Introducing a new cold metal forming method for producing a fold in the circular tubes with a certain wavelength

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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.