能量法理论与数值模拟相结合的方板对角拉伸临界起皱失稳预测

在薄板塑性成形起皱失稳预测研究领域,由于理论和试验研究方法各自的局限性,有限元模拟成为了不可或缺的研究手段。由于有限元法不包含起皱失稳判据,虽然利用引入初始缺陷等手段能够输出后屈曲形貌,但却无法界定确切的临界起皱时刻,且模拟结果会受到网格参数设置的影响发生波动。针对该问题,以吉田屈曲试验为验证试验,利用能量法理论与数值模拟相结合建立304不锈钢方板对角拉伸临界起皱判定线。探讨不均匀拉伸载荷性质下的临界起皱判定线的形貌及特征,讨论几何条件和边界条件对试件起皱失稳判定线的影响。通过全场应变测量系统VIC-3D拍摄的试验应变云图证实了所建立的起皱判定线的有效性。研究内容为板料起皱失稳极限图的建立提供了研究方法和部分数据支撑。     

NUSIMEP-7微粒比对分析

为提高含铀微粒中铀同位素比值的测定水平,欧盟联合研究中心 “标准物质和测量研究所” 定期组织微粒分析技术的国际比对。中国原子能科学研究院微粒分析实验室于2010年参加了“核特征的实验室间测量评价计划”（NUSIMEP-7）,旨在准确测量含铀微粒中同位素比值。本研究测量了单同位素、双同位素沉积碳片上含铀微粒的²³⁵U/²³⁸U、²³⁴U/²³⁸U、²³⁶U/²³⁸U,结果表明,90%以上的测量数据在“满意数值”范围内。     

等效拉深筋精细数值化模型与起皱模拟应用

提出一种等效拉深筋精细数值化模型。根据拉深筋与弹簧的作用效果相似的特点,在板料冲压成形动力显式算法中采用弹簧单元处理等效拉深筋模型,实时、同步地反映等效拉深筋模型的被动作用效果,能更准确地模拟冲压成形过程中板料的流动规律。实际汽车覆盖件实验结果表明,等效拉深筋精细数值化模型能够准确模拟拉延成形过程中局部起皱的产生、发展和最终起皱的形貌。     

应力加载路径对板壳塑性起皱失稳极限的影响研究

随着航天、航空、国防、汽车等制造技术领域对金属产品轻质化、强韧化的要求不断提高,塑性失稳成为制约其整体成形的难题之一,高效准确预测和预防薄壁构件成形过程中起皱失稳缺陷具有十分重要的意义.以楔形件拉伸试验作为研究对象,通过建立楔形件数值失稳起皱模型,结合分岔原理,提出一种板壳起皱极限图(Wrinkling limit diagram,WLD)数值求解及绘制方法.针对在金属板壳试件成形过程中应力加载路径对起皱失稳的显著影响,通过分析楔形件不同区域的应力加载路径,探究板壳变形时应力加载路径对临界起皱极限的影响.结果表明:对于存在彼此独立、不同受力状态起皱区域的试件,应对不同区域分别建立WLD.不同区域单元应力加载路径对WLD的影响可归纳为临界皱屈主应力比对主应变比的影响:当临界皱屈单元的主应力比越大时,临界皱屈主应变比就越大,即WLD斜率越大,工艺抗皱性越弱;同时,通过比较楔形件不同起皱失稳区域的承载变化过程可知,起皱单元的临界主压应力增加速率会影响板料的临界起皱失稳时刻.     

薄壁矩形管弯曲成形起皱变形的研究综述

失稳起皱是薄壁矩形管弯曲成形过程中的主要缺陷之一,严重制约着薄壁矩形管的弯曲成形质量和极限的提高。总结国内外学者和工程技术人员对薄壁矩形管起皱变形的研究现状,着重在实验研究、理论分析和有限元数值模拟三个方面进行讨论,介绍对现有弯曲工艺和模具结构做的研究和改进,最后提出薄壁矩形管弯曲成形起皱变形研究的发展趋势。     

