Influence of plastic anisotropy on the mechanical behavior of clinched joint of different coated thin steel sheets S. Saberi1, N. Enzinger1, R. Vallant1, H. Cerjak1, J. Hinterdorfer2, R. Rauch2 1
Institute for Materials Science and Welding, Kopernikusgasse 24, A- 8010 Graz, Austria e-mail:
[email protected];
[email protected]; URL: www.tugraz.at
[email protected];
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voestalpine Stahl GmbH, voestalpine-Straße 3, A- 4031 Linz, Austria URL: www.voestalpine.com e-mail:
[email protected];
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ABSTRACT: This work describes an experimental and numerical study of the TOX®- clinching process and its mechanical strength due to different surface conditions (blank, electro galvanized, corrosion protection primer (CPP) coated). The main purpose of using different types of surface conditions is to analyze their influences on the geometry and mechanical strength of the clinched joint. Finite element analysis (ABAQUSExplicit) is used to model the clinching process as well as the subsequent loading of the joint in the shear and cross tension test. The influence of the plastic anisotropy of the material is analyzed by evaluation of the punch force-displacement curve and the strength of clinched joint under these loading conditions. Key words: Thin steel sheets, TOX- Clinching process, plastic anisotropy, Numerical simulation
1 INTRODUCTION
2 CLINCH PROCESS
Clinching is a joining method in which sheet metal parts are deformed locally without the use of any additional element [1]. The first patent for clinching was granted in Germany in 1897 [2]. However, clinching was not used on the industrial scale until the eighties.This work describes an experimental and numerical investigation of TOX clinch process. Influence of different coatings on the mechanical strength of clinched joints is analysed for the commercial steel H180Y. The simulation of TOX clinch process is carried out using the finite element program ABAQUS. In order to avoid deformation locking under material incompressibility the penalty method is applied [3]. In the range of large plastic deformation (such as clinching) the phenomena of strain hardening occurs [4]. This hardening behaviour of the material plays an important role in numerical simulation and the flow curve from tensile test should be extrapolated by using some mathematically functions (e.g. Ludwik, Swift). Unbutton failure mode of clinched joints under quasi static loading is predicted numerically.
The clinching process is a combination of drawing and forming that locks together sheet metal layers [5]. The model consists of two sheets and one punch, one fixed die and one blank holder. The TOX clinch model is schematically shown in figure 1.
Fig. 1. Geometry model of TOX [6]
The blanks are plastically deformed and the shape of the tools remains theoretically unchanged during the clinching processes. The punch is movable, whereas the blank holder and the die are fixed during the process. The punch force needed for the joining depends on the thickness and the strength of the
materials to be joined, the size of the tools, friction coefficient and usually varies from 10 to 100 kN [7]. The main parameters describing a clinched joint are shown in figure 2.
In materials in which the properties depend on direction, the state of anisotropy is usually indicated by R- value [8]. Experiments show that R depends on the in-plane direction. The average of the Rvalues obtained for different directions (rolling 0°, diagonal 45°, and transverse 90°) in the plane of sheet represents the coefficient of normal anisotropy coefficient and can be determined from equation (1) [9].
R=
Fig. 2. Characteristics of Clinch Joint
R0 + 2 R45 + R90 4
(1)
A measure of the variation of normal anisotropy with the angel to the rolling direction is known as planar anisotropy and can be obtained from equation (2) [9] R0 − 2 R45 + R90 2
3 TOOL GEOMETRY
ΔR =
In this study the clinch joints are formed with TOX tools. The geometry of investigated clinch tools is shown in figure 3.
The R- values are shown in Table.1. All tests regarding the properties of H180Y were performed at voestalpine Stahl GmbH, Linz, Austria.
(2)
5 CORROSION PROTECTION PRIMER (CPP)
Fig. 3. TOX clinch tool
4 MATERIAL CHARACTERISTIC The investigated material H180Y is an interstitial free (IF) cold-rolled steel strip of higher strength, acc. to Standard EN10268:2006-10. The steel sheets are classified on the basis of surface conditions (B01: non-coated/blank; B02: electro galvanized 5.0 µm thick; B06: electro galvanized, pre-treated and CPP coated). The material properties of H180Y measured in the tensile test are shown in table1.
