International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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Fatigue analysis of CFRP composite laminates with circular cutouts
1
Abhilash S Mirji, 2P.K. Sahoo, 3B. Raghuvir Pai
M. Tech Graduate, MIT Manipal, Senior Scientist, STTD, CSIR-NAL, Professor, MIT, Manipal
Email: 1abhilashmirji002@gmail.com, 2pks@nal.res.in, 3raghuvir.pai@manipal.edu.
Abstract— The aerospace structural components, during
the service of aircraft, are subjected to fatigue loadings.
Accurate fatigue evaluations of those components are very
essential for assessing the structural integrity of the
aerospace structures. In the present investigation, fatigue
analyses of composite laminates with circular cutouts are
performed at stress ratio of 0.1 and frequency of 10Hz. The
composite laminates are made up of carbon-epoxy
AS4/3501-6 and of different stacking sequences such as
[0/902]s, [0/904]s, [02/90]2s and [45/-45]2s First, two
dimensional finite element analyses on such laminates have
been conducted. Then these stress analysis results are post
processed to predict fatigue life of laminates with cutouts
using ABAQUS and FE Safe finite element tool. The
analysis results were compared with results available in the
literature for all the laminate configurations. Both the
results are found to be in good agreement.
Index Terms— Composite materials; 2D finite element;
Stress analysis; Fatigue analysis.
I. INTRODUCTION
Composite materials have recently replaced conventional
metals in application to related industries. This class of
materials has offered substantial improvement over
conventional metal applications in advanced engineering
and high performance aerospace structures. Even though
being used extensively, they are not immune from
degradation and damage. Infact fatigue failure is one of
the main failure mechanism associated with composites.
Varvani-Farahani et al. distinguished damage regions as
the number of cycle propagates to failure i.e. matrix,
fiber- matrix interface and fiber regions. [1]
Lian and Yao performed 2D finite element analysis for
plain E-glass/ epoxy and simulated fatigue failure. The
'ABAQUS' FEA package was employed for this purpose
and individual plies were analysed. [4]
The above review of literature reveals the scarcity of 2D
finite element simulation of fatigue failure. In the present
work, fatigue life of carbon-epoxy laminates are predicted
taking into account stress analysis. The damage modes are
also obtained from FE Tool.
II. MULTICONTINUUM THEORY
The fundamental premise of continuum mechanics is that
any physical quantity of interest is evaluated at a material
point by averaging the quantity over a representative
volume element that surrounds the point of interest known
as representative volume element (RVE). In the case of a
typical unidirectional FRC, the fibers tend to have a
somewhat random spacing within the matrix material;
therefore, the RVE used to characterize a material point
must be large enough to contain numerous fibers in order
to provide an accurate statistical representation of any
quantity that is averaged over the RVE.
The concept of a multicontinuum simply extends the
notion of a continuum to reflect distinctly different
materials that coexist within the representative volume
element used to characterize a material point.
Unidirectional fiber-reinforced composite is viewed as
two interacting continua (Fiber and matrix continuum). In
such a representation, three different volume averages are
relevant to the mechanics of the composite material as
mentioned below
P. Papanikos et al [2] developed a fatigue damage model
for predicting the fatigue failure and life of carbon-epoxy
laminates with different stacking sequences. Analysis was
performed by developing 3D element in ANSYS FE code

Naderi and Maligno developed 2D finite element model
to simulate fatigue failure of different unidirectional and
multidirectional plies. [3]
Multicontinuum Theory traditional continuum mechanics
by expanding the focus to include two additional issues:


Composite average quantity or homogenized
average quantity indicated by superscript c.
Fiber average quantity indicated by superscript f.
Matrix average quantity indicated by superscript m.
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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
The development of relationships between the
various constituent average quantities of interest

Developing relationship that links the composite
average quantities to the constituent average
quantities.
3. Kinetic theory of fracture
V. FINITE ELEMENT ANALYSIS
The element type chosen is conventional shell element
S48- A four node doubly curved thin or thick shell with
six degrees of freedom at each node (three translations
and three rotations). The number of nodes and elements
for laminates with cutout it is 689 and 616 respectively.
The finite element model of laminate with circular and
with boundary conditions is shown in figure 1.
The kinetic theory of fracture (KTF) describes the process
of bond breaking via thermally activated processes.
Helius: Fatigue uses KTF to predict fatigue failure in the
matrix constituent, which translates to composite failure.
The bond breakage rate due to thermal energy is given by
Equation 1 [5]
Kb 
kT
 U 
exp  

h
 kT 
(1)
The effects of applied stress on bond breakage rate are
given by equation 2 [5]
Kb 
kT
 U   
exp  

h
kT 

(2)
(a) FE model
(b) FE model with BC’s
The base line equation for determining fatigue life of
composites is shown in equation 3. [5]
Figure 1: Finite element model with boundary conditions
applied on it
t
  eff    
kT

