In this chapter the primary aircraft aging mechanisms are discussed briefly in relation to their impact on aircraft structural health and longevity, and the associated technical issues and operational needs are identified. This is followed by brief discussions of technical issues and needs in the areas of nondestructive evaluation and maintenance and repairs. The focus of this chapter is on the degradation mechanisms for aluminum alloy airframe structures, which are predominant in current aging aircraft. Issues and recommendations concerning future aging aircraft, including composite primary structure are included in Chapter 10. The issues and needs identified in this chapter are the basis for the recommended engineering and management actions and near-term and long-term research and development presented in Part II of this report.
The three primary mechanisms that can affect the structural health and longevity of the Air Force's metallic aircraft structures are
stress corrosion cracking
fatigue cracking (including low-cycle and high-cycle fatigue)
Corrosion of airframe structure is the single, most costly maintenance problem for Air Force aging aircraft (SAB, 1994). Corrosion can occur in a variety of nonexclusive forms, including uniform or general corrosion, galvanic corrosion, pitting corrosion, fretting corrosion, crevice (filiform and faying surface) corrosion, intergranular (including exfoliation) corrosion, and stress corrosion cracking (ASM, 1987). Because of the potential structural effects, stress corrosion cracking is considered in greater detail in the following section.
Corrosion of aging aircraft results from a combination of factors, including
the use of aluminum alloys and tempers that are more susceptible to corrosion than currently available alternatives
inadequacy or deterioration of corrosion protection systems
exposure to various corrosive environments (e.g., humid air, saltwater, sump tank water, latrine leakage)
Control of corrosion is predicated on effective prevention, detection, and repair methods. Despite the best intentions of prevention and control practices, the complete elimination of corrosion is virtually impossible. In aging aircraft structures, corrosion protection and control systems deteriorate over time. The major concern with the deterioration of corrosion protection systems for aging aircraft structure is the resulting increase in maintenance costs because corrosion damage that is identified must be repaired. Based on current experience, this practice of identifying and repairing corrosion damage has been adequate for maintaining the integrity of aging structures. However, because corrosion damage is typically found by visual inspection techniques, and a fair amount of corrosion damage occurring on older Air Force aircraft is hidden from direct view, a significant amount of corrosion can remain undetected. Also, there can be a wide variation in extent and severity of corrosion damage among similar aircraft because of differences in environmental exposures and in the amount and type of maintenance that the aircraft may have received.
The different types of corrosion damage exhibit different characteristics and potential consequences with respect to both detectability and structural consequence. For example, exfoliation corrosion (severe intergranular corrosion where the buildup of corrosion products causes flaking and surface blisters) and pitting corrosion can be detected readily, depending on the accessibility of the damaged surface. Although these corrosion forms are evident as surface deterioration, they may not be found if the surface is inaccessible to visual inspection. Intergranular corrosion that propagates along grain boundaries away from exposed surfaces may be indistinguishable from the surface, challenging the reliability of nondestructive inspection (NDI) methods (Mindlin et al., 1996).
Undetected corrosion can progress significantly before being observed, leading to (1) increased maintenance costs and time in the depot for maintenance or (2) an increased risk that corrosion, in the presence of other forms of damage, may cause a more significant decrease in damage tolerance than otherwise estimated. This is discussed later in this chapter. Although corrosion can be very costly to repair, corrosion by