铝合金件超塑成形的数值模拟

近年来,数值模拟技术已广泛应用在金属成形工艺的设计和分析中,其中一个重要的功能是预测并预防所设计的工艺产生成形缺陷。在锻造成形中,常见的缺陷有开裂、模具未填充满、折叠和过烧等;在冲压成形中,常见的缺陷有拉裂、起皱、失稳及回弹等。也就是说,当模拟结果表明成形工艺存在缺陷时,可及时找出缺陷产生的原因,并提出防止缺陷产生的工艺措施,然后直接在计算机上修改工艺参数,再次进行数值模拟,直至达到工艺     

柔性压边和刚性压边技术在薄板类件多点成形中的对比分析

对比分析刚性压边和柔性压边两种不同工艺,探讨不同的成形工艺条件对起皱模拟、破裂以及成形极限的影响.用显式动力学商用软件LS-DYNA对两种厚度的马鞍型曲面件进行数值模拟.模拟结果表明,在刚性压边成形方式下,变形量较大时成形件起皱,压边力较大时成形件破裂;但在相同条件下,用柔性压边成形工艺的成形件结果良好,而且减小压边面的宽度到原来的1/2时,通过增加压边力的方法,还可获得良好的成形效果.柔性压边工艺可大大提高板料的利用率,改善板料的成形极限,减少各种不良缺陷发生.     

不同初始结构环壳液压成形理论分析与数值模拟

针对用环壳液压胀形工艺制作弯头过程中存在的失稳起皱问题,从理论上分析了环壳的初始形状对其成形的影响,并用动态有限元分析软件DYNAFORM对其成形进行了数值模拟,模拟结果与实验结果基本一致,同时对起皱原因进行了分析,研究结果表明,通过选择合适的胀前壳体结构,可以有效地改善胀形过程中壳体存在的失稳起皱问题。     

连续多点成形过程中起皱缺陷的有限元分析

连续多点成形中的皱曲是一重要的成形缺陷.阐述连续多点成形技术的原理,建立球形件的有限元模型,并采用数值模拟的方法研究连续多点成形中皱曲的情况.得到0.5 mm厚08Al钢板在成形过程中起皱缺陷的模拟结果.结果表明:皱曲主要产生在板材的两侧区域和两端区域.利用板材不同区域壳单元中面的主应变状态分析不同区域不起皱和起皱的原因.得到0.5 mm、1.0 mm、1.5 mm和2.0 mm厚08Al钢板在不同压下量下的端部起皱模拟结果.结果表明:板厚相同,压下量越大,起皱越严重;压下量相同,板厚越大,越不易起皱.分析端部皱曲的变化规律并做出合理解释,做了相关的试验验证,试验结果与模拟结果基本吻合.     

管材液压成形中的数值模拟方法

根据动力显式增量有限元方法和BT壳单元提出了液压胀形模拟算法.该算法充分考虑了管材胀形时的液体压力加载曲线、管材外壁与模具的润滑情况、轴向进给、径向进给等实际工艺条件对管材成形性的影响,能够预测零件成形过程中的开裂、起皱等缺陷,从而优化成形工艺参数,提高零件的成形工艺性.算法具有通用性,能够模拟轴线为直线、曲线以及带径向侧冲头的各种复杂管材的成形.将算法集成到FASTAMP软件中,通过模拟三台阶轴的液压胀形,并与实验数据比较,验证了算法的准确性与实用性.     

薄板液压成形起皱预测及控制研究进展

板材拉深成形广泛应用于航空航天、汽车制造、船舶工业等领域,是金属塑性加工领域的研究热点。随着工业生产向整体化、轻量化、高精度、低成本不断发展,以火箭燃料贮箱箱底为代表的大型曲面封头厚径比小于0.3%,起皱成为制约其拉深成形的主要缺陷之一,严重影响零件质量、模具使用寿命和工业生产的稳定性。综述板材拉深成形起皱理论预测、数值模拟和工艺试验的最新研究现状,重点介绍板材液压成形技术对于起皱控制的研究进展,表明通过合适的液压成形可以成形出无起皱缺陷、厚径比较小的零件。提出现有问题并对未来研究方向进行展望。     

