Indian Journal of Engineering & Materials Sciences
Vol. 24, August 2017, pp. 283-289
Tensile and compressive properties of epoxy syntactic foams
reinforced by short glass fiber
Wei Yu*, Hailong Xue & Meng Qian
Key Laboratory of Mechanical Reliability for Heavy Equipments and Large Structures of Hebei Province,
Yanshan University, Qinhuangdao 066004, China
Received 22 April 2015; accepted 29 March 2017
The hollow glass microsphere/epoxy resin syntactic foams reinforced by short glass fibers are fabricated. The
microsphere is constant 5% weight ratio to the epoxy resin matrix and the fibers with weight ratio to the resin matrix are 5%,
10%, 20% and 30%. Their mechanical properties are studied by uniaxial compression test and tension test. The compressive
deformation morphology and tensile fracture surfaces of syntactic foams are investigated. It is obtained that the compressive
and tensile strength of syntactic foams are enhanced by adding fibers, and the 10% fiber weight ratio is found to be much
more efficient, which shows 70% and 49% higher compressive strength and tensile strength respectively than that of
syntactic foams without fiber content. However, the strength decreases with further addition of glass fiber beyond 10%
weight ratio. The reason of strength enhancements is discussed. The compressive and tensile moduli are also enhanced by
adding fibers. The ductility of composites is found to decrease with larger fiber filling.
Keywords: Syntactic foams, Epoxy, Hollow glass microsphere, Glass fiber
The composite of hollow particle filled polymer is
called syntactic foams. Hollow particles are usually
made up of glass, ceramic, materials. The epoxy resin,
polyurethane, polyethylene, nylon, rubber, etc are
usually used as matrix. In recent years, the syntactic
foams attracted much attention by many scholars due
to its low weight, low moisture absorption, high
specific strength and good energy absorption1-5
, and
widely applied in the machinery industry,
transportation, construction, aerospace industry and
deep-sea engineering6-8
. As two kinds of commonly
used materials in the industry, the mechanical
properties and modification of epoxy resin and
polyurethane have been widely studied by many
scholars9-12
. Add higher volume fraction of hollow
particle can greatly reduce the density of syntactic
foams. However, the strength and stiffness of
composites greatly decrease because of adding higher
volume fraction of hollow particle10,11,13
.
Many studies reported in literature show that
adding fibers in syntactic foams is one of the methods
of improving the mechanical properties14-23
.
Karthikeyan et al.15
observed that adding of chopped
strand fibres into the syntactic foam system increased
the flexural strength. Wouterson et al.18
studied the
effect of fiber volume fraction and length on
mechanical properties and thermal properties by
experiment. It is shown that the tensile strength,
flexural strength and elastic modulus are enhanced by
adding fiber. Ferreira et al.20
presents the results of
the addition of short fibres on the fatigue behaviour of
syntactic foams. Their results show that the fatigue
strength more than 30% by the addition of small
percentages of glass or carbon fiber. Wang et al.22
investigated the flexural properties of syntactic foams
reinforced by fiberglass mesh and short glass fiber. On
the other hand, graphene platelets and nanofibers are
also used as reinforcements in these studies7,24-28
.
Numerical method is also used to investigate the effect
of fiber reinforcement on properties of syntactic foams29
.
From the above research, the increase on
mechanical properties of syntactic foams is not
obvious for the lower contenting of glass fiber. With
the increase of glass fiber filling amount, the
mechanical performance of syntactic foams increase.
However, how much glass fiber filled in syntactic
foams is reasonable, which has yet to see literature
report. Although the study of fiber reinforced epoxy
resin composite is more and the influence of fiber
content on the composite mechanical properties is
also analyzed, the fiber reinforced epoxy resin
containing hollow glass microspheres composites,
especially the fiber content is higher, has a little
research. The interactions of the fiber and _____________
*Corresponding author (E-mail: [email protected])
INDIAN J. ENG. MATER. SCI., AUGUST 2017
284
microsphere have important effect on the performance
of composites. Seeking a reasonable content of fibers
when the content of hollow glass microspheres and
fibers are higher is the purpose of this paper. In the
present study, the syntactic foams with 5% hollow
glass microsphere mass fraction and with four kinds
of glass fiber weight ratio in the range of 5%-30% are
prepared. The influence of glass fiber contents on
mechanical properties of composites are studied by
tensile and compressive experiments.
Experimental Procedure
Materials
Hollow glass microsphere with bulk density of 0.11
g/cm3, crushing strength of 3-5 MPa and average
diameter of 60 µm were used as the lightweight filler.
