Corrosion and fretting are two time dependent processes that frequently can result in either a significant fatigue life reduction or a significant stress allowable reduction when they occur conjointly with cyclic loads. Both of these processes acting alone can have a significant effect on the functional capability, performance, and structural integrity as well as the appearance. When acting in conjunction with cyclic loads, it is possible that cracks nucleate in regions of either corrosion damage or fretting damage or both. Thus, steps frequently must be taken to prevent either corrosion or fretting-especially if a “no-crack” design philosophy is employed.
Frequently, subsequent to crack nucleation in either the corrosion or fretting damage areas, the stress intensity is high enough to propagate the cracks. The nature of the environment and contact conditions in consort with the local strain (stress) may also cause the crack propagation to be significantly accelerated as well. Thus, the possibility of environmentally assisted fatigue crack propagation must be considered in a damage-tolerant fatigue design.
In formulating the detailed fatigue analysis and testing to provide input to the fatigue design methodology, it thus becomes extremely important to consider the potential for corrosion and/or fretting to occur. These potentially deleterious processes frequently occur at local geometric discontinuities (e.g. notches) and/or joints of all types. Thus, they become extremely important. If they occur, they can produce cracks when no cracks can be tolerated. Or, where cracks occur, the environment and/or contact conditions may accelerate the propagation of cracks by the cyclic loads.
This paper presents a brief introduction to both the corrosion (fatigue) and fretting (fatigue) processes. The potential effect of these phenomena on fatigue integrity is discussed. Methods of anticipating, preventing, alleviating, or estimating fatigue life are discussed in relation to maintaining the desired component integrity related to the overall design philosophy.