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Antonio Papangelo's blog

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Viscoelastic normal indentation of nominally flat randomly rough contacts

Viscoelastic materials are receiving increasing attention in soft robots and pressure sensitive adhesives design, but also in passive damping techniques in automotive and aerospace industry. Here, by using the correspondence principle originally developed by Lee and Radok and further extended by Ting and Greenwood, we transform the elastic solutions of Persson for contact of nominally flat but randomly rough surfaces to viscoelastic indentation. As an example, the cases of step loading and of the response to a single cycle of harmonic loading are studied.

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On the Interaction of Viscoelasticity and Waviness in Enhancing the Pull-Off Force in Sphere/Flat Contacts

Motivated by roughness-induced adhesion enhancement (toughening and strengthening) in low modulus materials, we study the detachment of a sphere from a substrate in the presence of both viscoelastic dissipation at the contact edge, and roughness in the form of a single axisymmetric waviness. We show that the roughness-induced enhancement found by Guduru and coworkers for the elastic case (i.e. at very small detachment speeds) tends to disappear with increasing speeds, where the viscoelastic effect dominates and the problem approaches that of a smooth sphere.

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Adhesion enhancement in a dimpled surface with axisymmetric waviness and rate-dependent work of adhesion

Surfaces showing macroscopic adhesion are rare in industry, but are abundant in Nature. Adhesion enhancement has been discussed mostly with geometrical systems (e.g. patterned surfaces), more rarely with viscoelasticity, and has the goal of increasing hysteresis and the detachment force at separation. Soft materials are common, and these have viscoelastic properties that result in rate-dependent increase of toughness.

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How to better grasp your spoon?

How can shear loading rate affect the soft adhesive contact area? A new blog post in discusses the problem following the paper "Papangelo, Antonio. (2021). On the Effect of Shear Loading Rate on Contact Area Shrinking in Adhesive Soft Contacts. Tribology Letters. 69." just published in Tribology Letters.

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Critical thresholds for mode-coupling instability in viscoelastic sliding contacts

Mode-coupling instabilities are known to trigger self-excited vibrations in sliding contacts. Here, the conditions for mode-coupling (or "flutter") instability in the contact between a spherical oscillator and a moving viscoelastic substrate are studied. The work extends the classical 2-Degrees-Of-Freedom conveyor belt model and accounts for viscoelastic dissipation in the substrate, adhesive friction at the interface and non-linear normal contact stiffness as derived from numerical simulations based on a boundary element method capable of accounting for linear viscoelastic effects.

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On the Effect of Shear Loading Rate on Contact Area Shrinking in Adhesive Soft Contacts

Adhesion and, its interplay with friction, is central in several engineering applications involving soft contacts. Recently, there has been an incredible push towards a better understanding on how the apparent contact area evolves when a shear load is applied to an adhesive soft contact, both experimentally and theoretically. Although soft materials are well-known to exhibit rate-dependent properties, there is still a lack of understanding in how the loading rate could affect the contact area shrinking.

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On the Effect of a Rate-Dependent Work of Adhesion in the Detachment of a Dimpled Surface

AbstractPatterned surfaces have proven to be a valuable design to enhance adhesion, increasing hysteresis and the detachment stress at pull-off. To obtain high adhesive performance, soft materials are commonly, used, which easily conform to the countersurface, such as soft polymers and elastomers. Such materials are viscoelastic; i.e., they show rate-dependent properties. Here, the detachment of two half spaces is studied, one being flat and the other having a dimple in the limit of short range adhesion and a power law rate-dependent work of adhesion, as observed by several authors.

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Stickiness of randomly rough surfaces with high fractal dimension: is there a fractal limit?

Two surfaces are ”sticky” if breaking their mutual contact requires a finite tensile force. At low fractal dimensions D, there is consensus stickiness does not depend on the upper truncation frequency of roughness spectrum (or ”magnification”). As debate is still open for the case at high D, we exploit BAM theory of Ciavarella and Persson-Tosatti theory, to derive criteria for all fractal dimensions. For high D, we show that stickiness is more influenced by short wavelength roughness with respect to the low D case.

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Experimental observations of nonlinear vibration localization in a cyclic chain of weakly coupled nonlinear oscillators

Experimental results on nonlinear vibration localization in a cyclic chain of weakly coupled oscillators with clearance nonlinearity are reported. Numerical modelling and analysis complements the experimental study. A reduced order model is derived and numerical analysis based on the harmonic balance method demonstrates the existence of multiple classes of stable spatially localized nonlinear vibration states. The experiments agree very well with the numerical results.

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Vibration localization due to flutter instability in a bladed rotor

The current push toward lightweight structures in aerospace and aeronautical engineering is leading to slender design airfoils, which are more likely to undergo large deformation, hence experiencing geometrical nonlinearities. The problem of vibration localization in a rotor constituted by N coupled airfoils with plunge and pitch degrees of freedom subjected to flutter instability is considered.

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Does roughness enhance or destroy adhesion????

Recently, Dalvi and co-authors have shown detailed experimental data of adhesion of soft spheres with rough substrates with roughness measured down to almost the atomic scale, finding that the Persson and Tosatti theory gave satisfactory predictions of the apparent work of adhesion during loading, once the increase of the surface area due to roughness is correctly computed at extremely small scales.

