Tension Fracture of Laminates for Transport Fuselage—Part III: Structural Configurations
Walker, T.H., Ilcewicz, L.B., Polland, D.R., Bodine, J.B., and Poe, C.C. Jr.
Fourth NASA Advanced Composite Technology Conference, NASA CP-3229, pp. 243-264, 1994.
A developmental program was conducted to investigate structural damage tolerance in stiffened fuselage crown structure representative of ATCAS advanced composite designs. These design concepts include tow-placed skins, drape-formed hat stringers, and braided/RTM frames with stringer mouseholes. Both circumferential and longitudinal notch orientations were evaluated considering axial body bending and pressure load conditions, respectively. Qualitatively, stiffening promotes stable growth and damage arrestment for both damage conditions. Significant notch-tip bending that occurs for longitudinal notches subjected to pressure depends on panel curvature in addition to the stiffening characteristics. Notched tests were conducted on large, structurally-configured panels to demonstrate LIMIT load capability with a severed structural unit (i.e., stiffener and skin bay). Axial damage tolerance testing was conducted on flat panels stiffened with 5 tear straps or 5 hat stringers. The tear strap panels confirmed similar strengths for identical panels with differing notch lengths. Both 5-stringer panels, identical except for material, exhibited stable damage growth and demonstrated sufficient axial damage tolerance capability. The 5-stringer panel fabricated from an intraply hybrid of AS4 and S2-glass had a 30% higher strength than the panel with all AS4 material, relating to the strength differences observed in large-notch tests of similarly sized unstiffened panels. Hoop damage tolerance was demonstrated with the testing of a curved 63″ x 72″ panel with hat stringers and mechanically attached J-section frames. LIMIT pressure-only and LIMIT pressure-with-flight-loads capabilities were both demonstrated with a notched test of the curved, stiffened panel subjected only to internal pressure (no applied axial load). An intense damage zone, consisting of extensive fiber failure and relatively compact delaminations, extended asymetrically from the notch tips to the fasteners closest to the mouseholes. Strain distributions in the skin and frames indicated the damage extension occurred in several jumps, implying arrestment capability in the configured structure. Finite element analysis under-predicted the mid-plane notch-tip strains prior to formation of additional damage, supporting previous findings on unstiffened panels that classical prediction techniques have difficulty predicting response in high gradient regions.