die clinching

ORIGINAL ARTICLE

Numerical and experimental investigations of extensibledie clinching

Xiaocong He & Fulong Liu & Baoying Xing &Huiyan Yang & Yuqi Wang & Fengshou Gu & Andrew Ball

Received: 19 December 2013 /Accepted: 16 June 2014# Springer-Verlag London 2014

Abstract With an increasing application of clinching in differ-ent industrial fields, the demand for knowledge of static anddynamic characteristics of clinching is increased. In the presentwork, the extensible die clinching process was numericallyinvestigated using finite element method. To validate the com-putational simulation of the extensible die clinching process,experimental tests on extensible die clinched specimens havebeen carried out. Good agreement is achieved between thepredictions and the experimental results. Monotonic tensile testswere carried out to measure the ultimate tensile strengths of theextensible die clinching joints and clinching-bonded hybridjoints. Deformation and failure of the extensible die clinchedjoints undermonotonic tensile loadingwere studied. The normalhypothesis tests were performed to examine the rationality of thetest data. This work was also aimed at evaluating experimentallyand comparing the strength and energy absorption of the exten-sible die clinched joints and clinching-bonded hybrid joints.

Keywords Extensible die clinching . Process simulation .

Finite element method . Load-bearing capacity . Energyabsorption

1 Introduction

Some relative new joining techniques have drawn more atten-tion in recent years because they can join advanced sheet

materials that are dissimilar, coated and hard to weld withconventional spot welding [15].

Many efforts have also been spent to develop hybrid join-ing techniques and alternatives for application into light-weight structures. Mucha et als paper [6] presented thepressed joint technology using forming process with or with-out additional fastener. The capabilities for increasing theload-carrying ability of mechanical joints by applying specialrivets and dies were presented. The joint forming was per-formed with the solid round die and rectangular split die forriveted joint forming. The effect of joint forming process onjointed material strain was compared by measuring the micro-hardness of the joints. Mucha andWitkowski [7] analyzed theshearing strength of double joints made of various joiningtechniques. The capabilities of S350 GD sheet metal joiningusing the ClinchRivet technique were presented. The resultsachieved for joints arranged in parallel and perpendicular tothe load for specified joint spacing were discussed. The as-sessment of joint effectiveness was performed for both ho-mogenous double joints and for various combinations of thesejoints. Neugebauer et al.s paper [8] showed the advantages ofthe two-piece dies especially in solid punch riveting of differ-ent materials with distinct differences in strength. The use ofthese dies effects convenient technological conditions and anextended range of application for solid punch riveting.

The use of clinching is of interest to different industriessuch as aerospace, automotive, packaging and domestic ap-pliance. This, together with increasing use of light-weightmaterials, has produced a significant increase in the useof clinching in light-weight structures in recent years[912].

In industrial applications of the clinched structures, knowl-edge of the mechanical characteristics of clinched joints isvery important. The static and dynamic behaviour of clinchedjoints has been the subject of a great amount of numerical andexperimental studies. Previous publications mostly focussed

X. He (*) : F. Liu : B. Xing :H. Yang :Y. WangInnovative Manufacturing Research Centre, Kunming University ofScience and Technology, Kunming 650093,Peoples Republic of Chinae-mail: [email protected]

F. Gu :A. BallCentre for Efficiency and Performance Engineering, University ofHuddersfield, Queensgate, Huddersfield, HD1 3DH, UK

Int J Adv Manuf TechnolDOI 10.1007/s00170-014-6078-y

on clinching with fixed grooved dies. An investigation onclinching mechanism has been conducted by Gao and Budde[13]. Some elementary terms were used to establish a basictheory for analyzing the clinching mechanism. The influenceof the clinching process parameters on the join-ability of high-strength steel was studied by Mucha [14] using finite element(FE) method. The results showed that some parameters, suchas die radius, die depth and die groove shape were mainlyaffected on the join-ability. Markowski et al. [15] presentedthe results of FE analysis for clinching joint machines C-frame. Several versions of frame geometry were accounted forwhen analyzing the straining of material, including the massreduction. The purpose of this FE simulation was to determinethe effect of mass reducing material recess on the structurerigidity. The suitability and economics of clinching processeswere studied by Varis [16, 17].

A dieless clinching process has been proposed byNeugebauer et al. [18]. Using the dieless clinching, it is

possible to produce a one-sided flat connection, which is notproducible with any other joining technology. Addition-ally, it is possible to enlarge the application potential ofmechanical joining technologies as for example semi-finished parts made of magnesium can be partially heat-ed and directly joined without an increase in processtime or a reduction in the process stability. The tools costs,the necessary tolerances and the tool wear are significantlyreduced.

