Issue 30

C. Putignano et alii, Frattura ed Integrità Strutturale, 30 (2014) 237-243; DOI: 10.3221/IGF-ESIS.30.30 239 ˆ h    , ˆ a a h  , ˆ P P Ebt  , 0 0 ˆ P P Ebt  , / ˆ Et      , ˆ G G Et  , we can estimate the dimensionless energy release rate:   2 2 0 0 1 co ˆ s sin ˆ ˆ ˆ ˆ ˆ sin 2 sin PP P P P G          (5) where we employ the condition   cos a a h L      (see Fig. 1) , leading to: 0 0 ˆ ˆ cos 1 sin 1 cos 1 ˆ ˆ ˆ sin P P a P P                                       (6) Notice that this equation is perfectly coherent with relation found by Chen in [3] for single pre-stresses tape. Finally, we can determine the vertical displacement:   0 0 sin sin 1 1 ˆ ˆ ˆ cos cot cos ˆ ˆ P P P P              (7) R ESULTS AND DISCUSSION ur analysis starts discussing the stability of the peeling. In detail, let us determine stable or unstable equilibrium conditions and what happens when it moves from unstable equilibrium. In this respect, in Fig. 2a, the dimensionless peeling force ˆ P is plotted as a function of the peeling angle  at equilibrium. Furthermore, in Fig. 2b, for a fixed load ˆ P P  , we show the relative dimensionless energy ˆ U as a function of the peeling angle. Notice that in this last case the peeling angle is referred also to conditions out of equilibrium. Now, it is possible to observe that, for any load smaller the critical peeling force, that is the maximum load the tape can sustain, two different equilibrium conditions exist. In the region h/a > 0 (corresponding to the tape configuration shown in Fig. 1a), the total energy ˆ U takes a local minimum at the peeling angles solving Eq. (4) and, therefore, in this case equilibrium (solid line in Fig. 2a) is stable. On the contrary, in the region h/a < 0 (corresponding to the configuration of Fig. 1b), we find a maximum for the total energy ˆ U and, therefore, unstable equilibrium conditions are present. It is noteworthy to understand now what happens when we move from non-equilibrium conditions. Given the pull-off load ˆ P P  , we focus our analysis on the starting configurations A, B, C, and D shown in Fig. 2a. Let us start from point A. We observe that the minimization of the total energy requires that the peeling angles decreases monotonically towards smaller and smaller values. The end of the process should correspond to vanish the peeling angle but, in this case, the tape, being completely attached to the substrate, would be forced to sustain the vertical load with an infinite stress. This is physically not possible and, therefore, failure will occur before the tape adheres to the substrate. On the other side, when the system starts from point B, the tape peeling angle increases until we have the complete detachment, which O