Identification of the steady-state creep behavior of Zircaloy-4 claddings under simulated Loss-Of-Coolant Accident conditions based on a coupled experimental/numerical approach

The secondary creep behavior of Zircaloy-4 (Zr-4) claddings under simulated Loss-Of-Coolant Accident (LOCA) conditions was investigated. A coupled experimental/numerical approach is developed to determine the steady-state creep constitutive law's parameters. The test matrix targets thermal mechanical conditions for which several creep mechanisms are expected to take place (phase transition zone of Zr-4). Ballooning tests were performed on tubular specimens. A heterogeneous thermal mechanical loading was applied to the specimen. The resulting thermal and kinematics fields were measured using full-field optical measurements. Then a Finite Element model was updated to determine a constitutive Norton creep law fitting the kinematic fields measured by way of digital image correlation (DIC). The following methodology enables a significant reduction in the number of tests used for the mapping of creep mechanisms depending on stress and temperature conditions. The results of nine experiments covering stresses from 7 to 45 MPa and temperatures varying from 745 to 850 degrees C are detailed. In the a-phase domain, the transition from low Norton exponents (similar to 1) to higher ones (similar to 4-5) is observed at a von Mises stress of 26 MPa. The activation energies are consistent in the a-phase domain. For modeling purposes, several tests (T < 810 degrees C) were then correlated. Three constitutive laws, depending on the stress and creep rate levels, were determined. Norton exponent and activation energy consistent with the superplasticity observed by Garde et al. were identified in the early stage of the two-phase domain. (C) 2017 Elsevier Ltd. All rights reserved.