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• May 19, 2016 Create Date
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Critical Joints in Large Composite Primary Aircaft Structures

Beyond the reasoning discussed above. composite materials are not well-suited to applications where
high out-of-plane forces are present. The tee-splice members in the joint concept of Figure I l are likely
to encounter such forces. the magnitude of which are very difficult to predict analytically or measure
experimentally. The fabrication of the corner fittings using composite materials would be impractical
for similar reasons as well as cost-prohibitive compared to the use of aluminum parts. While the splicr
plates may be slightly heavier due to the use of metal materials in these applications. any small extra
weight in the splices (or fasteners) is worth incurring to maximize the efficiency of the large heavy skins.
For a large transport, the weight of the splicing elements as a percentage of the total wing weight is small.
and splice efficiencies should not be evaluated solely on the basis of minimum splice plate and fastener
weight.
As a basic design philosophy, skin reinforcements or “pad-ups" were avoided wherever possible from
the standpoint of both cost and basic skin repairability. A skin pad-up in a bolted splice area where the
joint is working to its maximum efficiency implies that bolted joints or bolted repairs in a region outside
the pad-up would not be capable of restoring the ultimate strength of the structure. Thus, pad-ups are
allowed if warranted by other design considerations. but the joint itself must not be loaded to the point
that the surrounding structure would be unrepairable.
Having selected the critical-joint region for this investigation, a test and analysis development program
was formulated which w0uld ultimately demonstrate the technology required to design and build critical
composite wing joint structure.