Issue 45

G. Giuliano et alii, Frattura ed Integrità Strutturale, 45 (2018) 164-172; DOI: 10.3221/IGF-ESIS.45.14 165 strain measured at the beginning of the localized necking for all the possible values of the minor strain, as shown in Fig. 1. The comparison of the principal strains points location with the FLC allows to determine the defects onset in terms of necking (strain localizing) or fracture of the material. Finite element software uses FLC to examine the strains distribution in sheet metal parts. The FLC can be determined by experimental [4], theoretical [5-7] or hybrid methods [8] that combine experimental data with analytical or numerical approaches. An examination of different methods for determining FLC is presented in [9]. The theoretical methods are based on necking or material fracture criteria. Among the experimental methods, the most used is the Nakazima test for its simplicity. The test consists in making a hemispherical punch advance at a constant speed against a sheet bound between a die and a blank holder and allowing it to stretch until the necking or the fracture arises. This test is carried out on rectangular specimens with different widths so as to induce different strain states in the material, from the state of monoaxial stress to that of balanced stress. The maximum and minimum strains are detected by means of a reference grid drawn on the specimens before carrying out the test. Figure 1 : Typical FLC for sheet metal. Usually, the FEM evaluation of the feasibility of forming operations is obtained by comparing the estimated strains with the FLC. In the absence of this comparison, the numerical simulation can continue even after reaching the sheet fracture conditions. For this reason, some FEM software packages allow implementing criteria for fracture or strain localization. In this work, in order to validate the adopted FEM model, the results of an AA 6060 aluminum sheet forming process were compared with those resulting from numerical analysis. E XPERIMENTAL ACTIVITY he studied material consisted in a 1 mm thick sheet, made of an aluminium-magnesium-silicon alloy known as AA 6060 and characterized by the following chemical weight composition: Al-0.6%Si-0.3%Fe-0.1%Mn-0.6%Mg- 0.1%Cu-0.15%Zn-0.05%Cr-0.1%Ti. It was a general use alloy and it was characterized by a high corrosion resistance. In order to determine the material constitutive law, needed for describing the mechanical behavior of the material in the considered numerical model, tensile tests were carried out according to the relevant standard UNI EN 10002-1:2004. The tests were performed using a two-column universal electromechanical testing machine with a 100 kN capacity. The specimen was characterized by a resistant section of 20 mm 2 and a thickness equal to the sheet one. Fig. 2 shows the standard specimen and Fig. 3 the real stress-strain curve obtained from the tensile test. From the results of the tensile test it was possible to determine the material constants present in the constitutive equation expressed by the power law: T