Numerical and experimental analysis of wrinkling during the cup drawing of an AA5042 aluminium alloy

The recent trend to reduce the thickness of metallic sheets used in forming processes strongly increases the likelihood of the occurrence of wrinkling. Thus, in order to obtain defect-free components, the prediction of this kind of defect becomes extremely important in the tool design and selection of process parameters. In this study, the sheet metal forming process proposed as a benchmark in the Numisheet 2014 conference is selected to analyse the influence of the tool geometry on wrinkling behaviour, as well as the reliability of the developed numerical model. The side-wall wrinkling during the deep drawing process of a cylindrical cup in AA5042 aluminium alloy is investigated through finite element simulation and experimental measurements. The material plastic anisotropy is modelled with an advanced yield criterion beyond the isotropic (von Mises) material behaviour. The results show that the shape of the wrinkles predicted by the numerical model is strongly affected by the finite element mesh used in the blank discretization. The accurate modelling of the plastic anisotropy of the aluminium alloy yields numerical results that are in good agreement with the experiments, particularly the shape and location of the wrinkles. The predicted punch force evolution is strongly influenced by the friction coefficient used in the model. Moreover, the two punch geometries provide drawn cups with different wrinkle waves, mainly differing in amplitude.     

铝合金盒形件充液成形法兰变形特性及其失稳影响分析

通过有限元模拟和实验对铝合金盒形件充液成形过程进行研究,分析了铝合金盒形件充液成形法兰直边区和圆角区的变形特性,以及法兰变形对起皱和破裂等失稳形式的影响.结果表明:法兰圆角区变形复杂,存在径向压应力,根据变形特征可分为五个变形区;长直边法兰抗皱能力比短直边法兰小,容易起皱;短直边侧壁变薄量大,在与凸模圆角相切处的板料容易破裂;合理的压边间隙可以有效控制长直边法兰起皱和短直边侧壁破裂,实验结果证明了数值模拟结果的准确性.     

Investigation of Springback of Metallic Sheets at Small Strains

Abstract:  The bending process of an aluminium alloy and a high‐strength steel is analysed using the cylindrical bending test of 1 (Proceedings of the 5th International conference and workshop on numerical simulation of 3D sheet forming processes, 2002 , Jeju Island, South Korea). Despite its simplicity, it is now well known that this test is difficult to reproduce numerically. Indeed, it involves small plastic strains but large springback and exhibits complex contact boundary conditions providing severe benchmark characteristics. In order to obtain reliable results to be used for the validation of finite element models or simulations, particular attention has been paid to the fine measurement of several experimental parameters using a high‐resolution video camera. Several geometrical and contact parameters, as well as the springback angle, are determined. The springback results are compared with analytical results obtained using a classical bending model. It is shown that the agreement is good if the work‐hardening is identified within a small strain range, corresponding to the one covered during the test, as it mainly involves small deformations, pure bending and a weak anticlastic effect. Moreover, the decrease in the apparent modulus as a function of plastic strain leads to a more accurate measurement of the variation in the springback angle.     

拉深比对304不锈钢圆筒件残余应力的影响

304不锈钢圆筒拉深件在其口部易发生纵向开裂,外壁残余拉应力过大是造成其纵向开裂的根本原因之一.本文结合有限元法分析不同拉深比所得圆筒拉深件的残余应力,用纳米压痕试验研究了拉深比对304不锈钢圆筒拉深件筒壁残余应力的影响.结果表明:304不锈钢圆筒拉深件外壁的残余拉应力从筒底到口部先增大后减小,最大残余应力出现在筒壁中部约60%筒壁高度处;纳米压痕测得拉深比为1.43、1.54、1.67和1.82拉深圆筒件筒壁的最大残余应力分别为391.87、745.30、793.74和1 013.1 MPa;最大残余应力随拉深比的增大而增大.与其他文献对比分析,此研究结果是正确可靠的.     

