Failure Diagram for Chemically Assisted Crack Growth

A failure diagram that combines the thresholds for failure of a smooth specimen to that of a fracture mechanics specimen, similar to the modified Kitagawa diagram in fatigue, is presented. For a given material/environment system, the diagram defines conditions under which a crack initiated at the threshold stress in a smooth specimen becomes a propagating crack, by satisfying the threshold stress intensity of a long crack. In analogy with fatigue, it is shown that internal stresses or local stress concentrations are required to provide the necessary mechanical crack tip driving forces, on one hand, and reaction/transportation kinetics to provide the chemical potential gradients, on the other. Together, they help in the initiation and propagation of the cracks. The chemical driving forces can be expressed as equivalent mechanical stresses using the failure diagram. Both internal stresses and their gradients, in conjunction with the chemical driving forces, have to meet the minimum magnitude and the minimum gradients to sustain the growth of a microcrack formed. Otherwise, nonpropagating conditions will prevail or a crack formed will remain dormant. It is shown that the processes underlying the crack nucleation in a smooth specimen and the crack growth of a fracture mechanics specimen are essentially the same. Both require building up of internal stresses by local plasticity. The process involves intermittent crack tip blunting and microcrack nucleation until the crack becomes unstable under the applied stress.