Fatigue Strength Evaluation of the Clinch Joints of a Cold Rolled Steel Sheet

IJR International Journal of Railway

Vol. 2, No. 4 / December 2009, pp. 131-138

Vol. 2, No. 4 / December 2009 − 131 −

Fatigue Strength Evaluation of the Clinch Joints of a Cold Rolled Steel Sheet

Ho-Kyung Kim†

Abstract

Static tensile and fatigue tests were conducted using tensile-shear specimens to evaluate the fatigue strength of a SPCC

sheet clinch joint. The maximum tensile strength of the specimen produced at the optimal punching force was 1750 kN.

The fatigue endurance limit (=760 N) approached 43% of the maximum tensile load (=1750 N) at a load ratio of 0.1,

suggesting that the fatigue limit is approximately half of the value of the maximum tensile strength. The FEM analysis

showed that at the fatigue endurance limit, the maximum von-Mises stress of 373 MPa is very close to the ultimate ten-

sile strength of the SPCC sheet (=382 MPa).

Keywords : Clinch joining, Fatigue strength, FEM

1. Introduction

Type Spot welding is generally used as a joining method

of car parts that are mainly composed of low-carbon steel

sheets. However, the ground vehicle industry is gradually

pursuing the increased use of lighter materials such as alu-

minum and magnesium alloys. New and effective means

of joining of these materials are required as spot welding

is inappropriate for dissimilar materials or for materials

that have poor weldability. Clinch joining is considered to

be viable joining method for dissimilar materials [1-3]. In

addition, it is possible to join precoated metal sheets and to

make clean joints of a consistent strength using clinch

joining. Other advantages of the clinch joining in produc-

tion include the absence of joining components, no genera-

tion of poisonous gas, a relatively quite process, and low

energy consumption [3].

As shown in Fig. 1, the clinch joining process is a join-

ing method that folds deep drawing sheets into a cup shape

via punch and die tooling. The punch pushes the sheet

metals into the die with a grooved bottom perimeter until

the bottom sheet flows into the groove and the upper sheet

flows outward, forming a mechanical interlock. The joint

is similar to a button. A schematic diagram of the cross-

section of the joint is shown in Fig. 2.

In spite of the fact that clinch joining is widely used,

systematical investigations of the joints are seldom found.

Nearly all of the studies that do exist were conducted in

conditions that include a monotonic tensile strength and

FEM simulation of the clinch joining process [4-7]. Davies

et al. [4] proposed a method to predict the shear strength

using the mechanical properties of the metal sheet, the

sheet thickness, and the joining force. de Paula et al. [5]

conducted a simulation of the influence of changes in the

die and punch geometry on the material flow and result-

ing neck thickness and undercutting of clinch joints, which

directly affect the joint strength. Chung et al. [6] quanti-

fied the optimum joining condition of a clinch joint under

complex loading by optimizing the strength ratio with a

correlation between the diameter ratio (the button diame-

ter to the punch diameter) and the sheet thickness ratio

(the upper sheet thickness to the lower sheet thickness).

The clinch joints invariably contain stress concentrations

that are the major sites for the fatigue initiation. Only lim-

ited numbers of studies have been conducted on the

fatigue strength of clinch joints. For example, Carboni et

al. [8] investigated the fatigue strength of a tensile-shear

specimen produced by clinch joining for a zinc-coated

steel sheet. They found that the fatigue limit is close to

50% of the ultimate strength of the joint. However, they

correlated fatigue life using the applied amplitude load.

This method cannot predict the fatigue strength systemati-

cally, as the fatigue life of a clinch joint specimen is

expected to be governed not only by the load magnitude

but also by the loading type, button diameter, specimen† Department of Automotive Engineering, Seoul National University of technology

E-mail : [email protected]

− 132 − IJR International Journal of Railway

Ho-Kyung Kim

thickness, specimen width, and other factors.

Therefore, in this study, tensile and fatigue tests were

conducted on tensile-shear specimens of a cold rolled mild

steel sheet. The specimens were produced with optimal

joining force, and fatigue life of the clinch joint speci-

mens was evaluated. FEM analysis of the joint was con-

ducted to acquire the stress distribution at the fatigue limit

load.

2. Experimental procedures

2.1 Specimen preparation

The material selected for use in this study was cold

rolled mild steel (SPCC) with a thickness of 0.8 mm, as

used in the automotive industry. The mechanical proper-

ties of the steel were measured on an universal testing

machine (Instron 8516). The engineering stress-strain

curve for the steel is illustrated in Fig. 3, and the mechani-

cal properties of the steel are summarized in Table 1. The

chemical compositions of the sheets are shown in Table 2.

