Stringers are stiffening members of pressurized aircraft fuselage. They provide support to the fuselage’s skin. A new stringer grid concept is proposed for conventional aircraft fuselage. Optimization is used to find the hexagonal grid that best replaces the original while keeping the same total stringer length. A finite element model is built to analyze the optimal hexagonal grid stiffened structure and compare it with the original orthogonally stiffened structure in terms of eigenfrequencies and static response to external loading. The finite element model is validated through Flugge’s analytical expressions for stiffened shells. Results show that the hexagonal grid stiffened structure yields higher eigenfrequencies with stresses and displacements comparable with that of the original structure.

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The occurrence of cracks in aging aircraft fuselage is major problem in the airline industry. The remaining life of the aircraft is strongly dependent on the residual strength of its structure. Residual strength is affected by crack sizes and their growth rates. In the case of a longitudinal crack in a pressurized cylinder (as in the case of an aircraft fuselage), the geometry and loading conditions cause the edges of the crack to bulge out generating a complex stress field around the crack tips; this is known as the 'bulging effect'. The geometry of the shell, crack size and pressure contribute to this phenomenon. A proposed solution to reduce the effect of bulging for this type of crack is to apply a layer of polyisocyanurate (PIR) foam to the inner side of the fuselage near the crack site. This layer will bond to the shell and has the effect of reducing the bulge and consequently, the Stress Intensity Factor (SIF) at the crack tips. PIR foam is a lightweight material that adheres well to the shell and provides additional stiffness around the crack area. In the present study the effect of applying a PIR foam layer to a longitudinal T. crack in a pressurized cylindrical shell is assessed. Nonlinear Finite Element Analysis (FEA) is used in conjunction with the Modified Crack Closure Integral technique (MCCI) in order to evaluate the effect of bulging on the crack's SIF. Parameters considered in this study include shell radius, shell thickness, crack length, foam thickness and pressure. Numerical results are compared with existing experimental data and the effect of foam thickness for several shell configurations is presented. Results indicate that the bulge factor (BF) could be reduced by as much as 45% depending on shell configuration, foam thickness and pressure.

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