Numerical and experimental investigations of extensible die clinching ...

Numerical and experimental investigations of extensible die clinching

Xiaocong He, Fulong Liu, Baoying Xing, Huiyan Yang, Yuqi Wang, Fengshou Gu & Andrew Ball The International Journal of Advanced Manufacturing Technology ISSN 0268-3768 Volume 74 Combined 9-12 Int J Adv Manuf Technol (2014) 74:1229-1236 DOI 10.1007/s00170-014-6078-y

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Author's personal copy Int J Adv Manuf Technol (2014) 74:1229–1236 DOI 10.1007/s00170-014-6078-y

ORIGINAL ARTICLE

Numerical and experimental investigations of extensible die clinching Xiaocong He & Fulong Liu & Baoying Xing & Huiyan Yang & Yuqi Wang & Fengshou Gu & Andrew Ball

Received: 19 December 2013 / Accepted: 16 June 2014 / Published online: 27 June 2014 # Springer-Verlag London 2014

Abstract With an increasing application of clinching in different industrial fields, the demand for knowledge of static and dynamic characteristics of clinching is increased. In the present work, the extensible die clinching process was numerically investigated using finite element method. To validate the computational simulation of the extensible die clinching process, experimental tests on extensible die clinched specimens have been carried out. Good agreement is achieved between the predictions and the experimental results. Monotonic tensile tests were carried out to measure the ultimate tensile strengths of the extensible die clinching joints and clinching-bonded hybrid joints. Deformation and failure of the extensible die clinched joints under monotonic tensile loading were studied. The normal hypothesis tests were performed to examine the rationality of the test data. This work was also aimed at evaluating experimentally and comparing the strength and energy absorption of the extensible die clinched joints and clinching-bonded hybrid joints. Keywords Extensible die clinching . Process simulation . Finite element method . Load-bearing capacity . Energy absorption

1 Introduction Some relative new joining techniques have drawn more attention in recent years because they can join advanced sheet X. He (*) : F. Liu : B. Xing : H. Yang : Y. Wang Innovative Manufacturing Research Centre, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China e-mail: [email protected] F. Gu : A. Ball Centre for Efficiency and Performance Engineering, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK

materials that are dissimilar, coated and hard to weld with conventional spot welding [1–5]. Many efforts have also been spent to develop hybrid joining techniques and alternatives for application into lightweight structures. Mucha et al’s paper [6] presented the pressed joint technology using forming process with or without additional fastener. The capabilities for increasing the load-carrying ability of mechanical joints by applying special rivets and dies were presented. The joint forming was performed with the solid round die and rectangular split die for riveted joint forming. The effect of joint forming process on jointed material strain was compared by measuring the microhardness of the joints. Mucha and Witkowski [7] analyzed the shearing strength of double joints made of various joining techniques. The capabilities of S350 GD sheet metal joining using the ClinchRivet technique were presented. The results achieved for joints arranged in parallel and perpendicular to the load for specified joint spacing were discussed. The assessment of joint effectiveness was performed for both homogenous double joints and for various combinations of these joints. Neugebauer et al.’s paper [8] showed the advantages of the two-piece dies especially in solid punch riveting of different materials with distinct differences in strength. The use of these dies effects convenient technological conditions and an extended range of application for solid punch riveting. The use of clinching is of interest to different industries such as aerospace, automotive, packaging and domestic appliance. This, together with increasing use of light-weight materials, has produced a significant increase in the use of clinching in light-weight structures in recent years [9–12]. In industrial applications of the clinched structures, knowledge of the mechanical characteristics of clinched joints is very important. The static and dynamic behaviour of clinched joints has been the subject of a great amount of numerical and experimental studies. Previous publications mostly focussed

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Int J Adv Manuf Technol (2014) 74:1229–1236 Upper sheet