For the evaluation of the static and fatigue joining strength

Fig. 1 Clinch joining tool set.

Fig. 2 The dimensions of a cross-section of clinch joint.

Table 1. Typical span type on the high speed track in order of

distributed numbers

Materialσu

(MPa)

σy

(MPa)

E

(GPa)

εf

(%)

SPCC 382.1 247.1 210 31.8

Table 2. Chemical compositions of the SPCC (wt. %)

Material C Mn P S Al Fe

SPCC 0.04 0.25 0.01 0.005 0.05 bal.

Fig. 3 Engineering stress-strain relationship for the SPCC steel

sheet.

Fig. 4 Configuration of the tensile-shear specimen (L=100mm,

W=30mm).


Vol. 2, No. 4 / December 2009 − 133 −

of the joint, the geometry of the tensile-shear specimen, as

shown in Fig. 4, was adopted according to JIS Z3136 for

the spot welding specimen. Test specimens were prepared

using clinch joining 30 mm by 100 mm pieces of sheet

together with a 30 mm overlap. A servo-hydraulic univer-

sal testing machine (Instron 8516) with a capacity of

100 kN was adopted for the joining, tensile and fatigue

testing, as shown in Fig. 5. A punch with a diameter of

5.4 mm and a die with a diameter of 8.3 mm (TOX Cor-

poration Hamburg, Germany) were used to join the speci-

mens. A series of tensile tests were conducted on

specimens produced with different punch forces to acquire

the optimal punch force using the same universal testing

machine.

2.2 Tensile and fatigue tests

Tensile tests were carried out at a cross-head speed of

2 mm/min. In view of the static joining strength of the

specimen geometry, the optimal punch force was found to

be 70 kN according to the tensile test results for the speci-

mens produced with different punch forces. Therefore,

fatigue specimens, prepared with a punch force of 70 kN,

were tested under the load ratio R=(Pmin/Pmax)=0.1. The

applied cyclic loading waveform was sinusoidal and the

frequency of loading used varied from 5 to 20 Hz accord-

ing to the load amplitude. Failure was defined as the com-

plete separation of the specimens into two parts. The joint

of the specimen was cross-sectioned and successively pol-

ished with alumina polishing powders to 0.1 µm to mea-

sure the dimensions of the joint produced with the optimal

punch force. Measurements were made using an optical

microscope. The measured dimensions of the cross-sec-

tion of the clinch joint are summarized in Table 3.

2.3 Structural analysis of the joint

FEM analyses were carried out by means of ABAQUS

software (version 6.6) for the solver and HyperMesh soft-

ware (version 7.0) as the pre- and post-processor. A single

point lap joint was modeled using a HEXA element

(C3D8) and a PENTA element (C3D6). The model was

composed of 48,420 nodes and 42,956 elements. Contact

between the substrate faces was introduced by means of

the master-slave technique.

3. Experimental results and discussion

3.1 Optimal punch force for clinch joining

The joint strength of a clinch jointed specimen is typi-

cally affected by the sheet thickness, punch diameter, and

applied punch force. To determine the optimal punch force

for a steel sheet with a thickness of 0.8 mm and a punch

diameter of 5.4 mm, a series of tensile tests were per-

formed on the clinch jointed specimens as produced by

varying the punch forces. Fig. 6 shows the applied punch

force against the vertical displacement of SPCC sheets

during the clinch joining process. This curve can be

Fig. 5 A servo-hydraulic universal testing machine used for

clinch joining, tensile and fatigue tests.

Table 3. The dimensions of the cross-section of a clinch joint

produced at a punching force of 70 kN

Items Definition Size (mm)

BD Button diameter 8.3

CT Cap thickness 0.4

h Height 1.9

NT Neck thickness 1.4

PD Punch diameter 5.4

t1 Punch side metal thickness 0.8

t2 Die side metal thickness 0.8

Fig. 6 Punch force against displacement for a SPCC specimen

during clinch joining.


Ho-Kyung Kim

divided into three regions: a first region from the start to a

depth of 0.5 mm, a second region from 0.5 mm to 1.5 mm,

and a third region past a depth of 1.5 mm. In the first

region, the punch forces the upper and lower metal sheets

to adhere to the surfaces of the die, starting the pressing

process.

In the second region, the punch moves downward and

extrudes the sheets. This action requires more force for

plastic deformation, suggesting that the slope of region

two is steeper than that of region one.

