Issue 45

G. Giuliano et alii, Frattura ed Integrità Strutturale, 45 (2018) 164-172; DOI: 10.3221/IGF-ESIS.45.14 170 Figure 9 : AA6060 alloy forming limit curve. (a) (b) Figure 10 : Instability condition (FLP = 1) at the sheet pole: (a) using shell elements, (b) using axisymmetric elements. The experimental activity was conducted under three different lubrication conditions: - No lubrication; - Use of a sheet of polytetrafluoroethylene (PTFE), better known with the trade name Teflon ® ; - Use of grease containing graphite. The results in terms of the force-stroke curve of the punch are shown in Fig. 12. From Fig. 12, it can be seen that the fracture was determined after a stroke of about 28 mm in the case without lubrication. The displacement measured at the fracture point increases and reaches values of about 32 mm and 36 mm using graphite grease and PTFE as a lubricant,