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Numerical and experimental analysis of the bi-stable state for frictional continuous system

Friction induced vibrations are a very well known problem in engineering applications. Sometimes they are referred to be unpredictable, intermittent, capricious (!!!) and particularly experimentalists know this very well. Here, by means of numerical simulations and experimental measurements, we highlight how capriciousness may arise from the multistable behavior of frictional systems, i.e. their characteristic to show multiple stable solutions for the same set of governing parameters. Hence initial conditions and perturbations become crucial.

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How contact area decreases under shear load in soft bodies?

A very good paper has just been published in JMPS by  J.C. Mergel, J. Scheibert, R.A. Sauer "Contact with coupled adhesion and friction: Computational framework, applications, and new insights". it shows the results of numerical simulations with soft bodies sheared by a tangential force. There is a vivacious discussion in Literature if the contact area reduces due to adhesion or due to large deformation.

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Roughness-Induced Adhesion

Usually, roughness destroys adhesion and this is one of the reasons why the "adhesion paradox", i.e. a "sticky Universe", is not real. However, at least with some special type of roughness, there is even the case of adhesion enhancement, as it was shown clearly by Guduru, who considered the contact between a sphere and a wavy axisymmetric single scale roughness, in the limit of short-range adhesion (JKR limit).

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Electroadhesive sphere-flat contact problem

The electroadhesive contact between a conductive sphere with a rigid substrate, both coated with an electrically insulating layer is studied, by adopting two solution strategies: (i) a DMT approximation and (ii) an iterative finite element model which accounts for the effect of the electroadhesive tractions on the deformation of the elastic solids. The contact problem is solved by varying the applied voltage and the elastic modulus of the coating layer.

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On the Degree of Irreversibility of Friction in Sheared Soft Adhesive Contacts

A number of authors have experimentally assessed the influence of friction on adhesive contacts, and generally the contact area has been found to decrease due to tangential shear stresses at the interface. The decrease is however generally much smaller than that predicted already by the Savkoor and Briggs 1977 classical theory using “brittle” fracture mechanics mixed mode model extending the JKR (Griffith like) solution to the contact problem.

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Does the loading apparatus stiffness affect the equilibrium of soft adhesive contacts under shear?

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 bioin-spired 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.

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Interfacial Dissipative Phenomena in Tribomechanical Systems

Dear Colleagues,

In the last twenty years, tribology and nonlinear dynamics have included several major contributions related to key topics such as rough contact, friction, damping mechanisms, and dynamical behaviour of nonlinear systems, which are paving the way for future engineering challenges. The two fields are largely intertwined as, among the others, contact nonlinearities are almost omnipresent in any technical application ranging from the development of NEMS/MEMS to bioengineering, automotive, civil/mechanical industry, and aerospace.

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Self-excited vibrations due to viscoelastic interactions

Self-excited vibrations represent a big concern in engineering, particularly in automotive, railway and aeronautic industry. Many lumped models have been proposed over the years to analyze the stability of such systems. Among the instability mechanisms a falling characteristic of the friction law and mode coupling have been shown to give friction-excited oscillations. The mass-on-moving-belt system has been studied extensively in Literature, very often adopting a prescribed form of the friction law and linearizing the contact stiffness.

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Elliptical adhesive contact under biaxial stretching

Adhesive contact of the Hertzian indenter with an incompressible elastic substrate bi-directionally stretched along the indenter principal planes of curvature is considered in the Johnson–Kendall–Roberts theoretical framework. An approximate model is constructed by examining energy release rate conditions only on the edges of the minor and major axes of the contact ellipse. The effect of weak coupling between fracture modes I and II is introduced using a phenomenological mode-mixity function.

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Effect of Wear on the Evolution of Contact Pressure at a Bimaterial Sliding Interface

The eigenfunction method pioneered by Galin (J Appl Math Mech 40: 981–986, 1976) is extended to provide a general solution to the transient evolution of contact pressure and wear of two sliding elastic half-planes, under the assumption that there is full contact and that the Archard–Reye wear law applies. The governing equations are first developed for sinusoidal profiles with exponential growth rates.

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Can wear completely suppress thermoelastic instabilities?

ThermoElastic Instabilities (TEI) occur in sliding bodies at sufficiently high speed because a small thermoelastic disturbance tends to localize the contact, leading to “hot spots”. The role that wear plays in TEI has been studied briefly and only on highly idealized cases. We extend and complete in detail a model of Dow and Burton who studied the specific configuration of a blade sliding on a rigid halfspace normal to its line of contact. We find there is a limit value of wear coefficient, that can be estimated by simple equations, above which TEI is completely eliminated.

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Friction-induced energy losses in mechanical contacts subject to random vibrations

In this paper, we apply the previously developed Method of Memory Diagrams (MMD) to the description of an axisymmetric mechanical contact with friction subject to random vibrations. The MMD belongs to a family of semi-analytical methods of contact mechanics originating from the classical Cattaneo-Mindlin solution; it allows one to efficiently compute mechanical and energetic responses to complex excitation signals such as random or acoustic ones.

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On stickiness of multiscale randomly rough surfaces

A new stickiness criterion for solids having random fractal roughness is derived using Persson's theory with DMT-type adhesion. As expected, we find stickiness, i.e., the possibility to sustain macroscopic tensile pressures or else non-zero contact area without load, is not affected by the truncation of the PSD spectrum of roughness at short wavelengths and can persist up to roughness amplitudes orders of magnitude larger than the range of attractive forces.


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