The punch force increases abruptly past a depth of

approximately 1.5 mm, indicating that the metal sheets

reach the bottom of the die and continue to fill the die

impression gradually. At this point, much greater deforma-

tion force is necessary and the steepest slope is exhibited.

The maximum tensile shear strengths of the specimens,

produced under various punch forces, are shown in Fig. 7.

The tensile-shear specimen failed in the pull-out failure

mode before the punch force of 45 kN because the punch

force could not sufficiently join the upper and lower sheets

together. The failure mode was interface failure after a

punch force of 45 kN. The maximum tensile shear strength

increased continuously with the punch force up to a force

of 70 kN. However, the maximum strength decreased past

70 kN. Therefore, all of the fatigue specimens were pro-

duced at the optimal punch force of 70 kN. No gap existed

between the upper and lower sheets under a punching

force of 70 kN.

3.2 Static strength evaluation of the clinch

joint

Fig. 8 represents the applied load against the displace-

ment curve of a tensile-shear specimen produced using a

punch force of 70 kN. At a maximum applied load of

approximately 1750 N, cracking occurred at the neck of

the upper sheet (inner button part): consequently, the upper

and lower sheets unlocked, slipped away from each other,

and separated. Thus, some amount of applied load could

be maintained and decreased slowly without an abrupt

failure. This type of behavior is generally dependent on the

upper sheet. This behavior occurs more often when the

upper sheet is thicker and more ductile, as a thicker upper

sheet with more ductility has a greater capability to main-

tain the applied load at the neck, which is most critical in

the specimen [6]. In contrast, a thinner and more brittle

upper sheet has a thinner neck, giving it less capability to

maintain the applied load at the neck, eventually resulting

Fig. 7 Punch force against the maximum tensile-shear strength

for a SPCC specimen.

Fig. 8 Applied load-displacement curve of a clinch joint for

SPCC tensile-shear specimen.

Table 4. Summarized fatigue testing results

Max. Load (N) Nf (cycle) Failure Type

1891 289 pull-out

1862 346,050 interface

1813 263 pull-out





1705 5,383,800 non-failure

1666 1,705,800 interface

1617 2,549,000 interface

1568 1,682,800 interface


Vol. 2, No. 4 / December 2009 − 135 −

in an abrupt shear rupture [6].

3.3 Fatigue strength evaluation of a clinch

joint

Fatigue tests were conducted on specimens produced

using the optimal punch force. The results, including the

failure modes, are summarized in Table 4. Table 4 shows

that two different failure modes (interface failure and pull-

out failure) were observed, as shown in Fig. 9 (a) and (b).

In intermediate and high-cycle regions (Nf>1000), the

interface failure mode occurred, where a fracture occurred

at the neck of the upper sheet (inner button part), as shown

in Fig. 9 (a). The fatigue crack appeared to have initiated

at the neck. It then propagated through the sheet thickness

along the neck and finally led to the failure, as shown in

Fig. 9 (a). In contrast, in the low-cycle region (Nf<1000),

a pull-out failure mode occurred in which the failure was

caused by the separation between the sheets due to crack-

ing along the neck of the upper sheet. This resulted in a

loss of the capacity to support the applied load, as shown

in Fig. 9 (b).

The fatigue life of the clinch joints are plotted in Fig. 10

as a function of the applied load amplitude. The fatigue

endurance of the specimen at N=2.5×106 was approxi-

mately 760 N, which is nearly 50% of the maximum ten-

sile load (1750 N) of the specimen taking into

consideration a load ratio of 0.1. This suggests that the

fatigue behavior of a clinch joint is similar to that of steel,

which has a fatigue endurance that is generally close to

50% of its ultimate tensile strength. This result is similar

to that of Carboni et al. [8]. They reported that the fatigue

limit of clinch joint specimens is close to 50% of the max-

imum tensile load when using a zinc-coated 550 MPa-

grade constructional steel sheet. The fatigue endurance

value of 760 N can be converted into a maximum applied

load of 1670 N with a load ratio of 0.1. The ratio of

fatigue endurance to the static strength of the joint is 95%.

Although the clinch joint has poor static strength, the ratio

of fatigue endurance to the static strength of the joint is

excellent, relative to the spot weld joint, where the static

strength is excellent and the fatigue endurance strength is

less than 40% of the static strength [8]. This is due to the

fact that the spot weld joint inevitably has a high stress

concentration at the edge of the weld spot due to the pres-

ence of an inherent sharp notch at the interface between

the sheet metals. Moreover, the clinch joint has a much

lower stress concentration at the neck of the button, where

there is a critical point, resulting in a higher ratio of fatigue

endurance to static strength. The relationship between the

load amplitude and number of cycles parameter can be

expressed as ∆P =894.7−0.011.