薄壁件塑性成形失稳起皱的国内外研究进展

薄壁类零件由于其塑性成形容易达到对产品精密化、轻量化、强韧化及柔性化的要求 ,广泛应用于当代航天、航空、汽车、化工和其他高技术产业中 ,成为先进塑性加工技术面向 2 1世纪研究与发展的一个重要方向。受压失稳起皱则是影响薄壁件成形过程稳定性的最主要缺陷和障碍之一 ,决定着此类产品成形潜力大小 ,并严重影响着零件的成形质量、精度、模具的寿命及后续工序的完成。因此 ,研究各种不同变形状态下的失稳起皱机理和判断准则 ,准确有效预报和控制成形中失稳现象的产生 ,在薄壁类零件的塑性加工领域就显得极为关键。本文从试验研究、理论分析和数值模拟等 3个方面 ,介绍了国内外对薄壁件特别是管材成形中失稳起皱现象所作的工作和新进展 ,提出了该领域中亟待解决的瓶颈问题     

Numerical investigation of multi-point forming process for sheet metal: wrinkling, dimpling and springback

Multi-point forming (MPF) is a new flexible technique for manufacturing three-dimensional sheet metal parts. In this procedure, a pair of opposed matrices of punch elements substitute for the conventional fixed shape die sets, and the sheet metal can be formed rapidly between the matrices. Extensive numerical simulations of the processes for forming spherical and saddle-shaped parts were carried out by dynamic explicit finite element analysis. The contacting process between sheet metal and punch elements in MPF was investigated, and the variations of forming force with respect to the tool travel were analyzed. The wrinkling processes were simulated, and the MPF limit curves without wrinkles for spherical and saddle-shaped parts were obtained. Dimple is a particular defect in MPF, through the comparison of the thickness strains calculated by solid FE and shell FE, the finite elements appropriate for the numerical analysis of dimpling were detected, and the limit forming force without dimples was determined. Springback processes of MPF were simulated based on explicit-implicit algorithm. The springbacks and their distributions under different conditions were investigated.     

Substepping algorithms with stress correction for the simulation of sheet metal forming process

The finite element analysis of the sheet metal forming process involves various nonlinearities. To predict accurately the final geometry of the sheet blank and the distribution of strain and stress and control various forming defects, such as thinning, wrinkling and springback, etc., the accurate integration of the constitutive laws over the strain path is essential. Our objective in this paper is to develop an effective and accurate stress integration scheme for the analysis of three-dimensional sheet metal forming problems. The proposed algorithm is based on the explicit “substepping” schemes incorporating with the stress correction scheme. The proposed algorithms have been implemented into ABAQUS/Explicit via User Material Subroutine (VUMAT) interface platform. The algorithms are then employed to analyze a typical deep-cup drawing process and the accuracy of these algorithms has been compared with the implicit “return” algorithm and explicit forward algorithm. The results indicate that the explicit schemes with local truncation error control, together with a subsequent check of the consistency conditions, can achieve the same or even better level of accuracy as “return” algorithm does for integrating large plastic problems like sheet metal forming process.     

Formability analysis on the process of multi-point forming for titanium alloy retiary sheet

The multi-point forming (MPF) process of spherical surface parts of titanium alloy retiary sheet and titanium alloy sheet metal with different thickness and curvature radius was simulated by an explicit finite element software. Contradistinctive analysis between retiary sheet and sheet metal forming parts with different modes were done. The simulation results show that under the same forming conditions, titanium alloy retiary sheet is not easy to wrinkle and springback, whereas it is easy to form. The reason for differences in the formability of above-mentioned sheet metal is also analyzed. A non-wrinkling limited graph and a fracture critical graph for spherical surface parts of retiary metal sheet and metal sheet were obtained. Finally a forming test of titanium alloy cranial prosthesis was done in MPF press. Testing results indicate the customized 3D curved surface of prosthesis can be adequately shaped and the forming quality was guaranteed.     

Experimental and numerical study of stamp hydroforming of sheet metals

The objectives of this research was to experimentally and numerically study the stamp hydroforming process as a means for shaping aluminum alloy sheets. In stamp hydroforming, one or both surfaces of the sheet metal are supported with a pressurized viscous fluid to assist with the stamping of the part thereby eliminating the need for a female die. The pressurized fluid serves several purposes: (1) supports the sheet metal from the start to the end of the forming process, thus yielding a better formed part, (2) delays the onset of material failure and (3) reduces wrinkle formation. This paper focuses on the experimental and numerical results of the stamp hydroforming process utilizing a fluid pressure applied to one surface of the sheet metal. The effects of applying a constant, varying and localized pressure to the surface of 3003-H14-aluminum sheet alloy were evaluated. Experiments demonstrated draw depths improvements up to 31% before the material failed. A failure prediction analysis by Hsu was also carried out to predict an optimal fluid pressure path for the varying fluid pressure case. The commercial finite element analysis code Ls-Dyna3D was used to numerically simulate the stamp hydroforming process. Both isotropic and anisotropic material models were used and their predictions compared against the experimental results. The numerical simulations utilizing Barlat's anisotropic yield function accurately predicted the location of the material failure and the wrinkling characteristics of the aluminum sheet.     

