Finite element analysis of formability of a few kinds of special steel sheets

Deep drawing processes of various special steel sheets are simulated by the rigid-plastic finite element method. To predict the forming limit a criterion for ductile fracture is applied. From the histories of stress and strain in each element calculated by the finite element simulation, the fracture initiation site and the critical stroke are predicted by means of the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results show that the finite element analysis combined with the ductile fracture criterion is useful to predict the forming limit in a wide range of sheet steels.     

Formability of sandwich sheet materials in plane strain compression and rolling

The stress states developed during room temperature, plane strain compression modes of deformation of stainless steel clad aluminum and aluminum clad strainless steel sheets have been investigated in order to gain insight into the formability of bonded ductile sandwich sheet materials in primary metalworking processes. Assuming uniform, isostrain deformation in the component layers, sandwich compression stress-strain curves were predicted to be rule of mixtures averages of component compression stress-strain curves. These predictions showed good agreement with experimental data when friction and in-homogeneous deformation were taken into account. Since the through-thickness applied pressure can be assumed to be the same in both components of thin sandwich sheet materials, in-plane stresses which are tensile in the harder component and compressive in the softer component of a clad sheet are developed in order to satisfy the yield conditions. The nature of these in-plane stresses was confirmed by measurements of residual stress distributions in rolled clad sheet specimens, and it was shown how the tensile stress in the harder component may lead to unstable flow and failure of this component during forming. The observed failures were similar in both plane-strain indentation and rolling tests. Although the initiation of instability in symmetric clad sheet metals appears to be independent of the arrangement of the component layers, the process of final localization leading to fracture was observed to depend heavily on the layer arrangement. S. L. SEMIATIN, formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University.     

Formability Prediction of Two-Layer Sheets Based on Ductile Fracture Criteria

Two-layer sheets which consist of dissimilar metallic components are widely used in various industries. In this paper, formability of Al3105–St14 two-layer sheet is evaluated numerically and experimentally. Freudenthal, Cockroft & Latham, Oh and Brozzo ductile fracture criteria are implemented to predict forming limits in the numerical approach. Numerical FLDs and FLSDs are compared with experimental observations for Al3105–St14 two-layer sheet, and a good agreement is seen. The results show that the prediction of forming limits based on Oh and Brozzo ductile fracture criteria are more accurate than the predictions based on Freudenthal and Cockroft criteria.     

Comparative Study on Failure Prediction in Warm Forming Processes of Mg Alloy Sheet by the FEM and Ductile Fracture Criteria

An important concern in metal forming is whether the desired deformation can be accomplished without any failure of the material, even at elevated temperatures. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element (FE) method for predicting the onset of fracture in warm metal working processes of magnesium alloy sheets. The uniaxial tensile tests of AZ31 alloy sheets with a thickness of 3 mm and FE simulations were performed to calculate the critical damage values for three kinds of ductile fracture criteria. The critical damage values for each criterion were expressed as the function of strain rate at various temperatures. In order to find out the best criterion for failure prediction, Erichsen cupping tests under isothermal conditions were carried out at various temperatures and punch velocities. Based on the plastic deformation histories obtained from FE analysis of the Erichsen cupping tests and the critical damage value curves, the initiation time and location of fracture were predicted under bi-axial tensile conditions. As a result, Cockcroft–Latham’s criterion showed good agreement with the experiments.     

