FAA - EASA 09-2015
SYNOPSIS
The FAA assigned the Aviation Rulemaking Advisory Committee (ARAC) a new task to provide recommendations regarding revision of the damage-tolerance and fatigue requirements of Title 14, Code of Federal Regulations (14 CFR), part 25, including sub-parts C and E of 14 CFR part 26, and development of associated advisory material for metallic, composite, and hybrid structures.
Aging Composite Airframe FAA - EASA 09-2015 Presentation
The Aging Composite Airframe: A Detailed Discussion
SYNOPSIS
INTRODUCTION; Both
fail-safe and slow crack growth design concepts can (and have been)
defeated by durability-related fatigue and environmentally assisted
cracking in aging in CIVL & MIL metallic airframes.
Durability-related cracking manifests itself in the literature as the
onset of wide spread fatigue damage, WFD. Composite airframes
accumulate other types of damage over time; a potential for service
induced Wide Spread Damage, WSD. Techniques are needed for
assessing operational limits for composite and metallic aircraft
structural applications relative to the onset of WFD
(durability-related fatigue cracking) and service induced WSD that
can defeat the structure’s ability to carry its residual strength
requirement. Figure 1 illustrates the different perspectives
regarding the difference between the WFD and WSD terminology.
Figure1. The
Effect of Defects Distribution in Structural Integrity Planning; Ref
[1].
The distribution of
defect sizes in any given structure can be considered to consist of a
composite of the several distributions shown in Figure 1. The
material as received from the vendor will contain very small flaws or
defects such as inclusions, cracks, porosity and surface pits,
scratches, disbonds, delaminations, surface cuts or scratches and
machine marks. These inherent material flaws are considerably below
the detection capability of the non-destructive inspection (NDI) and
should be sufficiently small to not grow appreciably in service. A
distribution of larger defects can exist as a result of the
fabrication process or as large inherent flaws. The production
quality control process is designed to detect and eliminate as many
of these defects as possible but those that are not detected may
propagate due to fatigue mechanisms during service. The largest
defect size that could remain undetected in the newly fabricated
structure after the final inspection is designated as ao.
This defect dimension provides the starting point for damage growth
projections that demonstrate adequate service life or the necessity
for an in-service inspection and/or residual strength estimates. The
largest damage size that can remain undetected after an inspection is
designated as aNDI and becomes the
initial damage size for a usage period. The ”defects introduced in
service” are the result of discrete source events, as described in
AC25.571 and other sources. An accumulation of accidental damage can
also compromise both fail-safe and slow-growth design concepts.
The purpose of this
discussion is to provide a historical perspective for the evolution
of the certification/substantion concepts for composite airframes.
This understanding is fundamental as our community works to identify
and document the conditions that determine operational limits for
composite and hybrid (metallic and composite) airframes. This
understanding will assist in the development of potential updates to
AC 25.571 and AC 20-107.
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