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Item type:Publication, Access status: without files , A linear cohesive model of zero degree peeling of a viscoelastic tape from a substratePeeling in viscoelastic materials has been studied experimentally for many years mostly at 90 or 180 degrees angle, and typically the classical Rivlin energy balance equation is used to obtain a velocity-dependent work of fracture. The latter has been shown to be the product of an angular term and a velocity-dependent term, but there is no simple model to explain this behaviour: attempts have been made to generalize the Kendall elastic equation to viscoelasticity, but they lead to no velocity dependence (and infinite load) with frictional dissipation at zero angles, and in general at large angles. In the present model, we consider the original Kendall’s “sticking conditions,” for which a linear cohesive model is formulated for the viscoelastic tape as being on an elastic foundation, and peeling velocity is found to be proportional to the cubic power of the force for Maxwell material, or standard material with a large ratio between instantaneous and relaxed moduli. An explicit closed-form solution to this problem is first derived in this work. Experimental results on zero peel angle are scarce, and may be affected by the finite length of adhered and unadhered parts: hence, a complete picture of peeling behaviour at zero angles is elusive.Publicationtype: Journal Article26 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A comparison of crack propagation theories in viscoelastic materialsCrack propagation in viscoelastic materials cannot be understood with the use of classical fracture mechanics, which predicts no dependence on the speed of propagation, unless cohesive models like Barenblatt or Dugdale are introduced, as done by Knauss & Schapery first in the 1970s. However, there is another approach, suggested qualitatively by de Gennes in 1996, and quantitatively by Persson and Brener in 2005, which attempts an energy (power) balance by considering viscoelastic dissipation in the bulk of the material. Here, we revisit the main results of the two theories and show that they lead to approximately the same scaling laws not just for the standard material, but also for power law materials (which have a continuous spectrum of relaxation times). Recent findings by Schapery have concluded that the shape of the cohesive law results essentially in a shift in velocity which depends both on cohesive law shape and viscoelastic properties. Therefore, the Persson-Brener cutoff radius in the integral of dissipation can be chosen to fit approximately the cohesive model results to match the shift of the reference velocity.Publicationtype: Journal Article39 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the effect of the loading apparatus stiffness on the equilibrium and stability of soft adhesive contacts under shear loads(2020-11); ; The interaction between contact area and frictional forces in adhesive soft contacts is receiving much attention in the scientific community due to its implications in many areas of engineering such as surface haptics and bioinspired adhesives. In this work, we consider a soft adhesive sphere that is pressed against a rigid substrate and is sheared by a tangential force where the loads are transferred to the sphere through a normal and a tangential spring, representing the loading apparatus stiffness. We derive a general linear elastic fracture mechanics solution, taking into account also the interaction between modes, by adopting a simple but effective mixed-mode model that has been recently validated against experimental results in similar problems. We discuss how the spring stiffness affects the stability of the equilibrium contact solution, i.e. the transition to separation or to sliding.Publicationtype: Journal Article56 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , The Interaction of Frictional Slip and Adhesion for a Stiff Sphere on a Compliant Substrate(2020-03-01); ; ; How friction affects adhesion is addressed. The problem is considered in the context of a very stiff sphere adhering to a compliant, isotropic, linear elastic substrate and experiencing adhesion and frictional slip relative to each other. The adhesion is considered to be driven by very large attractive tractions between the sphere and the substrate that can act only at very small distances between them. As a consequence, the adhesion behavior can be represented by the Johnson-Kendall-Roberts model, and this is assumed to prevail also when frictional slip is occurring. Frictional slip is considered to be resisted by a uniform, constant shear traction at the slipping interface, a model that is considered to be valid for small asperities and for compliant elastomers in contact with stiff material. A simple model for the interaction of friction and adhesion is utilized, in which some of the work done against frictional resistance is assumed to be stored reversibly. This behavior is considered to arise from surface microstructures associated with frictional slip such as interface dislocations, where these microstructures store some elastic strain energy in a reversible manner. When it is assumed that a fixed fraction of the work done against friction is stored reversibly, we obtain good agreement with data.Publicationtype: Journal Article24 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the application of fracture mechanics mixed-mode models of sliding with friction and adhesion(2020); As recently suggested in an interesting and stimulating paper by Menga, Carbone and Dini (MCD), applying fracture mechanics energy concepts for the case of a sliding adhesive contact, imposing also the shear stress is constant at the interface and equal to a material constant (as it seems in experiments), leads to a increase of contact area which instead is never observed. We add that the MCD theory also predicts a size effect and hence a distortion of the JKR curve during sliding which is also not observed in experiments. Finally, a simpler example with the pure mode I contact case, leads in the MCD theory to an unbounded contact area, rather than a perhaps more correct limit of the Maugis-Dugdale solution for the adhesive sphere when Tabor parameter is zero, that is DMT's solution. We discuss that the MCD theory does not satisfy equilibrium, and we propose some more correct formulations, although they may be rather academic: recent semi-empirical models, with an appropriate choice of the empirical parameters, seem more promising and robust in modelling actual experiments.Publicationtype: Journal Article77