Aerospace
As prime contractor for the integration
of the composite bonded doubler technology to repair
metallic aircraft structures, Martec Limited was tasked
to develop the design and to perform the analysis
of a composite patch repair for the CF-18 Flight Station
(FS) 470.5 bulkhead at the X-19 cross section. The
repair was considered as a fatigue enhancement and
was required to delay the onset of a fatigue crack
initiation predicted to occur at the lower inside
pocket.
Based on parametric fatigue analysis received from
the Canadian Forces, a 25% to 30% load transfer through
the patch was required to achieve the desired fatigue
life enhancement. The final design consisted of 10
layers of graphite/epoxy composite woven material
bonded to the bulkhead inside pocket using FM-73 film
adhesive. The repair design difficulties were enhanced
by the presence of significant stress gradient through
the thickness of the bulkhead flanges and out-of-plane
deflections of the lower flange. The stress analysis
used for the composite design optimization involved
extensive FEA based methods using the global/local
approach.
The design loads were derived using full scale fatigue
test strain measurements performed at a large number
of locations in the affected area. The silane/BR127
metallic surface preparation was selected for the
planned installation. The patch installation process
specification was developed by the NRC Institute for
Aerospace Research.
From a cost and performance point of view, the composite
doubler repair proved to be more efficient than the
proposed classical mechanically fastened aluminum
bathtub fittings.
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