Test method for corrosion pit-to-fatigue crack transition from a corner of hole in 7075-T651 aluminum alloy

The paper presents a protocol to develop a corrosion pit of the order 120-180 μm and characterize the pit to fatigue transition at three stress levels 70, 120 and 180 MPa. The study was carried out using 7075-T651 aluminum alloy and the pit location was at the corner of the hole, simulating damage at the corner of a fastener hole. The study includes effort undertaken to develop micro pits of various size and consistency in obtaining a narrow size range. In addition, the paper presents analysis of the direct current potential drop (dcPD) data from a propagating fatigue crack to assess the crack nucleation and growth phenomenon from these corrosion micro pits. A stress ratio (R) of 0.65 was used for the above mentioned stress levels. Marker bands were used to determine the crack front as a function of cycles and to study the crack shape evolution from the post-test fracture surface analysis using a scanning electron microscope (SEM). The crack growth data from experimental crack dimensions “a” and “c” (where “a” is the dimension through the thickness and “c” the dimension along the specimen width) are in agreement with predictions using (AFGROW, 2014) [1]. Crack growth rates were also compared with that obtained from the Structural Integrity Prognosis System (SIPS) program (Papazian and Anagnostou, 2009) for similar test conditions, and the results are in good agreement.