In situ studies and modeling of the deformation and fracture mechanism for wrought Zircaloy-4 and Zircaloy-2 as a function of stress-state

In situ deformation and fracture studies were performed on wrought Zircaloy-4 and Zircaloy-2 to evaluate the mechanism for fracture initiation and propagation over a range of stress-state defined as triaxiality. High values of triaxiality (η) were achieved using the pre-cracked CT specimen (η ⩾ 1.7), intermediate values (η = 0.8-1.2) were produced for a symmetric notched specimen, lower values were initially observed for a smooth tensile specimen (η = 0.33), while the lowest initial triaxiality values were produced for the dual keyhole specimens (η = 0.0-0.16). Unstable crack extension is shown to occur under plastic constraint by a process of void nucleation, growth, and coalescence under all stress-states tested. A micromechanical model is developed for the mechanism of ductile tearing by void growth and coalescence. Excellent agreement between the model and experimental measurements of the critical strain for failure initiation and strain for fracture is observed.