Properties of Laser Welded SAF 2205 Duplex Stainless Steel Sheet

High rate welding methods for sheet material can offer significant cost reduction for mass production application in comparison with more conventional arc processes. Therefore, in this research, laser welds in SAF 2205 duplex stainless steel (DSS) sheets welded using different welding speeds were investigated. Metallography, texture measurements and mechanical testing were carried out on the weld joints. The corrosion properties were not evaluated. The base material was characterised by a bamboo‐like morphology and a ferrite volume fraction of 53 %. For all welding speeds, the ferrite level in the weld zone was higher than 85 % and the austenite showed an acicular morphology. Whereas in the base material a clear element partitioning existed between ferrite and austenite, no partitioning was observed in the welded zone. This is due to the very high cooling rates, which limit the amount of diffusion that can take place. Electron backscattering diffraction revealed that the texture of the cold rolled material was destroyed by the welding process. While the hardness of the base material was about 265 HV, the maximal hardness in the fusion zone exceeded 310 HV and increased with an increase of the welding speed. Yield and tensile stength were however not dramatically influenced. On the other hand, the formability properties were deteriorated by an increase of the welding speed. This behaviour can also be observed on the fracture surfaces of tensile specimens. The tensile tests on the welded sheet resulted in ductile fracture surfaces, but an easier void formation was observed in the laser welds. However, it has to be pointed out that formability of the laser welded DSS sheets is acceptable when a lower welding speed is used. This is also confirmed by the crack propagation observed during the Erichsen test. Therefore, the laser welding can be used as a joining operation for DSS sheet materials providing the corrosion requirements are fulfilled.     

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.     

Prediction and Experimental Validation of Forming Limit Curve of a Quenched and Partitioned Steel

Forming limit curve(FLC)is an effective tool to evaluate the formability of sheet metals.An accurate FLC prediction for a sheet metal is beneficial to its engineering application.A quenched and partitioned steel,known as QP980,is one of the 3rd generation advanced high strength steels and is composed of martensite,ferrite and a considerable amount of retained austenite(RA).Martensite transformation from RA induced by deformation,namely,transformation induced plasticity(TRIP),promotes the capability of work hardening and consequently formability.Nakazima tests were carried out to obtain the experimental forming limit strains with the aid of digital image correlation techniques.Scanning electron microscopy(SEM)was employed to examine the fracture morphologies of Nakazima specimens of the QP980 steel.The observed dimple pattern indicated that tensile stress was the predominant factor which led to failure of QP980 specimens.Therefore,maximum tensile stress criterion(MTSC)was adopted as the forming limit criterion.To predict the FLC of QP980 steel,Von-Mises yield criterion and power hardening law were adopted according to the tested mechanical properties of QP980 steel.Results were compared with those derived from other three representative instability theories,e.g.Hill criterion,Storen-Rice vertex theory and Bressan-Williams model,which shows that the MTSC based FLC is in better agreement with the experimental results.     

Incremental Sheet Metal Forming: Numerical Simulation and Rapid Prototyping Process to make an Automobile White‐Body

In order to make an automobile body structure, incremental sheet metal forming is introduced as a rapid prototyping process. Numerical modeling of the process is initially used to predict the deformation of the sheet metal to avoid failure during the incremental forming process using ABAQUS/Explicit finite element code and OYANE's ductile fracture criterion via a VUMAT user material. An automobile CAD model is then designed, and segmented into several parts in order to accommodate the working space of the CNC machine and formability of sheet metal. After that, CAM software is used to generate a tool‐path for making wooden‐dies and all small parts. Finally, a welding process is applied to join all parts which were cut by laser cutting after incremental sheet forming process.     

金属薄板面内压剪变形的损伤断裂行为

相变诱导塑性钢（TRansformation induced plasticity, TRIP）作为常用的先进高强钢在汽车等交通工具的轻量化方面有广泛的应用前景。而对于其复杂零件的成形过程,韧性断裂是不可忽视的问题之一。本文针对现有实验装置不易诱发薄板承受面内压剪时断裂失效,从而无法研究板料负应力三轴度区间断裂行为的问题,以高强钢TRIP800薄板为研究对象,设计了可在单向试验机完成压剪实验的试样和夹具。通过调整夹具旋转角度和试样装夹位置可以实现同一种试样在广泛的负应力三轴度范围内进行压剪断裂分析。基于ABAQUS/Explicit平台建立了三个典型加载方向20°、30°和45°对应的压剪过程有限元模型,分析表明:三种情况的试样局部变形区域的应力三轴度都小于0且断裂点的应力三轴度低至?0.485,验证了设计的装置可实现负应力三轴度区间的断裂失效分析,同时基于MMC断裂准则分析了不同应力状态的初始损伤情况及损伤扩展路径。      

