Numerical modeling of delayed hydride cracking in zirconium alloys

A mathematical model for the hydrogen embrittlement and fracture of hydride forming metals is presented The model takes into account the coupling of the operating physical processes of hydrogen diffusion, hydride precipitation, non-mechanical energy flow, hydride/solid- solution deformation and fracture. A finite element implementation of the model is used for the simulation of Zircaloy-2 hydrogen embrittlement and delayed hydride cracking initiation. Two cases are discussed: (i) a boundary layer problem of a semi-infinite crack, under mode I loading and constant temperature, and (ii) a cracked plate, under tensile stress and temperature gradient. The initial and boundary conditions in case (ii) are those encountered in the fuel cladding of light water reactors, during operation. The numerical simulation predicts hydride precipitation at a small distance from the crack tip. When the remote loading is sufficient, the near tip hydrides fracture. Thus a microcrack is generated, which is separated from the main crack by a ductile ligament, in agreement with experimental observations.