A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 56 2011 Issue 2

DOI: 10.2478/v10172-011-0047-3

T. BALAWENDER∗, T. SADOWSKI∗∗, M. KNEĆ∗∗

TECHNOLOGICAL PROBLEMS AND EXPERIMENTAL INVESTIGATION OF HYBRID: CLINCHED – ADHESIVELY BONDEDJOINT

ZAGADNIENIA TECHNOLOGICZNE I BADANIA EKSPERYMENTALNE W POŁĄCZENIACH HYBRYDOWYCH:KLINCZOWO-ADHEZYJNYCH

The choice of proper joining technology is an essential aspect of designing and manufacturing activity. New productdesigns enforce joining different materials in a new joining method. Clinching is a relatively new joining technology in whichsheet metal parts are deformed locally without use of any additional element. Combining clinching with adhesive bondingseems to be interesting hybrid joining method with new potential applications in manufacturing industry.

The paper deals with discussion of technological aspects and experimental investigations of clinched lap joints of differentmetal strips combined with their adhesive bonding, which can be applied for different branches of engineering. The main purposeof using different types of sheet metals was to analyze their influences on the geometry and mechanical strength of the clinchedjoint. The experiments with application of Digital Image Correlation (DIC) system ARAMIS allowed for exact monitoring ofthe deformation process of considered hybrid joint.

The clinched joint test specimens were carried out with laboratory test-stand which consisted of a pair of tools (punchand die) attached to C-frame hydraulic press.

Keywords: clinching, hybrid clinched – adhesive joints, digital image correlation system

Wybór właściwej technologii łączenia stanowi istotny aspekt projektowania i wytwarzania. Projektowanie nowych pro-duktów wymusza łączenie różnych materiałów za pomocą nowych technologii. Łączenie przetłaczaniem (klinczowanie) jestrelatywnie nową technologią, w której różne części metalowe łączone są poprzez lokalną deformację bez zastosowania do-datkowych elementów. Równoczesne zastosowanie klinczowania i klejenia łączonych elementów jest interesującą propozycjąhybrydowego połączenia z nowymi potencjalnymi możliwościami zastosowania w przemyśle wytwórczym.

W pracy przedyskutowano aspekty technologiczne i badania eksperymentalne klinczowych połączeń zakładkowych wy-konanych z różnych materiałów w połączeniu z techniką klejenia, które mogą być zastosowane w różnych gałęziach inżynierii.Głównym celem pracy było zastosowanie różnych materiałów metalicznych, aby przeanalizować ich wpływ na geometrię imechaniczną wytrzymałość połączeń hybrydowych. Badania wspomagane były przy pomocy cyfrowego systemu przetwarzaniaobrazów ARAMIS, który pozwala na dokładne monitorowanie całego procesu deformacji rozpatrywanego połączenia hybry-dowego.

Złącza klinczowe na próbkach stosowanych w badaniach wykonywano na laboratoryjnym stanowisku badawczym wypo-sażonym w prasę ramową typu C z zamontowanym zestawem narzędzi (stemplem i matrycą).

1. Introduction

Clinching (mechanical interlock) is a method ofjoining different metal parts (mainly sheets) by a processof local deformation without use of any additional join-ing elements with the application of a punch and a die[e.g. 1-3]. The mechanical interlock with application theclinching technique is shown schematically in Fig. 1.During clinching, a punch presses the joined sheets in-side a die cavity forming a shape that locks the sheets

together. The clinching technique is known from the endof the nineteenth century but it was not used on an in-dustrial scale until the eighties of the twenties century.Nowadays, clinching is used in the automobile, furnitureand computer industries. The most known examples ofclinching technology are joints in certain parts of an au-tomobile body (Mercedes–Benz S-class), different kindhousehold appliances, ventilation and air-conditioningproducts and plenty of others in sheet product manu-facturing.

