THEORETICAL ASPECTS OF FATIGUE AND CREEP CRACK GROWTH

Abstract

Theoretical aspects of crack growth behavior in fatigue and creep are examined in terms of micromechanisms of growth and macromechanical considerations. Fatigue crack growth curves in terms of threshold, near-threshold, Paris-law and rapid growth regimes, and various microprocesses that affect these regimes are discussed. Threshold and near-threshold regimes are affected by both intrinsic factors such as elastic modulus, slip character, slip reversibility and microstructural heterogeneities, and extrinisic factors involving crack path tortuosity and crack closure. In the near-threshold and rapid crack growth regimes, the growth occurs by the cumulative damage processes wherein damage accumulation during several cycles is required for each increment in crack length. In the intermediate Paris-law regime growth generally occurs by the plastic blunting process. Behavior of short cracks and non-propagating cracks are discussed briefly. Crack growth in creep is discussed in terms of relative kinetics of the stress relaxation processes involving creep of the material ahead of the crack tip as well as the crack tip blunting process, and the damage accumulating processes involving void nucleation and growth and formation of microcracks ahead of the main crack. Recent developments in the parametric analysis and in the theoretical modelling of creep crack growth are briefly outlined.