EFFECTS OF CRACK TIP STRESS STATES AND HYDRIDE-MATRIX INTERACTION STRESSES ON DELAYED HYDRIDE CRACKING

A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature. For test temperatures approached from below, factors such as the hydride-matrix accommodation energies, the stress state at the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward and inside the plastic zone boundary with increasing temperature. At low Kl values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition for KlH. For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above parameters become less evident.