L. Afferrante

Adhesion of Elastic Thin Films: Double Peeling of Tapes Versus Axisymmetric Peeling of Membranes

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

Interaction of thermal contact resistance and frictional heating in thermoelastic instability

\Updelta\gamma

Sliding thermoelastodynamic instability

Δγ. 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.

Biomimetic surfaces with controlled direction-dependent adhesion

This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

Microstructured superhydrorepellent surfaces: effect of drop pressure on fakir-state stability and apparent contact angles

Thermoelastic contact problems can posess non-unique and/or unstable steady-state solutions if there is frictional heating or if there is a pressure-dependent thermal contact resistance at the interface. These two effects have been extensively studied in isolation, but their possible interaction has never been investigated. In this paper, we consider an idealized problem in which a thermoelastic rod slides against a rigid plane with both frictional heating and a contact resistance. For sufficiently low sliding speeds, the results are qualitatively similar to those with no sliding. In particular, there is always an odd number of steady-state solutions; if the steady-state is unique it is stable and if it is non-unique, stable and unstable solutions alternate, with the outlying solutions being stable. However, we identify a sliding speed V(0) above which the number of steady states is always even (including zero, implying possible non-existence of a steady-state) and again stable and unstable states alternate. A parallel numerical study shows that for V>V(0) there are some initial conditions from which the contact pressure grows without limit in time, whereas for V

Adhesive contact of the Weierstrass profile

A 2D multilayered model has been considered to estimate the transient evolution of temperature and pressure perturbations in multi-disk clutches and brakes during operation. The model proposed by Decuzzi et al. [1] has been modified here to estimate the variation of b–perturbation growth rate—with V—relative sliding speed. It has been verified that the perturbation with the lowest critical speed has also the highest growth rate, and that low frequency perturbations are less critical than high frequency perturbations, at fixed critical speed. Therefore, when comparing perturbations with identical critical speed, those with higher wave numbers are responsible for more intense thermomechanical damages. Also, for perturbations with wave number smaller than the critical mcr, the temperature increases with m; vice versa for perturbations with wave number larger than mcr the temperature decreases with m. A reduction in thickness ratio a1/a2 between friction and metal disks has the effect of increasing the temperature and growth rate. An approximate formula for the temperature variation with time has been derived for a linearly decreasing engagement speed.

Fluid–structure interaction of deformable aortic prostheses with a bileaflet mechanical valve

Numerous mechanisms can give rise to instabilities and vibrations in sliding systems. These can generally be characterized as either elastodynamic (e.g. ‘brake squeal’) or thermoelastic. The time-scales of these processes differ considerably, so it is usual to neglect coupling between them, i.e. to neglect thermal effects in elastodynamic analyses and to use the quasi-static approximation in thermoelastic analyses. In the present paper, we consider the potential coupling between them in the simplest possible context—a thermoelastodynamic layer sliding against a rigid plane and constrained to one-dimensional displacements. The results show that although the coupling is extremely weak, it has a destabilizing effect on the natural elastodynamic vibration of the layer at arbitrarily low sliding speeds. A numerical solution of the transient equations below the quasi-static critical speed shows that an initial disturbance grows exponentially until periods of separation develop, after which the system approaches asymptotically to a steady state involving periods of contact and separation alternating at the lowest natural frequency of the elastodynamic system. With increasing sliding speed, the proportion of the cycle spent in contact is reduced and the maximum contact pressure increases. It is important to note that neither a quasi-static thermoelastic analysis, nor an elastodynamic analysis neglecting thermal expansion would predict instability in this speed range. Similar instabilities due to thermoelastodynamic coupling are almost certain to occur in more complex practical sliding systems such as brakes and clutches, implying the need for the incorporation of these effects in commercial analysis software. The proposed mechanism might also provide an explanation of reported experimental observations of vibrations normal to the contact interface during frictional sliding.

A re-examination of rolling contact fatigue experiments by Clayton and Su with suggestions for surface durability calculations

We propose a novel design of a biomimetic micro-structured surface, which exhibits controlled strongly direction-dependent adhesion properties. The micro-system consists of parallel elastic wall-like structures covered by a thin layer. Numerical calculations have been carried out to study the adhesive properties of the proposed system and to provide design criteria with the aim of obtaining optimized geometries. A numerically equivalent version of the double cantilever beam fracture experiment is, then, simulated by means of finite element analysis to investigate the anisotropic adhesion of the structure. We find that, because of inherent crack trapping properties of these types of structures, the wall-like geometry allows us to strongly enhance adhesion when the detachment direction is perpendicular to the walls. On the other hand, when the detachment occurs parallel to the walls, the system shows low adhesion. This controlled direction-dependent adhesive property of the proposed structure solves one of the key problems of biomimetic adhesive surfaces, which usually show very strong adhesion, even larger than biological systems, but are not suitable for object manipulation and locomotion, as detachment always occurs at high loads and cannot be controlled.