NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

Finite Element Modelling of Failure Behaviour in
Intercalated Epoxy-Clay Nanocomposites
Cecilia PISANO 1,a 1, b
, Łukasz FIGIEL
1
Department of Mechanical, Aeronautical and Biomedical Engineering/Materials & Surface
Science Institute, University of Limerick, Limerick, Ireland
a b
cecilia.psn@gmail.com, Lukasz.Figiel@ul.ie
Abstract
Failure behaviour in an intercalated epoxy-clay
nanocomposite is analyzed with a 2D finite element
(FE) model using the representative volume element
(RVE) concept. The intercalated morphology of the
nanocomposite is modelled with clay tactoids, randomly
distributed and oriented within the epoxy matrix.
Cohesive zone elements are used to model the gallery
failure within each tactoid. Effects of cohesive law
parameters (fracture energy), tactoid aspect ratio and
clay volume fractions fp on the macroscopic behavior of
the nanocomposite are investigated. The analysis shows
that the reduction of the nanocomposite strength and
strain to failure is associated with the gallery failure.
1. Introduction
Available experimental results show that intercalated
clay particles dispersed in a pure epoxy matrix can
reduce the strength of epoxy-clay nanocomposites. It
has been suggested that microcracks initiating within
tactoids (in so called galleries) can be the main failure
mechanism [1], leading to the strength reduction of the
nanocomposite. Hence, the main objective of this work
is to investigate that hypothesis using FE modelling.
2. Numerical model
Intercalated morphology of an epoxy-clay
nanocomposite is represented by clay tactoids,
randomly distributed and oriented in a 2D (plane strain)
RVE. Those clay tactoids are modelled as stacks of clay
platelets intercalated by galleries. The epoxy matrix and
clay platelets are discretized by continuum elements,
while cohesive elements are used to mesh the galleries.
Clay platelets are assumed to be elastic and isotropic
[2]. The epoxy matrix is modelled as a nonlinear
Ramberg-Osgood material. Linear traction-separation
laws are used to define the behaviour of the galleries.
Periodic boundary conditions (PBCs) are imposed on
the RVE, and the numerical homogenization is used to
determine the nanocomposite response [2].
3. Results and discussion
Fig. 1 shows simulated stress-strain curves for the
nanocomposites with different clay volume fractions fp
and fracture energies associated with the gallery
resistance to failure, GIC.. The stress-strain curves show
that the nanocomposite strength (defined as peak stress)
171
and strain to failure are reduced due to gallery failure.
The analysis shows that the gallery failure occurs earlier
for higher volume fractions of clay particles. The latter
is connected with enhanced interactions between
tactoids, and hence leads to the increase of stresses in
the galleries. It is worth noting that the increasing clay
loading suppresses plastic deformation in the matrix, on
the account of higher stresses in the galleries. Also, the
results predict that weaker galleries (i.e. galleries with
lower values of GIC) fail earlier, and lead to a significant
reduction of nanocomposite peak stresses and strains to
failure. Those results suggest that failure of the galleries
is a mechanism responsible for the reduction of the
nanocomposite strength and strain to failure. The
amount of energy dissipated due to the gallery failure
(defined as the difference between areas under stressstrain
curves for models including gallery failure, and
those for which failure was not considered, ‘NO FAIL’
curves in Fig. 1) increases with increasing volume
fraction of clay particles and with decreasing GIC.
True stress [MPa]
120
100
80
60
40
20
f P =3% NO FAIL
f P =5% NO FAIL
f P =3%;0.1J/m 2
f P =3%;0.01J/m 2
f P =5%;0.1J/m 2
0
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05
Applied strain
Fig. 1. Stress-strain curves for different volume fractions
of clay (f p),and fracture energies of the galleries (G IC)
4. Conclusions
FE-modelling of failure behaviour in an intercalated
epoxy-clay nanocomposite shows that the gallery failure
is the main mechanism for strength reduction in the
nanocomposite
5. References
[1] K.Wang et al., Macromolecules, 38, pp. 788-800, 2005.
[2] Ł.Figiel, C.P. Buckley, Computational Materials Science,
44, pp. 1332-1343, 2009.