∗ TECHNICAL UNIVERSITY OF RZESZÓW, 35-959 RZESZÓW, 8 POWSTAŃCÓW WARSZAWY STR., POLAND∗∗ TECHNICAL UNIVERSITY OF LUBLIN, 20-618 LUBLIN, 40 NADBYSTRZYCKA STR., POLAND

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Fig. 1. Stages of clinch joining process and main geometrical parameters of a clinched joint: th – the thinning of the upper sheet (the neckthickness), cl – the clinch lock (undercut), x – the axial thickness of the sheets (thickness of the indentation bottom)

The required tooling set consist of: a punch, a dieand a blank holder. The mechanical joining of two ormore blanks (e.g. metal sheets) is based on only theaccurate movement of the punch into the die; the sheetmetals are deformed locally without the use of any addi-tional elements. The joint strength is due to: force lock-ing, material locking and “S” shape locking (Fig. 2).The quality of the clinching process strongly depends onprecisely selected tools. The proper description of thisprocess should include: the tool geometries, parameteroptimisation and FEA simulation of the process [4-9].

Fig. 2. The mechanisms of locking of joining parts during clinchingprocess

The static strength of clinched joints is lower thanthat of other joints (e.g. self-pierce riveting and spotwelding), but the fatigue strength of clinched joints iscomparable to that of the other joints. Nowadays, manyresearches are looking for appropriate combination ofclinching tools to obtain the maximum load under sheartest of the clinched joint [e. g. 5, 6, 10-12].

Shortages of this cold-formed mechanical fasteningcan be also removed by hybrid joining, involving clinch-ing and adhesion techniques. It is a modern and innova-tive technology allowing connection of different types ofmaterials to create durable and reliable light construc-tions. However, its practical implementation is still verylimited. The application of clinching together with adhe-sive bonding leads to an improvement (in comparison toa simple joint): the quality, rigidity and the load capacity,dumping of noise and vibration, pressure tightness andcorrosion protection. This entails a significant increaseof:– long-term static strength– the amplitude of force under fatigue test– the energy required to the rupture of the hybrid joint

under static, dynamic and impact loading.The clinch-adhesive technique may be useful mainly

in the automotive industry. However, wider applicationin aerospace and aeronautical industries should be inves-tigated after an elaboration of the proper technology andan experimental verification of numerical models. Eachapplication of the clinch-adhesive technique leads to aformulation of the specific requirements to guarantee aproper design of the joint, and further a manufacture ofsafe and durable fastening. The basic technological prob-lem is the optimisation of the clinching process – coldforming operations – by the application of the suitableshape of the tools. The proper shape of the manufactur-ing process leads to improvements of joint strength underdifferent types of loading: mechanical, temperature andenvironmental or aggressive effects.

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2. Experimental work

The range of experimental investigations includedshear tests of single lap joints and visualisation of thedeformation process by the application of the DIC sys-tem ARAMIS. Single lap joints were manufactured andtested as clinched, adhesive bonded and hybrid joints.

Clinch lap joints were manufactured by a die and apunch arranged in an adopted stamping attachment set on120 kN in a C-frame hydraulic press. The maximal pressload was reduced by the overflow valve of the hydraulicsystem to about 90 kN. The tools were designed for join-ing of two sheets with 1 mm thickness. The position ofthe round indentation was symmetrical with respect tothe specimen axis. The nominal diameter of the clinchbulging measured on the side of the die was ϕ10 mm.The configuration of tools geometry was established toobtain good steel – steel joint.

Three sheet materials were used in the study:ETP-cooper, CuZn37 brass and low-carbon steel(deep-drawing steel). Thickness of all sheets was about1 mm. Mechanical properties of sheet materials weredetermined in uniaxial tensile tests. Because of sheetmaterial anisotropy, the tests were performed in threedirections in the sheet plane: 0◦, 45◦ and 90◦ accordingto the rolling direction. The results are shown in Table 1.

