The mechanism of detachment of thin films from a flat smooth rigid substrate is investigated. In particular, analytical solutions in closed form are proposed for the double peeling of an elastic tape as well as for the axisymmetric peeling of a membrane. We show that in the case of double peeling of an endless elastic tape, a critical value of the pull-off force is found, above which the tape is completely detached from the substrate. In particular, as the detachment process advances, the peeling angle is stabilized on a limiting value, which only depends on the geometry of the tape, its elastic modulus and on the interfacial energy. This predicted behavior agrees with the ‘‘theory of multiple peeling’’ and clarifies some aspects of this theory. Moreover, it is also corroborated by experimental results (work in progress) we are carrying out on a standard adhesive tape adhered to a smooth flat poly(methyl methacrylate) surface. In the case of the axisymmetric adhering membrane, a different behavior is observed. In such case, the system is always stable, and the detached area monotonically increases with the peeling force, i.e., the elastic membrane can sustain in principle any applied force. Results are validated by a fully numerical analysis performed with the aid of a finite element commercial software.

In this paper, we present numerical investigation of the contact between an elastic solid and a randomly rough surface. In agreement with recent results, we find that the contact area vs load relation depends on the statistical parameters only through the root mean square slope of the heights distribution. Such result extends to contact pressure regimes where the area/load relation is non-linear. Moreover, we show that fractal self-affine surfaces give a good representation of real surfaces from both topographical and contact mechanics points of view. Finally, we investigate how the network of non-contact areas evolves as the real contact area is increased, finding that the percolation threshold is smaller than the one predicted by Bruggeman's theory

La definizione della soluzione del problema di contatto è oggi richiesta da una sempre più ampia varietà di applicazioni. Le sollecitazioni, le deformazioni e la dissipazione all’interfaccia di contatto sono, nello specifico, parametri da settare nel design ottimizzato di tali componenti. In mancanza di soluzioni analitiche universali, si è fatto finora ampio ricorso ai Metodi FEM (Finite Elements Method): gli stessi tuttavia sono poco efficienti nella definizione di un’accurata soluzione di contatto. In particolare, ciò risulta vero quando si voglia tenere in conto la rugosità superficiale dei componenti a contatto. Al fine di superare tali limitazioni, sono stati sviluppate dagli autori metodologie agli Elementi di Contorno in grado di definire accuratamente i parametri di contatto più significativi: area di contatto, rigidezza, dissipazione ed attrito sono analizzate in funzione della configurazione di carico. Particolare attenzione è dedicata allo sviluppo di tecniche di mesh efficienti per la riduzione dei costi computazionali.

In the present paper we propose a generalization of the model developed in Afferrante, L.; Carbone, G.; Demelio, G.; Pugno, N. Tribol. Lett. 2013, 52, 439–447 to take into account the effect of the pre-tension in the tape. A detailed analysis of the peeling process shows the existence of two possible detachment regimes: one being stable and the other being unstable, depending on the initial configuration of the tape. In the stability region, as the peeling process advances, the peeling angle reaches a limiting value, which only depends on the geometry, on the elastic modulus of the tape and on the surface energy of adhesion. Vice versa, in the unstable region, depending on the initial conditions of the system, the tape can evolve towards a state of complete detachment or fail before reaching a state of equilibrium with complete adhesion. We find that the presence of pre-tension in the tape does not modify the stability behavior of the system, but significantly affects the pull-off force which can be sustained by the tape before complete detachment. Moreover, above a critical value of the pre-tension, which depends on the surface energy of adhesion, the tape will tend to spontaneously detach from the substrate. In this case, an external force is necessary to avoid spontaneous detachment and make the tape adhering to the substrate.

In this paper, we discuss the mechanism of detachment of thin pre-stressed films from a flat smooth rigid substrate. Indeed, we develop an analytical solution in closed form which shows how the critical value of the pull-off force strongly depends on the press-stress P0 . In detail, the critical pull-off force needed to detachment is shown to be higher for pre- stressed tapes. Furthermore, we notice that, when a high pre- stress is present, tapes may behave in different manner and spontaneously detach from the rigid substrate.

Two different aortic prostheses can be used for performing the Bentall procedure: a standard straight graft and the Valsalva graft that better reproduces the aortic root anatomy. The aim of the present work is to study the effect of the graft geometry on the blood flow when a bileaflet mechanical heart valve is used, as well as to evaluate the stress concentration near the suture line where the coronary arteries are connected to graft. An accurate three-dimensional numerical method is proposed, based on the immersed boundary technique. The method accounts for the interactions between the flow and the motion of the rigid leaflets and of the deformable aortic root, under physiological pulsatile conditions. The results show that the graft geometry only slightly influences the leaflets dynamics, while using the Valsalva graft the stress level near the coronary-root anastomoses is about half that obtained using the standard straight graft.

The simultaneous replacement of a diseased aortic valve, aortic root and ascending aorta with a composite graft equipped with a prosthetic valve is a nowadays standard surgical approach, known as the Bentall procedure: the Valsalva sinuses of the aortic root are sacrificed and the coronary arteries are reconnected directly to the graft. In practice, two different composite-material prostheses are largely used by surgeons: a standard straight graft and the Valsalva graft with a bulged portion that better reproduces the aortic root anatomy. The aim of the present investigation is to study the effect of the graft geometry on the the flowfield as well as on the stress concentration at the level of coronary-root anastomoses during the cardiac cycle. An accurate three-dimensional numerical method, based on the immersed boundary technique, is proposed to study the flow inside moving and deformable geometries. Direct numerical simulations of the flow inside the two prostheses, equipped with a bileaflet mechanical valve with curved leaflets, under physiological pulsatile inflow conditions show that, when using the Valsalva graft, the stress level near the coronary-root anastomoses is about half that obtained using the standard straight graft.