Critical parameters for fatigue damage

The fatigue damage analysis is examined from a historical perspective. The analysis indicates that some of the issues concerning the basic disparities between the experiment and model/interpretations. To help understand these issues, we have developed an approach with two driving force parameters to analyze the fatigue behavior. Such an approach helps in viewing the damage in terms of an intrinsic problem rather than an extrinsic one. In the final analysis one needs to unify the overall damage processes such that the description is complete from the crack initiation stage to short crack to long crack to final failure. In order to unify the damage process, three basic parameters are introduced for describing the overall fatigue process. These are ΔK, Kmax and internal stress contribution to Kmax. In addition, there are other effects from environment and temperature that can contribute to these parameters. In particular Kmax seems to play an important role in the overall damage process. We find that the internal stress is the missing link that can bridge the gap between the four main stages of damage that lies between the crack nucleation stage to final failure. Examples are sited in support of this view of explanation. Finally, it is suggested that systematic experimental data and analytical modeling to describe the internal stress gradients is required to help in forming a reliable life prediction methodology.