We’ll focus on question one here, and address question two next month.
The research program noted above gathered pertinent data by testing 8-in.-dia. circular blanks locked by a 5-in.-dia. circle of lock bead. Blanks were formed by various sizes of hemispherical punches, as large as 4-in. dia. Each blank was marked with a polar grid of 20 circles/in. for strain (stretch) measurements.
The first research phase attempted to identify how sheetmetal reacts when reaching a strain equal to the uniform elongation in the tensile test. Was there some type of visible diffuse neck, a termination of strain or load maximum? The answer to all three questions: No. The domes continued forming in a well-controlled manner to reach strain values well above the uniform elongation (Fig. 2). Measuring strain rate at the eventual failure site showed that a modest rate increase occurred at a strain approximating uniform elongation. Once increased, the new rate was maintained. Locations above and below the eventual failure site showed the same increase in strain rate when they reached strains equal to the uniform elongation.
How does stretching sheetmetal over a hemispherical punch differ from a tensile test? A tensile test is similar to stretching a length of chain or wire—one link can become weak and begin deforming independently. The rest of the chain will stop deformation as a maximum load forms, and eventually the weak link will fracture.
Stretching sheetmetal acts similarly to deforming a chain-link fence. The weak spot can only stretch as much as the areas surrounding it—no localization of strain can take place. The research showed that the diffuse neck did not terminate useful deformation when stretch forming sheetmetal.
| Fig. 2—Graph showing biaxial stretching of aluminum sheet does not terminate as a load maximum when uniform elongation is reached. (Trans ASM, Vol. 56, p. 35).
For biaxial stretchforming over a hemispherical punch, a local or thickness neck occurs when certain modes of deformation are allowed to form. The thickness neck is a highly localized thinning with no deformation along the neck. Therefore, with biaxial stretching the material on either side of the local neck remains rigid with no further deformation. This local neck usually starts with shear bands along the neck traversing at 45 deg. through the sheet thickness. The onset of the local neck formation (start of shear bands) defines when the sheetmetal fails. Fracture follows quickly (Fig. 2).
Having found the mechanism for the onset of failure, the third research phase studied several metal alloys in dead-soft, half-hard and full-hard condition. For the research to be useful, the final step was to determine if the strain at the onset of failure could be predicted. If so, maximum allowable combinations of useful strain would be available for part designers, troubleshooters, virtual press shops and other applications. We’ll present the results of this final research phase next month. MF
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