The effects of water aging on the mechanical properties of glass-fiber and kevlar-fiber epoxy composite materials/Sendinimo vandenyje itaka stiklo ir kevlaro pluosto epoksidiniu kompozitu mechaninems savybems.
Menail, Y. ; Mahi, A. El ; Assarar, M. 等
1. Introduction
Composite materials with organic matrix are widely used in the
international market as materials of choice: they are applied in the
fields of the aerospace, naval constructions, etc. This occurs due to
their competitive costs and high specific properties (e.g., an excellent
withstanding the corrosion). In spite of data shortage in long-term use
of these materials, their successful usage can not be proved, and the
main reason may be their durability.
In nowadays it is possible to find the reliability and visual
representation of composite materials durability using accelerated tests
with precise controlled conditions, which can be realized in the
environment of laboratory. The durability of composite materials
directly depends on the properties of individual structure elements [1,
2].
The humid environment (tap water) is acting in the long run on
these materials characterizing their physical-chemical characteristics
(temperature, composition, etc.). Its aggressive action appears under
several aspects of biological, chemical, physical, and mechanical
properties by altering materials in provoking a fail of the mechanical
characteristics in time. The interactions of water-polymer (distribution
of the water in the composites structures) and mechanical action (damage
of the structure) are the most critical factors.
The degradation of the composites is introduced as a phenomenon of
plastification or physical aging as the result from the distribution of
water in the polymer matrix at the level of the fiber-matrix interface
[3, 4]. The phenomenon of hydrolysis or chemical aging takes place in
the level of micromolecular chains, eventually generating damage by
osmotic fissuring [5].
The aim of this work is to estimate the effects of the tap water on
the behaviour of two composite materials composed of epoxy resin with
glass fiber and with Kevlar fiber. The effects of these materials
behaviour of the fatigue and the aging in the tap water are also
analyzed in this study.
Some tests were done to try the characteristics of non subjected to
fatigue and immersion the materials for various numbers of cycles in
order to determine the evolutions of damage under local interactions of
the humidity and fatigue. The analysed material was placed in the humid
environment for various durations in order to have a comparison for
determination of the aging effect on the fatigue process.
2. Materials and tests
Two types of materials with glass fiber and with Kevlar fiber were
produced at the LAUM (Acoustic Laboratory of the University of Maine).
The plates of composite were realized by vacuum molding using various
fabrics for the absorption of the resin excess and for their extraction.
This operation is done under a 0.3 bar vacuum for 6 hours, with the help
of vacuum pump, followed by 8 hours polymerization in 80[degrees]C
electric heater. Afterwards specimens are cut with the recommended
dimensions (200x20x1 mm), using a saw with diamond disk. Fiber's
mass rate in materials is determined by calculation and loss during the
process of burning. Tests were done by choosing three durations of
hydrothermal aging periods: 100, 500 and 1000 hours. This choice of the
aging durations is based on the well known experiments in order to
define the behaviour of materials in humid conditions. Damage by osmotic
cracking related to very important water absorption was identified
beyond 120 hours of aging. These results confirm the state of the
material damage [6]. The specimens reach quick balance (35 days) within
20[degrees]C and a model of Frick makes it possible to describe the
kinetics of absorption correctly in 0.35% of absorption [7].
The interactions between humidity and the damage by fatigue could
be done only on a local scale, after capillary diffusion of humidity in
the bottom of rapture at a temperature of 23[degrees]C [8].
The loss of properties is due to reversible phenomena of the matrix
plastification until 50[degrees]C [9].
The temperature of the medium of aging in this experiment is
particular that the tap water is equal to the ambient temperature
(15-25[degrees]C).
The protocol of the tensile tests of studied composite materials is
done in 3 stages (Fig. 1):
--the 1st stage of loading is done at a constant speed of lmm/mn
under the displacement controlled up to 50% of displacement to the
rupture in statics;
--the 2nd stage is a phase of fatigue with a form of sinusoidal
wave of 10 Hz frequency with the amplitude of 10% of displacement to the
rupture. The optionally ten numbers of fatigue cycles were chosen and
they are in a period from 100 to 50000 cycles;
--the 3rd stage is a phase of unloading of the specimens after
being subjected to fatigue with the imposed number of cycles.
