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Study on the characterization of the dynamic material properties of sheet metal for auto-body considering the pre-strain effect(예비변형률 효과를 고려한 차체용 박판의 동적 물성 특성 연구)
Because dynamic behaviors of metal material are different from static behavior, it is very important to ensure dynamic material properties of sheet metal for auto-body for accurate crash analysis. But most experiments to ensure dynamic material properties are carried out by using specimens directly gathered from sheet metal for auto-body without considering about forming effects. Most parts of auto-body are produced through the sheet metal forming process, and during this process they have change of thickness and residual stress and strain remain. Most studies of dynamic material properties related to this pre-strain effect were carried out at static or high strain rates over 1000 s-1. Because strain rate is under 500 s-1 when real auto-body crashes were occurred, in this case estimation factors of materials such as yield stress and ultimate tensile strength are took seriously in field of design and production part for auto-body. In this paper, to specify dynamic material properties considering pre-strain effect, sheet metal SPCEN, SPRC45E, SPRC35R and EZNCD were selected and were imposed 2, 5, 10 % pre-strains. After that using the high speed material testing machine, materials were tested at given strain rates of between 0.003 and 100 s-1. It was found from experimental results that in case of SPCEN, SPRC35R the yield stress and ultimate tensile strength were increased due to pre-strain effect at strain rate over 10 s-1 and over 1 s-1 for EZNCD. But SPRC45E was not affected by pre-strain effect. For materials having low yield stress in static test such as SPCEN, SPRC35R and EZNCD, dynamic material properties are affected by pre-strain effects at intermediate strain rates. Because of pre-strain effect, yield stress and ultimate tensile strength of materials are increased and in case of ultimate tensile strength it was found that according as an amount of pre-strain is increased ultimate tensile strength goes up.