Fatigue Crack Initiation and Early Growth Behavior of a Ferritic Stainless Steel in Laboratory Air and in 3%NaCl Aqueous Solution

摘要

This paper examines fatigue crack initiation and growth behavior in a ferritic stainless steel, SUS444. Fatigue tests were conducted in laboratory air and in 3%NaCl aqueous solution using an electro-hydraulic fatigue testing machine. Specimens with two different orientations, L and T directions, were evaluated. Cracks initiated from slip bands within ferritic grains regardless of direction and the effect of orientation on fatigue life could not be observed in either environment. An environmental effect could be identified, i.e. fatigue lives were shorter in the 3%NaCl aqueous solution than in laboratory air. In early crack growth regions, no discernible differences could be seen in either direction, but crack growth rates in the 3%NaCl aqueous solution were faster than in laboratory air. In long crack growth behavior, anisotropy could only be observed at low ΔK region, where the crack growth rates in L-T orientation were faster than in the T-L orientation. After allowing for crack closure, the difference between both orientations disappeared. An environmental effect could also be seen in the region of ΔK≥15 MPam~(1/2) for both orientations, where crack growth was enhanced in 3%NaCl aqueous solution.