Several investigators [9-10] have developed fatigue

models that predict the fatigue lives of spot welds through

the use of a stress intensity factor. In the present study, it is

worthwhile to predict the fatigue life of clinch joints based

on a fatigue crack growth approach in an effort to investi-

Fig. 9 Fatigue fractured specimens in (a) interface failure and

(b) pull-out failure modes.

Fig. 10 Load amplitude against the number of cycles for a

SPCC tensile-shear specimen.


Ho-Kyung Kim

gate the applicability of this model to clinch joints. The

interface failure appears to be dominated by the through-

thickness cracking on the upper sheet (inner button part)

according to an examination of the cross-sections of the

failed specimens under cyclic loading conditions. The

Paris law can describe the fatigue crack propagation as

(1)

where a is the crack length, N is the number of cycles, C

and m are material constants, and ∆Keq is the range of the

equivalent stress intensity factor. The stress intensity fac-

tor is assumed to remain nearly constant during the fatigue

crack growth process. The fatigue life of clinch joints can

then be obtained by integrating Eq. (1) as follows:

(2)

Here, the total crack growth distance is assumed to be

equal to the sheet thickness (=0.8 mm) because once a crack

passes through the thickness of upper sheet, the sheets sepa-

rate and the specimen fails, as shown in Fig. 9 (a).

The stress intensity factor for a clinch joint in tensile-

shear geometry is not available. Therefore, the stress inten-

sity factor for a spot weld was adopted. Although the nug-

get of a spot weld joint has a more severe stress condition

due to the presence of an inherent sharp notch in the inter-

face between the sheet metals relative to the button of the

clinch joint, its button is assumed to be identical to a nug-

get. Recently, Radaj and Zhang [9] derived a formula of

the equivalent stress intensity factor for the case of a ten-

sile-shear type specimen. The formula shown below was

applied to predict the fatigue lifetime for a fatigue speci-

men.

(3)

The effect of a load ratio R of 0.1 was neglected in this

investigation. To acquire the material constants C and m, a

fatigue crack growth test was conducted on a SPCC sheet

of identical thickness using compact tension specimen

geometry. The material constants C and m were deter-

mined as 2.3×10−12 MPa(m)0.5 MPa and 3.38, respec-

tively, as shown in Fig. 11~12 shows the predicted fatigue

lifetimes based on the fatigue model in Eq. (2) and on the

experimental results for the SPCC clinch joint in tensile-

shear geometry. As shown in this figure, the predicted

fatigue lifetimes based on the stress intensity factor solu-

tion of Radaj and Zhang [9] is in good agreement with the

experimental results only in the high-cycle regime. How-

ever, in the low-cycle regime, the predicted lifetimes do

not agree with the experimental results. This error could be

inferred from the inaccurate value of ∆Keq, as ∆Keq

applied only to the spot weld joint, not to the clinch joint.

Another error may be due to the change in the fatigue

crack growth properties (C and m) of the neck after being

subject to cold working conditions.

Furthermore the value of ∆Keq in the clinch joint and the

material constant values of C and m after cold working are

necessary for more accurate fatigue life predictions of

clinch joints.

3.4 Structural analysis of the clinch joint

Stress levels exceed the yield strength of the material

(equal to nearly 250 MPa) after linear elastic analysis of the

specimen geometry, suggesting that the use of non-linear

elasto-plastic analyses is necessary. For the structural analy-

sis, therefore, the work hardening slope for plasticity was

da

dN------- C K

eq∆( )

m=

Nt

C Keq

∆( )m

-----------------------=

Keq

∆0.694 F∆

πD t

---------------------=

m

Fig. 11 Fatigue crack growth behavior of the SPCC steel sheet. Fig. 12 Predicted fatigue lifetimes and experimental results for

a SPCC clinch joint in tensile-shear specimens.


Vol. 2, No. 4 / December 2009 − 137 −

defined using the true stress and strain relationships. Formaterials that behave in a ductile manner, the plastic strainsare large in comparison with the elastic strains. Thus, it isfeasible to convert the engineering stress-strain curve inthis study. The true strain ε was obtained from the engi-neering strain ε according to the following relationship:

(4)

The true stress σ was also converted by the engineeringstress S by the following equation:

(5)

The flow stress for this material can be expressed as ó =632.5ε0.195. The stress analysis results of the tensile-shearspecimen at an applied load of 1670 N (equal to a fatigueendurance limit value of 760 N with a load ratio of 0.1)are shown Fig. 13. The maximum von-Mises stress occurson the upper sheet neck where the upper and lower sheetsconnect together.

