1. EFFECT OF FLANGE LENGTH AND BLANK HOLDER FORCE ON CRACK INITIATION IN STRETCH FLANGING PROCESS  S. K. Panthia*, Yogesh Dewangb, M. S. Horab, Meraj Ahmeda aCSIR-Advanced Materials and Processes Research Institute, Bhopal-26, India bMANIT, Bhopal-03, India * Corresponding author- sanjay_
Need for the Study
Figures & Tables
Results & Discussion
INTRODUCTION
Flanging is one of the important sheet metal forming process which is used in automotive industry, aerospace industry, domestic appliance etc. to manufacture the various products or the part of any component. Stretch flanging is one of the flanging processes which are generally used to manufacture the concave shape products or part of the product with the help of tool setup. Crack in edge of the sheet during forming and thinning of sheet are two major failures in forming of such components. These failure depends on many geometric, material and process parameters such as radius of die, punch diameter, mechanical properties of material, friction, blank holder force, gap between die and punch, flange length etc. The design of such concave components considering the above parameters, which are also dependent on each other, are a very tedious, expensive and time consuming work by experimentation. But in increase in processing speed of computers and development of graphic oriented pre-processors has opened the door of the FEM to design such components and to predict the failures during forming. It is a very effective and less time consuming method. In this paper effect of blank holder force and flange length on crack initiation and propagation in stretch flanging process is investigated through FEM and experiments. Three different cases of blank-holder force (rigid fixed blank-holder, no blank-holder force and constant blank-holder forces of different intensities) and three different initial flange lengths (20 mm, 30mm and 40 mm) are considered for present investigations.
COMPARISON OF CRACK INITIATION AND PROPAGATION FOR INITIAL FLANGE LENGTH
The results of simulation are validated with experimental results in terms of edge crack initiation and propagation. Figure 5 shows the comparison of FEM simulation and experimental results in terms of edge crack location and its propagation in the flange. These are presented for flange length of 20 mm, 30 mm and 40 mm at punch-die clearance of 1 mm to find out the effect of flange length on crack. It is found that the crack originates from both ends along die profile radius in all cases. The crack length increases with increase in initial flange length because of increase in circumferential strain which, in turn, leads to edge crack along die profile radius. A good agreement is found between simulation and experimental results in terms of crack length in flange.
COMPARISON OF CRACK INITIATION AND PROPAGATION FOR BLANK HOLDING FORCE
The predicted crack initiation and propagation by finite element simulation is validated with the experimental results. Experiments are conducted at different blank holding force conditions .The comparison of edge crack location by simulation and experimental study for without blank holding force condition is shown in figure 6. It is found that due to absence of blank holding force stretch flange is not formed properly. It is observed from simulation and experimental results that the material accumulates along die profile due to absence of blank holding force and very less circumferential strain induced in the flange due to which edge crack not found in the flange.
Edge crack location for rigid fixed blank-holder case is also shown in figure 6. In this case blank is clamped tightly between blank holder and die. It can be seen that edge crack originates from both ends of flange and propagate towards center of flange. It is important to notice here that maximum edge crack propagation occurs in this case as maximum circumferential strain induced along die profile radius. A very good agreement is obtained between the results of FEM simulation and experiments in terms of edge crack location and crack propagation.
EFFECT OF FLANGE LENGTH
Flange length is an important process parameter which greatly influences the crack initiation and propagation in stretch flanging process. It also effect circumferential strain induced in stretch flange. Figure 7 shows the effect of flange length on maximum circumferential strain. It is found that maximum circumferential strain non-linearly with increase in flange length. An increment of approximately 19% is found in maximum circumferential strain with increase in flange length from 20 mm to 40 mm. In addition to this, it is also gathered from figure 5 that crack initiates from corner edge of flange and propagates towards center of sheet. It is found that higher edge crack propagation is observed with increase in flange length. The higher will be the flange length greater will be edge crack propagation as higher circumferential strain is induced in higher flange lengths.
EFFECT OF BLANK HOLDING FORCE
Figure 8 shows the variation in maximum circumferential strain with respect to blank holding force. The maximum circumferential strain at edge of the flange increases with increase in blank holding forces. The difference in circumferential strain at 1 KN and 5 KN is found approximately 15%. Figure 9 shows the contour plots of circumferential strain at different blank holding forces in a step of 1 KN for a range of 1 KN to 5 KN. It can also be seen through contour plots that circumferential strain increases with increase in blank holding force. It is also observed from this figure that the crack initiates from the edge of flange and then propagates towards from the corner edge and then propagates towards the centre of sheet. It is found that higher edge crack propagation takes place with increase in blank holding force. Besides, this edge crack propagation can also be quantified as circumferential strain increases appreciably upon increment of blank holding force. This shows that blank holding force has a great influence in forming of stretch flange.
