On mechanical response of Zircaloy-4 under a wider range of stress states: From uniaxial tension to uniaxial compression

Abstract

The present work summarizes the experimental campaign and computational effort performed on Zircaloy-4 material commonly used in nuclear applications. The Zircaloy-4 exhibits a mild strength differential response under compression as compared with tension. Because of the well-known twinning-slip mechanism usually activated in compression after yielding in HCP metals, distortional hardening is required to define plastic flow throughout fracture. A general asymmetric yield function with the first invariant to the Cauchy stress tensor and the third invariant to the deviator stress tensor are considered to model the pressure-sensitive response and the Strength Differential (SD) effect in HCP polycrystal materials. The proposed macroscopic yield function provides results that are in good agreement with experiments. The onset of fracture and subsequent crack propagation in upsetting tests is also investigated. The results show that failure is not only controlled by material properties but also by friction between contact surfaces of specimens and experimental setup. In addition, the concept of a fixed cutoff value for negative stress triaxiality (η=-1/3) condition is analyzed. It turns out that the specimen fails differently of what is observed experimentally when a constant cutoff value is set. However, a cutoff stress triaxiality function, yields a more realistic fracture pattern consistent with tests.