A MODEL TO DESCRIBE THE ANISOTROPIC VISCOPLASTIC BEHAVIOR 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 stud ied. The anisotropic behavior at a temperature of 350 degrees C has be en thoroughly quantified. The results obtained at room temperatures as well as the independence of the ratio R(p)=epsilon(theta theta)(p)/ep silon(zz)(p) with respect to the temperature would seem to indicate th at the set of anisotropy coefficients do not depend on the temperature . However, the viscosity of this alloy possesses a clearly marked mini mum in the neighborhood of 300 degrees C, a behavior which is probably due to the dynamic strain aging frequently observed in solid insertio n solutions. This material also presents a slight supplementary harden ing during out-of-phase cyclic loading (tension-torsion with a 90 degr ees phase lag). It is proposed to extend the formulation of a viscopla stic model, developed and identified elsewhere for other initially iso tropic materials, to the case of Zircaloy-4. Generally speaking, aniso tropy is introduced into this model through fourth order tensors which affect the flow directions [M], the linear kinematical hardening comp onents [N], as well as the dynamic and static recoveries, [Q] and [R] respectively, of the forementioned hardening variables. The identifica tion of this model is performed at 350 degrees C. It is shown that the formulation is capable of describing the set of mechanical characteri stics of this alloy.