Numerical and experimental study of the multi-axial quasi-static strength of clinched connections

Conventionally, the mechanical strength of clinched connections is determined by a single shear lap test and/or a pull-out test. However, in most practical applications a combination of shear and pull-out is exerted on the clinched joint. This paper deals with the development of an Arcan-like device which enables to introduce various shear/tensile ratios in a clinched assembly. An experimental survey of the multi-axial behaviour of a non-cutting single-stroke round clinched connection of two DC05 sheets, which is mild deep drawing steel, is conducted with this modified Arcan setup. These experimental results are used to check the validity of numerical models that predict the strength under multi-axial loading. Since the forming of a clinch is a fairly complex sheet metal operation, a good knowledge of the plastic material properties and the frictional behaviour is of the utmost importance to perform a sufficiently accurate FEA simulation. The impact of these factors on the multi-axial loading behaviour of the DC05–DC05 connection is investigated.     

三层板压印接头力学性能分析

本文的研究目的是分析三层板不同搭接方式的压印接头的力学性能,主要采用试验方法对其进行研究,具体方法如下:首先对1.5 mm AA5052+1.5 mm SPCC+1.5 mm AA5052三层板不同配置方式下的压印接头进行静态力学性能测试,分析三层板压印接头的静态失效模式及其静强度;再对静态力学性能最优组进行动态疲劳性能测试,分析疲劳失效模式及其疲劳强度,并采用最小二乘法拟合获得接头力-寿命(F-N)方程.试验结果表明:静态失效为上板颈部断裂、中板内锁拉脱和混合失效三种模式;T-A型接头静强度最低,T-C型接头静强度最高.其中,T-C型接头疲劳失效模式为上板颈部先断裂,之后下板出现裂纹导致接头失效;T-C型接头上板颈部疲劳性能较差,当应力水平为静强度40%时,疲劳循环最多只能达到15 965次,但是中板与下板连接可达到200万次.     

Design of mechanical clinching tools for joining of aluminium alloy sheets

The purpose of this study is to propose the design method of mechanical clinching tools, which is used to join aluminium alloy sheets in automobiles. Based on the analytical model used to predict the strength of the mechanical clinched joint, the design method of clinching tools that can satisfy the required joint strength is proposed. The analytical model has been defined as a function of the neck-thickness and the undercut, which are the presented mechanical clinched joint geometry. And joint geometry is defined by the shape parameters of the clinching tools. To obtain the desired joint strength, the required neck-thickness and undercut are inversely calculated from the analytical model. The mechanical clinching tools that join Al6063 alloy sheets are designed by the proposed method. FE-analysis and mechanical clinching test were performed for optimal conditions obtained from the proposed design method. The result of the FE-analysis and that of the mechanical clinching test showed good agreement of the clinched joint geometry. The H-type tensile test for mechanical clinched joint was carried out to evaluate the strength of the mechanical clinched joint designed by the proposed design method. The result of the tensile test showed that the fracture load satisfied the required joint strength. Also, the top-hat impact test was performed to evaluate the feasibility of using mechanical clinching for automotive component assembly. The result of impact test showed that mechanical clinching was able to produce similar crash resistance as self-piercing riveting.     

Rotation friction pressing riveting of AZ31 magnesium alloy sheet

A kind of joining method for magnesium alloys, rotation friction pressing riveting (RFPR), is proposed in this paper. In RFPR operation, a rivet with a plug rotating at high speed is brought to contact with the riveted sheets, generating frictional heat between the rivet and riveted sheets, which softens the sheet materials and enables the rivet to be drilled into the sheets under reduced force. When fully inserted, the rivet is stopped rotating, and the plug is immediately pressed into the shank of the rivet by a punch. The expansive deformation of the rivet shank occurs under the action of the plug, thereby forming a mechanical interlock between the rivet and the sheets to fasten the sheets together. The studies show that RFPR of AZ31 magnesium alloy sheet can be carried out at ambient temperature, and provides the joints with superior shear strength and fatigue property when compared with self-piercing riveting (SPR). The effects of the operating parameters of RFPR process on the quality of the joints were investigated in the study. The results shows that while the rivet rotation speed little affects the shear strength of RFPR joints, the punch pressure has a significant influence on the mechanical properties of the RFPR joints. A numerical analysis was also performed to understand the effect of the punch pressure on the interlock between the rivet and the sheets, and the stress and strain distribution inside the sheet materials around the rivet. The results show that the interlock increased with the punch pressure and there is residual compressive stress inside the sheet materials, which seems to explain the good fatigue property of RFPR joints observed.     

