Damage Mechanics and Metal Fatigue: A Discriminating Perspective

The effects of microstructure on the development and growth of small fatigue cracks in structural components play a strong role in dictating the class of mechanics approaches which are most viable in their treatment. The development of multiple small crack embryos associated with distributed cyclic plastic strain processes motivates a Continuum Damage Mechanics (CDM) description rather than a fracture mechanics treatment. The influence of material heterogeneity and the proximity to the free surface on the fatigue process in wrought alloys is significant and must be addressed in CDM. Sequential processes of early crack growth past heterogeneously distributed microstructural barriers, small crack coalescence and propagation of a dominant flow must be addressed through a consistent scheme which identifies damage driving forces as well as accessible propagation paths through the microstructure. Size effects in fatigue are related to the accessibility of low resistance microstructure paths to formation and progression of fatigue damage and the likelihood of coalescence of distributed crack systems. In this paper, we outline some of the key physical aspects of the problem and then discuss considerations for the application of damage mechanics concepts to the problem of fatigue crack nucleation and small crack growth.