Another clinching configuration has been developedinvolving an extensible die for improving the mechani-cal behaviour of clinched joints. The use of extensibledie clinching has increased in recent years. But a liter-ature survey on the extensible die clinching has showna very limited number of publications. In Zheng et al.spaper [19], the extensible die clinching process has beensimulated by FE method. The material flowing patternshave been compared between the fixed grooved die clinchingand the extensible die clinching. The influence of processparameters in extensible die clinching has been systematicallyinvestigated by Lambiase and colleagues [20, 21]. The exten-sible die clinched joints were produced under differentforming loads for evaluating the evolution of the joints profileexperimentally.

In the present study, the extensible die clinching processhas been computationally studied using FE analysis software.A two dimensional (2D) axisymmetric model was generatedbased on the Cowper-Symonds material models. An implicittechnique with Lagrange method and r-self-adaptivity wasused. To validate the computational simulation of the exten-sible die clinching process, experimental tests on specimens

Upper sheet

Lower sheet

20

20110

110

20

Fig. 1 A single lap clinched joint

(a) Fixed die clinching tools (b) Bottom view of fixed die

clinched joint

(c) Extensible die clinching tools (d) Bottom view of extensible

die clinched joint

Fig. 2 Comparison of tools andbottom views between fixed dieclinching and extensible dieclinching

Int J Adv Manuf Technol

made of aluminium alloy Al5754 were carried out. The struc-tural analysis has also been performed for comparing the

strength and energy absorption ability of the extensibledie clinched joints and clinching-bonded joints.

(a) Extensible die clinching Machine

Sliding sectorsDie anvil

Fixed die

Rubber spring

(b) Schemac of extensible die

(c) Geometrical dimensions (d) FE model

Punch Blank holder

Upper sheet

Lower sheet

Rubber spring

Sliding sectors

Fixed die

(e) Radial displacement of sliding sector in FE simulaon

s=0 mm s=0 mm s=0.2 mm s=0.8 mm

Fig. 3 FE simulation ofextensible die clinching process


2 Extensible die clinching process simulation

The single lap clinched joint comprises an upper sheet, lowersheet as shown in Fig. 1. The sheet materials tested wereAl5754 aluminium alloy plates of dimensions 110 mmlength20 mm width2 mm thickness and were clinched inthe central part of lap section. The mechanical properties ofthe aluminium alloy Al5754 were as follow: Youngs modu-lus, E=70 GPa; Poissons ratio, v=0.33.

Comparison of tools and bottom views between fixed dieclinching and extensible die clinching is shown in Fig. 2. Inthe fixed die clinching process, the interlock is produced bydriving the material towards the die groove. The extensible dieis composed of a series of sliding sectors. In the extensible dieclinching, material is spread radially rather than towards thedie groove, resulting in a better interlock than in the fixed dieclinching process. The extensible die clinched joints are char-acterized by different geometrical and mechanical propertiesas compared with fixed die clinched joints. In order to achievedesigned durability, the punch, blank holder, sliding sectorsand fixed die were made of high-strength steel materials. Therubber spring must be replaced at regular intervals.Figure 3a, b show extensible die clinching machine and sche-matic of extensible die. Figure 3c shows the basic geometry ofthe extensible die clinching model.

A 2D axisymmetric extensible die clinching process modelwas generated, as shown in Fig. 3d, using the commercial FEsoftware LS-Dyna. The model was meshed using the planeelement 2D Solid162, involving 5,427 elements with 5,905nodes in the model. The extensible die clinching process

Fig. 4 Cross-section comparison between simulations and tests of ex-tensible die clinching processes

(a) Monotonic tensile process of the clinched joints

(b) Monotonic tensile process of the clinch

(c) Failure mode of clinched and clinch-bonded hybrid joints

-bonded hybrid joints

Fig. 5 Monotonic tensile processand failure mode of the clinchedand clinch-bonded hybrid joints


involves a large deformation with high local plastic strains insheets, resulting in severe local mesh distortions. The ALEadaptive technique in ANSYS/LS-DYNA was used. ASS2Dsingle contact function was conducted to judge the contactsbetween the surfaces.