An experimental investigation of forming load and side-wall thickness obtained by a new deep drawing die

Deep drawing is an important process used for producing cups from sheet metal in large quantities. The deep drawing process is affected by many variables such as blank shape, punch and die radii, material’s formability characteristics, and many more. In order to obtain optimal process parameters with regard to part geometry, the blank and die geometry are particularly important factors. In this study, we investigated the effects of blank holder and die shapes, using six different blank holder and die shapes. We measured the distribution of blank holder force (BHF), the forming load at different drawing depths as well as thickness reduction of cup wall thickness for each set of die and punch geometries. The experimental study shows that the angle of blank and die surface influence the forming load and blank holder force distribution. Deep drawing dies with matrix and blank holder angle designed in this study provided deep drawing ratios that are about 25 % larger than those that usually can be obtained by a conventional die.     

OPTIMUM JOINING CONDITIONS IN A MECHANICAL PRESS JOINT

Mechanical press joining has been used in sheet metal work because it is a simple process and offers the possibility of joining dissimilar sheet metals, such as steel and aluminum alloy sheets. The mechanical press joint strength was found to vary with joining conditions, such as sheet thickness and punch diameter. The optimum joining conditions of the mechanical press joint under complex loading can be determined by correlating strength ratio with diameter ratio and sheet thickness ratio. The failure mode was considered during estimation of the joining strength. Under this experimental condition, the optimum strength ratio was acquired at a sheet thickness ratio of 1.0 and a diameter ratio of 1.683.     

压印连接接头的疲劳裂纹扩展分析及实验研究

研究了压印连接接头的强度性能,介绍了压印连接成形原理和工艺过程。分析了压印连接接头的表面裂纹的产生原因,通过有限元分析得出最大应力应变的位置并以实验加以验证。采用ANSYS/LS-DYNA有限元软件对压印连接接头疲劳裂纹扩展进行数值模拟。分析了平均应力、应力比和加载频率对压印连接接头的疲劳裂纹扩展速率的影响。     

Experimental determination of spring back and thinning effect of aluminum sheet metal during L-bending operation

In automotive industry, significant efforts are being put forth to replace steel sheets with aluminum sheets for various applications. Besides its higher cost, there are several technical hurdles for wide usage of aluminum sheets in forming. Major problems in aluminum sheet metal forming operations are deformation errors and spring back effect. These problems are dependent on the number of parameters such as die and tool geometry, friction condition, loading condition and anisotropic properties of the metal.To predict the exact shape, the geometry based punch contact program must be used. The shape changes once the punch is withdrawn, because of the materials elasticity. Prediction of such a spring back effect is a major challenging problem in industry involving sheet metal forming operations. It also needs applying appropriate back tension during the forming complex shapes. Slight deformation of the metal leads to non-axisymmetric loading. One can predict the residual stress by determining plastic and elastic deformation. Thus appropriate spring back effect can be investigated.The present investigation was carried out to determine the spring back and thinning effect of aluminum sheet metal during L-bending operation. Number of specimens with thickness varying from 0.5 mm to 3.5 mm were prepared. The experiments were conducted for different clearances between punch and die. It is observed that, beyond a particular clearance for each thickness of the sheet metal, the spring back and thinning effects were linearly increasing. However, below the critical clearance, scratches on the surface of the sheet metal were seen due to wear. The scratches were analyzed through Scanning Electron micrographs. As the clearance between punch and die reduces further, more wear on the punching surface was observed. And, as the clearance increases it leads to increase the spring back effect and fracture propagation.     

