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Correlation of Structural Analysis and Test Results for the McDonnell Douglas Stitched-RFI All-Composite Wing Stub Box

The objective of this paper is to present the correlation between the analytical and experimental
results for a full-scale graphite-epoxy wing stub box loaded in bending. The wing stub box
represents the inboard portion of a high-aspect-ratio wing box for a civil-transport-aircraft. This
wing box was designed and manufactured by the McDonnell Douglas Aerospace (MDA)
Company under the NASA Advanced Composites Technology (ACT) Program. The wing stub
box was fabricated using an innovative stitched/RFI (Resin Film Infusion) manufacturing
process which has the potential for reducing manufacturing cost and producing damage-tolerant
composite primary aircraft structure. This wing stub box was subjected to a series of tests at the
NASA Langley Research Center Structural Mechanics Test Laboratory. In the final test, the
wing stub box was loaded to failure after being inflicted with a 100 ft—lb impact damage at a
critical location. The final failure load (154 kips) is approximately 93% of the Design Ultimate
Load (DUL) of 166 kips.

Finite element analysis results obtained prior to testing, (as presented in references 1 and 2),
indicated that the wide bays outboard of the access door in the upper cover panel would not
deform nonlinearly until loading approached the DUL. However, in the test of the wing stub
box, documented in reference 3, large deformations occurred in this region at a load of
approximately 130 kips, which is significantly less than DUL. Following the test, a more
refined global finite element model of the wing stub box was developed in which a finer mesh
was used for the wide bays to better account for their nonlinear behavior.

In addition to the refined global analysis, a local analysis that was presented in reference 4 and
conducted prior to testing in order to study the splice joint between the stub box and the wing-tip
extension structure was re-examined to help determine why measured strains were substantially
greater than predicted. The abrupt termination of the stringers at the splice between the stub box
and the wing-tip extension structure causes stress concentrations in the skin of the upper cover
panel. Strain gages, located on the interior surface of the upper—cover—panel skin and at the base
of three stringer webs, recorded strains up to twice the allowable strain for the skin material. In
the pre-test local finite element analysis presented in reference 4, the predicted strain at the base
of one of these stringers was examined and was predicted to be less than half of the strain that
was recorded. Hence, post-test analytical study was conducted using a refined local model to
help identify the difference between test data and analytical results.