DP800 双相高强钢折弯及回弹研究

目的针对高强度钢板成形过程中的回弹问题,研究工艺参数对回弹的影响规律,优选工艺参数组合,以获得回弹较小的V形件。方法采用dynaform软件对V形件进行成形及回弹的数值模拟,以摩擦因数、模具间隙、冲压速度、凹模圆角半径等工艺参数为自变量,以回弹前后水平距离差最大值为因变量,设计了四因素三水平的正交试验方案,研究多个工艺参数对回弹影响的规律。结果实验结果表明,V形件回弹值大小随着摩擦因数的增大呈现减小趋势;随着模具间隙、凹模圆角半径的增大,回弹值呈现增大的趋势;而冲压速度对V形件回弹的影响较小,且工艺参数影响V形件回弹大小的主次顺序为模具间隙、摩擦因数、凹模圆角半径、冲压速度。结论优选工艺参数组合为:摩擦因数为0.2、模具间隙为2.6 mm、冲压速度为1200 mm/s、凹模圆角半径为12 mm,此时回弹水平距离差最大值为0.566 mm,最大减薄率为1.40%;实际生产可以忽略冲压速度对回弹的影响,仿真结果对实际生产具有指导意义。     

树脂复合减振钢板U型弯曲回弹实验与数值模拟研究

通过带法兰边的U型弯曲成形实验研究,考察了树脂复合减振钢板在不同压边力下的回弹特性.实验结果表明:压边力对树脂复合减振钢板回弹特性影响显著.较大的压边力有利于减小回弹缺陷.其次,考虑树脂层的粘弹性特性,采用非线性粘弹性模型来描述树脂层的力学变形行为,并采用Cohesive单元和固体壳单元分别对树脂层和表层钢板进行离散,进行了树脂复合减振钢板在不同压边力下的U型弯曲有限元数值模拟研究.和实验结果比较表明,所建立的有限元模型能够较好的模拟U型弯曲成形过程.最后,基于建立的有限元模型,考查了成形速度,树脂层厚度和表层钢板初始屈服应力对回弹的影响.参数分析结果表明:这三个参数对回弹角的影响显著.该研究对树脂复合减振钢板冲压工艺设计具有一定的指导意义.     

Application of upper bound method to the powder‐forging of cup‐shaped axisymmetric preforms for estimating punch load

This paper is concerned with the analytic derivation of the punch load required to forge cup‐shaped axisymmetric porous preforms. Splitting the two‐dimensional preform domain into three rectangular regions, we assumed the kinematically admissible velocity fields to apply the upper bound method. According to the resulting formulas, we developed an incremental numerical algorithm for computing time‐discretized step‐wise punch loads and preform volumes. In order to illustrate the validity of the proposed estimate, we compared the numerical results obtained by the present analysis with those by the finite element simulation. From the comparison made through different preform dimensions, we observed that the present analysis expects the reliable upper bounds of process step‐wise punch loads. Copyright © 2001 John Wiley & Sons, Ltd.     

柱面扁壳类覆盖件刚度的数值模拟研究

为更好地预测和评定板材覆盖件的刚度,进一步分析研究各影响因素对刚度的影响规律,以能代表汽车覆盖件曲面特点的柱面扁壳零件为研究对象,建立了柱面扁壳模拟件胀拉刚度的仿真分析模型.对柱面扁壳类覆盖件的成形、回弹以及刚度变形3个过程进行数值模拟研究,实验验证了本仿真系统的有效性和实用性,分析了压边力、回弹及约束形式对覆盖件刚度的影响规律.研究表明:增大压边力及回弹的减小都可以提高覆盖件的刚度;成形过程中,约束形式对刚度有重要影响,约束越大,刚度越大.     

Assessment of nonlinear exact geometry sampling surfaces solid-shell elements and ANSYS solid elements for 3D stress analysis of piezoelectric shell structures

In this work, the finite rotation exact geometry four-node solid-shell element using the sampling surfaces (SaS) method is developed for the analysis of the second Piola-Kirchhoff stresses in laminated piezoelectric shells. The SaS method is based on choosing inside the layers the arbitrary number of SaS parallel to the middle surface and located at Chebyshev polynomial nodes in order to introduce the displacements and electric potentials of these surfaces as fundamental shell unknowns. The outer surfaces and interfaces are also included into a set of SaS. To circumvent shear and membrane locking, the hybrid-mixed solid-shell element on the basis of the Hu-Washizu variational principle is proposed. The tangent stiffness matrix is evaluated by 3D analytical integration throughout the finite element. This novelty provides a superior performance in the case of coarse meshes. A comparison with the SOLID226 element showed that the developed exact geometry SaS solid-shell element allows the use of load increments, which are much larger than possible with existing displacement-based finite elements. Thus, it can be recommended for the 3D stress analysis of doubly-curved laminated piezoelectric shells because the SaS formulation gives the opportunity to obtain the 3D solutions of electroelasticity with a prescribed accuracy.     

