This thesis explores the mechanisms of short fatigue crack growth in polycrystal Zircaloy-4 with a blocky alpha microstructure. A full-field experimental study of cyclic plastic three-point bend beam loading has been conducted on Zircaloy-4 in order to assess quantitatively the mechanistic drivers for short crack growth.
The short crack growth is sensitive to the local microstructure with respect to grain crystallographic orientation and grain boundaries in Zircaloy-4. Crack paths are observed along crystallographic planes in polycrystal blocky alpha microstructures with predominant c-axis texture aligned out-of-plane or in-plane and normal to loading. Prismatic <a>-direction crack growth rate is remarkably lower than that for prismatic <c>-direction growth for the given loading direction. Microstructural obstructions such as grain boundaries can result in obvious crack retardation of short crack growth, especially when coupled with strong deflection in crack path. Short crack growth is coupled with slip activation which has a strong effect on crack path and crack growth rate. Prism slip is associated with prismatic <a>-direction crack growth, while prism, basal and <c+a> pyramidal slips are observed in the prismatic plane <c>-direction growth.
An in-situ optical digital image correlation (DIC) technology has been utilised to measure the strain distribution in the crack-tip area. When coupled with the crystallographic orientations obtained by electron backscatter diffraction (EBSD) measurement, the measured strain fields allow a series of micro-mechanical quantities including local stress, GND density, dissipated energy density and stored energy density to be addressed. Both the dissipated energy density and stored energy density well capture the fluctuation of crack growth rate (crack growth retardation) but the stored energy density provides better correlation. Thus, the cyclic stored energy density is found to be a potential driving force for short crack growth.