TABLE 1Averaged values of mechanical properties of materials used in the

tests (averaging formula xav=(x0+2x45+x90)/4)

Type of materialCuZn37brass ETP-copper

deep-drawingsteel

Yield stress R0.2

[MPa] 251 242 203

Ultimate tensilestress Rm [MPa] 389 273 325

Elongation to failureA [%] 39 20 29

Lankford’sparameter r 0,81 0,98 1,93

The lap shearing tests were performed on mechani-cal 100 kN screw testing machine. During the tests forceand ram displacement were recorded by the HBM am-plifier SPIDER and CATMAN software with the ramdisplacement rate equal 8 mm/min. The test specimenswere composed of two strips with three different metals(∼132 x 35 x 1mm) with an overlap of the length 35mm (Fig. 3.). It was assumed in the description that firstmaterial in specimen’s denotation (A) is deformed by a

punch, the second (B) is deformed by a die (i.e. A – Bclinch joint).

Fig. 3. Geometry of overlap joint specimen (A – B clinch joint)

The adhesive used in the tests was Dragon R© (prod-uct of Chemical Factory DRAGON, Krakow, Poland),a two-component epoxy destined for metal joining. Thecuring cycle of the adhesive is 4 hours at 20◦C and itcan be accelerated by warming. The full strength of thejoint is obtained after 24 hours. The adhesive is assignedto joining, in different combination, such materials as:steel, cast iron, aluminium, metal alloys, ceramics, plas-tics. Adhesive bonding joints were manufactured as over-lap joint shown in Fig. 4a and 4b. The lap surfaces ofsheets were cleaned and roughened with abrasive paperand then degreased with acetone. The adhesive, aftermixing components, was spread on both joined surfacesand parts were pressed with constant load and were leftfor curing for 24 hours at room temperature. The av-erage value of adhesive layer was about 0,15 mm. Thehybrid joint was created by the third stage of technologi-cal process (Fig. 4c) – after curing of the adhesive layer,clinching took place.

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Fig. 4. Methodology of adhesive bonded and hybrid specimens preparation: a) cleaning of the lap surfaces, b) adhesive bonding, c) clinchingprocess

3. Results and discussion

The experimental results concerning the pureclinched joints for three considered materials were pre-sented in Fig. 5. One can notice, that the maximum val-ue of the shear force was obtained for steel-steel sheetsjoint. The minimum value of the loading force was ob-served for brass – brass sheets joint, whereas mediumone for copper – copper samples.

Fig. 5. Experimental load-displacement curves obtained for clinchedjoints of tested materials

The example strength characteristics of hybrid jointscompared with the single joining techniques by pureclinching and pure adhesive bonding are shown in Fig.6 and 7. In case of different types of brass-brass joints(Fig. 6) one can observe that the hybrid joint and the pureadhesive joint have almost similar load-displacementscurves. At the maximum shear load both types of joints

suddely undergo failure, like in brittle materials. Theenergy absorption to the final failure of the sample isalmost the same. Comparison of the pure clinched andhybrid joints lead to the conclusion, that the load capac-ity increases approximately 6 times.

Fig. 6. Experimental load-displacement curves (for clinched, adhesiveand hybrid joints) obtained for brass – brass joints

Materials that create strong clinched joint, i.e. steel– copper, don’t create hybrid joint with high response toshear force (Fig. 7). But strength characteristics of thesematerials have a special feature – when adhesively bond-ed joint fails, the clinched joint still keeps the materialsconnected what results in long way of joint separationand high energy absorption. Comparison of the clinchedand hybrid joints leads to the conclusion, that the hybridjoint is 3 times stronger. It is also visible, that the pureadhesive joints have 33% higher energy absorption tothe final failure. The lower value of this energy in case

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of the hybrid joint is due to local damage introduced byclinching.

Fig. 7. Experimental load-displacement curves (for clinched, adhesiveand hybrid joints) obtained for steel – copper joints

The longitudinal cross sections of hybrid joints areshown in Fig. 8. Differences between the clinch jointand the hybrid joint are visible in the bottom corner ofthe joint cavity as a fold (particularly in Fig. 8a and 8c).This is due to fact, that the adhesive causes a buckling oflower material in the die groove. There is no differencein the shape of the upper material deformed by the punchin both cases: the pure clinched and the hybrid joints.The influence of the above buckling effect on the finalstrength of the hybrid joints should be investigated indetails in further experiments.

