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Unformatted text preview: Application of the Finite Element Method Using MARC and Mentat 81 Chapter 8: Aircraft Fuselage Window Keywords: 2D elasticity, plane stress, symmetry, superposition Modeling Procedures: ruled surface, convert, biased mesh 8.1 Problem Statement and Objectives An aircraft fuselage structure must be capable of withstanding many types of loads, and stress concentrations near cutouts are of particular concern. In this exercise, internal pressure in a structure similar to a Lockheed L1011 commercial aircraft fuselage is considered. The objective of the analysis is to determine the stress state and the factor of safety in a square fuselage panel containing a window cutout. The geometrical, material, and loading specifications for the panel are given in Figure 8.1. Geometry: Material: 2024T3 aluminum alloy Radius of Curvature: R=100” Yield Strength: 50 ksi Skin Wall Thickness: t=0.063” Modulus of Elasticity: 10.5 Msi Window Dimensions: a=10”, b=15” Poisson’s Ratio: 0.33 Window Corner Radius: r=2” Loading: Square Section Span Length: L=30” Internal Pressure: 9.0 psi Figure 8.1 Geometry, material, and loading specifications for an aircraft fuselage panel with a window cutout. L r b a R t Application of the Finite Element Method Using MARC and Mentat 82 8.2 Analysis Assumptions 1. The analysis model will consider only the fuselage structure with an open cutout. In other words, the analysis will be performed as if no window is present (yet pressurization is maintained). 2. The radiustothickness ratio (R/t) of the fuselage skin is approximately 1,587. Therefore, the effect of curvature can be ignored in the present analysis. 3. The spantothickness ratio (L/t) of the fuselage skin is approximately 476. Therefore, the structure is very thin and a state of plane stress can be assumed. 4. The panel edges are a sufficient distance from fuselage stiffeners, so very little bending will occur due to the applied internal pressure load. The effects of bending will thus be neglected. 8.3 Mathematical Idealization Based on the assumptions above, a 2D plane stress model of the structure will be developed. The loads can be obtained from cylindrical pressure vessel theory. In other words, the internal pressure will cause an axial and a circumferential stress in the cylinder, which can be applied at the edges of the structural section being modeled. The idealized model is the same as in Figure 8.1, except that the curvature is ignored. 8.4 Finite Element Model The finite element model of this structure will be developed using 2D plane stress fournoded quadrilateral finite elements. The present analysis can be greatly simplified by taking advantage of existing symmetries in the idealized model. Note that both horizontal and vertical mirror planes of symmetry exist, as shown in Figure 8.2a. The geometry, material properties, and loading conditions are all symmetric about these two planes. Therefore, the response of the structure (i.e., displacements, strains, and stresses) will also be symmetric about these two planes. Hence, it is necessary to model strains, and stresses) will also be symmetric about these two planes....
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This note was uploaded on 07/25/2008 for the course ME 424 taught by Professor Averill during the Spring '05 term at Michigan State University.
 Spring '05
 AVERILL
 Stress

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