81 results
Now showing1 - 10 of 81
- Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , An extended Griffith friction model for the transition to slip in the contact of graded-material spheresIn the present paper, we study the role of gradient in the material properties of contacting bodies in the difference between static and kinetic friction for a Hertzian geometry, according to the theory of "Griffith friction", for which the transition from stick to slip occurs as an elastic instability. We use the term "Griffith friction" to suggest an energy balance approach in mode II to derive stable and unstable equilibrium configurations, where in particular macroscopic sliding can occur by a global elastic instability, analogous to a Griffith crack which doesn’t arrest after reaching a critical size. The most important conclusion are that static friction coefficient: (i) is increased with harder surface; (ii) is increased for small normal loads and tends to infinity in the limit of zero load. These conclusions hold both in the case of a constant frictional fracture energy or a pressure-dependent frictional fracture energy at the interface.Publicationtype: Journal Article21 7 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Static friction coefficient depends on geometryClassical theories state that the friction coefficient is independent of geometry, depending only on the real contact area determined by roughness. However, experimental evidence shows significant differences between static and dynamic friction coefficients. Recent models introduce an energetic theory for friction, analogous to the Griffith theory for fracture, resulting in a constant dynamic friction coefficient at heavy loads but a larger static coefficient at low loads. We show that, for power-law punches, the ratio of static to dynamic friction coefficients at low normal loads is higher for flatter profiles and, in principle, grows unbounded at zero load. This may explain the observed variability in friction ratios. The model’s predictions align well with recent experimental results.Publicationtype: Journal Article16 8 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A JKR/Griffith model transition to slip in frictional contact between layered surfaces with roughness(American Institute of Physics (AIP), 2026-01-22); ; ; In a recent study, Liang et al. developed an analytical framework, termed the “Johnson–Kendall–Roberts (JKR)–Griffith model,” to describe how an energetic model of friction between nominally flat rough surfaces leads to the onset of slip governed by elastic instability. In the present study, this approach is extended to the case of a layered solid. By combining Persson’s contact mechanics theory, a JKR-type approximation, and the Cattaneo–Mindlin superposition principle, the model captures the transition from sticking to sliding under tangential loading. The analysis shows that the static friction can exceed the kinetic friction, with this enhancement depending on the ratio of elastic moduli, surface roughness, and normal load. The model further predicts that the maximum discrepancy between static and kinetic friction occurs at an intermediate layer thickness. This framework provides useful guidance for the design of layered surfaces to mitigate stick–slip phenomena, which are often responsible for undesirable machine vibrations and wear.Publicationtype: Journal Article14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adhesive contact of elastic spherical shells: Non-monotonic thickness dependence of pull-off forceThe adhesive contact of spherical shells is a critical problem across multiple length scales. In this study, we investigate the adhesive contact between an elastic spherical shell and a rigid plane. Two independent approaches are employed: the JKR theory from an energy perspective, and a simulation-based method incorporating a traction-separation law. Our results reveal a non-monotonic relationship between pull-off force and shell thickness. Specifically, for relatively thick shells, the pull-off force decreases with decreasing thickness; however, below a critical thickness, this trend reverses, and the pull-off force begins to increase. Furthermore, we explore the underlying mechanisms responsible for this behavior and examine the influence of adhesion parameters on the overall response.Publicationtype: Journal Article17 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A graded elastic modulus concept to eliminate stress or strain energy density singularity at sharp notches and cracks, with consequent elimination of size-scale effect on strengthIt has been recently suggested by the author that in the classical problem of a sharp wedge or crack loaded in plane (mode I and/or mode II), the stress singularity can be removed by grading the elastic properties of the underlying material from the notch tip by using a power law, E∼rβ. While the treatment is extended to the case of mode III (antiplane shear) which permits closed form results, we also discuss two ways to deal with the likely effect of material's grading on strength. In one, already explored in the previous paper, the strength is a power law of the modulus, and we suggest an “optimal” design by keeping the dominant stress constantly equal to the strength. In a second method, we propose to cancel the singularity in the strain energy density, which requires a much stronger grading, and we also possibly take into account that the critical strain energy density is a power law of the modulus. Noticing that only in the presence of a singularity a length scale can be defined experimentally by testing a very large notch and a very small one, according to the Theory of Critical Distances (TCD), the effect of cancelling singularity also implies independence