The punch, blank holder and die were modelled as rigidbodies, whilst the sheets were modelled as elasto-plastic ma-terials. The piecewise-linear plasticity material model whichadopts the Cowper-Symbols model to consider the influenceof strain rate was used. The relationship between the Cowper-Symbols model and yield stress is shown in the followingequation:

y 1 t

C

!1p

24

35 0 f Peff 1

where 0 is the yield stress in constant strain rate, t is theeffective strain rate and C and P are the parameters of strainrate; f(eff

P ) is the hardening coefficient which is based on theeffective plastic strain. Mooney-Rivlin elastic rubber modelwas used for the rubber spring.

Some criteria such as the von Mises yield criterion, thepiecewise-linear isotropic strain-hardening rule and the asso-ciated flow rule were adopted in simulations. The frictionbetween different parts in the model has an effect on the

profile of the extensible die clinched joints. In the lack ofexperimental data, tentative values of the Coulomb frictioncoefficient between different parts in the extensible dieclinching process model were assumed as follows: f=0.25punch-upper sheet, f=0.15 upper sheet-blank holder, f=0.15 upper sheet-lower sheet and f=0.25 lower sheet-die. These values were kept constant for the simulationsin this study.

To save simulation time, start the analysis at the momentwhen the punch was very close to the top surface of the uppersheet and apply a specified initial velocity to simulate theextensible die clinching process. The extensible die clinchingprocess is modelled by applying a downward initial velocityto every node within the punch. Figure 3e shows the radialdisplacement of sliding sector in the extensible die clinchingprocess FE simulation.

3 Extensible die clinching process tests

A clinching equipment RIVCLINCH 1106 P50 system wasemployed as clinching machine as shown in Fig. 3a. Allclinching joints were made with constant pre-clamp (4 kN)and setting load (50 kN). As shown in Fig. 3c, the diameter ofthe punch is 5 mm and all clinching joints were formed for the

Fig. 6 Force-displacement curves and tensile strengths normal probability density distributions of clinched joints and clinch-bonded hybrid joints


same depth sensor. The average value of the bottom thicknessis 1.4 mm. The cross-section comparison between simulationsand tests of extensible die clinching processes is shown inFig. 4. It is clear that the result obtained from tests agree fairlywell with the computational simulation. The results show thecapability of the FE model for simulating the extensible dieclinching process for different geometries and workconditions.

4 Deformation and failure of clinched joints

Clinching has found applications in heavy-duty situation, suchas car bodies. Load-bearing capacity and energy absorption(EA) are the two most important features in structural analysisof clinched joints. During the clinching process, the uppersheet undergoes a significant thinning near the punch cornerradius. The strength of an extensible die clinched joint de-pends on the joint profile and particularly on the neck thick-ness and the magnitude of the produced undercut.

In order to improve the mechanical properties of theclinched joints, it is also important for clinching to benefitfrom the advantages of other fastening techniques, for exam-ple adhesive bonding [22]. Adhesives are used to increase therigidity and tightness of the structure [23, 24]. It is commonlyunderstood that the addition of adhesive in clinched joints isbeneficial but it is not clear if there are negative effects onmechanical properties of clinched joints. Deformation andfailure of homogeneous clinched joints under tensile loadingwere investigated for validating the load-bearing capacity andEA of clinched joints and clinch-bonded hybrid joints.

The clinch-bonded hybrid joints were produced followingexactly the same procedure as the respective clinched joints.The adhesive used in the present study was two componentsacryloid cement. The mechanical properties of the adhesiveinvestigated were Youngs modulus 2 GPa and Poissons ratio0.30 which had been proved as an excellent adhesive property[25]. The adhesive was applied on degreased surfaces and thetwo sheets were pressed together in order to squeeze sufficientadhesive out to avoid undue quilting of the finished clinch-

bonded hybrid joints. The flow of the adhesive was removed.The clinching processes were then produced before adhesivecuring. The thickness of the adhesive layer was controlled bythe clinching process. The average values of the bottomthickness of the clinch-bonded hybrid joints is 1.5 mm thusthe thickness of the adhesive layer is estimated to be 0.1 mm.Thereafter, the adhesive was cured at room temperature for atleast 24 h. After curing, the adhesive layer can give strongadhesive forces between sheets. Figure 5 shows the clinchedjoints and clinch-bonded hybrid joints.

A servo-hydraulic testing machine was used for the mono-tonic tensile tests of the clinched joints and clinch-bondedhybrid joints. For each test, six samples were mechanicallytested. The distance between two grips was about 100 mm.The tests were performed with a constant displacement rate of1 mm/min and terminated when the sheets were separated orthe force drops to 20 % of the peak force value. Continuousrecords of the applied force-displacement curves were obtain-ed during each test. Figure 5a, b show the monotonic tensileprocess and failed joints separately. It is clear from Fig. 5a, bthat the failure modes of the clinched joints and clinch-bondedhybrid joints were neck fracture mode, as shown in Fig. 5c.

