Acoustic emission monitoring of damage mechanisms an aramid-epoxy composite after tensile fatigue and aging seawater.
Menail, Y. ; Mahi, A. El ; Assarar, M. 等
1. Introduction
The polymer matrix composite materials have gained much advance
compared with metals due to their very competitive features and
affordable costs depending on the consumer and the control of their
development. Their application areas are extensive (aerospace,
aeronautics, automotive, shipbuilding, oil industry, civil engineering,
etc.) and are becoming increasingly "innovative" every day
[1]. The critical role of reinforcement is still based on the specific
use of the material and their characteristics [2]. Aramid and carbon
fibers, besides their specific properties and their relatively high
costs, remain reserved in usage to particular fields. As for the glass
fibers in various forms, they are widespread due to their value.
The choice of the material (fiber and resin) that depends on the
application field, its life and cost of the raw material and the
production method, has led researchers to maximally optimize the
performance of these materials by simulations and practical researches
in situ [3]. In the marine environment, if the composites of glass
fibers are more or less controlled due to their very large use, the
aramid fiber composites are not [4]. This prompted us to contribute
through this research to better understand the effects of sea water on
the 171 Kevlar.
The purpose of this article is to determine the effect of sea water
on an epoxy- aramid composite simulated to fatigue in tensile for
different numbers of cycles and then immersed for different times. The
rupture of the plate after fatigue and aging was followed by acoustic
emission, for a better understanding of different phenomena (debonding,
delamination and fracture of matrix and fibers) that occur in the
material during the various tests [5-7].
2. Materials and methods
For this study, we opted for an aramid-epoxy composite. The
preparation of samples and the experimental design were conducted in the
laboratory LAUM (Acoustics Laboratory of the University of Maine) Le
Mans, France. The mixture prepared for the test pieces of our
realization is based on epoxy SR 1500 associated with an amine hardener
SD 2505 provided by Sicomin Composites. Aramid fibers are of Taffeta
171. The composite plates were prepared by hand lay-up; vacuum, for the
so-called "bag" technique. The polymerization was carried out
in an oven at 80[degrees]C for 6h. The cutting is achieved by an
articulated head saw with a diamond disk to give to 1 mm thick specimen,
20 mm wide and 200 mm long. Tensile tests were conducted on a universal
hydraulic machine INSTRON brand Model 8516 equipped with a load cell of
100 kN, shown by Fig. 1. The control and data acquisition were carried
out via computer to record the evolution of the stress in function of
the deformation, Fig. 2, and the tests were performed at ambient
temperature (15-25[degrees]C). The machine is driven with a constant
speed. This speed was determined after a series of preliminary tests,
which helped setting it to 1 mm/min for all sample types. The use of the
same speed, regardless of the type of test, eliminates the effect of
viscoelastic resins, when comparing results from different tests.
[FIGURE 1 OMITTED]
Monitoring and acoustic data acquisition, are done in parallel with
the mechanical tests with other software to another computer.
[FIGURE 2 OMITTED]
The experimental protocol was carried out the following way (Fig.
3):
* Loading at a constant speed of 1 mm/min under controlled
displacement of up to 50% of the static rupture displacement.
* Fatigue with a form of sinusoidal waves, a frequency of 10Hz with
an amplitude of 10% of the displacement at failure. We chose ten numbers
of fatigue cycles ranging from 100 to 50,000 cycles.
* Unloading the specimen after fatigue.
* Aging in sea water for a period of 100 h, 500 h and 1000 h, as
appropriate.
* Tensile strain with a moving speed of 1 mm/min.
[FIGURE 3 OMITTED]
3. Static tests
The results of the static tensile tests are given in Fig. 4 and the
values are recorded in Table 1.
[FIGURE 4 OMITTED]
The curve at the beginning of development and up to the maximum
strain is characterized by a brittle-type behavior of the material,
which is manifested by a substantially linear variation of the stress
compared with the deformation [8].
This behavior is the result of the progressive disruption of the
matrix; the weakest element of the composite, followed by debonding and
possible delamination. Once weakened, the material fails due to stress,
characterized by a sudden drop, which involves fiber breakage without
arriving at the total failure of the specimen.
[FIGURE 5 OMITTED]
Fig. 5 illustrates changes in the effect of traction on the
composite. The different mechanisms of damage encountered in this
material are visualized through the acoustic emission and are mainly the
matrix cracking, peeling at the fiber/matrix interface, the interlaminar
delamination and final rupture of the fibers, which leads to degradation
of the composite [9-11].
Each type of damage is characterized by its amplitude range. It is
noteworthy that these different ranges are difficult to be identified
accurately, since, from one side, they overlap each other and, on the
other hand, the different authors do not give the same amplitudes for
the same damage [12, 13].
In our case, it can be said that the matrix cracking starts from
the first 25 seconds for a range between 40 and 60 dB. Debonding and
delamination appear sporadically in the early solicitation to grow from
100 seconds (60-70 dB). As to fiber breakage it appears slightly
together to increase the delamination late (70-85 dB).
4. Fatigue tests in tension
To determine the effect of fatigue on the material, we have opted
for a series of numbers ranging between 100 and 50,000 cycles. Figure 6
gives an idea of the evolution of fatigue in a composite Kevlar 171.
The acoustic monitoring is a valuable contribution as it allows
visualizing the different damage under gone by the material. To
understand the emergence of different types of damage, it should be
known that the specimen undergoes a load equal to 50% of the
displacement at break static. Then fatigue starts at a frequency of 10
Hz, for an amplitude of 10% of the displacement at failure.
It is easy to distinguish the effects of both loading and fatigue.
The first step occurs at the outset by a break in matrix followed by
delamination and a fiber break, on a very short time which corresponds
to sudden loading of the specimen. Then begins the fatigue manifested by
a rupture of the matrix throughout the operation. Delamination is very
low and the fiber breakage is very rare.
