Modeling of hydrogen transport and elastically accommodated hydride formation near a crack tip

Transient hydrogen diffusion and elastically accommodated hydride formation coupled with material elastic deformation are studied in a hydride forming system. The constitutive behavior of the material is modeled as isotropically linear elastic and account is taken of the effect of the dilatational strain induced by the solute hydrogen and formed hydride. The concept of terminal solid solubility of hydrogen as affected by stress is described and the mode of hydrogen diffusion through the two-phase material (matrix + hydride) is discussed. Probabilistic precipitation of hydride is modeled in the neighborhood of a stationary sharp crack tip under mode I plane strain loading, fixed hydrogen concentration on the crack surfaces and the outer boundary, and a uniform initial hydrogen concentration below the stress-free terminal solid solubility. A full transient finite element analysis allows for numerical monitoring of the development and expansion of the hydride zone. Information about the shape, size and density of the hydride in the hydride zone is obtained. The mechanistic effects of the solute hydrogen and hydride formation on the stress intensity at the crack tip are analyzed and their consequence on the fracture toughness resistance of the material is discussed.