Fatigue life prediction of corrosion-damaged high-strength steel using an equivalent stress riser (ESR) model: Part I: Test development and results

The fatigue life of metallic aircraft structural components can be significantly reduced by environmentally induced corrosion. As part of a NAVAIR High Strength Steel Corrosion–Fatigue Assessment Program, methods were studied to predict the impact that corrosion-induced surface roughness has on the fatigue life of high-strength steel aircraft components. In order to adequately capture the corrosion damage features that cause fatigue cracking, a representative set of well-characterized corrosion–fatigue test results were generated to be used for model development. The test specimens fabricated for this program consisted of bare, unnotched AF1410 steel flat plates with a 25.4 mm diameter corrosion patch on one side. Two sets of test specimens were fabricated and tested, with one set abrasive blasted after heat treatment, and the other set hand polished after heat treatment. A method of growing corrosion in the laboratory was developed that consisted of filter paper soaked in a 3.5% NaCl solution and placed at the center of the test plate gage section, with a voltage applied across the filter paper to accelerate corrosion growth. High-resolution 3D surface topography data was collected from the corroded region on each test plate prior to fatigue testing using a commercial white-light interference microscope. Constant-amplitude fatigue tests were performed on corroded and uncorroded test plates at several different stress levels, for three different corrosion exposure levels. Post-test fractographic analysis of the corroded specimens indicate that all of the critical cracks originated from small corrosion notches on the order of 10–200 μm in width, 10–120 μm in height and 2–100 μm in depth. These notches were not considered to be pits in that the depth dimension was less than the surface dimensions. The repeatability of the fatigue initiating mechanism for corrosion damaged surfaces in this material indicates that it should be possible to develop a single modeling approach that reasonably captures the effects of corrosion notches in reducing fatigue life.