CONTINUUM DAMAGE MECHANICS ANALYSIS OF FATIGUE-CRACK INITIATION

The crack initiation period in an originally defect-free component can be a significant portion of its total fatigue life. The initiation ph ase is generally believed to constitute the nucleation and growth of s hort cracks, but the threshold crack length at which initiation occurs lacks a uniform definition. Moreover, available methods for predictin g fatigue damage growth usually require an existing flaw (e.g. Paris l aw) and may be difficult to apply to the initiation phase. This paper presents a continuum damage mechanics-based approach that estimates cu mulative fatigue damage, and predicts crack initiation from fundamenta l principles of thermodynamics and mechanics. Assuming that fatigue da mage prior to localization occurs close to a state of thermodynamic eq uilibrium, a differential equation of isotropic damage growth under un iaxial loading is derived that is amenable to closed-form solution. Da mage, as a function of the number of cycles, is computed in a recursiv e manner using readily available material parameters. Even though most fatigue data are obtained under constant amplitude loading conditions , most engineering systems are subjected to variable amplitude loading , which can be accommodated easily by the recursive nature of the prop osed method. The predictions are compared with available experimental results. (C) 1998 Elsevier Science Ltd. All rights reserved.