Table 2 presents the estimation of the energy ab-sorption (EA) of all tested types of the joints. Appli-cation of the adhesive causes the increase of the EAabout 3-4 times in relation to pure clinching of hybridjoints manufactured form the steel or steel-copper. Incase of the copper-copper, brass-copper and brass-brassjoints almost the whole energy accumulation capacitytakes place up to the maximum force P. The additionof the Dragon adhesive to the copper-copper clinchedjoint is the most effective as for EA increase. The pureclinch joints made of brass and brass-copper are veryweak as for capability of EA accumulation. Introductionof the adhesive bonding to clinched joints diametrical-ly changes mechanical performance of these joints. It isparticularly visible in case of the brass-brass joint, whereEA of the hybrid joint is approximately equal to 20 timesof the one of the clinched joint and is almost the samelike EA of the adhesively bonded joint.

TABLE 2Energy absorption of the clinched and the hybrid lap joints

types of joined materialsenergy absorption [J]

clinch joint adhesive joint hybrid joint

steel - copper 10,4 58,78 J 44,2

steel - steel 9,6 - 42,1

copper - copper 9,9 - 50,6

brass - copper 3,7 - 30,8

brass - brass 1,9 42,86 J 44,5

Fig. 8. The longitudinal section of hybrid clinched - bonded joint: a) brass – copper joint, b) steel – brass joint, c) steel – copper joint

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The experimental load-displacement hybrid curve ofcopper - steel joint, illustrated by specimen pictures ob-tained in ARAMIS test, is shown in Figs 9 and 10. Fig.9 presents all stages of the deformation process (the ver-tical strain εy and the shear force P) related to the timet. In order to monitor the whole deformation processthree vertical sections: “0”, “1” and “2” were specifiedalong the specimen axis. The most important is the sec-tion “1”, which contains clinched part of the joint. The

vertical strain εydistributions in sections “0” and “2” arealmost homogeneous during the whole loading process.A quite different distribution takes place along section“1”, where the very strong strain concentrations are vis-ible in the die groove.

The hybrid joint failure starts (Fig. 10), as the sheardamage of adhesive layer from the lap edge of the punchside material spreads to the clinch impression (this is vis-ible on the strength curve top as the small step). Then,clinch joint undergoes small deformation.

Fig. 9. The results of ARAMIS test obtained for copper – steel joint

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Fig. 10. Experimental load-displacement hybrid clinch – adhesive curve (copper – steel joint) illustrated by specimen pictures

and the shear damage of the rest of the adhesive lay-er underneath the clinch impression proceeds (the longvertical segment of the curve, picture 1). The last stepsdevelop when a gradual clinch joint failure occurs (pic-tures 2, 3, 4) and this involves the local shearing forcemaximum. The path of the clinch joint failure is longas well as the corresponding segment of the curve. Theshape of this segment depends on the clinch joint qualityand features described above.

4. Conclusions

The mechanical strength of the hybrid clinched –adhesively bonded joints strongly depends on adhesiveand adherents properties. When joined materials un-dergo the plastic deformation, the adhesive propertiesshould match these condition. In the experiments thebrittle and stiff adhesive Dragon was used. Thereforethe adhesive layer did not sustain the plastic deforma-tion during clinching process and the adhesive failureoccurred around clinching indentation. This additionallydemonstrates the influence of the processing technology,when the clinching process introduces the initial micro-damage into the adhesive interface. Local microdamagecauses the decreasing of the shear strength of tested hy-brid joints. Therefore the proper production of the hybridjoints is the crucial point for the further engineering ap-plications.