浮动凹模主动径向加压的筒形件拉深研究

提出了一种浮动凹模主动径向加压的工艺方法,分析了凸凹模夹紧坯料在拉深过程中的力学特点,说明了该方法能提高板料成形能力的技术关键,以带法兰的筒形件拉深为研究对象,运用LS-DYNA对采用该工艺方法和常规拉深进行有限元模拟并比较,结果表明,该工艺方法的拉深效果是显著的。     

钛合金压印接头疲劳性能与微观分析

压印连接是近年来新兴的连接方式,因其具有简单高效、低耗环保等优点,在连接应用方面越来越受到重视,而疲劳破坏是机械零件失效的主要形式。对钛合金压印接头的疲劳性能进行了实验研究,与母材的力学性能进行了对比分析,并对疲劳失效断口进行了断口分析和能谱分析。实验结果显示钛合金压印接头的平均拉伸-剪切强度约为同等尺寸材料拉伸-剪切强度的36.7%;钛合金压印接头的疲劳极限约为材料疲劳极限的46%,平均载荷约为接头最大静强度的42%。微观特征显示断口呈脆性疲劳断裂特征,由于微动磨损和氧化作用产生了成分为氧化钛的微动磨屑,且其硬度较高,因此在微动过程中起到磨粒的作用,从而加速了磨损和裂纹扩展,最终导致疲劳失效。     

Finite element modelling and experimental investigation of single point incremental forming process of aluminum sheets: influence of process parameters on punch force monitoring and on mechanical and geometrical quality of parts

New trends in sheet metal forming are rapidly developing and several new forming processes have been proposed to accomplish the goals of flexibility and cost reduction. Among them, Incremental CNC sheet forming operations (ISF) are a relatively new sheet metal forming processes for small batch production and prototyping. In single point incremental forming (SPIF), the final shape of the component is obtained by the CNC relative movements of a simple and small punch which deform a clamped blank into the desired shape and which appear quite promising. No other dies are required than the ones used in any conventional sheet metal forming processes. As it is well known, the design of a mechanical component requires some decisions about the mechanical resistance and geometrical quality of the parts and the product has to be manufactured with a careful definition of the process set up. The use of computers in manufacturing has enabled the development of several new sheet metal forming processes, which are based upon older technologies. Although standard sheet metal forming processes are strongly controlled, new processes like single point incremental sheet forming can be improved. The SPIF concept allows to increase flexibility and to reduce set up costs. Such a process has a negative effect on the shape accuracy by initiating undesired rigid movement and sheet thinning. In the paper, the applicability of the numerical technique and the experimental test program to incremental forming of sheet metal is examined. Concerning the numerical simulation, a static implicit finite element code ABAQUS/Standard is used. These two techniques emphasize the necessity to control some process parameters to improve the final product quality. The reported approaches were mainly focused on the influence of four process parameters on the punch force trends generated in this forming process, the thickness and the equivalent plastic deformation distribution within the whole volume of the workpiece: the initial sheet thickness, the wall angle, the workpiece geometry and the nature of tool path contours controlled through CNC programming. The tool forces required to deform plastically the sheet around the contact area are discussed. The effect of the blank thickness and the tool path on the punch load and the deformation behaviour is also examined with respect to several tool paths. Furthermore, the force acting on the traveling tool is also evaluated. Similar to the sheet thickness, the effect of wall angle and part geometry on the load evolution, the distribution of calculated equivalent plastic strain and the variation of sheet thickness strain are also discussed. Experimental and numerical results obtained allow having a better knowledge of mechanical and geometrical responses from different parts manufactured by SPIF with the aim to improve their accuracy. It is also concluded that the numerical simulation might be exploited for optimization of the incremental forming process of sheet metal.     