冷轧和镀锌汽车板的发展

冷轧汽车板应具备精确的尺寸，优良的板形、成型性能、抗凹陷性能、表面质量、涂装和焊接性能。介绍了冷轧汽车板和以冷轧板为基板生产的镀锌汽车板，包括热镀纯锌汽车板、合金化热镀锌汽车板和电镀锌汽车板的性能和发展情况，及我国冷轧、镀锌汽车板的发展展望。     

Experimental and Numerical Failure Criteria for Sheet Metal Forming

For sheet metal forming, often the forming limit diagram (FLD) is used as failure criterion as it can be derived easily in experiments. It is based on the assumption that localization of strain in the sheet plane is responsible for crack initiation, but application of FLD is limited to linear strain paths. Hence, only forming processes with approximately the same deformation history as the experiments carried out for FLD determination should be evaluated by this criterion. Forming limit stress diagrams (FLSD) do not exhibit such strict limitations. They are based on the assumption that principal stresses in the sheet plane are responsible for crack initiation. As these stresses are usually calculated by FE analysis using elastic plastic material laws, strain hardening is considered. Two‐step forming tests as application examples prove the FLSD to be adequate for evaluation of non‐linear forming processes with alternating forming directions. Nevertheless, FLSD are derived in extensive investigations which makes them unattractive for most industrial applications. Furthermore, both FLD and FLSD do not consider the physical background of ductile crack initiation which is provoked by an interaction of local stress triaxiality and equivalent plastic strain. Hence, a reliable failure criterion should concentrate on these two parameters. The Gurson‐Tvergaard‐Needleman‐ (GTN‐) damage model can predict crack initiation during sheet metal forming. Application of the GTN model to 2 step forming tests with the bake hardening steel H220BD+Z showed good agreement to experimental results although a sensitivity of the model to mesh size and stress triaxiality is observed.     

Improving the formability of low carbon sheet steel by control of interstitial carbon content and temperature

Several authors have shown a relationship between strain rate sensitivity and the formability of sheet metals. In this study it is shown that by cooling to subzero temperatures the strain rate sensitivity of low carbon sheet steel can be improved dramatically and that this increased strain rate sensitivity leads to improved formability. At laboratory strain rates the optimum conditions for improved formability, a temperature of ∼70 °C and an interstitial carbon content of ∼20 ppm, were determined experimentally. Optimization involves a tradeoff between strain rate sensitivity, strain hardening and fracture strain. Testing under these conditions leads to an improvement in formability of ∼13 pct.     

Effect of asymmetric rolling on the texture and mechanical properties of AA6111-aluminum sheet

Aluminum-alloy sheets are considered to be one of the high-potential substitutes for steel sheets, when considering the weight reduction of automobiles. However, aluminum-alloy sheets have inferior formability, mainly due to their lower plastic-strain ratios. The plastic-strain ratios of aluminum-alloy sheets can be increased with the formation of shear-deformation texture through the sheet thickness. Asymmetric rolling, in which the circumferential velocities of working rolls are different, is a favorable process for imposing shear-deformation texture through the sheet thickness. In this study, commercial AA6111-alloy sheets were used to compare the effects of conventional symmetric rolling and asymmetric rolling in controlling the shear textures and grain refinement, as well as the mechanical properties of the tensile and plastic-strain ratio.     

Ductile Fracture Prediction of 316LN Stainless Steel in Hot Deformation Process

A ductile fracture criterion of 316LN stainless steel, combined with the plastic deformation capacity of ma- terial and the stress state dependent damages, was proposed to predict ductile fracture during hot deformation. To the end, tensile tests at high temperatures were first performed to investigate the fracture behavior of 316LN stain- less steel. The experimental results show the variation of the critical fracture strain as a function of temperature and strain rate. Second, the criterion was calibrated by using the upsetting tests and the corresponding numerical simula- tions. Finally, the proposed fracture criterion was validated by the designed tests and the corresponding finite ele- ment （FE） simulation. The results show that the criterion can successfully predict the onset of ductile fracture at ele- vated temperatures.     

