In this study, the plastic deformation mechanism of a fully clamped beam under oblique loading at its free end is analyzed. Supposing the cross sections are variable along the beam length, a characteristic length LMP/NP, defined as the ratio between fully plastic bending moment MP and fully compression force NP, is employed to estimate the load carrying capacity of each cross section. By finite element (FE) simulations of the conical tubes, it is validated that if the initial failure positon locates in the middle of the beam, it will not change with the total beam length. Then, based on the analytical analysis and FE simulation, a progressive deformation mechanism triggered by bending, notated as progressive bending, is proposed for the first time. From the optimization result of maximizing loading force that the unit mass can withstand, the tubes with constant thickness are found to be better than tubes with graded thickness, when they are used as supporting structures. The multi-objective optimization for tubes with varying cross sections under oblique loading with different angles is also given. Then, two methods to improve the load carrying capacity of tubes are given: (1) to design the cross section of the tube, which is corresponding to let the critical loading force of all the cross sections be equal; (2) to optimize the initial failure point, so as to produce repeated failure modes.

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