Investigation of die radius arc profile on wear behaviour in sheet metal processing of advanced high strength steels

Advanced high strength steels (AHSS) are increasingly used in sheet metal stamping in the automotive industry. In comparison with conventional steels, AHSS stampings produce higher contact pressures at the interface between draw die and sheet metal blank, resulting in more severe wear conditions, particularly at the draw die radius. Developing the ability to accurately predict and reduce the potential tool wear during the tool design stage is vital for shortening lead times and reducing production cost. This paper investigates the effects of draw die geometry on the sheet metal tool wear distribution over the draw die radius using numerical and experimental methods. A numerical tool wear model is introduced and applied using the commercial software package Abaqus. Channel bend tests are carried out using an Erichsen sheet metal tester to verify the numerical model. Various geometries of radius arc profiles, including standard circular profiles, high elliptical profiles, and flat elliptical profiles, are numerically investigated, and the wear volume and contact pressure distribution along the radii are determined. The results show that the profile of the draw die radius has a significant effect on the wear distribution, and that a low contact pressure distribution can be achieved by using a combination of circular and high elliptical curved geometries.     

Mechanical behaviour of polymer–metal hybrid joints produced by clinching using different tools

In the present study, the mechanical behaviour of polymer–metal hybrid connections produced by clinching is investigated. Thin sheets were joined using different tools including grooved, split and flat dies as well as rectangular tools. The effect of the joining force on joinability was also analysed. Polycarbonate was used as the polymer partner because of its high strength and toughness, while aluminium alloy AA6082-T6, which is characterized by a high yield stress but low ductility, was used as the metal sheet. Mechanical characterization involved single lap shear tests and peeling tests. According to the achieved results, grooved dies are not suitable for joining polymers. Rectangular clinching tools required lower joining forces and produced the highest peeling performances; however, because of the low ductility of the aluminium alloy, the joints were partially damaged resulting in the weakest shear strength. Round clinching tools required higher joining force compared to rectangular ones. The joints produced by flat dies were characterized by higher shear strength; however, because of the small interlock produced, they were characterized by small values of peeling strength. Round split dies allowed producing the joints with the highest performances in shear and peeling tests.     

A numerical and experimental investigation of AZ31 formability at elevated temperatures using a constant strain rate test

The research activity reported in the present paper aims to evaluate the Forming Limit Curves (FLC) of the Mg alloy AZ31 in warm conditions (200 °C) while keeping the equivalent strain rate constant. Specific tools to carry out such a formability test were designed and created: a flat punch (in line with Marciniak’s test) embedding a heating system was adopted in order to heat the central part of the specimen both rapidly and uniformly, where ruptures were forced due to the presence of a driving sheet between the specimen and the punch. A Digital Image Correlation system was also embedded in the formability equipment in order to acquire major and minor strains continuously and evaluate the moment and location of failures. Finite Element simulations were run in order to define punch speed profiles (which differ according to the geometry of the specimen) that were able to keep a constant equivalent strain rate in the region where ruptures were forced. Experimental tests implementing the punch speed profiles were carried out in order to obtain temperature, load and strain data. FLCs at two different strain rate levels (0.02 s⁻¹ and 0.002 s⁻¹) both confirmed and allowed us to quantify the noticeable strain rate effect of such an alloy on the FLC at a temperature of 200 °C. The proposed approach for FLC evaluation is effective for materials whose properties are strongly influenced by the strain rate. Such FLC data can be usefully implemented in numerical simulations of sheet metal forming processes: while tensile tests can be used to determine variations in mechanical behaviour according to the strain rate, the FLCs evaluated in this work allow us to determine the occurrence of strain path-dependent critical conditions according to the strain rate.     

