Influence of corrosion damage on the initiation of fatigue cracks in high strength stainless steels

This research project investigates the influence of corrosion flaws on the initiation of fatigue cracks in high strength stainless steels, and more particularly in 15-5 precipitation hardening high strength stainless steel. Susceptibility of 15-5PH to localised corrosion was examined and pit-like corrosion flaws produced by pitting and crevice corrosion were introduced in fatigue specimens in order to measure the influence of these surface flaws on fatigue crack initiation. From the results and observations made during the experiments, models of crevice corrosion propagation and initial stage of fatigue were developed. Experimental testing revealed that 15-5 precipitation hardening stainless steel is more prone to crevice corrosion than pitting, and that crevice corrosion is thought to be the most likely cause of any pit-like flaws in this material. The first results of the modelling of the propagation of crevice corrosion in 15-5PH stainless steel showed that the initial growth across the metal surface was proportional t3/ 8 . It follows that the depth growth rate in that initial stage was proportional t5/ 8 . All pre-corroded fatigue specimens failed from fatigue cracks which initiated from crevice corrosion flaws during fatigue testing. After failure the shape and size of corrosion flaws where the cracks initiated were measured and their largest Kt values determined by finite element analysis. No general pattern linking total fatigue life and stress concentration factor value was found. However, it was shown that stress concentration factor Kt has an influence on the initiation and early crack growth behaviour, but has no effect on the life of longer cracks. In addition, it appeared that not considering the small excursions found at the bottom of corrosion flaws in the assessment of the stress concentration factor Kt underestimates the values of Kt.