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After being stamped, a split was found on the cup wall near to the top, as shown in Fig. 2. The major and minor strains of the deformed circles around the split, as shown in Fig. 6, were measured and plotted on the forming limit diagram, as shown in Fig. 7. As can be seen in this figure, the measured strains are either above or close to the forming-limit curve, and the failure is obviously due to stretching. since both the major and minor strains are positive. It is also noted that the strains are very close to the plane~strain failure mode, i.e., close to the axis on which the minor strain is zero.
The results of the circle-grid analysis indicate that the original design is very unstable. The FLD also indicates that the major strain is too large: this is consistent with the present authors' opinion that the considerable distance between the cup and one side of the blank-holder limits the flow of metal towards the cup area, resulting in the large strains. As discussed in the previous sections,the most effective method of decreasing the major strainis to provide more metal for the cup area.
Minor Strain(%)
Fig. 5. The forming-limit diagram for 0.7 mm DDQ steel sheet
Fig.6. Deformed circles on the sheet-blank
Minor Strain(%)
Fig.7. Measured strains around the split
3.2. Finite element analysis
To help further understanding of the deformation of the sheet-blank during the stamping process, a 3-D finiteelement analysis was performed for the first-draw operation of the original design. The explicit finiteelement code PAM-STAMP, which is capable of handling any arbitrary 3-D die shapes, was used to conduct the simulations. Since the 3-D die geometries, including the punch,the die cavity, the blank-bolder and the draw beads, are not simplified, the finite-element program is able to simulate the actual production processes more accurately.
In order to describe the geometry of the die components, a commercial CAD program was used to construct the surface models for these components. The mesh systems required by PAM-STAMP as the input data for the die geometries were then generated by the same commercial CAD program, as shown in Fig. 8. In earlier periods,it was very difficult, if not impossible, to generate the mesh system for a complicate 3-D die shape, such as the stamping dies. However, as CAD systems are being used with increasing popularity in the die and mould industry.the above procedures to generate the mesh systems for the die geometries become trivial. Since the PAM-STAMP code treats the die components as rigid bodies,the mesh systems are used only to describe the geometries of these components and not for stress analysis. In the present investigation, a mixture of 3-node triangular and 4-node rectangular elements is used to construct the mesh systems, The mesh system for the sheet-blank is also shown in Fig. 8. As seen in this figure, the mesh density is much higher in the cup area than 论文网elsewhere,since the cup area is the location where splitting occurs.The numbers of elements used in the analysis are summarized as follows: die: 9,910; punch: 5,499; blank-holder:4,411; sheet: 4,891; total: 24,711.
The material properties used for the finite-element analysis are the same as those given in the previous section, the other operational conditions being: blank-holder pressure: 57 kPa; punch velocity: 10 m/s; punch stroke: 895 mm; and coefficient of friction: 0.12. The CPU time spent on an HP735 work-station to run a single job is about 11 100 s, the simulation results being displayed on an SGI work-station.
One of the advantages of using finite-element analysis for stamping process is that the deformed shape of the sheet-blank can be monitored throughout the stamping process, which is not possible in the real production process. Amongst the deformed shapes, the binder-wrap is the most interesting to the die designer, since it can be used to determine if the blank-holder surface has been designed properly. In the present study, the main purpose of the finite-element analysis js to investigate the deformation of the sheet-blank and the location where the split is most likely to occur. Hence, the deformed shape and the thickness contour obtained from the finite-element simulations were investigated.