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In this article, a new manufacturing method is introduced to shape circular tubes into columns with triangular cross-section by the elastoforming process. Also, a theoretical analysis is performed to derive a theoretical formula for predicting total dissipated energy that is required for the forming process. For this purpose, V-shape dies with different angles are designed and some aluminum and brazen tubes with different characteristics are prepared. The circular tubes in the empty and filled conditions are compressed between a rigid V-shape die and a flat punch, and during the plastic deformation under the lateral loading, the tubes are shaped into the triangular sections. Considering different tube lengths, outer diameters and wall thicknesses, the specimens are categorized. Also, some of the samples are filled by cylindrical polyethylene Teflon with different thicknesses to investigate the effects of Teflon-filler on the shaping process of the triangular columns. The experiments show that using the cylindrical Teflon-filler, deformation mode of the triangular tubes improves, significantly. In addition, experimental observations of the deformation modes illustrate that there is an optimum value for wall thickness of cylindrical Teflon-filler and the tubes with the optimum Teflon-filler forms close to desirable triangular shape. The results show that by increasing tube wall thickness, probability of crack initiation and fracture reduces. Furthermore, comparison of estimations by the presented theory and the corresponding experimental measurements show an acceptable agreement, in both of empty and filled conditions.

A new theoretical model of plastic behavior of circular metal tubes during the contraction process as a metal forming and energy absorber processes is presented in this study. Some theoretical relations are developed to predict the axial load versus the axial displacement of the contraction process, based on the theoretical model. Some quasi-static experiments were performed on circular aluminum and brazen tubes to validate the theoretical analysis. In this research, effects of tube radius, contraction ratio, tube wall thickness and angle of conical die on energy absorption characteristics of circular tubes under axial quasi-static loading are investigated by the experimental and theoretical methods. Results show that there is an admissible agreement between the theoretical results and experimental measurements. Experimental results show that there is a steady force during the end zone of the process which is an important factor in viewpoint of the energy absorption. Also, the steady force increases by increment of die semi-angle, tube radius, friction coefficient and the contraction ratio.

This article introduces a new metal forming process for creating a fold with a controllable and certain wavelength into the circular metal tubes during the axial compression. Also, a new analytical model of plastic deformation of the circular metal tubes constrained between two rigid punches subjected to quasi-static axial loading is introduced. Based on the deformation model, some theoretical relations are derived to predict the instantaneous axial load of a complete fold creation into the tubes versus the axial displacement in the elastic and plastic zones. Four different mechanisms of energy absorption are considered in the present theoretical model: two kinds of bending mechanism around circular hinge lines, circumferential expansion and unbending mechanism corresponding to meridian strain. Theoretical prediction shows that the required axial load for creating a complete fold in a constrained metal tube between two rigid punches is dependent on the tube wall thickness, internal radius and initial free length of the tube and also tube material properties. Also, some quasi-static axial compression tests were performed on the circular tubes of brass and aluminum alloys between two rigid punches, and the results were compared with the theoretical predictions that showed a reasonable agreement. Furthermore, the experimental observations show that wavelength of a complete created fold into the circular tubes could be controlled using two rigid dies during the folding process and using the poly(vinyl chloride) Teflon as the filler, instead of the intrinsic wavelength of the tube.