Glass fiber with average length of 106 µm and
average diameter of 13 µm were used as reinforcing
phase, and its original yarn breaking strength is not
less than 0.28 N/Tex. Hollow glass microsphere and
glass fiber were supplied by Qinhuang Hollow
Glass Microsphere Co., LTD, China. The epoxy
resin
(E-44) and hardener polyamide (650#) were
supplied by Langfang Norsun Chemical Industry
Co., LTD.
Material processing and testing In this paper, hollow glass microspheres are added
in 10% weight ratio of the epoxy resin as well as
hardener in all syntactic foams. The short glass fibers
are added in the range of 5-30% weight ratio of the
resin matrix. The unreinforced syntactic foams are
also fabricated for comparison. Table 1 shows the
design proposal and the density. The specimen
preparation process is as follows. Firstly, the epoxy
resin and hardener were heated respectively in a water
bath pot at 80°C to be fully diluted. Then, the glass
fiber were firstly added to the epoxy resin and stirred
for 20 min to disperse the fibers fully. After that, the
hollow glass microspheres were added and stirred
additional 20 min. Then, the sufficient diluted
hardener was slowly mixed and stirred fully. And
then, the slurry was transferred to tabular and
cylindrical plastic moulds. Finally, the specimens
were cured at 40°C for 24 h and post-cured at 80°C
for 2 h.
Compressive and tensile tests were conducted in a
universal testing machine (WDW3100, Changchun
China) at room temperature. The dimensions for
compression specimens were Φ 19 mm × 20 mm, and
for tensile tests were 40 mm × 10 mm × 6 mm,
(Fig.1). Three specimens were tested in every series.
The load rate of 1 mm/min was maintained for all the
testing. The compressive deformation set as
50% strain.
Results and Discussion
Figure 2 shows the deformation morphology of
syntactic foams after compression. It is observed that
all the cylindrical specimens present the shape of
drum after compression because of the friction
between specimen and the pressure head. The
specimens without fiber or with low fiber content
have not obvious cracks on the surface by applying
50% strain. It indicates that the specimens are still
not completely destroyed. There are small cracks on
the surface of syntactic foams with 10% fiber
weight ratio. However, there are obvious large
oblique cracks in the specimens which fiber
contenting more than 10%. It is similar to that of
cast iron cylinder specimen compression damage. It
indicated that the composites with a lot of fiber
contents have brittle characteristics. The reason is
the brittle fracture of microspheres and fibers. In
addition, it can be seen from the Fig.2, the heights
of specimens are different, because of the different
resilient rate of specimen. The resilient rate
decrease with the increasing of fiber content.
The compressive and tensile stress-strain curves
are shown in Fig. 3. It is observed that the
Table 1 — Material composition
Specimen
No.
Epoxy resin
matrix (g)
Hollow glass
microsphere (g)
Glass fiber
(g)
Density ρ/
(g·cm-3)
1 200 10 0 0.927
2 200 10 10 0.940
3 200 10 20 0.955
4 200 10 40 0.991
5 200 10 60 1.003
Fig. 1 — Schemes of tensile specimens
Fig. 2 — Deformation morphology of compressive specimens
YU et al.: EPOXY SYNTACTIC FOAMS REINFORCED BY SHORT GLASS FIBER
285
compressive stress-stain curves show the same
trend, (see Fig. 3a). The first stage of the curves is
the elastic stage. The yield stage is begin when the
strain is about 4.5%, then the curves down slowly
and then slowly rising within a larger strain range.
The stress-strain curves rise faster when the strain
are beyond 25%. The figure is also shows that the
position of all the curves of fiber reinforced syntactic
foams are higher than that of plain syntactic foams. It
indicates that the mechanical properties of syntactic
foams had obvious enhancement by adding short glass
fibers. However, the three curves of syntactic foams
with fiber weight ratio 10%, 20% and 30% are close,
which suggests that their properties have a little
difference.
Figure 3(b) shows that the tension curve of
syntactic foams without fiber content has a larger
tensile fracture strain. Although the fracture strain
of syntactic foams decreases with the addition of
short glass fibers, it is still beyond 4% when the
fiber weight ratio is less than 10%. However, it
cannot reach 3% when the fiber content is larger.
The results indicate that the addition of fibers
decreases the ductility of the syntactic foams.
Figure 3(b) also shows that the tensile properties of
syntactic foams with 10% fiber weight ratio is
better than the others, and the tensile fracture strain
is about 5%. It indicates that adding short glass
fiber with 10% weight ratio can effectively enhance
the tensile properties of syntactic foams, and have
little effect on its ductility.
