23rd Congress of International Council of the Aeronautical Sciences, 8-13 September, 2002, Toronto, Canada
Paper ICAS 2002-R10

PRE-STRESSED SYMMETRIC COMPOSITE BEAMS IN AIRCRAFT - ANALYSIS AND DESIGN IMPLICATIONS

D. H. Chester
Israel Aircraft Industries Ltd., Israel

Keywords: beams, bending, composite materials, stress analysis, prestress, design, weight-saving

The uniform beam has a curved elastic core. It is reverse-bent prior to the attachment of the two stiffer outer-layers. When the bonds are dry and after the pre-loading bending-moment has been removed, the residual stresses modify the internal loads. From strain-compatibility at the interface, the allowable values of the associated stresses depend on the material properties and the geometry of the cross-section. Their limitations are determined by balancing the relaxation bending-moments, whilst equating the strains yields the beam curvatures used in its fabrication. The pre-stress is relieved before reaching the maximum load, when both materials are designed to arrive at their ultimate stresses together. Were it not for core shear flexibility, the values of these stresses would be limited solely by the ratios of Young's Modulus and the material's strength. The maximum relaxation stresses in the outer-layers are made equal to the opposite load-direction ultimate strengths. Then for optimal pre-stress, the residual stress and core flexibility are determinate. This analysis provides workable limits for the geometric and stiffness ratios.

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