Residual-Strength Assessment of Modified Transport Aircraft Fuselages
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In this study, several combinations of fail-safe features were investigated to assess their effect on the damage-tolerance characteristics and residual strength of a typical military transport aircraft fuselage section composed of floating-frame construction. Detailed nonlinear finite element models were developed for a key area of a representative aft fuselage. These local fuselage models were refined to include longitudinal lead and multiple-site-damage cracks of various sizes. A global-to-local modeling procedure was used whereby kinematic boundary conditions applied to the edges of the local fuselage models were developed using internal field quantities from a finite element simulation of a structurally complete (global) fuselage for a given load case. A residual-strength assessment of the relevant aft section for load cases involving a combination of limit internal pressure and flight loads was conducted using highly refined finite element models for cases involving 1) no fail-safe features (i.e., the basic fuselage configuration), 2) inclusion of shear ties as part of a modified fuselage design, 3) inclusion of tear straps as part a modified fuselage design, and 4) inclusion of both shear ties and tear straps. The influence of discrete source damage and small-scale cracking at fastener holes (i.e., multiple-site damage) was considered in the analysis. The critical crack-tip opening-angle fracture criterion and plane-strain core concept were employed in performing large-deformation elastic-plastic stable-tearing analyses using three-dimensional fracture-analysis code and structural analysis of general shells code to predict the fuselage section's residual strength. These results suggest that shear ties and tear straps may be effectively used to increase the structural integrity of the aft fuselage. Key results from this study may provide useful information regarding the efficiency of incorporating fail-safe design modifications in the existing aircraft to enhance the residual strength of cracked sections of the fuselage. Copyright 2008 by the American institute ot Aeronautics and Astronautics, Inc. All rights reserved.
