0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 ε n DP500G 10s-1 301LN 10s-1 T = 80°C Figure 6: Dynamic strain hardening coefficient versus true strain for 301LN and DP500G at 80°C CONCLUSION The split Hopkinson bar testing facility developed at the University of Ghent has proven to be a very valuable technique for the determination of the dynamic mechanical properties of a wide range of steel grades. This technique is complementary to hydraulic test-benches by enabling reliable results at ε& ≥ 1000s–1. Even the austenitic stainless steel, which has a dynamic elongation higher than 50%, can be tested to rupture. The analysis of the tensile tests performed at room temperature and up to ε& = 1100s-1 shows that strain rate sensitivity of the lower yield strength is strain rate dependent only for IFHSS260, ZStE420 and DP500G. A low temperature of –40°C leads to higher strength values but also to higher strain rate sensitivity due to a combined effect of temperature and strain rate. In contrast, the austenitic stainless steel 301LN shows a constant strain rate sensitivity of the yield strength over the whole range of strain rates. It has been shown that the study of combined temperature and strain rate effects is of great importance especially at 80°C where a decrease of the mechanical properties is observed. The austenitic steel is very sensitive to temperature changes and its mechanical properties are governed by the strain induced austenite to martensite transformation. The influence of strain rate is related to adiabatic heating during the deformation, which restricts the γ→α’ transformation. However, martensite forms more readily at low strain levels during high strain rate testing, which allows 301LN to absorb more energy in dynamic testing conditions. The need to characterise materials dynamically is well illustrated by the strain rate dependency of a crash relevant parameter like the energy absorption, which shows that austenitic stainless steel 301LN is more sensitive to strain rate than other steel grades, especially at 80°C despite a reduction of the transformed martensite volume fraction. The already high sensitivity to temperature of austenitic stainless steels could be even improved when considering it in a pre-hardened state. This will be an issue in the course of the European project “LIGHT&SAFE” coordinated by OCAS leading to broaden know-how on manufacturing techniques and durability of austenitic stainless steels for automotive applications. REFERENCES [1] Irving, B. (1998). Welding Journal. 11, 33. [2] Emmons, J.B. and Douthett, J. (1996). Automotive frames of stainless steel, Adv. Mat. & Proc. 8, 23. [3] Courbrough, G.J., Matlock, D.K. and Van Tyne C.J. (1993). SAE Special Publications, Sheet Metal and Stamping Symposium International Congress and Exposition, 279. [4] Follansbee P.S. (1986). In: Metallurgical applications of shock-wave and high-strain-rate phenomena, 451, Murr L.E. et al. (Eds). [5] Kolsky H.. (1949). Proc. Phys. Soc. Sec. B.62, pp. 676-700. [6] Hecker S.S. et al. (1982). Metallurgical Transactions A. 13A, 619.
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