| In this study, a series of monotonic tensile, compressive and uniaxial ratcheting tests are conducted at room temperature on Zircaloy-4(Zr-4) tubes which are widely used as fuel cladding in Pressurized Water Reactors (PWR) to investigate the mechanical behaviors under various loading conditions. The experimental results show that at room temperature (1) this material displayed viscoplastic characteristic: the stress-strain response was rate dependent;(2) it exhibits slightly anisotropic mechanical properties under monotonic tensile and compressive loading;(3) it exhibits obviously anisotropic ratcheting behavior under stress cycling with different mean stress directions: the ratcheting strain level of the test under tensile mean stress is higher than that under compressive mean stress;(4) the uniaxial ratcheting strain level increased with the increasing mean stress and stress amplitude, decreased with the increasing stress rate;(5) loading history had great influence on the ratcheting behavior: under the same loading condition, the ratcheting strain level of specimen with loading history is lower than that without history; lower stress level after higher stress history led to the shakedown of ratcheting and higher stress rate after loading history with lower stress rate also resulted in the shakedown of ratcheting. In the contrary cases, ratcheting strain continued to accumulate.The difference of mechanical properties under tensile and compressive loading, especially the yield stress, is presumably believed to contribute to the anisotropic ratcheting behavior of the material. Within the framework of unified viscoplasticity, an anisotropic constitutive model based on Ohno-Wang kinematic hardening rule is developed to describe the anisotropic ratcheting behavior of Zr-4tube at room temperature. The introduction of an initial back stress to the yield surface is determined according to a simple procedure. The predicted results of the model show reasonable agreement with the experimental data. |
