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SUMMARY:Plasticity Simulation based on the Dynamics of Full Dislocation-de
 nsity Functions
DTSTART:20151214T131500
DTEND:20151214T141500
DTSTAMP:20260509T054519Z
UID:27d21bbd18eb5a7406ab947331e71595d6116c1f8edeb9199e22223f
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Alfonso H.W. Ngan\, The University of Hong Kong\, P.R. C
 hina\nCurrent strategies of computational crystal plasticity that focus on
  individual atoms or dislocations are impractical for real-scale\, large-s
 train problems even with today’s computing power. Dislocation-density ba
 sed approaches are a way forward but most schemes published to-date give a
  heavier weight on the consideration of geometrically necessary dislocatio
 ns (GNDs)\, while statistically stored dislocations (SSDs) are either igno
 red or treated in ad hoc manners. In reality\, however\, the motions of GN
 Ds and SSDs are intricately linked through their mutual (e.g. Taylor) inte
 ractions\, and in fact\, GNDs and SSDs are indistinguishable on a microstr
 uctural level\, notwithstanding the fact that the GNDs are simply the port
 ion of dislocations associated with the overall shape change of the crysta
 l. A correct scheme for dislocation dynamics should therefore be the one c
 ommonly used in discrete dislocation dynamics (DDD) simulations\, namely\,
  an “all-dislocation” treatment that is equally applicable for all dis
 locations comprising both the GNDs and SSDs\, with a rigorous description 
 of the interactions between them.\nIn this seminar\, a new scheme for comp
 utational dynamics of dislocation-density functions\, based on the above 
 “all-dislocation” principle\, is discussed. The dynamic evolution laws
  for the dislocation densities are derived by coarse-graining the individu
 al density vector fields of all the discrete dislocation lines in the syst
 em\, without distinguishing between GNDs and SSDs. The mutual elastic inte
 ractions between dislocations are treated in full by generalizing the elas
 tic interactions between dislocation segments for dislocation densities\, 
 and reducing the Hirth-Lothe line-integral formulation into an algebraic f
 orm comprising only elementary functions which are straightforward enough 
 for efficient numerical implementation. Other features in the model includ
 e forest (Taylor) hardening\, stress due to dislocation curvature\, genera
 tion due to the connectivity nature of dislocations\, and dipole annihilat
 ion. Numerical implementation is by means of the finite volume method (FVM
 )\, which is well suited for high gradients often encountered in dislocati
 on plasticity.\nAs a first case study\, the model is utilized to predict v
 ibration-induced softening and dislocation pattern formation experimentall
 y observed in crystalline metals. The simulations reveal the main mechanis
 m for subcell formation under oscillatory loadings to be the enhanced elim
 ination of SSDs by the oscillatory stress\, leaving behind GNDs with low S
 chmid factors which then form the subgrain walls. The depletion of the SSD
 s also accounts for the softening\, and this occurs because the oscillator
 y loading brings reversals into the motions of SSDs which then increase th
 eir chance of meeting up and annihilation. This is the first simulation ef
 fort to successfully predict the cell formation phenomenon under vibratory
  loadings\, and this example highlights the importance of a rigorous “al
 l-dislocation” treatment since both the SSDs and GNDs have significant r
 oles to play.\nA second case study concerns size effects in crystal plasti
 city. The new model is found capable of capturing a number of key experime
 ntal features including the Hall-Petch relation in polycrystalline states\
 , and power-law relation between strength and size in micro-crystals. In t
 he former\, dislocation pile-ups at grain boundaries\, and in the latter c
 ase\, low dislocation storage and jerky deformation\, are predicted.\nKeyw
 ords: Crystal plasticity\; dislocations\; dislocation-density functions\; 
 stress-strain behavior.\nReferences:\n[1] H.S. Leung\, P.S.S. Leung\, B. C
 heng and A.H.W. Ngan\, (2015)\, “A New Dislocation-density-function Dyna
 mics Scheme for Computational Crystal Plasticity by Explicit Consideration
  of Dislocation Elastic Interactions”\, Int. J. Plasticity\, 67\, 1-25.\
 n[2] P.S.S. Leung\, H.S. Leung\, B. Cheng and A.H.W. Ngan\, (2015)\, “Si
 ze dependence of yield strength simulated by a dislocation-density functio
 n dynamics approach”\, Modelling and Simulation in Materials Science and
  Engineering\, 23\, 035001-1-27.\n[3] B. Cheng\, H.S. Leung and A.H.W. Nga
 n\, (2015)\, “Strength of metals under vibrations – dislocation-densit
 y-function dynamics simulations”\, Phil. Mag. 95 (Special Issue on Nanom
 echanical Testing)\, 1845-1865.\nBio: Professor Alfonso H.W. Ngan is curre
 ntly Kingboard Professor in Materials Engineering\, Chair Professor of Mat
 erials Science and Engineering\, as well as Associate Dean of Engineering\
 , at the University of Hong Kong. He obtained his BSc(Eng) degree from the
  University of Hong Kong in 1989\, and PhD from the University of Birmingh
 am in the U.K. in 1992. After a year of postdoctoral training at Oxford Un
 iversity\, he joined HKU as a Lecturer in 1993\, and was promoted through 
 the ranks to Chair Professorship in 2011.\nProfessor Ngan’s research wor
 k is focused on the microstructural basis of properties of engineering mat
 erials\, and\, in particular\, crystalline defects and their modeling\, an
 d more recently\, nanomechanics including applications to biological syste
 ms. He has published over 180 ISI papers\, and co-authored two books. His 
 research-related honours include the prestigious Rosenhain Medal and Prize
  from the Institute of Materials\, Minerals and Mining\, U.K.\, in 2007 
 – he is the only non-British national so far to receive this award since
  its establishment in 1951. He was also awarded a higher doctorate (DSc) f
 rom his alma mater the University of Birmingham in 2008\, and the Croucher
  Senior Research Fellowship in 2009 which is perhaps the highest honour aw
 arded to academics in Hong Kong. In 2014\, he was elected to the Hong Kong
  Academy of Engineering Sciences. He is a well sought-after journal review
 er and he won the Outstanding Reviewer Award of Scripta Materialia three t
 imes\, in 2006\, 2008 and 2011. He has organized a number of key conferenc
 es\, including Dislocations 2008 and Gordon Research Conference on Nanomec
 hanical Interfaces in 2013\, both held in Hong Kong. He will serve as one 
 of four Meeting Chairs in the Materials Research Society 2017 Spring Meeti
 ng to be held in the USA.
LOCATION:MXF 1 https://plan.epfl.ch/?room==MXF%201
STATUS:CONFIRMED
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