The aim of this article is the development for b.c.c. metals of a constitutive model based on physical considerations. Quasi static, dynamic and jump tests have been performed with a Split Hopkinson Torsion Bar at room temperature on high-purity polycrystalline tantalum. It has been noticed that shear strain is not a parameter that describes the physical state of the material since the value of the shear stress depends on the previous deformation conditions. Klepaczko's model based on the evolution of a single structure parameter (the total density of dislocations) has already shown a very good agreement with experimental results for f.c.c. metals. Although it is possible to apply this single parameter model to fit the experimental data with a reasonable agreement for b.c.c materials, the introduction of a second state parameter (the density of mobile dislocations) seems necessary for a better understanding of the phenomena occurring during plastic deformation of b.c.c metals.
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