A novel crack growth equation based on crack tip opening displacement variation

The crack tip opening displacement (CTOD) plays an important role in fatigue damage evaluation. An analytical approximation is employed in this paper to calculate the CTOD variation, which is inspired from the scanning electron microscopy (SEM) experiment. In this model, the crack closure effect is considered, which is determined by the plasticity in the vicinity of crack tip (forward and reverse plastic zone). Since the CTOD variation is a visual and traceable physical quantity and directly correlated to the crack growth behavior, it can be considered as the mechanical driving parameter of the fatigue crack growth. Therefore, a fatigue crack growth model is developed based on the above CTOD model. In this approach, a damage zone hypothesis is proposed to describe the transient influence of the previous loading sequence near the crack tip. The proposed model does not require cycle counting and can calculate the fatigue crack growth under arbitrary tension-tension variable amplitude loading. The interaction effect is taken into account, and the CTOD variation can be verified by microcosmic-test. For the model validation, some fatigue crack growth testing data are compared with the model prediction. Good agreements are observed. In addition, some observations of in-situ SEM experimentis are reviewed, in which the CTOD at each correspongding time instant in a loading cycle is quantified. The further discussion is conducted to investigate the CTOD mechanism under load sequence interaction effect. Several conclusions are given based on the current investigation.