Advances in the theory of delayed hydride cracking in zirconium alloys

Recent theoretical advances in understanding the phenomena of delayed hydride cracking (DHC) in structural materials such as zirconium alloys used in nuclear reactors are reviewed. The criteria for the initiation of DHC are established in terms of the thermodynamics of hydride formation, the micromechanics of hydride fracture, and the diffusion-controlled growth of a hydride at a flaw tip. Theoretical models for the critical hydride size for DHC initiation, threshold stress intensity factor at a sharp crack (or threshold maximum notch-tip stress at a blunt notch), diffusion-limited maximum hydride length, hydride thickness at a flaw, and DHC initiation and arrest temperatures during temperature transients are examined and compared to experimental observations. In the area of DHC propagation, computer simulation methods for the time-dependent hydride growth and fracture at a flaw have been developed and applied to study the effects on DHC velocity of temperature (and/or load) transients, mechanical properties of the hydride and matrix materials, hydrogen content and stress state.