In this paper, we revisit the formability of tube expansion by single point incremental forming to account for the material strain hardening and the non-proportional loading paths that were not taken into consideration in a previously published analytical model of the process built upon a rigid perfectly plastic material. The objective is to provide a new insight on the reason why the critical strains at failure of tube expansion by single point incremental forming are far superior to those of conventional tube expansion by rigid tapered conical punches. For this purpose, we replaced the stress triaxiality ratio that is responsible for the accumulation of damage and cracking by tension in monotonic, proportional loading paths, by integral forms of the stress triaxiality ratio that are more adequate for the non-proportional paths resulting from the loading and unloading cycles of incremental tube expansion. Experimental and numerical simulation results plotted in the effective strain vs. stress triaxiality space confirm the validity of the new damage accumulation approach for handling the non-proportional loading paths that oscillate cyclically from shearing to biaxial stretching, as the single point hemispherical tool approaches, contacts and moves away from a specific location of the incrementally expanded tube surface

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This study presents a numerical analysis of the tube expansion process by conventional tube-end forming versus single point incremental forming (SPIF) using DEFORM. The work includes the assessment of the strain paths within the principal strain space of these processes with respect to the formability limits as well as their evaluation within the equivalent strain versus stress triaxiality space. The results obtained demonstrated that the mechanics of tube flaring process in conventional and incremental forming are substantially different. This analysis of formability in the light of the accumulated equivalent strain and the average stress triaxiality allowed a better understanding of the differences between both processes in terms of the fracture limit strains.

Este articulo describe el proceso de aplicación de la deformación incremental para el conformado chapas de aluminio AA 1200 H14. En una primera parte se determinaron las variables más importantes del proceso, y la metodología para medir las deformaciones. Se realizó una simulación por elementos finitos del proceso de deformación en el régimen plástico a fin de determinar la fuerza requerida. Para la experimentación se diseñaron y construyeron los utillajes necesarios, y se programaron las trayectorias del centro de mecanizado para la producción de partes con geometrías irregulares. Varias pruebas experimentales se realizaron en chapas de aluminio de 1 mm de espesor utilizando tres diámetros de la herramienta. Se midió la fuerza requerida en la dirección normal al plano de la superficie de la chapa con una celda de carga y su relación con el tamaño y giro de la herramienta. Se encontró que el acabado superficial y la conformabilidad mejoran conforme el diámetro de la herramienta aumenta hasta cierto límite, y que la rotación de la herramienta disminuye la carga necesaria en el proceso.