Criteria for fracture initiation at hydrides in zirconium alloys I. Sharp crack tip

A theoretical framework for the initiation of delayed hydride cracking (DHC) in zirconium is proposed for two different types of initiating sites, i.e., a sharp crack tip (considered in this part) and a shallow notch (considered in part II). In the present part I, an expression for KIHis derived which shows that KIHdepends on the size and shape of the hydride precipitated at the crack tip, the yield stress and elastic moduli of the material and the fracture stress of the hydride. If the hydride at the crack tip extends in length at constant thickness, then KIHincreases as the square root of the hydride thickness. Thus a microstructure favouring the formation of thicker hydrides at the crack tip would result in an increased KIH. KIHincreases slightly with temperature up to a temperature at which there is a more rapid increase. The temperature at which there is a more rapid increase in KIHwill increase as the yield stress increases. The model also predicts that an increase in yield stress due to irradiation will cause an overall slight decrease in KIHcompared to unirradiated material. There is good agreement between the overall predictions of the theory and experimental results. It is suggested that more careful evaluations of some key parameters are required to improve on the theoretical estimates.