Lower sheet 110 20

20 20 110

Fig. 1 A single lap clinched joint

on clinching with fixed grooved dies. An investigation on clinching mechanism has been conducted by Gao and Budde [13]. Some elementary terms were used to establish a basic theory for analyzing the clinching mechanism. The influence of the clinching process parameters on the join-ability of highstrength steel was studied by Mucha [14] using finite element (FE) method. The results showed that some parameters, such as die radius, die depth and die groove shape were mainly affected on the join-ability. Markowski et al. [15] presented the results of FE analysis for clinching joint machine’s Cframe. Several versions of frame geometry were accounted for when analyzing the straining of material, including the mass reduction. The purpose of this FE simulation was to determine the effect of mass reducing material recess on the structure rigidity. The suitability and economics of clinching processes were studied by Varis [16, 17]. A dieless clinching process has been proposed by Neugebauer et al. [18]. Using the dieless clinching, it is

possible to produce a one-sided flat connection, which is not producible with any other joining technology. Additionally, it is possible to enlarge the application potential of mechanical joining technologies as for example semifinished parts made of magnesium can be partially heated and directly joined without an increase in process time or a reduction in the process stability. The tool’s costs, the necessary tolerances and the tool wear are significantly reduced. Another clinching configuration has been developed involving an extensible die for improving the mechanical behaviour of clinched joints. The use of extensible die clinching has increased in recent years. But a literature survey on the extensible die clinching has shown a very limited number of publications. In Zheng et al.’s paper [19], the extensible die clinching process has been simulated by FE method. The material flowing patterns have been compared between the fixed grooved die clinching and the extensible die clinching. The influence of process parameters in extensible die clinching has been systematically investigated by Lambiase and colleagues [20, 21]. The extensible die clinched joints were produced under different forming loads for evaluating the evolution of the joints’ profile experimentally. In the present study, the extensible die clinching process has been computationally studied using FE analysis software. A two dimensional (2D) axisymmetric model was generated based on the Cowper-Symonds material models. An implicit technique with Lagrange method and r-self-adaptivity was used. To validate the computational simulation of the extensible die clinching process, experimental tests on specimens

Fig. 2 Comparison of tools and bottom views between fixed die clinching and extensible die clinching

(a) Fixed die clinching tools

(c) Extensible die clinching tools

(b) Bottom view of fixed die clinched joint

(d) Bottom view of extensible die clinched joint

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

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

Die anvil

Fig. 3 FE simulation of extensible die clinching process

Sliding sectors

Fixed die

Rubber spring (a) Extensible die clinching Machine

(b) SchemaƟc of extensible die Punch

Blank holder Upper sheet

Lower sheet Sliding sectors Rubber spring Fixed die

(c) Geometrical dimensions

s=0 mm

s=0 mm

(d) FE model

s=0.2 mm

s=0.8 mm

(e) Radial displacement of sliding sector in FE simulaƟon


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

2 Extensible die clinching process simulation The single lap clinched joint comprises an upper sheet, lower sheet as shown in Fig. 1. The sheet materials tested were Al5754 aluminium alloy plates of dimensions 110 mm length×20 mm width×2 mm thickness and were clinched in the central part of lap section. The mechanical properties of the aluminium alloy Al5754 were as follow: Young’s modulus, E=70 GPa; Poisson’s ratio, v=0.33.

Comparison of tools and bottom views between fixed die clinching and extensible die clinching is shown in Fig. 2. In the fixed die clinching process, the interlock is produced by driving the material towards the die groove. The extensible die is composed of a series of sliding sectors. In the extensible die clinching, material is spread radially rather than towards the die groove, resulting in a better interlock than in the fixed die clinching process. The extensible die clinched joints are characterized by different geometrical and mechanical properties as compared with fixed die clinched joints. In order to achieve designed durability, the punch, blank holder, sliding sectors and fixed die were made of high-strength steel materials. The rubber spring must be replaced at regular intervals. Figure 3a, b show extensible die clinching machine and schematic of extensible die. Figure 3c shows the basic geometry of the extensible die clinching model. A 2D axisymmetric extensible die clinching process model was generated, as shown in Fig. 3d, using the commercial FE software LS-Dyna. The model was meshed using the plane element 2D Solid162, involving 5,427 elements with 5,905 nodes in the model. The extensible die clinching process

Fig. 5 Monotonic tensile process and failure mode of the clinched and clinch-bonded hybrid joints

