Aging Composite Airframe

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.  



The Aging Composite Airframe: A Detailed Discussion




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