Determination of minimum thickness of composite panels taking into account limits on stability and fatigue under postbuckling behavior

When designing the compressed thin composite skin of a multispar flap box of a small aircraft, its buckling due to compressive forces at loads below the operational level is admissible. This work considers smooth orthotropic rectangular panels loaded with longitudinal compressive strength. To determine the optimal parameters of the panels, a method based on a postbuckling state involving an analytical solution of geometrically nonlinear problems obtained by the Bubnov–Galerkin method was used. In this paper, methods for determining the minimum thickness of orthotropic panels, for which supercritical behavior is permissible when subjected to compressive forces, are developed. They include, firstly, the approaches using various strength criteria given the static loading, secondly, methods allowing for the variation of the width of the panel when considering two levels of loading: ensuring limitations of buckling at the first level of loading and static strength conditions in the case of geometrically nonlinear behavior at the second level of loading, and thirdly, methods taking into account the requirements of fatigue cycling. Here, also two levels of loading may be considered, at which stability and strength are ensured under postbuckling behavior, according to the parameters of fatigue loading and permissible stresses with respect to fatigue strength. These methods are reduced to solving analytical ratios with respect to the thickness shown on the example of hinge-supported composite panels. Since the obtained analytical relations are associated with the initial stage of postbuckling behavior, the paper provides an expression for compressive forces, at which the number of half-waves can be increased under geometrically nonlinear behavior. Of practical significance is the possibility of determining the optimal parameters of smooth orthotropic panels under static and fatigue loading at the early stages of design.