At the end of the 3rd stage the specimens can be tested in two
different stages:
--either they undergo aging (by immersion) with tap water
throughout 100, 500 or 1000 hours (4th stage, Fig. 1), then they are
broken by 1mm/min speed of tensile until rupture (5th stage, Fig. 1);
--either they are broken directly by 1mm/min speed of traction
without aging (5th stage, Fig. 1).
[FIGURE 1 OMITTED]
3 Results and discussion
3.1. Static tests
In order to follow the behaviour and the degradation of composite
materials studied in wet medium state, static tests in traction were
carried out with the goal to determine the forces and displacements with
the rupture. At least five specimens were tested in statics state for
each material. The obtained results are shown in Fig. 2. This represents
the evolution of the stress according to the strain of two studied
materials.
[FIGURE 2 OMITTED]
A quasilinear behaviour was observed until the rupture of fragile
type for both laminates. The value of laminate rapture stress with glass
fibers is higher than that of the fiber laminate of Kevlar owing to the
fact that the percentage of fibers in the first laminate is higher than
that of the second laminate. Table summarizes the results obtained in
static tensile on two materials.
3.2. Fatigue tests before aging
These tests are carried out by controlling displacement. Average of
displacement was being maintained as a constant. The evolution of the
maximum loading force [F.sub.max] according to the number of cycles N
was recorded during these tests. The results obtained in the case of a
fatigue test of both materials are shown in Fig. 3. It shows the
evolution of the [F.sub.max]/[F.sub.0max] ratio according to the number
of cycles of fatigue ([F.sub.0max] is the maximum load obtained in the
first cycle). The obtained results show that the loss of rigidity
(measured by the [F.sub.max]/[F.sub.0max] ratio) until the rupture of
the specimen proceeds in three stages: initially it appears by a brutal
reduction in the [F.sub.max]/[F.sub.0max] ratio as of the first cycles,
the reduction becomes very slow in the second phase corresponding the
near total durability of the specimen and finally in the third very
short phase where the loss of rigidity accelerates brutally until the
fracture of the specimen.
[FIGURE 3 OMITTED]
Three specific parts of the curve can be attributed to:
--the initiation and multiplication of transverse ply cracking in
the resin for the first part;
--the stable spread of this one as well as the initiation of
delaminations between adjacent plies during the second part;
--the accumulation of cracking involving delamination of the layers
and rupture of the fibers thus causing the final rupture of the specimen
for the last part.
The evolution of the rigidity in 3 stages in literature was
explained so: the first stage corresponds transverse ply cracking; the
second stage corresponds layers delamination and finally the third stage
corresponds fibers breakages [10-12].
It should be noted that the first stage constitutes only 10% of the
life expectancy while it corresponds 80% of the rate of damage.
The specimen of glass fibers brakes before reaching 50000 cycles,
whereas rupture of the specimen in Kevlar fibers is not reached at the
end of a million cycles.
3.3. Static strength and stiffness after fatigue loading
Monotonous static tests were carried out in order to evaluate
mechanical behaviour of both materials after fatigue with various
numbers of cycles, e.g. Fig. 4 show the stress-strain curves for the
Kevlar fibers laminate and glass fibers laminate for four numbers of
fatigue cycles (100, 1000, 10000 and 50000 cycles). From these results
it is clear the reduction in the ultimate stress and ultimate strain
when the number of cycles fatigue increases for the both laminates.
[FIGURE 4 OMITTED]
Fig. 5 shows the evolution of residual strength obtained in statics
according to the number of fatigue cycles for both laminates in a
semilogarithmic scale. These results show that the resistance strength
of material decreases with the increase in the number of fatigue cycles.
In the same way (Fig. 6), the delay of the residual stiffness evolution
(slope at the origin of the static test after fatigue) according to the
number of fatigue cycles is represented for the both laminates. It could
be noted that the module residual stiffness falls with the increase in
the number of fatigue cycles for both laminates (with glass fibers and
with fibers of Kevlar).
