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

Abstract

Crack initiation and crack growth behavior were studied on a ferritic stainless steel, SUS444, as a function of specimen orientation. Fatigue tests were conducted in laboratory air and in 3%NaCl solution using electro-hydraulic fatigue testing machine. Specimens with two different orientations, i.e. L and T direction, were prepared. In both orientations, cracks initiated from slip bands within ferritic grains. The effect of specimen orientation on fatigue lives was not observed in both environments. However environmental effect was found on fatigue lives, i.e. fatigue lives were shorter in 3%NaCl solution. In early crack growth region, while no discernible differences were found between both directions, crack growth rates in 3%NaCl solution were faster than in laboratory air. In long crack growth behavior, the anisotropy was only observed at low ΔK region, where the crack growth rates in specimens with L-T orientation were faster than those with T-L orientation. Taking account of crack closure, the crack growth in both orientations exhibited a similar behavior. Environmental effect was also found in the region of ΔK≥15 MPa √m for both orientations, where crack growth was enhanced in 3%NaCl solution.