Local buckling of aluminium members in the elastic-plastic range

Abstract

The ultimate behaviour of aluminium members is strongly affected by the interactive local buckling in the elastic-plastic range occurring in the element plates constituting the cross-sections and by the strain-hardening behaviour of aluminium material. In order to take into account these phenomena, the current design codes pursue the prediction approaches as well as adopted for the steel members. However, the theoretical results, in terms of ultimate resistance and deformation capacity, are often conservative in comparison to the real behaviour of members and, consequently, the use of aluminium as a structural material is not more competitive from an economic point of view. Therefore, the main aim of the present work is to provide specific methodologies for evaluating the real behaviour of aluminium members under uniform compression and non-uniform bending. Preliminarily, an experimental campaign is carried out on the square hollow sections made of aluminium alloy EN AW 6060 temper T6. In particular, nine stub tests and twelve three point bending tests are performed on four sections of width 40, 60, 80 and 100 mm with the nominal thickness of 2 mm. Subsequently, starting from the J2 deformation theory of plasticity, the theory of plastic buckling of plates is presented including also the variability of the Poisson’s ratio depending on the stress levels. The differential equation of the plates at the onset of buckling is developed and the corresponding solution is provided. This is an innovative step compared to the theoretical solutions currently existing in the technical literature because the variability of the Poisson’s ratio in the elastic-plastic range is commonly neglected. Starting from the obtained closed-form solution, the interactive buckling either in the elastic or in the plastic range of box and H-shaped sections under uniform compression is analysed. Then, the attention is devoted to the prediction of the ultimate response of aluminium alloy beams subjected to a moment gradient. To this scope, a wide parametric analysis has been performed by the ABAQUS computer program to investigate the response parameters characterising the ultimate behaviour of members. The parameters affecting the ultimate performances of aluminium alloy beams subject to local buckling under non-uniform bending are the flange slenderness, the flange-to-web slenderness ratio, the non-dimensional shear length and the Ramberg-Osgood coefficient which governs the strain hardening behaviour. The flange slenderness parameter is directly related to the occurrence of local buckling either in the elastic or in the plastic range. The flange-to-web slenderness ratio accounts for the interactive buckling of the plate elements constituting the member section. The non-dimensional shear length accounts for the longitudinal stress gradient due to non-uniform bending. These non-dimensional parameters define entirely the geometry of the three-point bending scheme usually adopted for evaluating the rotation capacity of metal members. The final goal is the setting up of formulations to predicting both the maximum bending resistance and the rotation capacity of aluminium beams. Moreover, some advances concerning the use of the effective thickness approach, currently adopted by Eurocode 9, are also proposed and herein presented for the first time both in the case of compression and bending. The accuracy of the previous methodologies is pointed out by comparing the buckling resistance and the deformation capacity obtained by the theoretical approaches and the mathematical relations with those provided by the experimental tests presented in this work and in the scientific literature. [edited by Author]