Corrosion protection primer is provided in cooperation with Chemetall GmbH, Frankfurt/Main. The CPP used for this investigation (B06) has a thickness of 2.5 to 4.0 µm and consist of a high percentage of electrical conductive Zinc-pigments, corrosion protection pigments (amorphous silica), a few percentages of Tungsten pigments, insulation polymer matrix for bonding and some other organic pigments. Since the CPP contain Zn-pigments in a polymer matrix, the system is soft and the pigments are extremely deformed during the clinching process. The cross section of investigated CPP coated blank is shown in figure 4.
Table1. Material properties for H180Y
Fig. 4. Cross section of CPP, schematically [10] and as observed in LOM
6 EXPERIMENTAL PROCEDURE
7 NUMERICAL SIMULATION
The TOX clinch tests are carried out for different coated steel thin sheet H180Y, as described. The effect of coating on the punch force and geometry of clinched joint are experimentally analysed. More than 90 experiments have been done on the basis of different bottom thickness (0.60-0.35 mm) for six different coating surfaces. Experimental results show that the maximum punch force to complete the clinching process is a little bit lower than non-coated for coated blanks. The experimental punch- force curves for B01/non-coated, B02/electrogalvanized and B06/ CPP are shown in Figure 5.
7.1 Simulation of TOX- clinching process
Modified Experimental Punch forceDisplacement diagram
Fig. 7. 2D- FE Model for TOX clinching process
Punch force (kN)
60 50
B01_Mod B06_Mod B02_Mod
40 30 20 10 0 0
1 2 Punch displacement (mm)
Finite element simulation of TOX®- clinching process carried out by using ABAQUS- Explicit. The simulation model is shown in figure 7.
3
As the clinching process requires a large plastic deformation, it is necessary to extrapolate the true stress-strain curves up to at least 200% strain in order to simulate the clinching process using ABAQUS program. Flow curve for FE- simulation are extrapolated based on Hollomon and Ludwik [11]. Figure 8 shows a good agreement between simulation and experimental results of TOXclinching process.
Fig. 5. Punch- force displacement curve of TOX- clinching test for different coated sheets (final bottom thickness 0.35mm) (B03)
The Punch force-displacement curves are very similar and the maximum punch forces show small differences within the range of measurement accuracy. Experimental shear tension tests are also carried out for the three different surface conditions. The results in figure 6 show that the CPP coated clinched joints have a relative lower maximum shear forces than blank joints.
(B06)
Fig. 8. Comparison of experimental result and simulation for TOX- clinching process
7.2 Influence of anisotropy on clinched joint
Fig. 6. Experimental Shear force- Bottom thickness curve
Hill’s anisotropic plasticity potential is defined in ABAQUS from user input consisting of ratios of yield stress in different directions with respect to a reference stress [12]. In the simulation both normal and planar anisotropy have been considered, but the planar anisotropy ΔR has more effective influence on the simulation results. Thus the ΔR consideration in the simulation gives better results if compared to the experiment. The ΔR effects on the geometry of clinched joint are also analyzed and simulation results show good agreement with experiment.
7.3 Simulation of TOX- clinched joint under quasi static loading FE- Simulation of clinched joints under quasi static loading is carried out. Considering mass scaling technique simulation gives very good results. Simulation model and numerical results are compared with experimental data in figures 9-11.
force for coated sheets is a little bit lower than for non-coated sheets what proofs the effect of frictional coefficient µ. Therefore the ΔR-effect on the maximum shear force for coated sheet may be reduced by reducing the friction force due to the surface conditions and lower predicted shear force. 8 CONCLUSIONS
Fig. 9. Simulated TOX- clinched joint (von Mises Stress distribution)
Three different coated thin steel sheets have experimentally been investigated in the TOX- clinch process. It has been found that there are minor influences of the coating system on the punch force and joining parameters. The influence of planar anisotropy is analyzed by means of a FE- simulation. It has been found that anisotropic behavior of the material in the simulation leads to a higher maximum shear force, which is close to the maximum shear force of TOX-joints for blank sheet. The predicted geometry of clinched joints considering the planar anisotropy of the material gives very similar results if compared to the experiments. ACKNOWLEDGEMENTS
Fig. 10. Simulated TOX- clinched joint under quasi static shear tension (von Mises Stress distribution)
This work is financial supported by the K-net JOIN - Network of Excellence for Joining in Austria. REFERENCES 1.
2. 3.
4. 5.
Fig. 11. Comparison of simulated shear force- displacement curve with experimental results
Predicted shear force considering the planar anisotropy ΔR of the material gives a higher maximum force in compare to isotropic behaviour. The experimental results show that the maximum
6. 7. 8. 9. 10. 11. 12.
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