Engineering Perspective: Analysis of Pull Bead Restraint for Hybrid Single Bead/Dual Bead Designs

In sheet metal stamping, drawbeads are a key element in controlling the inflow of sheet metal to form large panels.Most studies have focused on a single-bead design, which provides limited binding; only a few studies have covered multiple pull-beads or other geometries.”Drawing Weld Bead Constraints in Sheet Metal Drawing Operations,” an article on single-bead design published Nov/Dec.STAMPING Journal 2020, explains that binding can be increased to some extent by increasing the penetration depth of the male bead and making the radius of the bead more pointed.
The sharper radius increases the deformation of the sheet metal as it bends/straightens with each step, while it flows through the drawbead.For materials with limited ductility, such as aluminum alloys and advanced high-strength steels, minimizing the deformation level per bending/non-bending cycle by using larger weld bead radii can help prevent sheet metal cracking.Rather than making these radii sharper, the restraint can be increased by increasing the number of bending/straightening steps (see Figure 1).
The purpose of this study was to introduce a hybrid single-bead/dual-bead design and analyze the performance of this configuration in terms of its achievable binding force.The proposed dual bead design has three additional sequences of bending and straightening, and more friction than a single adjustable bead.This results in a higher binding force for the same bead penetration or the ability to reduce bead penetration to minimize sheet deformation.
Aluminum AA6014-T4 specimens were tested to determine how the center bead penetration and the gap between the adhesive affect the binding force.The test samples used for this study were 51 ± 0.3 mm wide, 600 mm long, and 0.902 ± 0.003 mm thick.Clean and properly lubricate sheet samples and inserts with 61AUS Grinding Oil.Drawbead inserts are machined from D2 tool steel and heat treated to HRC 62.
Figure 2 shows the components of the tunable double bead used in this study.The same drawbead simulator and hydraulic cylinder system was used in the study discussed in the previous article, which presents the system design in more detail.The entire drawbead simulator assembly is mounted on a steel table within the frame of the Instron tensile testing machine, and the adjustable dual-bead inserts are mounted in the drawbead simulator.
During the experiment, a constant clamping force of 34.2 kN was applied to keep the gap between the upper and lower parts of the drawbead consistent when the sheet was pulled over the drawbead.The gap between the upper and lower parts of the drawbead is always greater than the thickness of the sheet, and is adjusted by a shim set.
The test procedure is similar to that used in the monotunable bead test described in the previous article.Use a calibrated spacer to create the desired gap between the blades and use a feeler gauge to verify the accuracy of the gap.The upper clamp of the tensile testing apparatus clamps the upper end of the sheet, while the lower end of the strip is clamped between the inserts.
Numerical models of drawbead experiments were developed using Autoform software.The program uses an implicit integration method to simulate forming operations, allowing easy modification of the simulation model without significantly affecting computation time.This procedure simplifies mold tryout and shows good correlation with experimental results.Details of the numerical model are provided in the previous article.
Experiments were conducted to determine the effect of center bead penetration on drawn bead system performance.Tested with 6mm, 10mm, 13mm center pass penetration and no center pass while maintaining the gap between the insert and the lath at 10% of the test specimen thickness.Three tests were performed for each geometric configuration to ensure consistent results.
Figure 3 shows the repeatability of the experimental results for 6 mm bead penetration in three specimens, with an average standard deviation of 0.33% (20 N).
Figure 1. In a hybrid pull bead design, the adjustable penetration of the bead provides greater restraint.Retracting the bead converts this pull bead into a traditional single bead configuration.
Figure 4 compares the experimental results (no center bead and 6, 10 and 13 mm penetration) with the simulation results.Each experimental curve represents the mean of three experiments.It can be seen that there is a good correlation between the test and simulation results, with an average difference in the results of about ±1.8%.The test results clearly show that increasing bead penetration leads to an increase in binding force.
In addition, the effect of gap on the restraint force was analyzed for the double-bead configuration of aluminum AA6014-T4 with a center bead height of 6 mm.This set of experiments was performed for gaps of 5%, 10%, 15% and 20% of the specimen thickness.A gap is maintained between the flange of the insert and the specimen.The experimental and simulation results in Fig. 5 show the same trend: increasing the gap may lead to a significant reduction in drawbead restraint.
The friction coefficient of 0.14 was chosen by reverse engineering.A numerical model of the drawbead system was then used to understand the effect of the gap between the sheet and flange for 10%, 15% and 20% sheet metal thickness gaps.For a 5% gap, the difference between the simulated and experimental results is 10.5%; for larger gaps, the difference is smaller.Overall, this discrepancy between simulation and experiment can be attributed to through-thickness shear deformation, which may not be captured by the numerical model in the shell formulation.
The effect of a gap without a central bead (one wide bead) on binding was also investigated.This set of experiments was also performed for gaps of 5%, 10%, 15% and 20% of the sheet thickness.Figure 6 compares the experimental and simulation results, showing good correlation.
This study demonstrated that the introduction of a center bead was able to change the binding force by a factor of more than 2.For the aluminum AA6014-T4 billet, a trend was observed to decrease the restraining force as the flange gap was opened.The developed numerical model of sheet metal flow between drawbead surfaces shows an overall good correlation with the experimental results and can certainly make the tryout process easier.
The authors would like to thank Dr. Dajun Zhou of Stellantis for his valuable advice and helpful discussion of the project results.
STAMPING Journal is the only industry journal dedicated to serving the needs of the metal stamping market.Since 1989, the publication has been covering cutting-edge technologies, industry trends, best practices and news to help stamping professionals run their business more efficiently.
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Post time: May-23-2022
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