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The mechanical size effect as a mean-field breakdown phenomenon / Eralp Demir in Acta materialia, Vol. 58 N° 5 (Mars 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 5 (Mars 2010) . - pp. 1876–1886 Titre : The mechanical size effect as a mean-field breakdown phenomenon : Example of microscale single crystal beam bending Type de document : texte imprimé Auteurs : Eralp Demir, Auteur ; Dierk Raabe, Auteur ; Franz Roters, Auteur Année de publication : 2011 Article en page(s) : pp. 1876–1886 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : Size  effect  Electron  backscattering  diffraction  (EBSD)  Bending  Copper  Crystal  plasticity Résumé : Single crystalline copper beams with thicknesses between 0.7 and 5 μm are manufactured with a focused ion beam technique and bent in a nanoindenter. The yield strengths of the beams show a mechanical size effect (smaller-is-stronger). The geometrically necessary dislocation (GND) densities estimated from misorientation maps do not explain the observed size effect. Also, accumulation of GNDs principally requires pre-straining. We hence introduce a mean-field breakdown theory and generalize it to small-scale mechanical tests other than bending. The mean-field breakdown limit is defined in terms of a microstructural correlation measure (characteristic dislocation bow-out length) below which the local availability of dislocation sources and not the density of GNDs dominates the mechanical size effect. This explains why a size dependence can occur for samples that are not pre-strained (by using a very small critical strain to define the yield strength). After pre-straining, when GNDs build up, they can contribute to the flow stress. The mean-field breakdown theory can also explain the large scatter typically observed in small-scale mechanical tests as the availability of sufficiently soft sources at scales around or below the correlation length does not follow statistical laws but highly depends on the position where the probe is taken. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645409008106 [article] The mechanical size effect as a mean-field breakdown phenomenon : Example of microscale single crystal beam bending [texte imprimé] / Eralp Demir, Auteur ; Dierk Raabe, Auteur ; Franz Roters, Auteur . - 2011 . - pp. 1876–1886. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 5 (Mars 2010) . - pp. 1876–1886 Mots-clés : Size  effect  Electron  backscattering  diffraction  (EBSD)  Bending  Copper  Crystal  plasticity Résumé : Single crystalline copper beams with thicknesses between 0.7 and 5 μm are manufactured with a focused ion beam technique and bent in a nanoindenter. The yield strengths of the beams show a mechanical size effect (smaller-is-stronger). The geometrically necessary dislocation (GND) densities estimated from misorientation maps do not explain the observed size effect. Also, accumulation of GNDs principally requires pre-straining. We hence introduce a mean-field breakdown theory and generalize it to small-scale mechanical tests other than bending. The mean-field breakdown limit is defined in terms of a microstructural correlation measure (characteristic dislocation bow-out length) below which the local availability of dislocation sources and not the density of GNDs dominates the mechanical size effect. This explains why a size dependence can occur for samples that are not pre-strained (by using a very small critical strain to define the yield strength). After pre-straining, when GNDs build up, they can contribute to the flow stress. The mean-field breakdown theory can also explain the large scatter typically observed in small-scale mechanical tests as the availability of sufficiently soft sources at scales around or below the correlation length does not follow statistical laws but highly depends on the position where the probe is taken. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645409008106

Transverse diffusive mixing of solutes in pressure driven microchannels: a Lattice Boltzmann study of the scaling laws / Segun Ayodele in La Houille blanche, N° 6 (2009) 

