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Mike Ciavarella's blog

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friction and persson's theory for fractal roughness

dear colleagues   I recommend reading this recent paper Chadha, V., Randolph, A. B., Reifler, K., Jacobs, T. D., & Beschorner, K. E. (2025).

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suggestions to please editors....

Dear Friends  I send you a nice short paper with indications on how to "please" editors.  Nothing new for senior people, but maybe useful to give to your young phd students.regards, 

 

MC

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comment on a recent Nature paper on nucleation of sliding by Fineberg's group

Dear colleagues  As fellows in fracture mechanics, you may be interested in these comments on a recent very nice Nature paper by the group of Jay Fineberg, making clever experiments to measure nucleation of small cracks in frictional interfaces of PMMA, and their propagation velocity during nucleation and later rapid acceleration.   Fineberg derives a threshold shear stress for nucleation which depends on the specimen width which seems new in fracture mechanics to me, but does not entirely convince me, see in the text.   If you have any comments please let me know, I would be glad to discus

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Leonardo's universal friction coefficient is found to be universal after all!

It turns out that friction coefficient 0.25 suggested as universal by Leonardo da Vinci more than 500 years ago has some universaility, as minimum friction coefficient for any granular material: it makes me proud as italian ;) I guess it would be interesting to show this experimental result also theoretically or numerically, any interest? [HTML] 

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A new analytical model for fibrillar viscoelastic adhesion using the Schapery or the Shrimali-Lopez-Pamies nucleation models

Hello:  I would be interested in any comment about this preprint on fibrillar viscoelastic adhesion, originally devised by Schargott Popov and Gorb, where we use for the first time not only the Schapery model for nucleation of cracks, but also the Shrimali and Lopez Pamies, which leads to quite stronger enhancement of adhesion (the limit is the square of the Schapery one), and pull-off with no real prior propagation phase.  Propagation with Schapery nucleation criterion is found to be qualitatively similar to the Schapery and Persson-Brener propagation theories, except that the reference ve

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A new interesting theory by Persson on wear commented

A very interesting wear theory by Persson and coworkers has been recently proposed (in fact, it is still under review!). 

I make some comments and simplifications here to show general trends

https://www.researchgate.net/publication/387143610_Some_comments_on_a_re...

Wear is a complex problem, and I don't think this theory will give easily quantatitative predictions, but it certainly deserves attention. 

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Finite strains explain the non-monotonic change of contact area in soft rubber contacts loaded in torsion?

In unidirectional sliding of rubber contacts on smooth hard surface it has been found that contact shrinks largely in longitudinal directions, and generally much less in the transverse direction, and two explanations have been suggested to explain this: one is the effect of mixed mode fracture mechanics in the presence of adhesion (with mode II reducing adhesion and mode III less clear), and another uniquely based on finite strain effects even for a simple material model as neo-Hookean hyperelastic material.

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On friction effects and the conditions of failure of adhesion in punch shaped pillars and mushrooms

Considerable 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 :10235-43, 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.

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classical rate-independent models for viscoelastic fracture do not work!

It is 50 years that people use models of viscoelastic fracture which assume the fracture energy in the process zone is independent on the crack speed. This has been assumed in the cohesive models by Knauss and Schapery and then by the dissipation theory of de gennes and Persson-Brener.  Researchers like to use this model because it is simple.   Unfortunately, it does not work, as we show in adhesion experiments in the recent JMPS paper attached.   We obtain nice results for a broad band power law material, but these results only work at very low speeds.

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The Fiorillo index -- a new index to attempt to spot citation gaming

An index has been proposed by Fiorillo (document attached).   

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On productivity, big multicenter grants, and paper-mills: the pursuit of an academic career leads to scientific progress?.

 

We are in the time where big multi-centre projects with an ever-increasing number of scientists involved seem to multiply, and single scientist projects seem to die, apart from the happy island of ERC funding in Europe (probably NSF career funding in US?). I believe that with ERC funding and the direct call of ERC winners some European universities have taken a path of excellence to follow, while on multicentre big money projects they are less effective.

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can we stop cracks due to elastic modulus changes ahead of crack tips?

Dear Colleague

   It is known that ahead of a crack subject to static or fatigue loading microcracking and damage makes the material soften (of smaller elastic modulus) but also its strength degrades (in composite materials, there are so called “wearout models” which associate strength reduction exactly to the reduction of modulus).

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hyper and probably over-prolific scientific authors (Nature article)

dear friends

Nature reported in Dec.2023 that up to four times more researchers pump out more than 60 papers a year than less than a decade ago. Saudi Arabia and Thailand saw the sharpest uptick in the number of such scientists over the past few years, according to a preprint posted on bioRxiv on 24 November.

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Cancelling the effect of sharp notches or cracks with graded elastic modulus materials

Dear colleague

   I wonder if you could be interested in this idea of "Cancelling the effect of sharp notches or cracks with graded elastic modulus materials", particularly if you know who could help doing some experiments. Thanks for any comment, since this is a preprint it would be useful to discuss to make improvements.    RegardsMike    Cancelling the effect of sharp notches or cracks with graded elastic modulus materials

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Friction for a sliding adhesive viscoelastic cylinder: Effect of Maugis parameter

Prediction of friction is one of the nigthmares of tribologists!   For elastomers, friction may be due to shear stresses or to dissipation and adhesion hysteresis.  Here we consider the two effects in rolling/ sliding a viscoelastic cylinder. We find that at low speeds the numerical bem results confirm Persson/Brener theory for crack propagation at large Tabor parameter.

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apparent paradox: for a sliding flat punch on a viscoelastic halfspace, friction is zero or not?

dear Imechanica friends here is an apparent paradox: for a sliding flat punch on a viscoelastic halfspace, friction is zero or not?  Certainly there seems to be viscoelastic dissipation, but the pressure is also normal to dispacements, so am I missing something obvious here?

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2023 career elsevier top scientists rankings --- congratulation to the Harvard professors

I have extracted in attachment the top scientists according to the Elsevier October 2023 data-update for "Updated science-wide author databases of standardized citation indicators", see https://elsevier.digitalcommonsdata.com/datasets/btchxktzyw/6

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On the Shrimali and Lopez-Pamies theory on viscoelastic fracture mechanics

dear collegues

 

   I have been attracted recently by a theory from Shrimali and Lopez-Pamies which is based on experimental evidence obtained from Suo's group in Harvard in 2012 in a (limited, but significant) set of experiments on rubber.   The theory assumes that the stretch to nucleating a crack is constant and independent on stretch rate.  The theory is then built on that, to predict the increase of fracture energy with rate.

 

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Prof. Huajian Gao elected FRS

Eighty outstanding researchers, innovators and communicators from around the world have been elected as the newest Fellows of the Royal Society, the UK’s national academy of sciences and the oldest science academy in continuous existence.

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A few PhD positions are available at PoliBA (Italy) in adhesion regulated by microvibrations

Call for PhD student grants and applications

 

Location: Polytechnic University of Bari, Department of Mechanics Mathematics and Management (DMMM), Via Orabona 4 - 70125 Bari – Italy

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Fretting fatigue --- is there really an important effect of friction coefficient??

dear Colleagues

In writing a review of models on fretting fatigue, I am looking at the effect of friction coefficient.   Obviously, if friction is zero there is no fretting fatigue!   This goes without saying.

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