Figure 4 shows the relationship of the strength
and modulus versus the fiber weight ratio. In most
cases, the three data points of a same proportion are
close in addition to the individual specimen. From
Fig. 4(a), it is observed that all the compressive
strengths of fiber reinforced syntactic foams are
higher than that of syntactic foams without fiber
reinforced. The maximum value is at 10% glass
fiber weight ratio. Although the compressive
strength slight decreases with further addition of
glass fiber beyond 10% weight ratio, the strengths
are still obviously larger than that of syntactic
foams without glass fiber content. Figure 4(a) also
shows that the tensile strength increase with the
Fig. 3 — Stress-strain curves of syntactic foams (a) compression
and (b) tension
Fig. 4 — The strength and modulus versus the fiber weight ratio
INDIAN J. ENG. MATER. SCI., AUGUST 2017
286
increase of glass fiber content when the fiber weight ratio
less than 10%, then it decreases with further addition of
glass fiber beyond 10% weight ratio. Specially, there is a
large decrease when the fiber weight ratio more than
20%. It indicates that adding a certain amount of glass
fiber can enhance the strength of composite, but the
enhancement effect is more when fiber contents are larger
instead of smaller. The reason is that there are the air
bubbles and clustering of fibers and hollow glass
microspheres in the resin. With the increase of glass fiber
content, the viscosity of resin matrix is increased, so the
difficulty of dispersion fibers is harder.
The clustering of fibers and microspheres will
decrease the strength of composites, because the
clustered regions would serve as crack initiation sites
due to stress concentration30
. The air bubbles caused
by stirring are also difficult to eliminate completely
before the resin curing21,22
. Obviously, the influence
of fiber clustering and air bubbles on compressive
strength is less than that of tension. The tension
properties are sensitive to these defects in resin, for
the cracks will produce from the defects and rapid
development when applied tensile load. This is the
reason why the tensile strength decreases more by
adding larger fibers, whereas the compressive strength
does not. The compressive and tensile modulus of
syntactic foams with fiber reinforced are larger than
that of syntactic foams without fiber contents as seen
in Fig. 4(b). Ignore the individual data points, the
modulus values of fiber reinforced syntactic foams are
higher than that of syntactic foams without fiber
reinforced. The compressive modulus values are close
to that of tension.
Table 2 shows the average strength and modulus.
Among them, the minimum data of the elastic
modulus value with fiber weight ratio 20% and the
strength value with fiber weight ratio 5% are ignored.
From Table 2, it is observed that the maximum
average value is more than 70% in strength at 10%
glass fiber weight ratio, compared with syntactic
foams without glass fiber reinforced. The addition of
10% weight ratio glass fiber produces of 49%
improvement in tensile strength, compared with
syntactic foams without glass fiber reinforced. The
maximum value of compressive modulus is more than
39% at 10% weight ratio of glass fiber, compared
with syntactic foams without fibers. The compressive
modulus decreases with further addition of glass fiber
beyond 10% weight ratio, but the variation is smaller.
The reason of modulus decrease is also the clustering
of fibers and air bubbles in the resin. Nevertheless, the
tensile modulus does not display the same trend. The
maximum value appears at the 30% weight ratio of
glass fiber. The reason of this trend may be the effect
of clustering of fibers and air bubbles is smaller at the
initial tension loading phase.
Figure 5 shows the relationship of the specific
strength and specific modulus versus the fiber weight
ratio. From Fig. 5(a), it is observed that the specific
compressive strength and specific tensile strength
increase with the increasing content of glass fiber
when the glass fiber weight ratio is less than 10%,
Table 2 — Average strength and modulus
Specimen
No.
Yield
limit
(MPa)
Tensile
strength
(MPa)
Compressive
modulus
(MPa)
Tensile
modulus
(MPa)
1 18.7 15.8 772 778
2 28.2 18.3 832 905
3 31.7 23.5 1075 939
4 31.2 17.3 1052 1020
5 28.6 15.5 1041 1037
Fig. 5 — (a) Specific strength and (b) specific modulus versus the
fiber weight ratio
YU et al.: EPOXY SYNTACTIC FOAMS REINFORCED BY SHORT GLASS FIBER
287
then decrease with further adding fibers. Figure 5(b)
shows that all the specific modulus of fiber reinforced
syntactic foams are higher than that of syntactic
foams without fiber, and the specific compressive
modulus of syntactic foams with 10% fiber weight
ratio has the maximum value, while the specific
tensile modulus shows the maximum value at the 30%
fiber weight ratio with a little higher than that of 10%
fiber weight ratio.
