Time‐dependent creep–fatigue crack growth (CFCG) is a major consideration in estimating the remaining life of elevated temperature components. Fracture mechanics approaches have proven useful in providing a framework for characterizing crack growth under service conditions, and in defining safe operating conditions and selecting inspection criteria and intervals. Experimental and analytical approaches have been developed to characterize crack growth under combined creep and fatigue loading conditions using (Ct )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 modelling crack tip deformation, and thus, accurately estimating the (Ct )avg value. A new creep‐reversal parameter, CR , is defined in this study to quantify the extent of creep–fatigue interaction at the crack tip, and is used in an analytical scheme, suitable for components, for calculating (Ct )avg . This approach does not rely on any simplifying assumptions regarding the extent of reinstatement of Ct , which is dependent on the amount of creep reversal due to cyclic plasticity, during the unloading part of a trapezoidal loading waveform cycle. The (Ct )avg values calculated by this approach compare well with the experimentally obtained values for compact type (CT) specimens, thus providing an experimental verification of the approach.
The Ct parameter has proven very useful in correlations of creep crack growth behaviour under both transient and steady state creep conditions. In this paper, we investigate the implications of adopting a different definition of the creep zone based on an absolute measure of creep strain rather than a measure that is relative to the elastic strain. The analytical equations for small‐scale creep and transition times from small‐scale creep to extensive creep conditions are derived for the Ct parameter using this alternative definition, considering both primary and secondary creep strain. It is shown that this alternative definition gives a reasonable starting point for the theory, approximately in accordance with the transient evolution of the Ct parameter based on the more conventional definition of the creep zone, and removes the somewhat artificial limit of applicability imparted by the classical creep zone definition used in creep fracture mechanics. Accordingly, diffusional creep processes with a power law exponent n ≤ 3 are admitted in the Ct description.
Crack growth histories for creep‐brittle aluminium alloy 2519‐T87 are simulated by controlling the rate of release of finite element nodes along the crack growth path using a variable time‐step, nodal release algorithm. While earlier experimental studies established little or no correlation between time‐dependent fracture parameters and the crack growth rate, a˙, during creep‐brittle fracture, the numerical results presented here indicate excellent correlation of the Ct parameter with a˙ during the quasi‐steady state crack growth regime. Differences in the experimental and numerical determination of time‐dependent fracture parameters are likely to be due to difficulties in experimental determination of the creep component of the load line deflection rate, V˙c , during creep‐brittle crack growth. A new quantity, K/rqc, is derived from time‐dependent fracture parameters to predict crack growth for transient, quasi‐steady state and steady state crack growth. However, Ct and K/rqc should only be employed as parameters for predicting creep‐brittle crack growth with an understanding of the couplings which exist between these parameters and the crack growth rate.
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