Widespread fatigue damage (WFD) is considered a primary threat to structural safety on aircraft.  The National Materials Advisory Board report on the aging of USAF aircraft [Tiffany, et al., 1997] summarized this with the statement -  “The onset of WFD in a structure is characterized by the simultaneous presence of small cracks in multiple structural details; where the cracks are of sufficient size and density, the structure can no longer sustain the required residual strength load level in the event of a primary load-path failure or a large partial damage incident.”  Thus, the presence of small cracks can reduce the safe load carrying capability of a fail-safe structure below its design requirement.

The objective of WFD studies is to determine when (in-service time) the crack population reaches the size and density to invalidate the initial design assumptions. Most older transport aircraft were designed (or later checked) using fail-safe damage tolerant design assumptions whereby if a discrete event (major local damage by fatigue or ballistic penetration) caused a rather large crack to form in the structure.  And then the design loads were set to preclude loss of the aircraft due to the nature of the redundant structure.   The assumption was that the discrete damage could occur anytime during the design lifetime of the aircraft.  The discrete damage was assumed to be of such a size that it would be evident either in flight or during routine inspections.  The design rules required that the structure could withstand this level of damage (with some growth) during an additional period of operation that was based on some multiple of the inspection period.  This design approach assumed that only the discrete damage was present and that only this damage was allowed to grow.  If the crack population in the surrounding structure could influence the stress intensity factors associated with this discrete damage event, then the initial design considerations were violated and it would be necessary to determine when this crack population became a threat to the behavior of the discrete damage.

Subsets of WFD are Multi-Site Damage (MSD) and Multiple-Element Damage (MED).  MSD refers to the cracking scenario where cracks are developing in the same structural element (fuselage joint) and MED refers to the cracking scenario where cracks are simultaneously developing in several elements (skin, spars, etc.) in a structural component (wing).  Multi-Site Damage has been found to be an important consideration in the continued safe operation of aircraft [Steadman, et al. 1999].