저자명 이희종 
년도 2006 
In order to achieve the light-weight and safe design of auto-body structures, the crash analysis of the high speed deformation has to be carried out with the accurate stress-strain curves at the high strain rate. The flow stress of a material generally increases as the strain rate increases and decreases as the temperature increases, that is regarded as the inherent characteristics of a material. Accurate crash simulation needs accurate quantitative information of material data at the strain rate up to several hundred per second since the strain rate of the local deformation in the crash reaches up to the range of 300/sec ~ 500/sec. Particularly, material properties of steel sheets are very important to the car crash since 70 % of an auto-body is composed of steel sheets and deformation of steel sheets is more sensitive to the strain rate and the temperature than that of other materials and the strain rate sensitivity varies with the operating temperature. Steel sheet for an auto-body has not been sufficiently investigated over a range of intermediate strain rates and operating temperatures of the car.
This paper is concerned with the thermo-mechanical behavior and temperature-dependent strain-rate sensitivity of steel sheets for an auto-body. In order to identify the temperature dependent strain rate sensitivity of SPRC35R, SPRC45E and TRIP60, uniaxial tensile tests have been performed with the variation of strain rate from 0.001/sec to 200/sec and the variation of the environmental temperature from -40℃ to 200℃. The thermo-mechanical response at the quasi-static state is obtained with the static tensile test and that at the intermediate strain rate is obtained with high speed tensile test. Experimental results show that the strain rate sensitivity increases at low temperature and decreases at high temperature. It is noted that as the strain rate increases, the variation of flow stress becomes sensitive to the temperature. The results also indicate that the material properties of SPRC35R are more dependent on the changes of strain rates and temperature than those of SPRC45E and TRIP60.
In the past, the material properties at the intermediate strain rate have been obtained from interpolation of results obtained from the quasi-static test and the high strain rate test such as a split Hopkinson bar, but this interpolation cannot describe the behavior of steel sheets at the intermediate strain rate. So a new material constitutive equation is needed to represent the variation of accurate stress at the intermediate strain rates and operating temperatures of the car. In order to represent the variation of accurate stress, a new material hardening equation is suggested by modifying the well-known Khan-Huang model. The hardening equation suggested gives good correlation with the experimental results at the various intermediate strain rates and temperatures. In order to verify the effectiveness and accuracy of suggested model quantitatively, the standard error between the experimental result and fitted one is compared with the other well-known constitutive model such as the Johnson-Cook and Khan-Huang models. The comparison demonstrates that the suggested model gives the relatively well description of the experimental results at the various strain rates and temperatures.

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