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MEchanics GAthering -MEGA- Seminar: Talk1 - The density of interacting quasi-localised modes in amorphous solids; Talk2 - How inertia can facilitate friction; Talk3 - Sudden failure in amorphous materials during quasistatic loading

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Date and time 22.11.2018 16:1517:30  
Place and room
Speaker  Wencheng JiTom de GeusMarko PopovicPCSL, EPFL
Category Conferences - Seminars
The density of interacting quasi-localised modes in amorphous solids by Wencheng JiPCSL, EPFL
Abstract Amorphous solids are very common materials in our daily life, such as glass, toothpaste, mayonnaise, coffee foam, and soya beans. Unlike crystals, amorphous solids do not present topological defects due to their lack of long-range order. Instead they display excitations where a group of particles can rearrange. These essentially local excitations lead to a dipolar change of stress in the medium, which can effectively couple them. One physical quantity related to the local low-energy excitations is a quasi-localized mode whose density follows D(ω)~ω4 [2-3] in glass different from Debye theory, where ω is the vibrational frequency of the quasi-localized modes.
Here, we provide a theory for the density of quasi-localized modes for classical systems at zero temperature, which takes their interactions into account and clarifies their relationship with shear transformations [4-5]. We confirm this relationship by using the molecular dynamics simulations of quasi-statically sheared glasses. 
[1] ArXiv prepring arXiv:1806.01561.
[2] V. Gurevuch, D. Parshin, and H. Schober, Physical Review B 67, 094203 (2003).
[3] E. Lerner, G. During, and E. Bouchbinder, Physical Review Letters 117, 035501 (2016).
[4] A. Argon, Acta Mettalurgica 27, 47 (1979).
[5] J. Lin and M. Wyart, Physical Review X 6, 011005 (2016).

How inertia can facilitate friction by Tom de GeusPCSL, EPFL
Abstract We study the nucleation of slip between two sliding  solids, whereby we focus on a mesoscopic level where the disorder, introduced by the surface roughness, matters. It is at this scale that we can study how different contacts interact through the bulk’s elasticity. A result of this interaction is that the detachment of one asperity can trigger that of other contacts in its vicinity. An interesting question is if such collective effects organise into depinning-like avalanches. Vice versa this system allows the clarification of the debated roled of inertia on an avalanche-like response [1-3]. We argue that, due to the presence of rare weak sites, the response is smooth in the thermodynamic limit. At the same time we find this mechanism not to be efficient, leading to a stick-sliip response in finite systems.
[1] D.S. Fisher, K. Dahmen, S. Ramanathan, Y. Ben-Zion, PRL 78(25), 4885-4888 (1997).
[2] J.M. Schwarz, D.S. Fisher, PRE, 67(2), 021603 (2003).
[3] K. Karimi, E.E. Ferrero, J.-L. Barrat, PRE, 95(1), 013003 (2017).

Sudden failure in amorphous materials during quasistatic loading by Marko PopovicPCSL, EPFL
Abstract The response of amorphous materials to an applied strain can be continuous, or instead display a macroscopic stress drop when a shear band nucleates. Such discontinuous response can be observed if the initial configuration is very stable. We study theoretically how such brittleness emerges in athermal, quasi-statically driven, materials as their initial stability is increased. We show that this emergence is well reproduced by elasto-plastic models and is predicted by a mean field approximation, where it corresponds to a continuous transition. In mean field, failure can be forecasted from the avalanche statistics. We show that this is not the case for very brittle materials in finite dimensions due to rare weak regions where a shear band nucleates. We build an analogy with fracture mechanics predicting that their critical nucleation radius follows ac~(Σ- Σb)-2  where Σ is the stress a shear band can carry.
 

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  • General public
  • Free

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  • MEGA.Seminar Organizing Committee

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Solids Structures Fluids

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