Structural integrity evaluation of KC-135 fuselage structure

In this study, extensive numerical simulations were conducted in order to assess the damage tolerance and residual strength capability of the KC-135 fuselage's original design using state-of-the-art fracture mechanics numerical methods (FRANC3D/STAGS) and to evaluate the impact of multiple site damage on the fuselage's structural integrity. Geometric nonlinear fracture mechanics analyses were conducted on detailed “local” models of three key KC-135 fuselage areas in the forward, center, and aft fuselage sections subjected to internal pressure loads and simulated flight loads. For the damage tolerance assessments, stress intensity factors were determined for longitudinal skin cracks under realistic loading conditions while accounting for such factors as the actual built-up structure, load redistribution, and large displacements. Elastic-plastic stable tearing analyses, which apply the critical crack tip opening angle fracture criterion, were conducted on the center and aft fuselage areas in order to predict their residual strength. These nonlinear fracture mechanics analyses indicated that the aft fuselage area may have a lower damage tolerance capability than the other areas because of its thinner 7075-T6 skin material. Residual strength predictions showed that the aft fuselage crown skin had the lowest load-carrying capability; it was only able to sustain a longitudinal skin crack that spanned approximately one-third of a frame bay under limit load conditions without failure of the skin panel. The initial structural integrity evaluations provided the foundation for conducting a thorough damage tolerance and residual strength assessment of the KC-135 aft fuselage that accounted for varying degrees of multiple site damage. A parametric study considering the influences of various combinations of lead cracks and multiple site damage on the fuselage's structural integrity was conducted on an aft fuselage lap joint. Analysis results provided an estimate of the failure threshold where the fuselage structure is able to sustain damage of a smaller size but runs the risk of catastrophic failure with larger damage. Under limit load conditions, the aft fuselage lap joint was able to sustain a crack that spanned across five fasteners with smaller multiple cracks at fastener locations in front of the lead crack, but larger lead cracks could not be sustained without the potential for catastrophic failure. (Abstract shortened by UMI.)...