失效模型拓展、应变速率与温度效应对材料断裂的影响等3个方面的研究现状及应用效果进行了分析介绍,其中,MMC模型、Lou-...     

Failure of high strength steel sheets: Experiments and modelling

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft–Latham and Bressan–Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force–displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLCs). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.     

考虑材料韧性损伤的中厚板半冲孔成形过程分析

为研究半冲孔成形过程中韧性损伤的演化以及部分工艺参数对成形质量的影响规律,本文在ABAQUS有限元模拟软件中建立了半冲孔轴对称有限元模型,并通过VUMAT用户子程序引入GTN(Gurson-Tvergaard-Needle-man)损伤模型,结合同时考虑空穴形状与体积变化影响的韧性断裂准则,进行弹塑性大变形有限元分析.基于该有限元模犁,预测了半冲孔工艺中静水压力、等效应力、等效应变、应力三轴度以及损伤断裂的产生和发展趋势,分析了反顶力、压边力和冲裁间隙对零件的影响规律,并与实验结果进行对比分析,验证了数值模拟的准确性.     

Study of ductile fracture and preform design of upsetting process using adaptive network fuzzy inference system

This paper combines adaptive-network-based inference system (ANFIS) and elasto-plastic finite element to predict the ductile fracture initiation and the preform shape of the upsetting process. From the hybrid-learning algorithm in ANFIS, it can efficiently construct rule database and optimal distribution of membership function to solve the punch stroke which causes the ductile fracture, and the preform shape which results a desired cylindrical workpiece after forming in the upsetting process. As a verification of this system, the punch stroke for ductile fracture initiation and the free boundary radius of the billet after forming are compared between ANFIS and FEM simulated results. In the ductile fracture prediction, it is proved that ANFIS can efficiently predict the ductile fracture initiation successfully for arbitrary friction coefficient and aspect ratio. In the preform shape prediction, the simulated cylindrical radius shows good coincidence with the desired radius after forming. From this forward and inverse investigation, the ANFIS is proved to supply a useful optimal soft computing approach in the forming category.     

The application of a ductile fracture criterion to the prediction of the forming limit of sheet metals

To predict accurately the forming limit in sheet metal forming, the combination of FE simulation with tension tests is adopted in this paper to determine the material constants p and C in a ductile fracture criterion (DFC), which is advanced by the author. Forming limits of bore-expanding, hemispherical punch bulging and deep drawing (cylindrical, square-cup parts) are predicted by means of the DFC. Comparison of the results predicted by the DFC with experimental values shows that the precision of forming limit predicted by material constants obtained by the combination method is more accurate than that predicted by material constants obtained by the tension method, and that the critical punch stoke and the fracture initiation position in forming processes above mentioned are predicted accurately by the DFC.     

Slight clearance punching of ultra-high strength steel sheets using punch having small round edge

A slight clearance punching process of ultra-high strength steel sheets using a punch having a small round edge was developed to improve the quality of the sheared edge. No crack from the edge of the punch was generated by relaxing concentration of deformation with the punch having a small round edge, and the fracture was delayed. A small edge radius of 0.13 mm was effective for improving the quality of the sheared edge of ultra-high strength steel sheets, the increase in shiny burnished surface. The delayed fracture was prevented by the increase in compressive residual stress for the punch having the small round edge. For 1000 strike punching of an ultra-high strength steel sheet having a tensile strength of 1200 MPa, a sheared edge of high quality was produced with a TiAlN-coated punch having the small round edge. In addition, the chipping of the punch edge was prevented even for a slight clearance by the small round edge. It was found that both small round edge and slight clearance are indispensable for high quality punching of ultra-high strength steel sheets having low ductility.     

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.     

