Section 1.4.1. Widespread Fatigue Damage
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].