薄壁深抛物线形零件固体颗粒介质成形工艺

薄壁抛物线形壳体成形过程为拉深和胀形两种变形模式的复合,极易发生起皱和破裂。固体颗粒介质成形是采用固体颗粒代替刚性凸模或凹模(或弹性体、液体)对板料进行成形的工艺。板材在颗粒介质内压的作用下成形,可以有效防止抛物线形件拉深成形过程中侧壁的起皱;由于颗粒内压是非均匀分布的,故可以有效控制抛物线形件成形过程中的破裂,提高板材的成形极限。根据固体颗粒介质成形工艺的特点,提出了两次成形薄壁深壳体零件的工艺,建立了数值分析模型,通过数值模拟和试验对该成形过程和工艺参数进行了分析。结果表明,采用固体颗粒介质成形工艺过程简单、成形工件壁厚分布均匀、表面质量好、回弹小。     

计算机仿真中板料冲压成形压边力的优化

通过运用板料拉伸、压缩失稳理论和有限元计算方法，提出了新的起皱临界应力计算方法，对板料冲压成形中各处的稳定性进行数值化描述，从而确定冲压工艺的合理性，并优化压迫力，有效地防止拉裂和起皱的发生，解决了工程分析中的一个难题。通过实例的计算对比，表明该方法是正确有效的。     

On the prediction of side-wall wrinkling in sheet metal forming processes

Prediction and prevention of side-wall wrinkling are extremely important in the design of tooling and process parameters in sheet metal forming processes. The prediction methods can be broadly divided into two categories: an analytical approach and a numerical simulation using finite element method (FEM). In this paper, a modified energy approach utilizing energy equality and the effective dimensions of the region undergoing circumferential compression is proposed based on simplified flat or curved sheet models with approximate boundary conditions. The analytical model calculates the critical buckling stress as a function of material properties, geometry parameters and current in-plane stress ratio. Meanwhile, the sensitivities of various input parameters and integration methods of FEM models on the prediction of wrinkling phenomena are investigated. To validate our proposed method and to illustrate the sensitivity issue in the FEM simulation, comparisons with experimental results of the Yoshida buckling test, aluminum square cup forming and aluminum conical cup forming are presented. The results demonstrate excellent agreements between the proposed method and experiments. Our model provides a reliable and effective predictor for the onset of side-wall wrinkling in sheet metal forming processes.     

The effect of nanoparticulate loading on the fabrication and characterization of multi-doped Zn-Al2O3-Cr2O3 hybrid coatings on mild steel

In this paper, an improved approach is proposed to determine the optimal profiles of two controllable process parameters (hydraulic pressure and blank holder force), which improve the forming condition and/or make better use of forming limits in hydromechanical deep drawing (HMD) process. A method based on adaptive finite element analysis coupled with fuzzy control algorithm (aFEA-FCA) was developed using LS-DYNA to determine the optimal loading profiles and thus to maximize the limiting drawing ratio (LDR). Maximum thickness reduction, maximum wrinkle height in the flange region of the sheet metal blank, and position of the nodes in the unsupported portion of the sheet metal blank between punch and die were used as criteria in the fuzzy control algorithm. Different rule-based matrices were compared by considering the maximum thinning occurred in the sheet metal blank, and thus, the most accurate matrices were determined for the control algorithm. The optimal loading profiles could be determined in a single FEA, thus reducing the computation time. The proposed approach enables determining the optimal loading profiles and also could be applied to complex parts easily. In addition, effects of initial blank diameter and coefficient of friction between the sheet-blank holder and sheet-die on the optimal loading profiles were investigated. An attainable LDR of 2.75 for AA 5754-O sheet material in hydromechanical deep drawing process was proven experimentally using the optimal loading profiles determined by adaptive FEA.