The application of small overloads for fractography of small fatigue cracks initiated under constant-amplitude loading

Fatigue crack initiation and crack growth are relevant issues for various structures. Initially small fatigue cracks usually are a part-through crack. The growth rate is small and the initial crack growth life covers a significant part of the fatigue life. Prediction on crack growth of part-through cracks under constant-amplitude (CA) loading is still difficult. Experimental verification occurs by fractographic examination. It requires marker load cycles inserted between the CA cycles to delineate the shape of the crack front. In the present investigation it is proposed to adopt small overloads for this purpose. This can introduce crack growth delays. However, it may be insignificant for small OL's and large blocks of the CA baseline cycles. Furthermore, OL's should produce striations which are more easily detected in the electron microscope. These questions are the major topic of the present investigation. Crack growth tests are carried out on specimens of aerospace aluminium alloys with different marker load sequences. Crack fronts and crack growth rates of small part-through cracks at the edge of a tapered hole in a thick plate specimens are obtained. At the grain boundaries a chaotic crack surface is observed associated with different crystallographic orientations of the adjacent grains. The analysis of this issue has led to the conclusion that 2-dimensional models for crack extension in a load cycle along the entire crack front is unrealistic. It is a 3-dimensional phenomenon.