Numerical modeling of advanced materials

The finite element (FE) method is widely used to numerically simulate forming processes. The accuracy of an FE analysis strongly depends on the extent to which a material model can represent the real material behavior. The use of new materials requires complex material models which are able to describe complex material behavior like strain path sensitivity and phase transformations. Different yield loci and hardening laws are presented in this article, together with experimental results showing this complex behavior. Recommendations on how to further improve the constitutive models are given. In the area of damage and fracture behavior, a non-local damage model is presented, which provides a better prediction of sheet failure than the conventional Forming Limit Diagram.     

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。     

FE analysis on Deformation and Temperature Nonuniformity in Forming of AISI-5140 Triple Valve by Multi-Way Loading

In triple valve forming process by multi-way loading severely nonuniform deformation and temperature distributions are prone to occur, which may lead to poor forming quality and macro-micro defects. A 3D coupled thermo-mechanical rigid-viscoplastic finite element (FE) model for multi-way loading forming of AISI-5140 steel equal diameter triple valve was developed based on DEFORM-3D. Through comprehensive simulation and analysis, the influences of main process parameters on the forming process and nonuniformity of deformation and temperature were studied. The results showed that: (1) the degree of deformation nonuniformity decreased with the increase of the punch loading speed, initial temperature of billet, or the decrease of the friction factor; (2) the average temperature of forming body increased as the punch loading speed, initial temperature of billet and the friction increased, while the degree of temperature nonuniformity decreased with the increase of punch loading speed or decrease of initial billet temperature.     

Prediction of damage in small curvature bending processes of high strength steels using continuum damage mechanics model in 3D simulation

Sheet metal bending of modern lightweight materials like high-strength low-alloyed steels (HSLA) is one major challenge in metal forming, because conventional methods of predicting failure in numerical simulation, like the forming limit diagram (FLD), can generally not be applied to bending processes. Furthermore, the damage and failure behaviour of HSLA steels are changing as the fracture mechanisms are mainly depending on the microstructure, which is very fine-grained in HSLA steels composed with different alloying elements compared to established mild steels. Especially for high gradients of strain and stress over the sheet thickness, as they occur in small curvature bending processes, other damage models than the FLD have to be utilised. Within this paper a finite element (FE) 3D model of small curvature bending processes is created. The model includes continuum damage mechanics model in order to predict and study occurring failure by means of ductile coherence loss of the material and crack formation with respect to influencing process parameters. Damage parameters are determined by inverse numerical identification method. The FE-model is strain based validated considering the deformation field at the outer bending edge of the specimen by using an optical strain measurement system. The Lemaitre based damage model is calibrated against the experimental results within metallographic analysis adapting the identified damage parameters to the bending process und thus adjusting the crack occurrence in experiment and simulation. Using this model the bendability of common HSLA steel, used for structural components, is evaluated with respect to occurring damage and failure by numerical analysis.     

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。     

A New Method to Calculate Threshold Values of Ductile Fracture Criteria for Advanced High-Strength Sheet Blanking

A new approach is presented in this paper to calculate the critical threshold value of fracture initiation. It is based on the experimental data for forming limit curves and fracture forming limit curves. The deformation path for finally a fractured material point is assumed as two-stage proportional loading: biaxial loading from the beginning to the onset of incipient necking, followed plane strain deformation within the incipient neck until the final fracture. The fracture threshold value is determined by analytical integration and validated by numerical simulation. Four phenomenological models for ductile fracture are selected in this study, i.e., Brozzo, McClintock, Rice-Tracey, and Oyane models. The threshold value for each model is obtained through best-fitting of experimental data. The results are compared with each other and test data. These fracture criteria are implemented in ABAQUS/EXPLICIT through user subroutine VUMAT to simulate the blanking process of advanced high-strength steels. The simulated fracture surfaces are examined to determine the initiation of ductile fracture during the process, and compared with experimental results for DP780 sheet steel blanking. The comparisons between FE simulated results coupled with different fracture models and experimental one show good agreements on punching edge quality. The study demonstrates that the proposed approach to calculate threshold values of fracture models is efficient and reliable. The results also suggest that the McClintock and Oyane fracture models are more accurate than the Rice-Tracey or Brozzo models in predicting load-stroke curves. However, the predicted blanking edge quality does not have appreciable differences.     