筒形件毛坯热冲压成形过程数值模拟

针对某厚壁深孔筒形件,借助三维有限元模拟软件Deform-3D,对其成形的压型、冲盂和拔伸等3个工步进行模拟,分析得到各工步变形特点。结果表明,压型即将结束时载荷急剧升高,坯料底部转角处和凸台未完全充满,未充满部位在冲盂工步即将结束时得以充满;冲盂时凸模端部以下一部分锥形金属由于受摩擦作用不参与变形,随凸模刚性下移;拔...     

Modeling of hybrid vibration control for multilayer structures using solid-shell finite elements

A self-control method of vibrations is presented in this paper. This method combines the passive damping capabilities afforded by viscoelastic materials with the active control properties associated with piezoelectric materials. Active control is introduced, using the piezoelectric properties, in order to improve the reduction in vibration amplitudes that can be obtained by viscoelastic passive damping alone. To this end, a filter has been mounted between the sensors and the actuators. The resulting nonlinear problem is discretized using the recently developed solid-shell finite element SHB20E, due to the advantages it offers in terms of accuracy and efficiency as compared to standard finite elements with the same geometry and kinematics. In order to solve the discretized problem, a resolution method using DIAMANT approach is developed. A set of selective and representative numerical tests are performed on multilayer plates to demonstrate the interest of the proposed damping model.     

Springback reduction with control of punch speed and blank holder force via sequential approximate optimization with radial basis function network

This paper proposes a springback reduction technique with the control of punch speed and blank holder force (BHF) via sequential approximate optimization (SAO). Springback is one of the major defects in sheet forming and its reduction is a crucial issue for improving product quality. Computer-aided-engineering is one of the helpful tools for predicting springback and widely used in automotive industries. Various approaches are considered for springback reduction, and we optimize the punch speed as well as BHF (variable BHF). Sheet forming simulation is generally costly and time-consuming, and the SAO with the radial basis function network is used to determine the optimal punch speed and variable BHF. The U-shaped forming in NUMISHEET’93 is used in the numerical simulation. The standard deviation of the bending moment is minimized subject tearing evaluated with the forming limit diagram. The punch speed and the variable BHF are taken as the design variables. The validity is examined through numerical simulation.     

板料V形弯曲回弹的动力烛式有限元分析

板料成形后的回弹对精度影响较大，在数值模拟时对回弹进行精确预测显得非常重要。基于连续介质力学及有限变形理论，建立了适合于三给板料成形分析的显式算法的有限元数学模型，采取集中质量矩阵，用动力显式积分的方法，使位移计算显式化，避免了由材料、几何、边界条件等高度非线性因素引起的计算收敛问题。根据该模型开发了动力显式算法的板料成形过程模拟的有限元分析程序DESSFORM3D，应用该软件模拟了包括回弹在内的整个板料V形弯曲的成形过程。通过3个不同凸模行程时计算与实验的板料几何形状对比以及计算结果与实验结果对比，验证了软件计算结果的准确性。     

Adaptive through‐thickness integration for accurate springback prediction

Accurate numerical prediction of springback in sheet metal forming is essential for the automotive industry. Numerous factors influence the accuracy of prediction of this complex phenomenon by using the finite element method. One of them is the numerical integration through the thickness of shell elements. It is known that the traditional numerical schemes are very inefficient in elastic–plastic analysis and even for simple problems they require up to 50 integration points for an accurate springback prediction. An adaptive through‐thickness integration strategy can be a good alternative. The main characteristic feature of the strategy is that it defines abscissas and weights depending on the integrand's properties and, thus, can adapt itself to improve the accuracy of integration. A concept of an adaptive through‐thickness integration strategy for shell elements is presented in this paper. Its potential is demonstrated using two examples. Calculations of a simple test—bending a beam under tension—show that for a similar set of material and process parameters the adaptive rule with seven integration points performs significantly better than the traditional trapezoidal rule with 50 points. Simulations of an unconstrained cylindrical bending problem demonstrate that the adaptive through‐thickness integration strategy for shell elements can guarantee an accurate springback prediction at minimal costs. Copyright © 2007 John Wiley & Sons, Ltd.     