Fig.1. Definition of various portions of stretch flange
Fig. 2. True stress-true strain curve for AA 5052
Table . 1. Chemical composition of AA 5052
Die
Punch
Blank-holder
Element Si Fe Mg Mn Cu Cr Al
Wt.%
0.25
0.40
2.8
0.10
0.35
95.70
Table . 2. Mechanical Properties of AA 5052
Mechanical Property
Material AA 5052
Mass density (ρ)
2680 kg/m3
Young’s Modulus(E)
70.3 GPa
Poisson’s ratio(υ)
0.33
Sheet metal blank
Fig. 3. FEM simulation model of stretch flanging
Methods
MATERIALS BEHAVIOR
Uniaxial tensile testing of the aluminum alloy 5052 sheet has been prepared as per ASTM E8 standard and it is shown in the figure 2. The tensile samples are tested on a computerized universal testing machine INSTRON at a strain rate of per second for AA 5052 at room temperature along rolling direction. After testing the samples the following true stress-true strain curve is obtained as shown in figure 2 below. Table 1 and Table 2 show the chemical composition and mechanical properties of AA 5052 of thickness (t) = 0.5mm.
EXPERIMENTS OF STRETCH FLANGING PROCESS
In order to validate the results of finite element simulation, experimental results of stretch flanging process are obtained using hydraulic press equipped with a load cell of 2 KN. The experimental tool set up used in the present study is shown in Figure 4. The tool setup comprises of punch, die and blank-holder with a rectangular sheet metal blank. The experiments are carried out by clamping the work piece between blank-holder and die by using four bolts of M10 size. A constant rate of displacement 10 mm/min was applied to punch for formation of flange over the die. The punch displacements and punch load were continuously recorded in a data acquisition system. The experimental results are obtained considering similar geometrical and process conditions as that for simulation for validation.
Fig. 4 Experimental tool and workpiece
(a) (b) (c) (d) (e) (f)
Fig.5. Comparison of edge crack initiation and propagation in stretch flange at different initial flange length (i) FEM simulation :(a) L = 20 mm (b) c =30 mm (c) c = 40 mm (ii) Experiments: (d) L = 20 mm (e) L= 30 mm (f) L= 40 mm
Analysis
FINITE ELEMENT MODELING AND SIMULATION
The explicit finite element code ABAQUS /Explicit is used to simulate the stretch flanging process in this study. Figure 3 indicates the FEM model of the stretch flanging process. Three dimensional C3D8R element type is used for modelling the sheet metal blank. Punch, die and blank-holder are the tool components were treated as rigid bodies and are modelled with using 4-noded bilinear quadrilateral discrete rigid 3D (R3D4) elements. The blank material is assumed to be an elasto-plastic material with isotropic hardening. The material properties of the blank used in the simulation is taken as per Table 2.Friction is modelled between the blank and the tool interfaces by using the Coulomb assumption in all cases as μ = 0.1. The damage initiation criteria (D) are defined as function of equivalent plastic strain for prediction of failure in flange. The geometrical dimensions of punch, die, holder and sheet metal blanks are taken similar as that of experiments. In the present study there are three different cases of blank holding conditions which are considered. In all three cases die remains fixed and punch travels along vertical downward direction for formation of stretch flange.
First of all the case of rigid fixed blank-holder is considered which consists of rigidly clamping the workpiece between die and blank holder .The model of rigid fixed blank-holder case was obtained by restricting the upward motion of the blank holder along punch travel direction which in turn also restricts the motion of workpiece. The model of without blank-holder force was made by applying a blank holding force of zero intensity on the blank holder and now by allowing the displacement of blank holder along punch travel direction. Finally, the model, was obtained by applying various constant blank holding forces (1 KN,2KN,3KN,4 KN and 5KN) on blank holder in order to obtained change in thickness without restriction of displacement of blank-holder along punch travel direction.
Conclusions
(a) (b) (c) (d)
Fig. 6. Comparison of edge crack initiation and propagation (i) Without blank-holder force (a) FEM simulation (b) Experiment (ii) Rigid fixed blank-holder (c) FEM simulation (b) Experiment
In the present work, the finite element simulation and experimental study have been carried out for forming the curved flange using stretch flanging process to predict the crack initiation and its propagation in the flange. The crack length in flange during forming increases with increase in initial flange length of blank and it is due to increase in stretching of sheet in circumferential direction which leads to increase in strain and results in crack initiation at edge of the sheet and this crack propagates towards the centre of sheet. Crack propagation in the flange increases with increase in blank holder forces and maximum crack propagation is obtained for the fixed blank-holder condition. Therefore, this study concludes that stretch flange can’t be formed of desired shape without using the blank holder force but it should be in balanced condition.
ACKNOWLEDGEMENTS
Authors would like to thank to Director CSIR-AMPRI, Bhopal for permission to attend the conference and SERB, New Delhi for the financial support to carry out the research work.
Fig.7. Effect of flange length on maximum circumferential strain
Fig.8. Effect of blank- holder force on maximum circumferential strain
References
(a) BHF= 1 KN (b) BHF= 2 KN (c) BHF = 3 KN
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(d) BHF = 4 KN (e) BHF = 5 KN
Fig.9 .Contour plots of circumferential strain at blank holding force of (a) 1 KN (b) 2KN (c) 3 KN (d) 4KN (e) 5 KN