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
3.4. Strength and stiffness after fatigue and water aging
The application of a sinusoidal mechanical load on a material
generates damage. This last increases with the number of fatigue cycles
involving an increase in the quantity of water absorbed by the material.
The rate of saturation in a composite material depends on the intensity
of the stress, of the direction of the request compared to the
reinforcement, the chemical nature of the resin and the temperature of
the medium of absorption.
[FIGURE 7 OMITTED]
After fatigue tests with various numbers of cycles, the test-
specimens were immersed to a tap water for three different periods of
time, in order to make them undergo various levels of aging. Then they
were tested in static tensile, e.g. Fig. 7 gives the static test results
after fatigue to 10.000 cycles and for three durations of aging (100,
500, and 1000 hours). This figure presents the stress-strain curves for
both materials. The analysis of these results shows that the behaviour
remains quasilinear until the specimen rupture, which is of fragile
type. The ultimates stress and strain decrease when the time of
immersion increases.
The results obtained in static test after fatigue and aging are
presented in a semilogarithmic scale in Fig. 8, and Fig. 9. Fig. 8
represents the evolution of the residual strength according to the
number of fatigue cycles o in both materials and for various durations
of aging. In the same way, Fig. 9 gives the evolution of the residual
stiffness obtained in static tests according to the number of fatigue
cycles in both materials and for the various durations of aging. It
could be noted also that strength (Fig. 8) and stiffness (Fig. 9)
decrease when the number of fatigue cycles increase.
[FIGURE 8 OMITTED]
These results highlight the effect of the damage by fatigue and the
duration of aging on the behaviour of studied composite materials. The
degradation of both materials depends on the number of fatigue cycles
applied and the time of immersion. The rate of absorption of water is
strongly influenced by the level of damage and water absorption clearly
involves the reduction of strength and stiffness. The low interfacial
resistance of composite materials to the aggression due to the
absorption of water severely compromises the advantage of using a matrix
practically insensitive for humidity [13].
[FIGURE 9 OMITTED]
4. Conclusions
This work comprises a significant number of results concerning
aging in tap water for the time of immersion going up to 1000 hours. The
both studied materials (glass fibers and Kevlar fibers laminates)
present similarities in the fatigue behaviour, but differ in tensile
strength.
The knowledge and prediction of the fatigue behaviour of composite
materials with organic matrix and Kevlar and glass fibers, aged in wet
environment state, require thorough studies, since they depend on
several parameters, in fact, the technique of implementation,
environment of aging, the tests, etc.
The specimens subjected to the tap water, are more fragile than
those which were preserved in the air. The difference in resistance of
materials probably is due to the propagation of water in the capillary
left after the traction tensile fatigue effect (chemical and physical
aging).
The influence of tap water on both materials (glass and Kevlar
fibers) appears by a clearer degradation in wet medium state than in dry
medium state, and it is more aggressive with the Kevlar fibers than with
glass fibers. The number of fatigue cycles applied has quite a visible
effect on the resistance of composite materials.
Received March 03, 2009
Accepted April 17, 2009
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Y. Menail *, A. El Mahi **, M. Assarar ***, B. Redjel ****, A.
Kondratas *****
* University Badji Mokhtar, Sidi Ammar, BP 12, 23000--Annaba,
Algeria, E-mail: menailyounes@yahoo.fr
** University of Maine, av. O. Messiaen, 72085 Le Mans, France,
E-mail: abderrahim.elmahi@univ-lemans.fr
*** University of Maine, av. O. Messiaen, 72085 Le Mans, France,
E-mail: mustapha.assarar.etu@univ-lemans.fr
**** University Badji Mokhtar, Sidi Ammar, BP 12, 23000 Annaba,
Algeria, E-mail: bredjel@yahoo.fr
***** Kaunas University of Technology, K. Donelaicio g. 73, 44029
Kaunas, Lithuania, E-mail: alvydas.kondratas@ktu.lt
Table
Static test results
Mechanical characteristics Glass fiber Kevlar fiber
Surface mass, g/[m.sup.2] 300 170
Fiber, % 65 42
Longitudinal module, GPa 16 16.5
Transversal module, GPa 16 16.5
Stress of the rupture, MPa 380 305
Deformation of the rupture, % 3.5 2.7