Reinforced syntactic foams with containing of 10%
glass fiber weight ratio shows 70% and 49% higher
compressive strength and tensile strength, as well as
65% and 45% higher specific compressive strength and
specific tensile strength, than that of syntactic foams
without fiber reinforced. In addition, the compressive
modulus and tensile modulus show 39% and 21%
higher than that of specimen without fibers also. The
density of reinforced syntactic foams is only 3% higher
than that of plain syntactic foams. Although the tensile
modulus and specific tensile modulus of syntactic foams
with 30% fiber weight ratio have the maximum values,
the value is only a little higher than that of the composite
with 10% fiber weight ratio. It indicates that the
syntactic foam with 10% glass fiber weight ratio is the
better ratio in these four ratios.
Figure 6 shows the SEM photos of tensile fracture
surfaces. From Fig. 6(a), it is found that the
microspheres are uniformly distributed in the resin
matrix. There are amount of debonding and fracture
of microspheres at the fracture surface. Figure 6(b-d)
shows the fracture surfaces of syntactic foams with
fiber reinforced. It is found that the interfaces between
the matrix and glass fiber/microspheres have a good
bonding effect, but some of fibers are debonded
because of the crack propagation along with the
fibers. From Fig. 6(b), it can be seen clearly that the
resin has ladder-like fracture morphology and there is
an obvious discontinuous at the fiber, and the fiber is
pulled out and the interface between fiber and resin
matrix is debonded. The reason is believed that the
crack propagation is restricted by fibers, then it
propagates along with the fibers. Some of the fibers
are fracture, and amount of fibers are debonded and
pulled out. More energy could be dissipated for the
Fig. 6 — Fracture surfaces of syntactic foams (a) without fiber, (b) 10% fiber weight ratio (c), 20% fiber weight ratio and (d) 30% fiber
weight ratio
INDIAN J. ENG. MATER. SCI., AUGUST 2017
288
fiber debonding and pull out, which will lead to
increase the fracture strength of composites. Some of
the fibers oblique to the tensile loading direction are
fracture, which also leads to increase the fracture
strength of syntactic foams because of the higher
strength of fiber than that of resin matrix. However,
adding large amount of fibers leads to the difficult
dispersion of fibers and the fibers clustering and air
bubbles are generated. Figures 6(c) and 6(d) are the
fracture surface micrographs of syntactic foams with
20% or 30% fiber weight ratio. From Fig. 6(c-d), it
can be seen that there are microbubbles and other
defects in the fracture surface.
Figure 7 shows the fracture surface photos of
syntactic foams with the other magnification. Besides
of the microspheres, there are a lot of fibers
debonding and pull-out at the fracture surfaces. The
fibers in fracture surface of syntactic foams with 10%
fiber weight ratio (Fig.7a) are less than that of
syntactic foams with 20% or 30% fiber weight ratio,
Fig.7(b-c), and there are not obviously fiber
clustering, whereas the fiber clustering is seen in
Fib.7(b) and (c). In addition, a lot of fiber filling in
resin matrix will lead to the decreasing of ductility for
the brittle fracture of fibers. The microbubbles and
fiber clustering are the defects of syntactic foams. The
defect in syntactic foams increases with the increase
of fiber content. The crack generated firstly from the
defect when the syntactic foams under load, and led to
fracture of specimen eventually. This is the reason why
the strength decreases when the containing of fibers
weight ratio beyond of 10%.
Conclusion
In this paper, the compressive and tensile
properties of hollow glass microsphere/epoxy resin
syntactic foams with short glass fiber content and
without fiber content are investigated. The fiber
weight ratio to the resin matrix is in the range of 5-
30%. The results show that the compressive and
tensile strength of syntactic foams are enhanced by
adding short fibers. The glass fibers hinder crack
propagation and their debonding, pull-out and fracture
are the significant reason for the tensile strength
enhancements. The modulus and specific modulus of
syntactic foams is also larger enhanced by adding
fibers. The syntactic foams with 10% glass fiber
weight ratio shows 70% and 49% higher compressive
strength and tensile strength respectively than that of
syntactic foams without fiber content, and its density
is only 3% higher than that of plain syntactic foams.
The fiber clustering and microbubbles are seen in the
fracture surfaces of syntactic foams with larger fiber
content. That is the reason of their strength less than
that of syntactic foams with 10% glass fiber weight
ratio. In addition, the ductility of syntactic foams
decreases with larger fiber filling in it, and the brittle
fractures are existed in compressive specimen.
Fig. 7 — Fracture surfaces of syntactic foams (a) 10% fiber
weight ratio, (b) 20% fiber weight ratio and (c) 30% fiber
weight ratio
YU et al.: EPOXY SYNTACTIC FOAMS REINFORCED BY SHORT GLASS FIBER
289
Acknowledgments The research work is supported by the Natural
Science Foundation of Hebei Province (No.
A2014203051).
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