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. Because sheet thicknesses can be reduced, a 10% reduction in vehicle weight can be anticipated. This technology will also enable the use of lower-grade lower-formability materials so that reductions in material costs can be anticipated.