DIFFUSION NEAR A TENSILE CRACK (EMBRITTLEMENT, FRACTURE, HYDROGEN, STEEL, STRESS)

In inhomogeneously stressed solid solutions, a force is exerted on each solute atom. The magnitude and direction of the force depend on the solute atom's chemical potential gradient. The chemical potential, in turn, depends on the local solute concentration and local stress state. The forces exerted on the solute atoms cause them to be redistributed within the stressed body. This diffusion of solute atoms is described by Fick's laws. Near a tensile crack, the primary component of interstitial diffusion is directed radially inward, toward the crack tip. Solute atoms accumulate there. Such accumulation is necessary in order for embrittlement to occur. Embrittlement is most acute at a particular temperature, the one at which the radial component of solute mobility is at a maximum. By introducing a zone of plastic deformation around the crack tip and by assuming that solute atoms do not interact, one obtains a critical material strength below which no embrittlement can occur. This critical strength has no explicit dependence on microstructure, applied stress or shape of the cracked body. Calculated values of the critical material strength and the temperature of most acute embrittlement are in reasonable agreement with experimental values, for the case of hydrogen in steel. Solute accumulation within a particular microstructural feature leads to fracture of that feature, followed by repeated solute accumulation and fracture. An average crack growth rate can be calculated for this discontinuous process. The calculated growth rate curves, as a function of applied stress intensity, exhibit three distinct regions that are also observed in experimentally measured growth rate curves