Models for Small Crack Growth under Creep-Fatigue in Austenitic Steels

Low cycle fatigue (LCF) endurance data have a valuable part to play in the lifetime assessment of components and structures. These data comprise the initiation and growth stages, but the growth relations themselves and their practical use are not as familiar as those employed for deeper cracks. Early work modelled continuous-cycling fatigue crack growth by assuming a succession of miniature LCF specimens at the crack tip, the field then being extended by investigators examining behaviour at high temperatures. Models were developed allowing for the concomitant contribution of creep damage for comparison with continuous-cycling properties where striation spacings recorded cyclic crack progress. Alongside such modelling, empirical laws were deduced describing the progress of short cracks. Expressions may be derived linking LCF with linear-elastic fracture-mechanics (LEFM) crack growth, using the parameter ΔJ (equivalent stress-intensity parameter). However, the purpose of this review is to compare and contrast those models which employ an easily measured surface parameter (such as total or plastic strain range) as the governing variable. Crack growth normally adopts an exponential form so that the rate of growth per cycle accelerates as the crack deepens. The distinguishing feature is that the process zone at the crack tip is itself surrounded by cyclically yielding material, in contrast with LEFM. Energy methods may also be employed, where the process zone at the crack tip fails when the accumulated energy density reaches a critical value. An upper bound relation is provided, accounting for the deleterious effects of creep-fatigue-oxidation interaction, if empirical data are not to hand. A conservative assessment may thus be made of cyclic crack growth rate at a specified depth. This review examines the capability of each model to allow for such creep-fatigue effects.