Buckling And Ultimate Resistance Of High-strength Aluminum Thin-walled Stiffened Beams

Aluminum thin-walled stiffened beams are major load-bearing members for airframe, whose static failure is commonly caused by structural buckling phenomena. Buckling problems of thin-walled beams involve too complex interaction between geometric nonlinearity and elastoplasticity to be analytically solved; it further complicates the buckling issue that flexible structural design always induces comprehensive local-global interaction; high sensitivity of buckling behavior of thin-walled beam provides challenge for accurate numeric simulation. Moreover, when compared with traditional stiffened beam design, the utilization of high-strength material may reduce structural size to an extent beyond the limit of traditional design specification. This requires validation and necessary modification of traditional design method in greater range for high-strength stiffened beam design. Therefore, investigation of buckling behavior of high-strength thin-walled stiffened beams possesses important theoretical and practical value. Taking into account practical engineering technical requirements, this paper combines analytical solution, finite element method and experimental approach to investigate resistance of high-strength aluminum thin-walled stiffened beams under combined action of bending and shear, with the aims of exploring resistance mechanism of thin-walled beam and proposing design guidance. The primary work and achievements of the Ph D thesis are as follows:Recent studies roughly revealed that the elastic stability of web panel is greatly reduced by its significant non-uniform shear stress distribution, with respect to the traditional loading of uniform shear. Elastic buckling behavior of web panels of thin-walled beams with weak flanges is studied, aiming at a buckling coefficient formula unifying the effect of both weak and strong flanges. A new parameter, the flange-to-web ratio of moment of inertia, is proposed to characterize the effect of flanges. Then, a semi-analytical method is applied to investigate the buckling behavior of simply supported web panels, in two cases, inclusive or exclusive of effect of the moment of inertia of flanges. It is revealed that elastic buckling load, in particular, the buckling coefficient of web panel is a function of two key parameters, web aspect ratio and flange-to-web ratio of moment of inertia. Meanwhile, a finite element analysis(FEA) model allowing for the sensitivity of boundary conditions is validated by comparing with the semi-analytical solution to the case exclusive o f effect of the moments of inertia of flanges. Next, numerical results are utilized to show the fact that buckling capacity of web panel can be overestimated by the traditional assumption of uniform shear with a maximum of 100%. Besides, it also verifies that for the same flange-to-web ratio of moment of inertia, the buckling behavior of square web panels is closer to the uniform shear buckling than other rectangular web panels. Finally, an accurate design formula is proposed to calculate buckling coefficient of web panel.An accurate interaction design method is proposed to calculate buckling load of web panel where non-uniform shear stress co-exists with bending stress. A new loading component termed pure non-uniform shear is identified to exist in web panel. Then, the combined shear and bending load applied on the web panel is considered as a superposition of three basic loading components which are pure bending, uniform shear and pure non-uniform shear. While design formulae have been proposed for individual loading and combination of both uniform shear and pure bending, interaction equations for pure non-uniform shear and pure bending or uniform shear are presented based on the curve-fitting of the corresponding FEA results. Finally, an interaction equation is proposed for all the three basic loading components. The equation is entirely consistent with interaction expression under combinations of any two among the three basic loading components and it s solution agrees with the FEA result. Also, the pure non-uniform shear results in linearly varying bending moment along the longitudinal direction. Then, an exploration is done whether traditional interaction equation for combined uniform shear and pure bending is applicable to the actual non-uniform shear loading case.Experimental investigation is performed on high-strength aluminum thin-walled stiffened beams under interaction of bending moment and shear force. Compared with previous static test of thin-walled beams, the most important achievement of this test is the revelation of new resistance interaction between plate elements, that local capacity of a plate element can be affected by other remote elements. Previous research believes that stiffeners can disconnect plate elements on both sides efficiently so that local capacity of a plate element is not affected by plate elements beyond stiffeners. This experimental study verifies limitation of the previous argument and expands knowledge of beam bearing mechanism. First, a test rig design to meet specific loading requirements is developed. Then, the test specimens buckle and possess considerable post-buckling capacity, showing complex interaction between geometric nonlinearity and elastoplasticity. Finally, resistance and failure pattern of test specimens with different configurations(including web opening and stopping-crack stringer) and constraints are acquired. Furthermore, influence of initial imperfection on specimen resistance is explored.A nonlinear numerical study is carried out on resistance of thin-walled stiffened beams under combination of bending and shear. First, experimental results are used to validate two finite element models, which explicitly exhibit difference of both ultimate capacity and collapse response resulting from resistance interaction. Then, mechanism analysis is conducted and reveals that resistance interaction between plate elements separated by stiffener is developed through the corresponding change of global deformation resulting from geometric variation of plate element. Next, parametric analysis is performed and verifies that: 1) the interaction between plate elements grows with yield strength of material; 2) interaction between plate elements leads to sharp variation of beam resistance; 3) resistance interaction can be activated by reasonable determination of web configuration such as cutout dimension, cutout position, sub-panel thickness reduction and inwards mid-surface offset. Finally, the energy concept is adopted to study structural stiffness response during loading, considering the fact that structural failure as a result of stiffness reduction may occur before ultimate capacity. On this basis, this paper presents a failure criterion reflecting the effect of stiffness.