铝/钢薄板无铆连接接头成形过程及形貌控制

目的 研究铝/钢薄板无铆连接过程中接头形貌的影响因素。方法 采用有限元模拟方法分析了铝/钢薄板在平底模具下接头形貌的成形过程。结合变形区域的金属流动情况,解释了互锁结构的形成机理,并分析了不同工艺参数对接头形貌的影响。结果 接头互锁结构主要是依靠铝板填充钢板的凹陷部位而形成的,抑制钢板与模具接触一侧的金属流动有助于接头底部和侧壁的钢板拉薄,进而形成内部凹陷,促进互锁结构的形成。摩擦因数对接头形貌参数影响较大,增大摩擦因数可显著提高接头的互锁量。结论 冲头半径、冲头圆角、凹模深度以及摩擦因数对颈厚值和互锁量均有显著影响。通过控制这些影响因素,可以得到良好的接头。此外,接头的失效形式以铝板颈厚较薄处的剪断失效为主,因此对于采用铝上钢下的无铆连接,保证颈厚值相对于互锁量更加重要。     

Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement

This experimental work concerns the study of the preforming of a specific highly double curved geometry with a triple point (case corner) by the sheet forming process using powdered interlock reinforcement (G1151®). Three different punches (square box, prism, tetrahedron) were used in this study, each of them presenting highly double curved geometry with a case corner. A specific sheet forming device specially designed for the preforming of textile reinforcement was used. The expected shapes with the three punches have been obtained with an optimized blank-holder pressure. No classical defaults such as wrinkling or yarn damage are present in the useful zone of the preforms. However, a new default, not observed for spherical or hemispherical shape has been identified. It concerns the out of plane buckling of yarns. This phenomenon not observed on the square box is visible on some faces and edges of the prismatic and tetrahedron shapes. For the square box, it is easily possible to control the orientation of the yarn within the preform in the faces, whereas this is not possible for triangular faces of the prismatic and tetrahedron shapes. The square box punch is therefore more adapted to preform the highly doubled curved shape with the case corner.     

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.     

Formability improvement with independent die and punch temperature control

A combined experimental and numerical study of the effects of die and punch temperature on the formability of a modified AA3003 aluminum alloy sheet for a case study sample is presented. Here, the non-isothermal deep drawing of a cup-like feature in a thin gauge aluminum automotive component is considered. An experimental forming setup that incorporates both heated dies and a cooled punch has been developed. A parametric study of the effects of die temperature, punch temperature, and blank holder force on the formability of the part is conducted. Numerical simulations of the warm forming process are performed using a coupled thermo-mechanical finite element model. The temperature-dependant material model combines the Bergstrom hardening rule with Barlat’s YLD2000 yield function and was implemented in LS-DYNA as a user-defined material model. Selected experimental cases were modelled numerically and compared to experiments. The FEA model was validated against experimental results by comparing measured and predicted punch force versus displacement as well as trends in the formability as a function of die temperature.     

粘接剂对压印连接强度的影响及数理统计分析

作为替代连接技术,压印和粘接表现出独特的优势。以压印和压印-粘接复合接头为对象,采用数理统计分析检验试验数据的合理性,进而对比不同材料组合及连接方式来分析粘接剂对压印连接接头强度的影响。试验表明,压印连接的剪切强度比剥离强度高,异种材料组合、压印-粘接复合能获得更有效的连接结构;胶层厚度为0.1mm时获得较好效果。     

An expert system for process planning of sheet metal parts produced on compound die for use in stamping industries

Process planning of sheet metal part is an important activity in the design of compound die. Traditional methods of carrying out this task are manual, tedious, time-consuming, error-prone and experience-based. This paper describes the research work involved in the development of an expert system for process planning of sheet metal parts produced on compound die. The proposed system is organized in six modules. For development of system modules, domain knowledge acquired from various sources of knowledge acquisition is refined and then framed in form of ‘IF-Then’ variety of production rules. System modules are coded in AutoLISP language and user interface is created using visual basic (VB). The system is capable to automate various activities of process planning including blank modeling, blank nesting, determining punch force required, selection of clearance between punch and die, identifying sheet metal operations, and determining proper sequence of operations for manufacturing the part. The proposed system can be implemented on a PC having VB and AutoCAD software, therefore its low cost of implementation makes it affordable even for small scale sheet metal industries.