Fig. 7 Energy absorption normal probability density distributions of clinched joints and clinch-bonded hybrid joints

Maximum Force

[kN]

EA

[J]

Fig. 8 Intercepts of strength and EA for clinched joints and clinch-bonded hybrid joints


Such failure of the clinched joints and clinch-bonded hybridjoints could be attributed to a too small clearance of the toolsdiameters or a too deep die.

Under the tensile-shear load, the neck of the upper sheetbear a main shear load by geometrical interlocking. When theshear stress reaches the yield criterion of aluminium alloyAl5754, a crack is initiated from the interfacial surface of theupper sheet and grows into the upper sheet thickness. Afterrowing into the upper sheet, crack kinks towards the buttoncentre and then propagates along the circumference of thebutton neck of the upper sheet. Finally, the inner button issheared off at the neck. In Fig. 5a, sheets were separated forfive samples and not completely separated for one sample.

Figure 6 shows the force-displacement curves of theclinched joints and clinch-bonded hybrid joints. In the caseof the clinched joints, after the peak, the force decreasesgradually. In the cases of the clinch-bonded hybrid joints,however, after the peak, the force suddenly drops. It is clearfrom Fig. 6 that the load-bearing capacity of clinch-bondedhybrid joint is higher than that of the clinched joint. It is alsoclear from Figs. 5 and 6 that the repeatability of the clinchedjoints and clinch-bonded hybrid joints are big though therepeatability of the adhesive joints was not very big [5].

To examine the rationality of the test data, the normalhypothesis tests were performed using MATLAB 7.0. Theresults indicated that the tensile strengths of all the clinchedjoints and clinch-bonded hybrid joints follow normal distribu-tions. The mean values () and standard deviations () havethe following numerical values: for the clinched joints C=1,895.30 N, C=43.81; for the clinch-bonded hybrid jointsCB=2,022.50 N, CB=49.41. All test data fitting the regionwas estimated by the degree of confidence of 95 %. Thetensile strengths normal probability density distributions ofthe clinched joints and clinch-bonded hybrid joints are alsoshown in Fig. 6.

5 EA of clinched joints and clinch-bonded hybrid joints

The normal hypothesis tests were performed to examine therationality of the EA values of the clinched joints and clinch-bonded hybrid joints. The results show that the EA values ofall the clinched joints and clinch-bonded hybrid joints follownormal distributions. For the clinched joints EAC=1.28 J,EAC=0.04; for clinch-bonded hybrid joints EACB=1.37 J,EACB=0.16. All test data fitting the region was estimated bythe degree of confidence of 95 %. The EA values normalprobability density distributions of the clinched joints andclinch-bonded hybrid joints are shown in Fig. 7.

Fig. 8 shows the intercept for load-bearing capacity and EAof the clinched joints and clinch-bonded hybrid joints. It isclear that both the maximum load and EAvalues of the clinch-bonded hybrid joints are higher than that of the clinched joint.

This means that the addition of adhesive resulted in an in-crease in both the load-bearing and the energy absorptioncapacities of the clinched joints.

6 Summary

The extensible die clinching process has been computationallyinvestigated in this paper using the commercial FE softwareLS-Dyna. Experimental tests on the extensible die clinchedjoints made of aluminium alloy Al5754 have been carried outto validate the numerical simulation of the extensible dieclinching process. The result obtained from tests agreed fairlywell with the computational simulation.

Deformation and failure of homogeneous clinched jointsunder tensile loading were investigated for validating the load-bearing capacity and EA of the clinched joints and clinch-bonded hybrid joints.

As mentioned above, the clinched joints were producedbefore adhesive curing. In the extensible die clinching pro-cess, adhesive layer can be fully sandwiched between twosheets. After curing, the adhesive layer can increase thestrength of the clinched joints due to the adhesion mechanism.However, after the peak load, the failure of adhesive layeroccurs in a brittle manner. In this case, though the clinch stillkeeps the sheets connected, the joint can only bear low load,resulting in some more elongation.

Acknowledgments Financial support of the National Natural ScienceFoundation of China (Grant No. 50965009) is gratefully acknowledged.

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Numerical and experimental investigations of extensible die clinchingAbstractIntroductionExtensible die clinching process simulationExtensible die clinching process testsDeformation and failure of clinched jointsEA of clinched joints and clinch-bonded hybrid jointsSummaryReferences

die clinching

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