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Fig. 9. Fracture strains in SPIF and Stretch-bending for a tool diameter of 10mm. Two interesting results can be pointed out from Fig. 9. On the one hand, it can be seen that the fracture strains in the Stretch-bending tests with cylindrical punch of 10 mm diameter are quite similar to that obtained with Nakazima tests (FFL), and only in some cases placed slightly above this fracture limit. This denotes a relatively insensitivity of fracture strains with the bending effect for this material, in which failure is mainly controlled by the maximum shear stresses, as discussed in Vallellano et al. (2008). On the other hand, it has been found that fracture strains in SPIF are clearly above both the FFL and as well above the fracture strains in Stretch-bending. This seems to indicate that the enhancement on formability obtained with SPIF might not be only explained via the effect of bending induced by the tool radius. Other parameters such as shear stresses, the contact stresses, the cyclic straining and the hydrostatic stresses Emmens and den Boogaard (2009) may be affecting the overall spifability of the AA2024-T3. The analysis of these parameters inpidually would require a detailed experimental and numerical study to characterize the effect of each one of these variables independently. 4. Conclusions This paper analysed experimentally the formability of AA2024-T3 metal sheets under different forming processes: stretching, Stretch-bending and single point incremental forming. The conventional formability limits were correctly set from a series of normalized Nakazima tests by using different specimen geometries. The feasibility of these conventional forming limits, and especially the fracture forming curve, was put to the test by evaluating the failure strains of stretch-bending and SPIF experiments carried out using tool of relative small diameters. The results seem to show that the bending effect, controlled by the t0/R ratio, might not be enough to explain the enhanced formability registered in SPIF. In this sense, other parameters should were pointed out to have an influence in the phenomenon captured in relation with the spifability of this low-ductility aluminium alloy. In any case, the authors would propose a more insistent experimental plan in order to further analyze this phenomenon, their causes and to fully confirm its experimental occurrence. Acknowledgements The authors would like to thank the Spanish Government for its financial support thought the Project No. DPI 2009-13335. The authors would also like to thank to GREP at the University of Girona, especially to I. Bagudanch and M.L. Garcia-Romeu, for borrowing their facilities to carry out the SPIF tests. References Centeno, G., Vallellano, C., Vázquez, J., Morales,
D., Martínez-Donaire, A.J., García-Lomas, F.J., 2012. Numerical analysis of the deformation mechanisms in incremental forming of AA2024-T3 sheets. AIP Conference Proceedings 1431, pp. 740-747. Centeno, G., Doblas, F.J., Martínez-Palmeth, L.H., Martínez-Donaire, A.J., Vallellano, C., 2012. FEA of the Bending Effect in the Formability of Metal Sheets via Incremental Forming. Steel Research International, Special Edition: 14th Metal Forming International Conference 2012, pp. 447-450. Centeno, G., Silva, M.B., Cristino, V.A.M., Vallellano, C., Martins, P.A.F., 2012. Hole-flanging by incremental sheet forming. International Journal of Machine Tools and Manufacture 59, pp. 46-54. Emmens, W.C., van den Boogaard, A.H., 2009. An overview of stabilizing deformation mechanisms in incremental sheet forming. Journal of Material Processing Technology 209, pp. 3688–3695. ISO 12004-2:2008, Metallic Materials-Sheet and Strip-Determination of Forming Limit Curves in Laboratory. Martínez-Donaire, A.J., Vallellano, C., Morales, D., García-Lomas, F.J., 2010. Experimental Detection of Necking in Stretch-Bending Conditions: a Critical Review and New Methodology. Steel Research International 81, pp. 781-784. Pérez-Santiago, R., Bagudanch, I., García-Romeu, M.L., 2011. Force Modeling in Single Point Incremental Forming of Variable Wall Angle Components. Key Engineering Materials 473, pp. 833-840. Silva, M.B., Nielsen, P.S., Bay, N., Martins, P.A.F., 2011. Failure mechanisms in single point incremental forming of metals. International Journal of Advanced Manufacturing Technology 56, pp. 893-903. Stoughton, T.B., Yoon, J.W., 2011. A new approach for failure criterion for sheet metals. International Journal of Plasticity 27, pp. 440-459. Vallellano, C., Morales, D., García-Lomas, F.J., 2008. A study to predict failure in biaxially stretched sheets of Aluminum alloy 2024-T3. Materials and Manufacturing Processes 23, pp. 303-310. Vallellano, C., Morales, D., Martínez-Donaire, A.J., García-Lomas, F.J., 2010. On the Use of Concave-Side Rule and Critical-Distance Methods to Predict the Influence of Bending on Sheet-Metal Formability. International Journal of Material Forming 3 S1, pp. 1167-1170.