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Guidelines for Analysis, Testing, and Nondestructive Inspection of Imapct-Damaged Composite Sandwich Structures

This report contains a summarized version of the results obtained over several years of research
into the damage tolerance of composite sandwich structures. The research was spurred by the
use of sandwich structure in general aviation (small airplanes and business jets), where newly
certified aircraft are entering service and by the maintenance difficulties encountered with
existing commercial transport aircraft parts. The research was in three areas: testing, analysis,
and nondestructive inspection (NDI).

Testing was performed under static and fatigue compressive loads of impacted 8” x 10” panels.
The results characterized the damage resistance and tolerance of flat and curved sandwich
panels. Response surfaces for damage resistance and damage tolerance were drawn from the
testing data in order to extend the database to other than tested configurations. Field level NDI
was performed to find internal damage for both honeycomb and foam cores. Analyses of
impacted panels were used to determine damaged sandwich response, predict final failure, and
correlate with tests. Analysis, whether performed by closed form solutions or finite element
models, can be useful in design and certification of impact-damaged composite sandwich
structures. The state of the art is such that analysis cannot stand by itself but can be useful in
directing and analyzing test results and expanding test data to untested configurations by
semiempirical methods. The analytical work described in this report showed that careful
modeling can describe the structural response of the impacted panel under compressive load, but
some adjustments were needed to predict failure. The analysis performed also showed that
damage progression capability is vital to the fidelity of the analysis.

Inferences from the research helped formulate guidelines for testing, analysis, and NDI, which
characterized the effects of impact damage on sandwich panels. These guidelines should prove
useful for development and certification efforts. A need to extend the current effort to larger
structures was identified. The full—scale tests will be able to interrogate the effects of rogue
damage on nonpressurized and pressurized fuselage structures that include curvature.