The influence of environment upon the accumulation of damage under corrosion fatigue conditions

There is an increasing demand for an understanding of the effects of complex load history on fatigue lifetime since it is realised that constant-amplitude S-N endurance data may not be appropriate for the prediction of the lifetime of components subject to variable amplitude loading. However the amount of damage accumulated within a component during its lifetime is a function, not only of its' load history, but of the synergistic effects of stress and operating environment. In this respect the ability of a material to resist the damaging effects of electrochemical corrosion can play a major role in determining the ultimate lifetime of a component; the effect of corrosion being dependent upon the specific combination of material/environment, the rate of application of load and the duration of metal/environment contact. For conditions where applied loads are below the fatigue or endurance limit the corrosion processes can lead to the development of defects which, under benign conditions, would not normally form or if developed would attain a non-propagating condition. The paper presented here includes the results of studies derived from two-stage intermittent air/environment fatigue cycling and cumulative damage corrosion fatigue loading of a medium carbon steel in two microstructural conditions (i) normalised and (ii) quenched and tempered. From the results of these studies it is shown that a simple damage accumulation model is insufficient to satisfactorily predict ultimate lifetime and consideration should be given to processes such as multiple crack initiation, pitting, Stage I and Stage II environment-assisted crack growth.