The role of stress-state on the low cycle fatigue resistance of non-hydrided beta-treated Zircaloy-4

Reactor operation results in cyclic stresses that produce fatigue. Increased strain to failure under monotonic loading has been observed for Zircaloy-4 under a stress-state of shear defined by a low triaxiality. Triaxiality (η) is defined as the ratio of hydrostatic stress to von Mises stress and typically ranges from values of -0.33 for compression, to 0 for torsion loading and to 0.9 for notched, axial loaded specimens. Low Cycle Fatigue (LCF) testing of Zircaloy-4 was performed under uniaxial and torsion loading in air at room-temperature. Both smooth and notched specimens were tested to vary the value of positive triaxiality. An increase in LCF resistance with less notch sensitivity is observed under torsion loading. Cyclic stress-strain curves are developed that show a higher strain to failure and lower flow stress under torsion loading. This suggests a lower triaxiality produces increased strain to failure capability under both cyclic and monotonic loading that is shown to generally follow a Rice-Tracey model for void nucleation, growth and coalescence. Fractography examinations are used to understand the mechanism for crack extension.