(a) Monotonic tensile process of the clinched joints

(b) Monotonic tensile process of the clinch-bonded hybrid joints

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


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involves a large deformation with high local plastic strains in sheets, resulting in severe local mesh distortions. The ALE adaptive technique in ANSYS/LS-DYNA was used. ASS2D single contact function was conducted to judge the contacts between the surfaces. The punch, blank holder and die were modelled as rigid bodies, whilst the sheets were modelled as elasto-plastic materials. The piecewise-linear plasticity material model which adopts the Cowper-Symbols model to consider the influence of strain rate was used. The relationship between the CowperSymbols model and yield stress is shown in the following equation: 2

ε˙ t σy ¼ 41 þ C

! 1p 3 5 σ0 þ f εP eff

ð1Þ

where σ0 is the yield stress in constant strain rate, ε˙t is the effective strain rate and C and P are the parameters of strain rate; f(εPeff) is the hardening coefficient which is based on the effective plastic strain. Mooney-Rivlin elastic rubber model was used for the rubber spring. Some criteria such as the von Mises yield criterion, the piecewise-linear isotropic strain-hardening rule and the associated flow rule were adopted in simulations. The friction between different parts in the model has an effect on the

profile of the extensible die clinched joints. In the lack of experimental data, tentative values of the Coulomb friction coefficient between different parts in the extensible die clinching process model were assumed as follows: f=0.25 punch-upper sheet, f=0.15 upper sheet-blank holder, f= 0.15 upper sheet-lower sheet and f=0.25 lower sheetdie. These values were kept constant for the simulations in this study. To save simulation time, start the analysis at the moment when the punch was very close to the top surface of the upper sheet and apply a specified initial velocity to simulate the extensible die clinching process. The extensible die clinching process is modelled by applying a downward initial velocity to every node within the punch. Figure 3e shows the radial displacement of sliding sector in the extensible die clinching process FE simulation.

3 Extensible die clinching process tests A clinching equipment RIVCLINCH 1106 P50 system was employed as clinching machine as shown in Fig. 3a. All clinching joints were made with constant pre-clamp (4 kN) and setting load (50 kN). As shown in Fig. 3c, the diameter of the 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


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

same depth sensor. The average value of the bottom thickness is 1.4 mm. The cross-section comparison between simulations and tests of extensible die clinching processes is shown in Fig. 4. It is clear that the result obtained from tests agree fairly well with the computational simulation. The results show the capability of the FE model for simulating the extensible die clinching process for different geometries and work conditions.

4 Deformation and failure of clinched joints Clinching has found applications in heavy-duty situation, such as car bodies. Load-bearing capacity and energy absorption (EA) are the two most important features in structural analysis of clinched joints. During the clinching process, the upper sheet undergoes a significant thinning near the punch corner radius. The strength of an extensible die clinched joint depends on the joint profile and particularly on the neck thickness and the magnitude of the produced undercut. In order to improve the mechanical properties of the clinched joints, it is also important for clinching to benefit from the advantages of other fastening techniques, for example adhesive bonding [22]. Adhesives are used to increase the rigidity and tightness of the structure [23, 24]. It is commonly understood that the addition of adhesive in clinched joints is beneficial but it is not clear if there are negative effects on mechanical properties of clinched joints. Deformation and failure of homogeneous clinched joints under tensile loading were investigated for validating the load-bearing capacity and EA of clinched joints and clinch-bonded hybrid joints. The clinch-bonded hybrid joints were produced following exactly the same procedure as the respective clinched joints. The adhesive used in the present study was two components acryloid cement. The mechanical properties of the adhesive investigated were Young’s modulus 2 GPa and Poisson’s ratio 0.30 which had been proved as an excellent adhesive property [25]. The adhesive was applied on degreased surfaces and the two sheets were pressed together in order to squeeze sufficient adhesive 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 adhesive curing. The thickness of the adhesive layer was controlled by the clinching process. The average values of the bottom thickness of the clinch-bonded hybrid joints is 1.5 mm thus the thickness of the adhesive layer is estimated to be 0.1 mm. Thereafter, the adhesive was cured at room temperature for at least 24 h. After curing, the adhesive layer can give strong adhesive forces between sheets. Figure 5 shows the clinched joints and clinch-bonded hybrid joints. A servo-hydraulic testing machine was used for the monotonic tensile tests of the clinched joints and clinch-bonded hybrid joints. For each test, six samples were mechanically tested. The distance between two grips was about 100 mm. The tests were performed with a constant displacement rate of 1 mm/min and terminated when the sheets were separated or the force drops to 20 % of the peak force value. Continuous records of the applied force-displacement curves were obtained during each test. Figure 5a, b show the monotonic tensile process and failed joints separately. It is clear from Fig. 5a, b that the failure modes of the clinched joints and clinch-bonded hybrid joints were neck fracture mode, as shown in Fig. 5c.

