A model to describe the anisotropic viscoplastic behaviour of Zircaloy-4 tubes

With the aid of multiaxial tests under monotonic and cyclic loadings (tension, torsion and internal pressure), the anisotropic viscoplastic properties of Zircaloy-4 tubes in two metallurgical states can be studied. The anisotropic behavior at a temperature of 350 degrees C has been thoroughly quantified. The results obtained at room temperatures as well as the independence of the ratio R(p)=epsilon(theta theta)(p)/epsilon(zz)(p) with respect to the temperature would seem to indicate that the set of anisotropy coefficients do not depend on the temperature. However, the viscosity of this alloy possesses a clearly marked minimum in the neighborhood of 300 degrees C, a behavior which is probably due to the dynamic strain aging frequently observed in solid insertion solutions. This material also presents a slight supplementary hardening during out-of-phase cyclic loading (tension-torsion with a 90 degrees phase lag). It is proposed to extend the formulation of a viscoplastic model, developed and identified elsewhere for other initially isotropic materials, to the case of Zircaloy-4. Generally speaking, anisotropy is introduced into this model through fourth order tensors which affect the flow directions [M], the linear kinematical hardening components [N], as well as the dynamic and static recoveries, [Q] and [R] respectively, of the forementioned hardening variables. The identification of this model is performed at 350 degrees C. It is shown that the formulation is capable of describing the set of mechanical characteristics of this alloy.