ABSTRACT
In recent years several fatigue models have been developed within the framework of continuum damage mechanics ( C D M ) (Bhattacharya and E l l ingwood , 1999; Chaboche and Lesne, 1988; Chow and Wei , 1994; Mar igo , 1985; Papa, 1993; Pass et al., 1993). Wi th in this framework, internal state variables are introduced to model the fatigue damage. Degradation of material response under cycl ic loading is simu lated using constitutive equations which couple damage cumulation and mechanical responses. The microcrack initiation and growth are lumped together in the form of the evolution of damage variables from zero to some critical value. Mos t of the existing C D M based fatigue damage models are based on the classical (local) con tinuum damage theory even though it is wel l known that the accumulation of dam age leads to strain softening and loss of ellipticity in elasto-statics and hyperbolicity in elasto-dynamics (Bazant, 1991; Bazant and Pijaudier-Cabot, 1988; Belytschko and Lasry, 1989; Geers, 1997). To alleviate the deficiencies inherent in the local C D M theory, a number of regularization techniques have been devised to l imit the
size of the strain localization zone, including the nonlocal damage theory (Bazant and Pijaudier-Cabot, 1988) and gradient-dependent models (Geers et a/., 1998). Recent advances in C D M based theories (Mazars and Pijaudier-Cabot, 1996; van Vroonhoven and deBorst, 1999) revealed the intrinsic links between the nonlocal C D M theory and fracture mechanics providing a possibility for building a unified framework to simulate crack initiation, propagation and overall structural failure under cycl ic loading.
