THE USE OF A FAILURE ASSESSMENT DIAGRAM FOR INITIATION AND PROPAGATION OF DEFECTS AT HIGH-TEMPERATURES

The use of a failure assessment diagram, of the type in the R6 defect assessment procedure, is investigated for creep crack growth under steady loading. While a detailed approach based on C* remains attractive for appreciable creep crack growth, it is shown that a simplified approach can be formulated for limited crack extension. A failure assessment diagram is derived based on the option 2 curve of R6 using isochronous stress-strain data. The inclusion of elastic strains in the isochronous data covers stress redistribution effects. Equations are given which enable a toughness, K(mat) for assessments at temperatures in the creep range to be evaluated from creep crack incubation and growth data presented in terms of C*. The toughness, K(mat), replaces the fracture toughness used in low temperature R6 assessments. Thermal stresses can be included in assessments by evaluating the stress intensity factor for the combined thermal and mechanical loading. A formula is given which enables the effect of thermal stresses to be reduced when creep strains are sufficient to relax out part or all of the thermal stress.