冷轧超低碳搪瓷钢板的试验研究

研究了冷思超低碳搪瓷钢板的力学性能和贮的贮氢性能。研究结果表明,以超低碳为基础,添加适量的合金元素,采用合适的工艺制度生产的冷轧钢板既具有优良的成形性能,又具有优良的贮氢性能。其断裂伸长率达５２％。平均ｒ值达１．８,ｎ值达０．２５,氢渗透时间达到７．７ｍｉｎ,具有优良的抗鳞爆性能。     

Finite Element and Experimental Analyses on the Formability of Steel Sheets Produced by Compact Endless Cast and Rolling

Although the compact endless cast and rolling mill (CEM) is a promising candidate as a next-generation energy-saving steel process, due to its short history, the formability of the steel sheet produced by the CEM process are not known yet. Herein, drawability and stretchability of low-carbon steel sheets produced by the CEM process are investigated and compared with those of conventional hot-rolled low-carbon steel sheets, to estimate its applicability to industrial parts. Finite element analyses using the Gurson-Tvergaard-Needleman damage model were conducted and compared with the experimental results. Homogeneous microstructure and relatively strong textures of {111}||ND γ-fibers and 〈110〉||RD α-fibers were developed in the CEM-processed steel in comparison with the conventional hot-rolled specimen. The drawn cup of the CEM specimen showed weak earing phenomena, while having higher limiting drawing ratios (2.0 and 1.95 in the experimental and numerical simulation, respectively). Furthermore, a difference in limit dome height between the two specimens is negligible. Therefore, it is confirmed that CEM-processed steels have comparable properties of strength and formability, provide an effective manufacturing process, and exhibit good potential as a next-generation energy-saving process.     

一个基于孔洞演化机制的韧性断裂预测模型

在韧性断裂中微观孔洞演化机制的基础上,提出了一个基于孔洞演化机制的非耦合型韧性断裂预测模型.模型充分考虑了两种典型的孔洞演化机制:孔洞的长大机制和孔洞的拉长扭转机制.该模型引入了三个具有不同物理意义的材料参数:材料对不同孔洞演化机制的敏感度、应力状态敏感度系数和材料的损伤阈值,并使用等效塑性应变增量表征其对韧性损伤累积过程的驱动作用.为了使模型可以更好地反映三维应力状态对材料韧性断裂性能的影响,将该模型从主应力空间转换到由应力三轴度、罗德参数和临界断裂应变构成的三维空间,得到了由模型确定的三维韧性断裂曲面,并研究了相关参数对三维韧性断裂曲面及平面应力二维韧性断裂曲线的影响.利用5083-O铝合金、TRIP690钢和Docol 600DL双相钢三个典型的轻质高强板材的韧性断裂数据验证了该模型对不同材料和不同应力状态的适用性和准确性.     

汽车用自润滑合金化板的综合应用性能

对自润滑合金化板及普通合金化板的成形性能、焊接性能、化成性能、电泳涂装性能和耐蚀性能等应用性能进行了对比研究.结果表明:相比普通合金化板,自润滑合金化板的成形性能显著提高,钢板表面动摩擦因数由0.16降低到0.12是导致成形性能提高的主要原因;自润滑合金化板的焊接性能、化成性能及涂装性能与普通合金化板均在同等水平,耐蚀性能相比普通合金化板得到进一步提高.自润滑合金化板的综合应用性能优良,对当前汽车厂的生产工艺具有完全适应性,可随时切换而无需变更生产工艺.     

The effects of strength ratio on the forming limit diagram of tailor-welded blanks

In the present study, the influences of strength ratio (SR) are studied on the formability and forming limit diagram (FLD) of tailor-welded blanks (TWBs). AISI 340, St 12 and St 14 steel sheets with equal thickness of 1?mm were used as different kinds of steel to make TWB with different SR. TWBs were obtained by CO₂ laser welding of different steel sheets. Limit strength ratio (LSR) is introduced as a new useful factor to predict the FLD of TWB with different SR. Results of this research show that with increasing of difference of TWB’s SR and LSR, formability and the level of FLD will decrease. By SR increasing, limit dome height decreases and some defects such as weld line movement increase. The experimental findings show that the SR of TWB can effect on the position of fracture in the TWB products.