Buckling analysis of stiffened panels in creep age forming

Abstract

Integrally stiffened panels have been widely applied in transportation applications because of the advantages of lightweight and high bending stiffness. During the forming process, buckling may occur on the stiffened panel due to the high compression concentrating on the top of the stiffener, which is one of the challenges in the forming of integrally stiffened panels. This thesis presents experimental, semi-analytical and numerical investigations on the buckling behaviour of integrally stiffened aluminium panels in creep age forming (CAF). The experimental programme includes buckling tests of stiffened panels of aluminium alloy 7050 under bending at room temperature and buckling tests with different loading degrees in CAF including the loading, heating and creep-ageing stages. The buckling mode, the strain distribution and the buckling strain of the stiffened panel under bending have been obtained from the experiments for the first time. It has been found that the buckling mode varies from one half-wave cosine mode to three half-waves cosine mode with the increase of buckling stress from the elastic to plastic region. During CAF, buckling mainly occurs and grows in the heating process. A new semi-analytical method has been proposed in this study for the elastic-plastic buckling analysis of stiffened panels under bending to predict the buckling stress at room temperature and ageing temperature. In this method, simplified models of the blade-stiffened panels and orthogrid-stiffened panels have been built with the application of two theories of plasticity, the incremental theory (IT) and the deformation theory (DT). The governing differential equations have been solved using the differential quadrature (DQ) method and the iteration process has been adopted due to the non-linearity of material properties in the elastic-plastic buckling analysis. A non-linear finite element (FE) model has also been developed and simulations of stiffened panels at room temperature and ageing temperature have been carried out, using the commercial software Abaqus/Standard. The semi-analytical results of buckling strain based on IT and the FE results both show good agreements with experimental results, verifying their effectiveness. Based on the proposed semi-analytical method and FE method, the effects of geometric parameters of stiffened panels (stiffener thickness to height ratio, stiffened panel length to stiffener height ratio, stiffened panel width to stiffener height ratio, and skin thickness to stiffener thickness ratio) and temperature on buckling behaviour in the elastic-plastic region have been investigated and discussed. It is found that increasing stiffener thickness to height ratio enhances the buckling stress of stiffened panels under bending, while the effects of stiffened panel length to stiffener height ratio, stiffened panel width to stiffener height ratio and the thickness ratio are insignificant in the plastic region. The proposed semi-analytical method and FE method provide accurate results of buckling stress and formability limits for cold forming and CAF processes, which can be used to guide the parameter optimisation in the structural design of stiffened panels.