STRAIN GRADIENT PLASTICITY – NUMERICAL MODELLING AND IMPLICATIONS FOR FRACTURE AND HYDROGEN EMBRITTLEMENT

Series: 
CCMMP Seminars
Speaker: 
Emilio Martínez Pañeda
Host: 
David Dunstan
Date: 
January 23rd, 2018 at 14:00
Room: 
GO Jones Room 610
Abstract: 
STRAIN GRADIENT PLASTICITY – NUMERICAL MODELLING AND IMPLICATIONS FOR FRACTURE AND HYDROGEN EMBRITTLEMENT
Emilio Martínez Pañeda
University of Cambridge. Department of Engineering
 
Abstract
Experiments have consistently shown that metallic materials display strong size effects at the micron
and sub-micron scales (smaller is stronger). As a result, a significant body of research has been
devoted to model this size-scale dependent plastic phenomenon. At the continuum level,
phenomenological strain gradient plasticity (SGP) formulations have been employed to extend
plasticity theory to small scales. Grounded on the physical notion of geometrically necessary
dislocations (GNDs, associated with non-uniform plastic deformation), SGP theories relate the plastic
work to both strains and strain gradients, introducing a length scale in the constitutive equations. While
modelling size effects in metal plasticity was motivated at first by growing interest in micro-technology,
the use of SGP formulations has been extended to a wide range of applications where GNDs are
expected to play a major role. Particularly, gradient-enhanced modelling of fracture and damage
appears imperative (independently of the size of the specimen) as the plastic zone adjacent to the
crack tip is physically small and contains strong spatial gradients of deformation.
 
The speaker and his collaborators have been actively engaged in the investigation of gradient effects
in fracture and damage. Strain gradient plasticity predictions reveal that GNDs close to the crack tip
promote local strain hardening, leading to a much higher stress level relative to conventional plasticity.
The analysis of stationary and propagating cracks shows that gradient plasticity models provide a
sound rational basis to mechanistically interpret (i) cleavage in the presence of plasticity and (ii)
hydrogen assisted cracking. Gradient-enhanced predictions proved to be particularly relevant in
hydrogen embrittlement models due to the essential role that the hydrostatic stress has on both
interface decohesion and hydrogen diffusion. Encouraging agreement with experimental data has
been obtained by incorporating the GND effect in the modelling of hydrogen transport and
environmentally assisted cracking. The promising results achieved have attracted the interest of
industrial partners and technical standards organizations, ending with a scientific/engineering
handshake a journey that began from fundamental micromechanics.
 
Short Bio
Emilio Martínez Pañeda works as Research Associate with Norman Fleck and Vikram Deshpande at
the University of Cambridge. Before, he was an H.C. Ørsted Fellow at the Technical University of
Denmark (DTU), where he conducted research in the group of Christian Niordson and Viggo
Tvergaard. Emilio earned an Industrial Engineering (B.Eng+M.Eng) degree from the University of
Oviedo (2011), an MSc degree in Structural Engineering from the University of Granada in (2012) and
a PhD in Mechanics of Materials from the University of Oviedo (2013-2016). During his PhD, Martínez
Pañeda conducted research under the supervision of Covadonga Betegón in the fields of mechanics
of materials and computational solid mechanics. Emilio spent a large part of his PhD studies abroad,
holding visiting scholar positions at the Technical University of Denmark (host: C. Niordson), theUniversity of Luxembourg (host: S. Bordas), the University of California Santa Barbara (host: R.
McMeeking) and the University of Cambridge (host: N. Fleck). His work on the mechanics of solids
has been disseminated in numerous journal publications and has been recognized through several
awards such as the Acta Materialia Student Award, the Springer PhD Thesis Prize or the Best PhD
Thesis Award (Univ. Oviedo).