 U 
n(t )  n0  n01  (1   ) exp    exp 
d  (3)
h
 kT  0  kT  

VI. RESULTS AND DISCUSSIONS
PROBLEM DEFINITION
Composite laminates made of CFRP AS4/3501-6 material
of dimensions 50*10*0.127 (each lamina thickness) and
following stacking sequences of [0/902]S, [0/904]S,
[02/902]S, and [45/-45]2s is considered to perform fatigue
analysis. The properties are tabulated in table 1. The
multidirectional lamina properties shown in table
indicates that lamina is transversely isotropic because
elastic properties in second and third direction is same i.e.
E2=E3 and also G12=G13. But laminate is orthotropic,
because elastic properties in all three directions are
different.
Stress analysis for composite laminates was carried out in
ABAQUS and the results are as shown in figure 2 (a)- 2(d)
below.
(a) [0/902]s
(b) [0/904]
Table 1: Material properties of AS4/ 3501-6 [3]
E1 (MPa)
E2 (MPa)
E3 (MPa)
G12 (MPa)
G13 (MPa)
G23 (MPa)
ν12
ν 23
ν 13
138*103
9*103
9*103
5.7*103
5.7*103
3*103
0.3
0.45
0.3
(c) [02/902]s
(d) [45/-45]2s
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ISSN (Online): 2347-2820, Volume -2, Issue-5,6, 2014
84
International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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Figure 2: Stress analysis results for laminates with circular
cutout
The above stress results are post processed in FE safe to
obtain fatigue life of composite laminates with cutout as
shown in figure 3(a)- 3(d) respectively.
(c) [02/902]s
(d) [45/-45]2s
Figure 4: S-N plots for different laminates with circular
cutout
CONCLUSIONS:
(a) [0/902]s
(b) [0/904]s
(c) [02/902]s
(d) [45/-45]2s
Figure 3: Fatigue analysis results for laminates with
circular cutout.
Similarly fatigue life cycles to failure were obtained at
different stress levels by changing the boundary
conditions optimistically to obtain S-N plots for above
laminates. These results were compared with results
obtained from literature. Both are found to be in good
agreement.
Fatigue failure analysis is carried out for AS4/ 3501-6
carbon/ epoxy laminates subjected to tensile cyclic
loading conditions with stress ratio as 0.1 and frequency
as 10 Hz. Fatigue life for different stacking sequences is
predicted and compared with available results from
literature [3] as in figure 4(a)-4(d) . It was found that
stress values were high at cutout sections were failure is
possible very easily and as well as magnitude of fatigue
cycles to failure was also high at cutout region. This
observation concludes that fatigue crack is initiated at
cutout region and this crack will propagate to failure as
number of cycle increases. S-N curves were plotted for
different stress levels and were compared with results
available in literature [3]. Both were found to be in good
agreement.
REFERENCES
[1]
Varvani-Farahani, H. Haftchenari, M. Panbechi.”
An energy-based fatigue damage parameter for
off-axis unidirectional FRP composites”.
Composite Structures 79 (2007) 381–389.
[2]
P. Papanikos et al. “Modelling fatigue damage
progression and life of CFRP composites”.
ISTARM, Institute of structure and advanced
materials, Patron-Attinan. Greece
[3]
M. Naderi and A.R. Maligno. “Fatigue life
prediction of carbon/epoxy laminates by stochastic
numerical simulation”. Composite structures
94(2012)1052-1059.
[4]
Wei Lian, Weixing Yao.”Fatigue life prediction of
composite laminates by FEA simulation method”.
International Journal of Fatigue 32 (2010)
123–133.
[5]
FE Safe composite theory manual, edition 2010.
(a) [0/902]s
(b) [0/904]s
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ISSN (Online): 2347-2820, Volume -2, Issue-5,6, 2014
85