on size/scale and constant strength. It is concluded that the technique is much more powerful than drilling a hole or rounding the tip of the notch/crack. Moreover, if a “smart” material could be designed to damage itself as to reduce its modulus when near a high stress concentration according to our prescriptions, it would naturally self-heal, opening up interesting applications.Publicationtype: Journal Article33 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the scaling of strength of flat and mushroom-shaped ends of microstructured adhesivesConsiderable research has been conducted on shape of pillar ends for optimal adhesion. In experiments with elastomers, it has been found that mushrooms-ended ones are superior to flat-ended ones, but early experiments have suggested an extremely strong scaling in strength with pillar radius (del Campo et al. Langmuir 23:10235–10243, 2007). We discuss various theories and experimental results on scaling of strength, and in particular, we elaborate recent experiments on single pillars with mushroom ends finding that the scaling on strength is much less surprising. When the ratio of tip to shaft radii increases toward an optimal value, the mode of failure changes from edge to center defects, and the scaling of strength changes from that associated with the presence of the classical linear elastic fracture mechanics stress singularity at the corner of the flat-ended punches caused by large friction at the interface, with that caused by the population of defects immersed on a nearly uniform stress field in the mushroom center. Strong scaling of strength therefore is also accompanied with large scatter of strength.Publicationtype: Journal Article13 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Wear due to fatigue initiationPersson and coauthors have recently proposed an extension of the Rabinowicz idea for fatigue wear at different scales of roughness, where Paris’ crack growth law is applied to ”potential” wear particles. However, Persson's theory suffers from the fact that initial size of defects is unknown and fatigue life is not entirely due to propagation, so we investigate a different formulation, where a law for initiation of cracks is used for a specimen with initial roughness of engineering interest. We find that results (in particular dependence on amplitude of roughness, and on friction coefficient) are qualitatively similar to the original Persson and coworkers’ theory, but may differ substantially quantitatively. As the assumption of a constant fatigue threshold may be incorrect for short cracks, both fatigue limit and fatigue threshold are made dependent of crack size, using the Murakami formulation as one of the possible alternatives. This makes wear rate be sensitive to the fine scale details of the roughness spectrum, which has an effect on increasing wear rate and small particles emission. The model seems to have qualitative trends in agreement with experiments, but obviously wear is a very complex phenomenon and many factors may be not captured.Publicationtype: Journal Article13 24 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , An asymptotic model of vibroadhesionA compliantly fixed hemispherical indenter in adhesive contact with an elastic sample firmly bonded to a rigid base is considered under the assumption that the rigid base undergoes small-amplitude high-frequency normal (vertical) oscillations. A general law of the rate-dependent JKR-type adhesion is assumed, which relates the work of adhesion to the contact front velocity. Using the Bogoliubov averaging approach in combination with the method of harmonic balance, the leading-order asymptotic model is constructed for steady-state vibrations. The effective work of adhesion is evaluated in implicit form. A quasi-static approximation for the pull-off force is derived. The case of rigid fixation of the indenter is considered in detail.Publicationtype: Journal Article10 8 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Optimal grading of elastic modulus at contact corners with friction with application in fibrillar adhesivesRecent studies have reported that micropillars composed of graded materials exhibit high adhesion performance in fibrillar adhesive systems. To uncover the origin of this enhanced performance and further promote the application of graded materials, we investigate the general contact mechanics of wedges composed of power-law graded materials. Our findings show that the stress singularity at the apex of the wedge can be eliminated by increasing the exponent of the power-law modulus. For cases with perfectly bonded interfaces, frictionless interfaces, and interfaces with Coulomb friction, the critical conditions for eliminating stress singularities are presented, thereby suggesting how to obtain optimal designs.Publicationtype: Journal Article19 61 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Dynamic fracture in DCB specimen loaded by end momentsDynamic fracture depends on a delicate interplay of the dynamic Energy Release Rate (ERR) Gd which is the static value Gs as reduced by inertia terms depending on crack speed and sometimes acceleration, and the dynamic toughness Γd which generally varies with crack speed. We review some basic results on Double Cantilever Beam specimen, and make a simple analysis of a DCB under remote bending loads applied slowly. We comment that smooth propagation ensues here contrary to the case discussed in the literature where the loading the end by a constant displacement rate, a very complex non-linearly oscillating velocity results, possibly leading to crack arrest (stick–slip) and which is only partly captured by present models, making difficult the determination of the dynamic toughness.Publicationtype: Journal Article14 32