Creep crack growth under small-scale creep conditions.

Two different constitutive models are employed to describe creep crack growth under small-scale creep conditions theoretically. The first model combines elastic/nonlinear viscous deformation fields with a critical-strain criterion to be satisfied at a structural distance ahead of the crack. At small stress intensity factors, crack growth is found to become irregular and to tend to instabilities. This is a consequence of the properties of the Hui-Riedel (HR) field, which dominates near the growing crack tip. In the second (damage mechanics) model creep strain rates are modified by a Kachanov-type denominator with a damage parameter. Solutions of these constitutive equations for crack geometries contain crack growth automatically. If a solution for steady-state crack growth exists, which could not finally be decided, the growth rate must be a oc K sub I high 2 for dimensional reasons in contrast to a oc K sub I high n in the first model. For non-steady growth, the crack tip and the proce ss zone may either be contained within the creep zone or may grow outside the creep zone. Corresponding to these cases, two similarity solutions can be obtained, in which the the crack length, the process zone size and the creep zone size increase smoothly as a function of time with not tendency instabilities. This difference to the first model is a consequence of the fixed structural length in the first model, whereas in the second model the process zone is variable and does not leave a range of validity for the HR field