tensile and compressive properties of epoxy syntactic...

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


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


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


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


289

Acknowledgments The research work is supported by the Natural

Science Foundation of Hebei Province (No.

A2014203051).

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