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When the strain is temporarily stopped during tensile testing of a metal, a stress relaxation phenomenon is known to occur whereby the stress diminishes with the passage of time. This phenomenon has been explained as the change of elastic strain into plastic strain. A technique was devised for deliberately causing strain dispersion to occur by applying the stress relaxation phenomenon during stamping. A new step motion that pause the die during forming was devised; it succeeded in modifying the deep-draw forming limit by a maximum of 40%. This new technique was verified through tensile and actual stamping tests. It was confirmed that the use of step motion causes the strain to disperse, thereby modifying the deep draw forming limit. The degree to which the forming limit is modified is dependent on the stop time and the temperature. Step motion technology increases the stampability of high-strength, forming-resistant materials and allows for expanded application of these materials.

This paper describes a new approach and specific design procedure for more lightweight automotive Body in White (BIW) design. For a BIW structure with a target value for static stiffness, joints with high stiffness sensitivity are selected and a lightweight BIW joint design chart is produced from calculations combining topology optimization and thickness optimization. Using this chart made it possible to theoretically assign an optimum spring value to the BIW structure for the purpose of lightweight design, enabling substantial weight reduction in the base model while maintaining the same stiffness performance.