Influence of strain hardening, strain aging and the Bauschinger effect on steel tubular strut load capacity

A nonlinear finite element model for tracing the inelastic pre- and postbuckling load-deformation path of tubular struts has been developed. The analysis accounts simultaneously for both the geometrical and material nonlinearities. The influence of strain-unloading is included. In particular, the model takes into account a generalized stress-strain relationship and the influence of strain hardening. Several stress-strain relationships of the material are assumed with respect to as-received material, material that is prestrained in tension and fully-aged, and material that is prestrained in tension and unaged. The structure tangent stiffness matrix is obtained by using a series of transformation matrices to update the element geometry. A so called arc-length iterative numerical procedure is combined with a modified Newton-Raphson method to obtain a solution to the incremental equations of equilibrium. The procedure has been applied to trace out the inelastic load-deflection paths of both as-received and prestrained pin-ended struts. Material and strut tests have been performed on circular tubular sections of 60.3mm diameter and wall thickness 2.3mm. Several material effects (strain hardening, strain aging, the Bauschinger effect and residual stresses) have been included in the tests. The significant influence of these effects have been investigated by applying a two-way analysis variance to the strut test results. It has been found that the combined influence of strain aging, strain hardening and the Bauschinger effect have a dominant role in the test results referred to herein, with load capacity reductions up to 38%. Strain aging has been seen to be more significant than strain hardening for the rimming steel used in enhancing the capacity of the prestrained struts. The experimental results are also compared with the current column design curves from ECCS, SSRC and existing and proposed Australian column curves. By comparing the theoretical results with the experimental results of the as-received struts and the struts prestrained in tension, both fully-aged and unaged, it shown that the developed theoretical model can accurately predict the experimental maximum loads and the post-buckling load-deflection paths of steel tubular struts