Public ISBD [article]  in La Houille blanche > N° 6 (2009) . - pp. 93-100 Titre : Transverse diffusive mixing of solutes in pressure driven microchannels: a Lattice Boltzmann study of the scaling laws Titre original : Mélange transverse du type convection à diffusion de solutés dans les micro canaux sous pression: une étude Lattice Boltzmann des lois d'échelle Type de document : texte imprimé Auteurs : Segun Ayodele, Auteur ; Fathollah Varnik, Auteur ; Dierk Raabe, Auteur Article en page(s) : pp. 93-100 Note générale : Hydraulique Langues : Anglais (eng) Mots-clés : Microfluidique  Micro-canaux  Lois  d'échelle  Lattice  Boltzmann Index. décimale : 551.4 Résumé : We study scaling laws characterizing the inter-diffusive zone of two miscible fluids flowing side by side in a Y-shape laminar micromixer using the lattice Boltzmann method. The lattice Boltzmann method solves the coupled 3D hydrodynamics and mass transfer equations and incorporates intrinsic features of 3D flows related to this problem. We observe the different power law regimes occurring at the center of the channel and close to the top/bottom wall. The extent of the inter-diffusive zone scales as square root of the axial distance at the center of the channel. At the top/bottom wall, we find an exponent 1/3 at early stages of mixing as observed in the experiments of Ismagilov and coworkers [Appl. Phys. Lett. 76,2376 (2000)]. At a larger distance from the entrance, however, the scaling exponent close to the walls changes to 1/2. For a channel with infinite aspect ratio (width/height), a criterion for this cross over can be given using the homogenization of the tracer concentration field along the shortest dimension (height) of the channel [J.-B. Salmon et al J. Appl. Phys.101, 074902 (2007)]. We analyze the effect of finite aspect ratio and volumetric flow rate on the extent of diffusive broadening. Interestingly, we find the same scaling laws regardless of the channel's aspect ratio. However, the point at which the exponent 1/3 characterizing the broadening at the top/bottom wall reverts to the normal diffusive behavior downstream varies with the aspect ratio. DEWEY : 553.7 ISSN : 0018-6368 En ligne : http://www.shf-lhb.org/index.php?option=article&access=standard&Itemid=129&url=/ [...] [article] Transverse diffusive mixing of solutes in pressure driven microchannels: a Lattice Boltzmann study of the scaling laws = Mélange transverse du type convection à diffusion de solutés dans les micro canaux sous pression: une étude Lattice Boltzmann des lois d'échelle [texte imprimé] / Segun Ayodele, Auteur ; Fathollah Varnik, Auteur ; Dierk Raabe, Auteur . - pp. 93-100. Hydraulique Langues : Anglais (eng) in La Houille blanche > N° 6 (2009) . - pp. 93-100 Mots-clés : Microfluidique  Micro-canaux  Lois  d'échelle  Lattice  Boltzmann Index. décimale : 551.4 Résumé : We study scaling laws characterizing the inter-diffusive zone of two miscible fluids flowing side by side in a Y-shape laminar micromixer using the lattice Boltzmann method. The lattice Boltzmann method solves the coupled 3D hydrodynamics and mass transfer equations and incorporates intrinsic features of 3D flows related to this problem. We observe the different power law regimes occurring at the center of the channel and close to the top/bottom wall. The extent of the inter-diffusive zone scales as square root of the axial distance at the center of the channel. At the top/bottom wall, we find an exponent 1/3 at early stages of mixing as observed in the experiments of Ismagilov and coworkers [Appl. Phys. Lett. 76,2376 (2000)]. At a larger distance from the entrance, however, the scaling exponent close to the walls changes to 1/2. For a channel with infinite aspect ratio (width/height), a criterion for this cross over can be given using the homogenization of the tracer concentration field along the shortest dimension (height) of the channel [J.-B. Salmon et al J. Appl. Phys.101, 074902 (2007)]. We analyze the effect of finite aspect ratio and volumetric flow rate on the extent of diffusive broadening. Interestingly, we find the same scaling laws regardless of the channel's aspect ratio. However, the point at which the exponent 1/3 characterizing the broadening at the top/bottom wall reverts to the normal diffusive behavior downstream varies with the aspect ratio. DEWEY : 553.7 ISSN : 0018-6368 En ligne : http://www.shf-lhb.org/index.php?option=article&access=standard&Itemid=129&url=/ [...]