Modelling the effect of creep-fatigue interaction on crack growth

Time-dependent creep-fatigue crack growth (CFCG) is a major consideration i n estimating the remaining life of elevated temperature components. Fractur e mechanics approaches have proven useful in providing a framework for char acterizing crack growth under service conditions, and in defining safe oper ating conditions and selecting inspection criteria and intervals. Experimen tal and analytical approaches have been developed to characterize crack gro wth under combined creep and fatigue loading conditions using (C-t)(avg) as the crack tip parameter. The analytical approaches that have been proposed to characterize CFCG are limited in their application because they do not completely account for the effect of creep-fatigue interactions in modellin g crack tip deformation, and thus, accurately estimating the (C-t)(avg) val ue. A new creep-reversal parameter, C-R, is defined in this study to quanti fy the extent of creep-fatigue interaction at the crack tip, and is used in an analytical scheme, suitable for components, for calculating (C-t)(avg). This approach does not rely on any simplifying assumptions regarding the e xtent of reinstatement of C-t, which is dependent on the amount of creep re versal due to cyclic plasticity during the unloading part of a trapezoidal loading waveform cycle. The (C-t)(avg) values calculated by this approach c ompare well with the experimentally obtained values for compact type (CT) s pecimens, thus providing an experimental verification of the approach.