A theoretical model for delayed hydride cracking velocity considering the temperature history and temperature gradients

Delayed hydride cracking (DHC) threatens the safety of Zircaloy components. The DHC velocity in the stage of stable crack growth needs to be studied. Based on the recent research, with a new idea the theoretical model is developed for the DHC velocity considering the temperature history and temperature gradient. The stress field ahead of the crack tip is calculated based on the fracture mechanics theory of second-order estimate of plastic zone size, and the redistributed stresses are statically equivalent with the stresses obtained from Linear Elastic Fracture Mechanics. It is confirmed that a simple linear function of the equivalent axisymmetric temperature distribution can be effectively and easily calibrated for predictions of the DHC velocity at different temperature gradients along the cracking direction. The developed model is validated to be effective because the predicted velocities agree well with some experimental results for the cases with and without temperature gradients, whether the test temperature is approached by heating or cooling. The theoretical results indicate that (1) with a higher positive temperature gradient along the cracking direction, the DHC velocity increases and a higher crack arrest temperature will occur; while a negative temperature gradient will remarkably depress DHC process; (2) The total hydrogen concentration in the hydrogen source has an important effect on the DHC velocity, and the crack arrest temperature can be evidently lowered for the cases with a small quantity of hydrogen atoms there;(3) At higher temperatures, negative flux contribution from the hydrogen concentration gradient in the hydrogen source is found to be the main mechanism of crack arrest.