Silvester John Noronha, Ph. D
Rate Theory - Finite Element coupled approach to plastic instability in irradiated materials:
Strain localization and dislocation pattern formation are typical features of
plastic deformation in metal and alloys. A dynamical approach (or complex system
theories) is used to study the instabilities arising during deformation.
A recent study by us led to characterizing a unique crossover in the dynamical
states (from chaotic to power-law critical state) of the Portevin-Le Chatlier (PLC)
effect as the strain rate is varied[8, 9]. This crossover was successfully modeled
using a rate equations involving spatial dependence. Recently we developed a
non-linear dynamical model involving the evolution of different species of
dislocations and other defects predominant in irradiated materials. We found
instabilities in the model lead to state where patterning (or clear channels)
appear in irradiated copper. This is the first fully dynamical model proposed
for the instabilities in irradiated materials [12]. Further analysis and immediate
extension of the model is poised to substantially enhance our understanding of
the dynamics of defects in irradiated materials. The dynamical model is used as
constitutive model in cystal plasticity simulation using Finite Element Analysis
through using user subroutine UMAT in ABAQUS. The approach is a huge step in
science based designing of structural components of nuclear reactors that will
undergo constant irradiation.
Fracture prediction in Ferritic steels with temperature & irradiation:
Fracture prediction in steels is an important issue; statistical models relying
on “best fit” to experimental data are now used to predict the fracture behavior
of structural integral applications (e.g. nuclear pressure vessel).
These models does not account for the microstructural behavior of the material.
Recently we extended the dislocation simulation approach to include the crack-tip
blunting and was quite successful in predicting the sharp upturn of fracture toughness
with temperature (Brittle-Ductile Transition) [1,3]. The model has also been successful
in predicting the shift in ductile-brittle transition temperature (DBTT) with
irradiation dose [2]. However, our current model does not take in the account the
dynamic interaction between microcracks and the macrocrack. There is increasing
evidence that most macroscopic cleavage fracture
(even in non-crystalline brittle materials) occur by the linking of microcracks
to the macrocrack. Work is underway to implement the dynamic interaction of
microcracks with macrocrack in our model. We are developing a full dislocation
dynamics model, which includes the main crack, and secondary cracks.
The secondary cracks distributed around the main crack maps the microstructure;
their effect on the propagation of the main crack (or brittle failure of materials)
will be studied. This dislocation-level model of crack-tip plasticity could then
be used to check the validity of statistical models and/or to get better
prediction of fracture behavior of materials where secondary cracks are detrimental.
The extended model would also be used to further our statistical study on the
factors [2, 11] that contribute to the scatter in experimental measurement of
fracture toughness.
Multiscale simulation of plasticity and fracture in nanostructured materials:
With the rapid advancement of nanoscience and technology in the recent years,
there has been significant interest for the development of nano materials and
structures with extraordinary strength, toughness, resistance to creep and fatigue,
etc. The mechanical properties of these materials, especially the deviations from
known grain size scaling relations, have opened up a wide range of questions.
These arise mainly due to microstructural and dimensional constraints appear
at the nanoscale. The fracture behavior is strongly influenced by the mechanical
deformation and plasticity in a highly localized scale. The mode of crack
propagation by brittle or ductile tearing can be traced to the materials
deformation response within a few nanometers of the crack tip. The
understanding of plasticity and fracture behavior at this scale is very
important in predicting the reliability of micro/nano machines (e.g. MEMS/NEMS).
The main drawback of the molecular dynamics technique is the short time scales.
For the studies of deformation this means that the deformation process is carried
out at very high strain rates. Since the strain rate may affect the deformation
mechanism, it follows that molecular dynamics, although very useful as a
technique can give only part of the picture. For the study of fracture this
means that cracks are followed at extremely fast speeds and strain rates.
This problem could be overcome by coupling the atomistic simulations with
discrete dislocation (DD) and/or continuum simulations. The idea would be to
feed in the output from atomistic simulation to the DD code to simulate the
long-term (long time scale) behavior [5, 6]. Also the DD simulations can feed
back the atomistic simulation by providing the proper boundary conditions. The
proposed study will use atomistic simulation methods, discrete dislocation
simulation and continuum methods. It is only recently these simulation methods
have developed to a stage where microstructural details can be fed in and also
the computer power increased to a point such that these simulations could be
carried out in a reasonable rime. Thus it is now possible for the first time,
to link different length scales of fracture modeling: the atomistic, dislocation
and continuum models.
References:
1. S. J. Noronha and N. M. Ghoniem, Modeling the brittle-ductile transition in Ferritic Steels: Dislocation Simulations, Int. J. Mech. Mat. Des. (2007, in press)., (2006, in press).
2. S. J. Noronha, N. M. Ghoniem, Brittle-ductile transition in F82H: Effects of irradiation, J. Nucl. Mater. (2006, in press).
3. S. J. Noronha, N. M. Ghoniem, Discrete dislocation simulation of brittle-ductile transition in ferritic steels, Metall. & Materr. Trans. A, 37 (2006) 539.
4. S. J. Noronha, J. Huang, N. M. Ghoniem, Multiscale modelling of brittle to ductile transition, J. of Nuclear Materials, 329 (2004), 1180.
5. S. J. Noronha and D. Farkas, Effect of dislocation blocking on fracture behavior of Al and -Fe: a multiscale study, Mat. Sci. Engg. A, 365 (2004), 156.
6. S. J. Noronha and D. Farkas, Effects of dislocation pinning on fracture behaviour: atomistic and dislocation simulation, Phys. Rev. B 66 (2002), 132103.
7. S. G. Roberts, S. J. Noronha, A. J. Wilkinson and P. B. Hirsch, Modelling the initiation of cleavage fracture of ferritic steels, Acta. Mater. 50 (2002), 1229-1244.
8. G. Ananthakrishna, S. J. Noronha, C. Fressengeas and L. P. Kubin, From Crossover in the dynamics of Portevin-Le Chatelier effect from Chaos to SOC, Mat. Sci. Eng. A, 309, (2001), 316-319.
9. G. Ananthakrishna, S. J. Noronha, C. Fressengeas and L. P. Kubin, Crossover from chaotic to self-organized critical dynamics in jerky flow of single crystals, Phys. Rev. E. 60, (1999), 5455.
10. S. J. Noronha, L. Quaouire, G. Ananthakrishna, C. Fressengeas and L. P. Kubin, Chaos in the Portevin Le-Chatelier Effect, Int. J. Bifurcation Chaos Appl. Sci. Eng., 7 (1997), 2557.
11. S. J. Noronha and N. M. Ghoniem, Modeling the brittle-ductile transition in Ferritic Steels: II. Application to Scatter in Fracture toughness measurements Int. J. Mech. Mat. Des. (2007, in press)
.
12. S. J. Noronha, G. Ananthakrishna, N. G. Ghoniem, A dynamical model for Clear Channel-formation in irradiated materials (in preparation Phys. Rev. Letts).
13. S. J. Noronha and N. M. Ghoniem, The nature of elastic-plastic stress fields in discrete dislocation model of crack-tip plasticity (to be submitted to J. Mech. Phys. Solids)