This location coincides with the region where crackswere observed on the neck, which is also the region of thethinnest part of the sheet. The maximum von-Mises stressof 373 MPa at the fatigue endurance limit is close to theultimate tensile strength of the SPCC sheet (=382 MPa).This implies that the true tensile strength of the neck issignificantly improved through cold working during clinchjoining. This idea is supported by the maximum true ten-sile strength of 435 MPa. Finally, FEM analysis could sup-port the experimental observations by showing a highstress concentration in the critical region where failuresoccurred and predicting the stress distribution of the clinchjoint correctly.

4. Conclusion

Static tensile and fatigue tests were conducted using ten-

sile-shear specimens for an evaluation of the fatiguestrength of clinch joint specimens created from a coldrolled mild steel sheet. The following conclusions can bedrawn from the experimental work and FEM analysis.

1) The optimal applied punch force was found to be70 kN to achieve maximum tensile load resistance duringthe joining process for SPCC steel sheets considering thesheet thickness and the punch diameter.

2) For the tensile-shear test, cracking occurred at theneck of the upper sheet (inner button part). As a result, theupper and lower sheets unlocked, slipped away from eachother, and separated at a maximum applied load of approx-imately 1750 N.

3) The fatigue endurance limit (760 N) approached 43%of the maximum tensile load (1750 N) at a load ratio of0.1, suggesting that the fatigue limit is nearly half of thevalue of the maximum tensile strength. A relationship wasfound between the applied load amplitude ÄP and the life-time of the cycle N. The relationship can be expressed as∆P = 894.7N-0.011.

4) The FEM analysis shows that the maximum von-Mises stress of 373 MPa at the fatigue endurance limit isclose to the ultimate tensile strength of the SPCC sheet(382 MPa). This implies that the true tensile strength ofthe neck is significantly improved through cold workingduring clinch joining.

1. Sawhill J.M. and Sawdon S.E.(1983), “A new mechanical

joining technique for steel compared with spot welding,”

SAE paper; 830128, pp. 1-12.

2. Larsson J.K.(1994), “Clinch joining-A cost effective joining

technique for body-in-white assembly,” Advanced Technolo-

gies & Processes. IBEC'94, pp. 140-145.

3. Pedreschi R.F. and Sinha B.P.(1996), “The potential of press-

ε 1 e+( )ln=

σ S 1 e+( )=

Fig. 13 (a) Analysis result and (b) detail of the tensile-shear specimen under an applied load of 1670 N.


Ho-Kyung Kim

joining in cold-formed steel structures,” Construction and

building materials, Vol. 10, pp. 243-250.

4. Davies R., Pedreschi R. and Shiha B.P.(1996), “The shear

behavior of press-joining in cold-formed steel structures,”

Thin-Walled Structures, Vol. 25, pp. 153-170.

5. de Paula A.A., Aguilar M.T.P., Pertence A.E.M. and Cetlin

P.R.(2007), “Finite element simulations of the clinch joining

of metallic sheets,” Journal of Materials Processing Technol-

ogy, Vol. 182, pp. 352-357.

6. Chung C.S., Cha B.S. and Kim H.K.(2001), “Optimum join-

ing conditions in a mechanical press joint,” Materials and

Manufacturing Processes, Vol. 16, pp. 387-403.

7. Varis J.P.(2003), “The suitability of clinching as a joining

method for high-strength structural steel,” Journal of Materi-

als Processing Technology, Vol. 132, pp. 242-249.

8. Carboni M., Beretta S. and Monno M.(2006), “Fatigue

behavior of tensile-shear loaded clinched joints,” Engineer-

ing Fracture Mechanics, Vol. 73, pp. 178-190.

9. Radaj D. and Zhang S.(1997), “Stress intensities at spot

welds,” International Journal of Fatigue, Vol. 88, pp. 167-185.

10. Lin S.H., Pan J., Wung P. and Chiang J.(2006), “A fatigue

growth model for spot welds under cyclic loading condi-

tions,” International Journal of Fatigue, Vol. 28, pp. 792-803.

Fatigue Strength Evaluation of the Clinch Joints of a Cold Rolled Steel Sheet

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