Following these results, we can say that the material has poor
resistance to sudden efforts and adapts better to a long and constant
fatigue.
[FIGURE 6 OMITTED]
As the material undergoes two successive stresses, fatigue then
aging and their effects are superimposed. To distinguish them from one
another, we conducted two series of tests. The first is a constant
immersion (1000 h) and variation of the number of cycles for monitoring
fatigue and constant fatigue (50 000 cycles) with variation of immersion
for monitoring the aging periods.
5. Effects of fatigue
After different numbers of fatigue cycles (500, 1000, 10000 and
50000), the specimens are immersed in sea water for 1000h to undergo the
same aging water. After that they undergo a static tension until
failure. The obtained results, plotted on Fig. 7 are compared with each
other to highlight the effect of fatigue on the aramid composite.
[FIGURE 7 OMITTED]
They show that the behavior of the material is the same in the case
of the static test of material characterization of the Fig. 4. It is of
brittle type for the series of tested specimens. First, quasi-linear
variations of the stress in function of the deformation, then a sudden
drop in this latter. This drop is due to fiber rupture that causes the
total failure of the specimen. We note that the maximum values of the
stress and the corresponding stress decreases when the number of fatigue
cycles increases, except for the curve 1000 cycles which is superimposed
on that of the 500.We can deduce that the effect of fatigue is the same
for the range 500-1000 cycles.
6. Aging effects
After different numbers of fatigue cycles, we immersed the samples
in sea water for three different durations in order to subject them to
different levels of aging. Then, they were tested in static tension.
Fig. 8 shows the results of static tests after fatigue at 50,000 cycles
and for the three aging times (100, 500 and 1000 h). This figure shows
the evolution of the stress versus strain. The analysis of these results
shows that the behavior is quasi-linear until failure of the specimen of
brittle type [14-16].
The strain and displacement at the rupture decrease slightly when
the immersion time increases, except for the 500 h. This shows that the
immersion times must be increased so that monitoring becomes more
representative.
[FIGURE 8 OMITTED]
7. Double effect: fatigue and aging
The effects of fatigue and aging are combined and shown in Fig. 9.
This figure represents a static test after 1000 cycles of fatigue
followed by immersion in sea water for 100 hours of 171 Kevlar compared
to Fig. 4, which shows a static test without fatigue or aging. The
results of Fig. 8 show that there are many changes occurring on the
behavior of the specimen under the same test. The material is weakened
because it begins to degrade from the first few seconds of the test,
while the test piece of Fig. 4 begins to deteriorate only after the 25th
second. Matrix rupture and sudden delamination appear due to the aging
effects.
From the first 50 seconds, the various degradations begin to
gradually appear intensively. From the 50th to 100th second, we notice a
matrix rupture and delamination more or less diffuse. From the 100th to
the end, we obtain a matrix rupture and delamination with more intensive
development of fiber breakage.
[FIGURE 9 OMITTED]
8. Conclusion
Although the objective of this work is more or less achieved, it
has opened lots of opport unities that will allow us, if explored, to
enhance our results and to better assimilate the various phenomena
observed in this study.
The results of tensile tests in static and fatigue on an aramid
epoxy-amine composite in a humic environment are convincing in most
cases.
Knowledge and prediction of the behavior of these composite
materials require more extensive studies, since they depend on several
parameters, namely, implementation technique, testing themselves, the
means of investigation etc.
The obtained results allowed us to notice that fatigue affects
significantly the mechanical properties of the material, and the more
the number of fatigue cycles is higher, the maximum load that can be
support by the material decreases.
Similarly to the aging effect, the results have shown that the
absorption of sea water by the composite affects negatively its
mechanical behavior. The maximum load that the material can withstand
decreases with the increase in the immersion time.
Hence, it can be said that the combination of two constraints only
weakens the material, which was revealed by the acoustic monitoring. The
emergence of various degradations, matrix rupture delamination and fiber
breakage in the final phase, according to fatigue and aging is
indicative of the behavior of this material and its characteristics.
Acknowledgements
It is my pleasure to take this opportunity to thank the director of
LAUM Laboratory, University of Maine, for welcoming me into his
structure, and to express my gratitude to the "composite
materials" team leader for guiding me, and for everything he did to
make this work successful.
I cannot end without thanking also the whole acoustic steam to
which I have been introduced for the handling of the wonderful tool.
A thank you all!
Received April 30, 2015
Accepted January 06, 2016
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Y. Menail *, A. El Mahi **, M. Assarar ***, B. Redjel ****
* LR3MI, University Badji Mokhtar, Sidi Amar, BP 12, 23000--Annaba,
Algeria, E-mail: menailyounes43@gmail.com
** University of Maine, LAUM, CNRS UMR 6613, avenue Olivier
Messiaen, 72085 Le Mans Cedex 9, France, E-mail:
abderrahim.elmahi@univ-lemans.fr
*** University of Reims Champagne-Ardenne, LISM, EA 4695, IUT de
Troyes, 9 rue de Quebec, 10026 Troyes Cedex, France, E-mail:
mustapha.assarar@univ-reims.fr
**** University Badji Mokhtar, Sidi Amar, BP 12, 23000 Annaba,
Algeria, E-mail: bredjel@yahoo.fr
cross ref http://dx.doi.org/10.5755/j01.mech.22.1.12176
Table 1
Static test results of Kevlar 171
Mechanical characteristics Kevlar taffetas 171
Surface mass, g/[m.sup.2] 170
Fiber, % 42
Longitudinal module, GPa 16.5
Transversal module, GPa 16.5
Stress of the rupture, MPa 305
Deformation of the rupture, % 2.7
