Issue 49

C. Bellini et alii, Frattura ed Integrità Strutturale, 49 (2019) 791-799; DOI: 10.3221/IGF-ESIS.49.70 795 in which v r is the relative sliding speed and R sv is the relative sliding speed below which the friction force tends to vanish. The value of the friction coefficient μ varies between 0 and 0.3. In particular, the friction coefficient assumes the values of 0, 0.025, 0.05, 0.1, 0.2 and 0.3. A user-defined subroutine is used to introduce the most suitable FLC in the model. A NALYSIS OF NUMERICAL AND EXPERIMENTAL RESULTS he FEM modelling and the experimental tests conducted on the AA6060 aluminium alloy produce the following results: 1. The FLC is reached through a deformation path that varies according to the adopted friction coefficient; 2. The distance between the symmetry axis and the point where the FLP is equal to 1 decreases as the friction coefficient decreases from the value 0.1 to the value 0, while it is almost constant for a value of the friction coefficient including between 0.1 and 0.3; 3. The experimental results determined in the fracture condition [19] confirm what was found in point 2; 4. The stroke of the punch (measured as the condition FLP = 1 is reached) is maximum for a value of the friction coefficient different from zero, that is when the principal strains measured on the sheet surface are unbalanced. Fig. 3b shows, in yellow, the achievement of the instability condition (FLP = 1) near the sheet symmetry axis and in perfect lubrication conditions (μ = 0). In order to confirm what stated in point 1, Fig. 4 shows the deformation paths followed in the different friction conditions simulated in the work. Figure 4 : Deformation paths followed in different friction conditions and FLC relevant to the AA6060 aluminium alloy represented in the positive half-plane. In Fig. 5, the achievement of the condition FLP = 1 in the various friction conditions is observable in yellow. It is possible to point out that the yellow zone moves from the centre to the periphery of the specimen as the friction coefficient increases from 0 to 0.1. Fig. 6 shows the distance between the centre of the specimen and the fracture line in a sheet subjected to the experimental test conditions. In [19] it has been observed, through experimental tests, that the fracture line approaches the sheet symmetry axis if the lubrication conditions between the punch and the sheet are improved (theoretically approaching friction coefficient values equal to 0). Moreover, the punch stroke measured at the fracture attainment varies from about 26 mm, in the condition of lubrication absence, to about 30 mm, in the case a polytetrafluoroethylene sheet (PTFE) is used as a lubricant. Fig. 7 shows the punch stroke trend, measured when the FLP = 1 condition is reached, as a function of the friction coefficient. From the figure it is possible to highlight what stated in point 4. This result is a direct consequence of the followed deformation path varying the friction coefficient, reported in Fig. 4. In fact, in the positive half-plane, the FLC can be represented by the curve passing through the points A-B-C-D. The A-B section of the FLC curve is characterized T