Coupled finite element simulation and optimization of single- and multi-stage sheet-forming processes

This article presents the development and application of a coupled finite element simulation and optimization framework that can be used for design and analysis of sheet-forming processes of varying complexity. The entire forming process from blank gripping and deep drawing to tool release and springback is modelled. The dies, holders, punch and workpiece are modelled with friction, temperature, holder force and punch speed controlled in the process simulation. Both single- and multi-stage sheet-forming processes are investigated. Process simulation is coupled with a nonlinear gradient-based optimization approach for optimizing single or multiple design objectives with imposed sheet-forming response constraints. A MATLAB program is developed and used for data-flow management between process simulation and optimization codes. Thinning, springback, damage and forming limit diagram are used to define failure in the forming process design optimization. Design sensitivity analysis and optimization results of the example problems are presented and discussed.     

Numerical validation of double punch test for the assessment of tensile strength in natural fiber reinforced concrete

In this paper, the results of Finite Element Analysis on concrete specimens reinforced by plant fiber and undergone to double punch tests are presented. In finite element analysis, the constitutive law of the material was calibrated by referring to tensile strength obtained by the Chen formulation and fractural energy derived by the double punch test. The analysis was carried out accounting different volume content of natural fibers derived from a very common plant of Sicily (Agave), to ensure the repeatability of the test. Then, the failure load can be determined by finite element analysis and compared with the experimental one. A good accordance between numerical and experimental data permits us to support the use of the double punch test as a more attractive testing method for the evaluation of the mechanical proprieties of the materials. Double punch test, in fact, is much simpler to perform than the split cylinder test method.     

Enhanced finite element formulations on the numerical simulation of tailor-welded hydroformed products

This work aims to assess the influence of different finite element formulations in the performance and quality of solution obtained by numerical simulation in the analysis of tailor-welded hydroformed tubular parts. Tube hydroforming represents a cost effective forming process for high-strength, low weight products on, as an example, automotive and airspace applications. On the other side, the use of tailor-welding in order to obtain custom-made combinations of thicknesses and materials - leading to a wide variety of user-defined products - can be introduced into conventional tubular hydroforming processes in order to further improve the applicability range of the later process. The main goal of the present work is to describe the state-of-the-art in the field, focusing on distinct finite element formulations and providing guidelines for the simulation of tubular hydroforming process combined with tailor-welded joining techniques. Hexahedral solid and solid-shell enhanced assumed strain elements, either with reduced and full numerical integration procedures, are analyzed in order to infer about the potentialities of the combined forming technology. Material characterization of the heat affected zone is included and the influence of finite element modeling on defects onset and prediction during forming is considered.     

板料弯曲回弹影响因素的有限元模拟研究

通过静态力学、动态力学实验方法,研究了热致性液晶聚合物(LCP)的种类对环氧树脂共混物在不同温度下的拉伸强度和应力-应变曲线的影响,通过TEM观察了共混物的相形态结构.结果表明,反应型液晶聚合物(LCPU)比其它种类的液晶聚合物对环氧树脂的改性效果更好;在不同温度下,其拉伸强度和应力～应变行为均比其它材料优越;固化物的动态力学结果表明:反应型的液晶聚合物键入了固化网络,出现新的松弛,TEM结果表明,反应型的液晶聚合物在基体材料中形成大小在nm数量级的液晶聚集微区,没有反应基团的液晶聚合物PHBHT在10%的加入量下,与环氧的共混物结构也有液晶聚集微区产生,但是聚集区大小在微米量级.     

Solid-mechanics finite element simulations of the draping of fabrics: a sensitivity analysis

This paper covers numerical investigations of the draping of woven fabrics into a “hat” shape, combining a hemispherical cup with a wide flat rim. A mechanical approach is adopted using finite element analysis (FEA) methodology. In this, the fabric is considered as a solid sheet with mechanical properties and friction properties. In this study, a linear elastic anisotropic material model describes the deformation of fabrics. An explicit dynamic finite element analysis is applied and systematic parametric numerical studies are presented, which incorporate investigations of the effects of numerical parameters, material properties and processing conditions on the draping of fabrics. More specifically, the effects of the following variables and parameters are included: number of elements, number of time increments in the dynamic FEA analysis, punch speed, shear and tensile moduli of fabric, coefficient of friction for all interfaces and level of load on the fabric holder.