Micromechanics theory of fatigue crack initiation and propagation

Fracture properties of Zircaloy-4 were determined as a function of the energies required for failure. Charpy V-notch fracture samples were prepared from nuclear grade Zircaloy-4. The fracture mechanism was studied as a function of orientation, loading rate, and temperature. Yield stress, tensile streets, and integrated energy values were determined from the resulting fracture force-displacement curves. Fracture toughness values were found to be independent of orientation but strongly dependent upon temperature and loading rate; K-values decreased with increasing temperature, dynamic values were higher than static. Structure of the force-displacement curves revealed four distinct segments: (a)elastic deformation, (b)plastic deformation, (c)fracture, and (d)tearing after crack arrest. Elastic energy (a)values decreased with temperature, while plastic energy (b)values increased with temperature. Cracking energy (c)values decreased with increasing temperature for statically loaded specimens but increased with increasing temperature for dynamically loaded specimens. Tearing energy (d)values were strongly dependent upon temperature and orientation. Two orientations (T-S and L-S)exhibited a gradual increase in tearing energy with increasing temperatures very characteristic of ductile materials. The other two orientations (T-L and L-T)had a temperature transition curve reminiscent of a ductile-brittle transition material. Sharp temperature transition resulted from the onset of crack arrest. A sudden transition dip was observed in the tearing energy which was a function of loading rate and orientation.