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The issue of the effects of corrosion on structural integrity of aircraft has been a question of concern for some time [3-38]. The potential effects are many and they can be categorized as follows. In the discussion below the use of the terms global and local refers to the likely extent of the corrosion on the surface of a component. Global means the corrosion would be found on much of the component whereas local means the corrosion may be localized to only small, local areas.
Reduce the section with a concomitant increase in stress. Global or local.
Produce a stress concentration. Local.
Nucleate cracks. Local, possibly global. Source of Multiple-site cracking.
Produce corrosion debris. This may result in surface pillowing by various means, which may significantly change the stress state and structural behavior. Local and global.
Create a situation that causes the surfaces to malfunction. Local and global.
Cause environmentally assisted crack growth (EACG) under cyclic (Corrosion fatigue) or sustained loading (SCC) conditions. Local.
Create a damage state that is missed in inspection when the inspection plan was not developed for corrosion or when corrosion is missed. Local and global.
Change the structurally significant item due to the creation of a damage state not envisioned in the structural damage analysis or fatigue and strength analysis. If the SSI is specified, for example, by location of maximum stress or strain, then the corrosion may cause another area(s) to become significant. Local or global.
Create an embrittlement condition in the material that subsequently affects behavior. Local or Global.
Create a general aesthetic change from corrosion that creates maintenance to be done and does damage to the structure. Local or global.
Corrosion maintenance does not eliminate all the corrosion damage and cracking or the repair is specified improperly or executed improperly thus creating a damage state not accounted for in the design. Local or global.
Generate a damage state that alters either the durability phase of life or the damage tolerant assessment of the structure or both.
Create a widespread corrosion damage (WCD) state or a state of corrosion that impacts the occurrence of widespread fatigue damage (WFD) and its concomitant effects. (See references 3, 5, 6, 15, 17, 28, 29, 30, 34, 35, 36, 37, 38 for more information).
The question of whether corrosion, corrosion fatigue and stress corrosion cracking are safety concerns or just maintenance/economic concerns has been a point of discussion related to aircraft structural integrity for over 30 years. Nonetheless, a great deal of the aircraft structural integrity community believes that corrosion related degradation is just an economic concern. It was with this situation in mind that Campbell and Lehay [14] and Wallace, Hoeppner and Kandachar [15] pursued the presentation of technical facts and knowledge to illustrate the potential for a safety issue as well as a maintenance/economic issue. Finally, Hoeppner et al [30] at the 1995 ICAF meeting in Melbourne, Australia reviewed failure data obtained from the USAF, USN, US Army, FAA, and the NTSB related to aircraft incidents and accidents in the USA between 1975-1994 to evaluate further the potential for corrosion and fretting related degradation to be significant safety issues. Recently, several instances of pitting corrosion in aircraft and helicopter components have been identified as critical safety issues as discussed in the following section.
Review of Pitting Corrosion Fatigue Models, D.W. Hoeppner, V. Chandrasekaran, and A.M.H. Taylor, University of Utah and FASIDE International Inc.