Maximum Force [kN]

EA [J]

Fig. 8 Intercepts of strength and EA for clinched joints and clinchbonded hybrid joints


Such failure of the clinched joints and clinch-bonded hybrid joints could be attributed to a too small clearance of the tools’ diameters or a too deep die. Under the tensile-shear load, the neck of the upper sheet bear a main shear load by geometrical interlocking. When the shear stress reaches the yield criterion of aluminium alloy Al5754, a crack is initiated from the interfacial surface of the upper sheet and grows into the upper sheet thickness. After rowing into the upper sheet, crack kinks towards the button centre and then propagates along the circumference of the button neck of the upper sheet. Finally, the inner button is sheared off at the neck. In Fig. 5a, sheets were separated for five samples and not completely separated for one sample. Figure 6 shows the force-displacement curves of the clinched joints and clinch-bonded hybrid joints. In the case of the clinched joints, after the peak, the force decreases gradually. In the cases of the clinch-bonded hybrid joints, however, after the peak, the force suddenly drops. It is clear from Fig. 6 that the load-bearing capacity of clinch-bonded hybrid joint is higher than that of the clinched joint. It is also clear from Figs. 5 and 6 that the repeatability of the clinched joints and clinch-bonded hybrid joints are big though the repeatability of the adhesive joints was not very big [5]. To examine the rationality of the test data, the normal hypothesis tests were performed using MATLAB 7.0. The results indicated that the tensile strengths of all the clinched joints and clinch-bonded hybrid joints follow normal distributions. The mean values (μ) and standard deviations (σ) have the following numerical values: for the clinched joints μC = 1,895.30 N, σC =43.81; for the clinch-bonded hybrid joints μCB =2,022.50 N, σCB =49.41. All test data fitting the region was estimated by the degree of confidence of 95 %. The tensile strengths normal probability density distributions of the clinched joints and clinch-bonded hybrid joints are also shown in Fig. 6.

5 EA of clinched joints and clinch-bonded hybrid joints The normal hypothesis tests were performed to examine the rationality of the EA values of the clinched joints and clinchbonded hybrid joints. The results show that the EA values of all the clinched joints and clinch-bonded hybrid joints follow normal distributions. For the clinched joints μEAC =1.28 J, σEAC =0.04; for clinch-bonded hybrid joints μEACB =1.37 J, σEACB =0.16. All test data fitting the region was estimated by the degree of confidence of 95 %. The EA values normal probability density distributions of the clinched joints and clinch-bonded hybrid joints are shown in Fig. 7. Fig. 8 shows the intercept for load-bearing capacity and EA of the clinched joints and clinch-bonded hybrid joints. It is clear that both the maximum load and EA values of the clinchbonded hybrid joints are higher than that of the clinched joint.

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This means that the addition of adhesive resulted in an increase in both the load-bearing and the energy absorption capacities of the clinched joints.

6 Summary The extensible die clinching process has been computationally investigated in this paper using the commercial FE software LS-Dyna. Experimental tests on the extensible die clinched joints made of aluminium alloy Al5754 have been carried out to validate the numerical simulation of the extensible die clinching process. The result obtained from tests agreed fairly well with the computational simulation. Deformation and failure of homogeneous clinched joints under tensile loading were investigated for validating the loadbearing capacity and EA of the clinched joints and clinchbonded hybrid joints. As mentioned above, the clinched joints were produced before adhesive curing. In the extensible die clinching process, adhesive layer can be fully sandwiched between two sheets. After curing, the adhesive layer can increase the strength of the clinched joints due to the adhesion mechanism. However, after the peak load, the failure of adhesive layer occurs in a brittle manner. In this case, though the clinch still keeps the sheets connected, the joint can only bear low load, resulting in some more elongation. Acknowledgments Financial support of the National Natural Science Foundation of China (Grant No. 50965009) is gratefully acknowledged.

