Damage Tolerance of Composite Fuselage Structure

Walker, T., Scholtz, D., Flynn, B., Dopker, B., Bodine, J., Ilcewicz, L., Rouse, M., and McGowan, D.

Sixth NASA/DOD/ARPA Advanced Composite Technology Conference, NASA CP-3326, 1996.

A developmental program was conducted to investigate damage tolerance of skin/stringer and sandwich composite fuselage structural configurations. Unconfigured laminate and sandwich center-notched specimens were tested in uniaxial tension and compression to characterize the effect of material, layup, processing, and specimen geometry variables on material response. For tension, a relatively strong sensitivity to variables was observed, with a general inverse relationship between small- and large-notch strengths. Specimen finite-width effects and pre-growth notch-tip strains that differed from classical predictions were observed. For compression, responses were less sensitive to material and layup variables than in tension, and consistently demonstrated notch-length sensitivities below that predicted by linear elastic fracture mechanics. Unlike tension, this response was accompanied by classical specimen finite-width effects. Thickness was found to be a more dominant variable than material, layup, and/or construction, with increasing thickness providing improved notched strengths. Physical evidence suggests that strain-softening material response is active, and finite element models incorporating these laws have successfully predicted the observed responses. Non-local material response also appears active. Preliminary attempts to attain strain-softening laws directly from crack-opening displacements were unsuccessful, but improved specimen geometries are being evaluated.

Large-scale structural tests confirmed adequate crown (skin/stringer) and keel (sandwich) damage tolerance capability for the perceived critical load cases. Predictions based on the material characterization tests and metallic elastic/plastic correction factors compared favorably with the experimental results, and demonstrated the need for consideration of non-linear material response to properly capture the load redistribution during damage growth. Finite element analyses with strain-softening laws developed from unconfigured small-notch tests successfully predicted responses of crown and keel structural tests. Alternate crown design concepts with intraply S-glass hybridized skins demonstrated superior capability, both for axial and hoop damage tolerance.