Plastic flow of a ferritic mild steel and a high strength austenitic steel under dynamic biaxial loading

Uniaxial and biaxial experiments performed at room temperature at different stein rates on mild steel Ck 34 and austenitic steel X2 CrNiMnMoNNb 21 16 5 3 have shown that: Under biaxial tension-tension loading on mild steel there is a close agreement between the yield loci and the Mises yield criterion at elastic strain rate of 10 high minus 5 s high minus 1 and 10 high minus 1 s high minus 1. On the contrary the yield loci which result from the tension-torsion loading fulfil the Tresca yield criterion for both the quasistatic and the dynamic tests. Consequently the yield criterion changes with the nature of the load. The yield loci of the quasistatic (10 high minus 3 s high minus 1) and dynamic (10 high 2 s high minus 1) combined tension torsion-tests of austenitic steel are described by ellipses which correspond to different proof strain definitions of yielding. For low (plastic) strains and strain rates the yield loci are in close agreement with the von Mises yield criterion whereas for large strains or high strain rates the yield loci tend towards the Tresca yield criterion. Therefore the size and the shape of the yield loci are functions of strain and strain rate. The quasistatic and dynamic tension-torsion behaviour of the austenitic steel is described by Perzyna's constitutive equation. There is good agreement between measured and claculated results if a yield criterion as a function of strain and strein rate, and a formula which contains the dependence of flow stress on strain rate based on thermal activation, are included.