Plasticity of New Steels Used in Automotive Industries - Temperature Measurements by Thermo-Vision 2005-01-1325

This paper presents an original experimental set-up to determine the thermoviscoplastic behavior of two steels: Dual-Phase sheet steel DP 600 and sheet steel with phase TRansformation Induced by Plasticity TRIP 800. Measurement of temperature gradients during shear and tensile tests at different strain rates varying from 10^-4s^-1 to 10s^-1 and up to large plastic strains in shear, γ ≈ 1, has been performed. The temperature increase ΔT due to thermal coupling is measured by an infrared camera which records the infrared radiation emitted from the shear or tensile zone during the process of plastic deformation. By combination of the hydraulic servo-controlled universal machine, the direct impact technique with Hopkinson tube as a measuring device and the infrared camera, it is possible to find the stress evolution versus plastic strain and the local temperature in the elastic range (thermo-elastic effect) as well as during plastic deformation. It was also possible to estimate appearance of the adiabatic shear bands (ASB) observed at elevated strain rates. Original constitutive relations published earlier [1, 3,4,49] which take into account effects of strain hardening, strain rate and temperature sensitivities on the flow stress for DP 600 2.0 mm steel sheet, including small and large strains, were verified with data already obtained for several mild steels. In this paper a comparison is discussed between the modified Johnson-Cook model and the constitutive relations mentioned above for DP steel. The energy absorbed by a material during plastic deformation for both approaches is also considered. In conclusion, this paper may be used as an experimental data base useful to verify some models taking into account effects of strain hardening, strain rate and temperature, (ε,ε̇,T) at low and high strain rates. The discussion is focused on new generation of steel sheets used in automotive industries. The strong temperature coupling can be verified only by development of more original experimental techniques, for example infra-red thermo-vision, as discussed in this paper.