Issue 45

G. Giuliano et alii, Frattura ed Integrità Strutturale, 45 (2018) 164-172; DOI: 10.3221/IGF-ESIS.45.14 166 n K    (1) where σ and ε are the stress and the strain, respectively; while K and n are characteristic constants of the material. Figure 2 : Geometrical characteristic of the tensile test specimen. Figure 3 : Real stress-strain curve definition obtained by tensile test (AA 6060 alloy). In order to validate the FEM model, a forming test was carried out, schematically similar to the Nakazima test, but with larger geometric dimensions. In particular, the punch was characterized by a diameter of 60 mm. To perform this test, special equipment designed and built at the Laboratory of Technology and Manufacturing System of the University of Cassino and Southern Lazio was used. Fig. 4 shows an operating schema of the equipment. The system regulating the vertical translation of the punch was based on a trapezoidal screw jack with an external female thread. The rotation of the screw, driven by an electric motor through a mechanical transmission system, dragged the external female thread, integral with the crosshead on which a punch coupled to a load cell was mounted, with an alternative straight upward or downward movement. The punch speed, equal to 3 mm/min, was selected by appropriately acting on the electric motor through an electronic apparatus (inverter). In the same schema of Fig. 4 it is also possible to observe the equipment capable of evaluating the force trend applied by the punch on the sheet during the test. It was constituted by a multimetre and a power supply, connected to the load cell placed under the punch. Fig. 5 shows the technical solution adopted for mounting the load cell on the movable crosshead. The sheet was clamped on the equipment by means of a steel plate, by tightening the apposite screws with a closing torque of 45 Nm (Fig. 6). Further details on experimental equipment are