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SUMMARY:CESS Seminar: Fundamentals of brittle failure at the atomic scale
DTSTART:20190322T121500
DTEND:20190322T130000
DTSTAMP:20260427T201433Z
UID:a9155a794b95bcbb072161ade7d0d84f2b5e3c671513589a2664cc34
CATEGORIES:Conferences - Seminars
DESCRIPTION:Dr. Laurent Brochard\, researcher in Civil Engineering at Lab
 oratoire Navier (ENPC\, CNRS\, IFSTTAR)\nAbstract\nBrittle failure is ubiq
 uitous in civil engineering materials from concrete to rocks and faults. A
 nd yet\, how brittle failure initiates is still debated. While the failure
  of pre-cracked bodies is predicted by an energy criterion (fracture mecha
 nics)\, that of flawless materials is usually given by a stress criterion\
 , and no clear scientific consensus exists about intermediate cases (e.g.\
 , notch). In this work\, we use atomistic simulation techniques to investi
 gate the elementary mechanisms behind brittle failure. A difficulty\, thou
 gh\, is that the process zone size of the studied material must be nanomet
 ric to comply with the computational limits of molecular simulations. Very
  few materials exhibit such a small process zone (e.g.\, the process zone 
 of rocks is typically 10-100 mm) and the candidate material we study is gr
 aphene. We also investigate a fictitious material (2D triangular lattice w
 ith harmonic interactions between closest neighbours).\nInvestigating the 
 failure behaviour of graphene in various configurations\, we characterize 
 the transition from the energy criterion of fracture mechanics to the stre
 ss criterion. Interestingly\, one particular situation exhibits an unexpec
 ted result: when the distance between two crack tips approaches the proces
 s zone size\, the average stress in-between the tips exceeds the strength.
  While most macroscopic theories of initiation would not expect this behav
 iour\, Leguillon’s criterion does [1]. Leguillon’s criterion is a fini
 te fracture mechanics approach requiring both energy and stress criteria t
 o be fulfilled over an initiation length. The peculiarity of our situation
  is that energy is the limiting factor since stress is highly concentrated
  between the tip while only little mechanical energy is available in the m
 aterial [2].\nTo further investigate the atomic processes of failure\, we 
 consider the athermal limit (0K). Since atomic interactions are conservati
 ves\, failure can be viewed as an instability arising when one of the eige
 nvalues of the hessian matrix becomes negative. And the associated eigenve
 ctors provides a description of the elementary mechanism of failure. Inter
 estingly\, failure of flawless materials exhibits infinite failure bands t
 he width of which recalls the process zone size\, i.e.\, a property that o
 ne usually gets from a cracked material. The corresponding eigenvalue are 
 highly degenerated\, whereas for flawed materials\, the eigenvalues have l
 ittle or no degeneracy and failure involves localized atom moves only.\nAt
  non-zero temperature\, failure is no more deterministic because of therma
 l agitation. We conduct an extensive study over many time and length scale
 s and identify a temperature-time-size equivalence that can be formalized 
 by a universal scaling law of strength and toughness which extends Zhurkov
 ’s theory [3] to size effects [4]. Interestingly\, the scaling of streng
 th and toughness differ only regarding the scaling in size\, and therefore
  was not previously identified in Zhurkov’s work. One can formally relat
 e this difference to the degeneracy of the negative eigenvalues in the ath
 ermal limit. Such scaling law is also of very practical interest to relate
  failure properties at different length and time scales.\n \n[1] Leguillo
 n\, D. (2002). Strength or toughness? A criterion for crack onset at a not
 ch. European Journal of Mechanics\, A/Solids\, 21(1)\, 61–72.\n[2] Broch
 ard\, L.\, Tejada\, I. G.\, & Sab\, K. (2016). From yield to fracture\, fa
 ilure initiation captured by molecular simulation. Journal of the Mechanic
 s and Physics of Solids\, 95\, 632–646.\n[3] Zhurkov\, S. N. (1984). Kin
 etic concept of the strength of solids. International Journal of Fracture\
 , 26(4)\, 295–307.\n[4] Brochard\, L.\, Souguir\, S.\, & Sab\, K. (2018)
 . Scaling of brittle failure: strength versus toughness. International Jou
 rnal of Fracture\, 210(1-2)\, 153–166.\n\nBio\nLaurent Brochard is a res
 earcher in Civil Engineering at Laboratoire Navier (ENPC\, CNRS\, IFSTTAR)
  since 2012. He received his M.S. and Ph.D. from Ecole des Ponts ParisTech
  in 2008 and 2011. He is also engineer from Ecole Polytechnique (France) a
 nd from École des Ponts ParisTech (France). His research focuses on multi
  scale approaches for the study of the physics and mechanics of materials 
 with emphasis on phenomena that have their origin at the molecular scale: 
 adsorption and poromechanics\, fracture mechanics and failure initiation\,
  thermo-mechanical couplings. Targeted applications are mostly in geomecha
 nics (CO2 sequestration\, nuclear waste storage\, unconventional oil and g
 as\, cementitious materials).\n 
LOCATION:GC B3 30 https://plan.epfl.ch/?room=GCB330
STATUS:CONFIRMED
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