Experimental and numerical investigations on buckling behaviour of stiffened panel during creep age forming

Experimental and numerical studies on the buckling behaviour of stiffened panels under four point bending during creep age forming (CAF) have been carried out in this study for the first time. The experimental programme comprised buckling tests of five different sizes of stiffened panels of aluminium alloy 7050 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 strain evolution of the stiffened panel were obtained from the experiments, using digital image correlation (DIC) for the room temperature tests and strain gauges for the CAF tests. The effect of stiffener height and stiffener thickness and the effect of the heating and creep-ageing stages in CAF on the buckling behaviour have been investigated and discussed. It was found that buckling mode varied from one half-wave cosine mode in the elastic loading to three half-wave cosine mode with the increase of buckling stress from elastic to plastic region, and during CAF buckling mainly occurred and grew in the heating process. The corresponding non-linear finite element (FE) simulations of stiffened panels at room temperature and ageing temperature (160 degrees C) have also been carried out, and the FE results of buckling strain and buckling mode shape show a good agreement with experimental results. The non-linear FE method can provide accurate results of buckling strain and formability limits for cold forming and CAF processes, which can be used to guide the structural design of stiffened panels.