Abstract:
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.