Introduction of the adhesive into the clinched jointessentially changes the mechanical strength of the joint.Taking into account the wide range of commercial ad-

hesives for professional usage (from stiff and strong toflexible and ductile) the adhesive choice should be care-fully balanced. The choice criterion of adhesive for thehybrid clinched – adhesive joint seems to be similar tomixed adhesive joint. The clinch joint is like a strongadhesive in the middle of the overlap and a flexible ad-hesive should be used in the neighbourhood. The futureinvestigations should focused on determination of suchjoint composition, likes a clinch – flexible adhesive.

The hybrid clinched – adhesively bonded joining isvery new assembly technology and there is lack of itspractical usage for nowadays. Technology is intensive-ly investigated because of expected advantages concern-ing low costs, flexibility and its environmental compat-ibility. The low mechanical strength of simple clinchedjoints (resistance to shear or tensile loading) compared toequivalent spot welds, forces to looking for a new join-ing solutions. The clinched – adhesively bonded joiningseems to be very good combination which can enhanceadvantages of these both methods.

Acknowledgements

Financial support of Structural Funds in the Operational Pro-gramme – Innovative Economy (IE OP) financed from the EuropeanRegional Development Fund – Project ”Modern material technologiesin aerospace industry”, No POIG.0101.02-00-015/08 is gratefully ac-knowledged.

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REFERENCES

[1] T.A. B a r n e s, I.R. P a s h b y, Joining techniques foraluminium spaceframes used in automobiles. Part II –adhesive bonding and mechanical fasteners. J Mater ProcTechn 99, 72-79 (2000).

[2] J.P. V a r i s, J. L e p i s t o, A simple testing – basedprocedure and simulation of the clinching process usingfinite element analysis for establishing clinching para-meters. Thin-Walled Struct. 41, 691-70 (2003).

[3] J. V a r i s, Ensuring the integrity in clinching process.J Mater Proc Techn 174, 277-285 (2006).

[4] N. N o n g, O. K e j u, Z. Y u, Q. Z h i y u a n, T.C h a n g c h e n g, L. F e i p e n g, Research on pressjoining technology for automotive metallic sheets. JMater Proc Techn 137, 159-163 (2003).

[5] M. O u d j e n e, L. B e n - A y e d, On the parametricalstudy of clinching joining of metallic sheets using theTaguchi method. Eng Struct 30, 1782-1788 (2008).

[6] M. O u d j e n e, L. B e n - A y e d, Shape optimizationof clinching tools using the response surface methodol-ogy with Moving Least-Square approximation. J MaterProc Techn 209, 289-296 (2009).

[7] V. J a y a s e k a r a, K.H. M i n, J.H. N o h, M.T. K i m,J.M. S e o, H.Y. L e e, B.B. H w a n g, Rigid-plasticand elastic-plastic finite element analysis on the clinch-ing joint process of thin metal sheets. Met Mater Int 16,339-347 (2010).

[8] Ch.J. L e e, J.Y. K i m, S.K. L e e, D.Ch. K o, B.M.K i m, Design of mechanical clinching tools for joiningof aluminium alloy sheets. Mat Design 31, 1854-1861(2010a).

[9] Ch.J. L e e, J.Y. K i m, S.K. L e e, D.Ch. K o, B.M.K i m, Parametric study on mechanical clinching processfor joining aluminium alloy and high-strength steelsheets. J Mech Sci Technol 24, 123-126 (2010b).

[10] X. H e, Recent development in finite element analysisof clinched joints. Int J Adv Manuf Technol 48, 607-612(2010).

[11] M. C a r b o n i, S. B e r e t t a, M. M o n n o, Fatiguebehavior of tensile-shear loaded clinched joints. EngFract Mech 73(2), 178-190 (2006).

[12] A.A. d e P a u l a, M.T.P A g u i l a r, A.E.M P e r -t e n c e, P.R. C e t l i n, Finite element simulations ofthe clinch joining of metallic sheets. J Mater ProcessTechnol 182(1-3), 352-357 (2007).

Received: 10 January 2011.