Ductile damage and simulation of fine-blanking process by FEM

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切角板坯对纯铜薄板矩形盒拉深影响的试验及数值分析

在大量纯铜薄板矩形盒拉深试验的基础上,结合有限元计算,分析了矩形盒的拉深特性。指出,矩形切角板坯使法兰曲边变形分布得以改善,非拉深变形抵抗弱化,提高了拉深成形性。有限元模拟结果显示矩形板坯拉深断裂点产生在凸模肩部转角附近,且始终承受两向不等拉伸,应变计算值与实际拉深测定值相符;切角板坯的拉深断裂点则转移至接近凹模口的侧壁处,且始终处于压剪组合变形状态,断裂时板厚应变相当小,认为倾向于剪切断裂。切角板坯断裂点的拉、压应力组合效应使该点板厚几乎不变,是提高拉深极限的重要因素之一。     

Characterization of mechanically sheared edges of dual phase steels

In recent years advanced high strength steels (AHSS) received increased interest for light structures with improved performance, but they are often sensitive to edge cracking during sheet metal forming. In this study mechanically sheared edges were characterized for three dual phase steels (DP600, DP780 and DP980), sheared with three die clearances (5%t, 10%t, 15%t) and along rolling and transverse directions. Microstructures of the materials were provided first, and then the sheared edges were examined by optical microscopy and scanning electron microscopy that reveal the morphology and random feature of the sheared edges. A factorial analysis was performed to reveal the general trends of the processing parameters on four edge zones. A new strain measurement method was used for characterizing strain distribution in the sheared region, which shows the peak strain to be higher than 3. The strain quickly decreases from sheared edge to interior, leaving a shear-affected zone of about 500 μm or 31% of the thickness. The fracture processes and involved mechanisms were discussed.     

小模数齿形精冲的数值模拟及参数优化

文章运用有限元分析软件对小模数齿形件精冲过程进行数值模拟,研究冲裁间隙、压边圈尺寸、压边力、反压力、凹模刃口圆角等主要工艺参数对成形结果的影响,得出了各工艺参数与冲裁剪切面光洁程度的关系。同时引入Cockcroft&Latham韧性断裂准则预测裂纹出现和扩展的时间及位置,探讨了成形过程中各种缺陷(撕裂带、裂纹等)产生的原因,并通过实验验证了部分模拟分析结果。     

Microstructures,Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components

Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.     

Damage and Failure Analysis of AZ31 Alloy Sheet in Warm Stamping Processes

In this study, a combined experimental-numerical investigation on the failure of AZ31 Mg alloy sheet in the warm stamping process was carried out based on modified GTN damage model which integrated Yld2000 anisotropic yield criterion. The constitutive equations of material were implemented into a VUMAT subroutine for solver ABAQUS/Explicit and applied to the formability analysis of mobile phone shell. The morphology near the crack area was observed using SEM, and the anisotropic damage evolution at various temperatures was simulated. The distributions of plastic strain, damage evolution, thickness, and fracture initiation obtained from FE simulation were analyzed. The corresponding forming limit diagrams were worked out, and the comparison with the experimental data showed a good agreement.     

圆管内翻边成形的数值模拟及参数优化

以摩托车减震器一翻边成形部件为研究对象,建立圆管内翻边的有限元模型,利用数值模拟方法,分析翻边过程中应力、应变和冲击力的变化。该部件在翻边过后,圆管端缘存在由于变形堆积而起皱的缺陷,针对该缺陷选取圆管不同的结构参数以及凸模的外形,建立不同工况下的有限元模型并对比分析翻边结果。结果表明,当圆管壁厚减小值d取0. 25 mm、凸模角度θ为75°时,翻边后的圆管端缘起皱情况得到较好改善,凸模冲击力降低。最后,根据参数化分析结果确定凸模上移余量在0~0. 03 mm范围内时,损耗比可控制在5%以下。