References 1. He X, Gu F, Ball A (2014) A review of numerical analysis of friction stir welding. Prog Mater Sci 65:1–66 2. Mucha J (2013) The effect of material properties and joining process parameters on behavior of self-pierce riveting joints made with the solid rivet. Mater Des 52:932–946 3. Mucha J (2014) The numerical analysis of the effect of the joining process parameters on self-piercing riveting using the solid rivet. Archive Civil Mech Eng 4. He X, Gu F, Ball A (2012) Recent development in finite element analysis of self-piercing riveted joints. Int J Adv Manuf Technol 58: 643–649 5. He X, Xing B, Zeng K, Gu F, Ball A (2013) Numerical and experimental investigations of self-piercing riveting. Int J Adv Manuf Technol 69:715–721 6. Mucha J, Kascak L, Spisak E (2013) The experimental analysis of forming and strength of clinch riveting sheet metal joint made of different materials. Adv Mech Eng, Article ID 848973 7. Mucha J, Witkowski W (2013) The experimental analysis of the double joint type change effect on the joint destruction process in uniaxial shearing test. Thin-Walled Struct 66:39–49 8. Neugebauer R, Jesche F, Israel M (2010) Enlargement of the application range of solid punch riveting by two-piece dies. Int J Mater Form 3:999–1002

Author's personal copy 1236 9. He X (2010) Recent development in finite element analysis of clinched joints. Int J Adv Manuf Technol 48:607–612 10. Mucha J, Witkowski W (2014) The clinching joints strength analysis in the aspects of changes in the forming technology and load conditions. Thin-Walled Struct 82:55–66 11. Lee C, Kim J, Lee S, Ko D, Kim B (2010) Design of mechanical clinching tools for joining of aluminium alloy sheets. Mater Des 31(4):1854–1861 12. He X, Zhao L, Yang H, Xing B, Wang Y, Deng C, Gu F, Ball A (2014) Investigations of strength and energy absorption of clinched joints. Comput Mater Sci 13. Gao S, Budde L (1994) Mechanism of mechanical press joining. Int J Mach Tools Manuf 34(5):641–657 14. Mucha J (2011) The analysis of lock forming mechanism in the clinching joint. Mater Des 32(10):4943–4954 15. Markowski T, Mucha J, Witkowski W (2013) FEM analysis of clinching joint machine’s C-frame rigidity. Eksploatacja Niezawodnosc-Maint Reliab 1:51–57 16. Varis JP (2003) The suitability of clinching as a joining method for high-strength structural steel. J Mater Process Technol 132(1–3): 242–249 17. Varis JP (2006) Economics of clinching joint compared to riveted joint and example of applying calculations to a volume product. J Mater Process Technol 172(1):130–138

Int J Adv Manuf Technol (2014) 74:1229–1236 18. Neugebauer R, Todtermuschke M, Mauermann R, Riedel F (2008) Overview on the state of development and the application potential of dieless mechanical joining processes. Archive Civil Mech Eng 8(4): 51–60 19. Zheng J, He X, Xu J, Zeng K, Ding Y, Hu Y (2012) Finite element analysis of energy saving jointing method base on energy materials: clinching. Adv Mater Res 577:9–12 20. Lambiase F, Di Ilio A (2013) Finite element analysis of material flow in mechanical clinching with extensible dies. J Mater Eng Perform 22(6):1629–1636 21. Lambiase F (2012) Influence of process parameters in mechanical clinching with extensible dies. Int J Adv Manuf Technol 66:2123– 2131 22. He X (2011) A review of finite element analysis of adhesively bonded joints. Int J Adhes Adhes 31:248–264 23. Bartczak B, Mucha J, Trzepiecinski T (2013) Stress distribution in adhesively-bonded joints and the loading capacity of hybrid joints of car body steels for the automotive industry. Int J Adhes Adhes 45:42–52 24. He X (2012) Numerical and experimental investigations of the dynamic response of bonded beams with a single-lap joint. Int J Adhes Adhes 37:79–85 25. He X, Ichikawa M (1993) Effect of thickness control of adhesive layer on strength of adhesive joints. In: Proceedings of the 70th JSME Spring Annual Meeting, Tokyo, Japan, 166-168, (in Japanese)