Materials Sciences and Applications, 2015, 6, 953-962
Published Online November 2015 in SciRes.
How to cite this paper: Afrough, M., Pandya, T.S., Daryadel, S.S. and Mantena, P.R. (2015) Dynamic Response of Pultruded
Glass-Graphite/Epoxy Hybrid Composites Subjected to Transverse High Strain-Rate Compression Loading. Materials Sciences
and Applications, 6, 953-962.
Dynamic Response of Pultruded
Glass-Graphite/Epoxy Hybrid Composites
Subjected to Transverse High Strain-Rate
Compression Loading
Mohammad Afrough*, Tejas S. Pandya, Seyed Soheil Daryadel, Prabhakar Raju Mantena
Composite Structures and Nano-Engineering Research, Department of Mechanical Engineering, University of
Mississippi, Oxford, USA
Received 18 September 2015; accepted 14 November 2015; published 17 Nove mber 2015
Copyright © 2015 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
In a previous study, the energy absorption and dynamic response of different combinations of cy-
lindrical fiber-reinforced pultruded hybrid composite samples made of unidirectional glass and
graphite fiber/epoxy, were investigated under longitudinal compression loading. It was found that
placing glass fibers in the inner core of composites resulted in a higher ultimate compressive
strength and specific energy absorption. In this study, the dynamic responses of pultruded glass-
graphite/epoxy hybrid specimens with rectangular cross-section subjected to transverse com-
pression loading are reported. Crack initiation and propagation was monitored using a high-speed
video camera, and the effects of hybridization were analyzed. It was found that the location of
glass or graphite fibers inside the pultruded composites has no significant effect on the ultimate
compressive strength under such transverse compression loading. The energy absorption in all
the hybrid specimens was almost identical. Graphite/epoxy composite showed higher specific
energy absorption due to its lower density, and glass/epoxy composite had the lowest specific
energy absorption.
Pultruded Composites, High Strain-Rate Compression Loading, SHPB, Energy Absorption,
Transverse Loadings
Corresponding author.
M. Afrough et al.
1. Introduction
Composites are utilized because they have desired properties which cannot be attained by other types of consti-
tuent materials. Fibrous composites, including reinforcing fibers embedded in a matrix material, are commonly
used in different applications. Fiber-reinforced composite materials present different features in terms of stiff-
ness, specific strength, deformation etc. Their usage encompasses a wide range of applications in automotive,
aerospace and marine.
The two most common reinforcing fibers are glass and graphite. Glass fibers have high tensile strength and
lo w tens ile mod ulus. O n the othe r hand, graphit e fiber s possess very hi gh tensi le modulus, lo w wei ght and low
impact resistance. A number of investigations have been carried out on pultruded composites [1]-[13]. The ir re-
sults illustrate that hybridization with different percentages of glass and graphite within the same epoxy matrix
has a significant effect on the mechanical properties.
In many applications, composite materials are subjected to dynamic loading, which is produced by vibration
or wave propagation [14]. Therefore, it is necessary to investigate the strain rate sensitivity, failure modes and
other be havio rs und er dyna mic loa ding. Sever al s tudie s ha ve been performed on strain rate sensitivity [15]-[17].
Ochola et a l. [18] tested a glass fiber reinforced polymer at different strain rates. The results show that the value
of mean ultimate compressive stress for this composite increased by 20.9% as the strain rate is increased from
103 to 450 s1. Vari ous set -ups li ke d ro p -weight and Sp lit H o pki nson P re ssur e B a r (S HP B ) have b ee n u se d [19]
Furthermore, direction of loading and fiber orientation play an important role in dynamic behavior of compo-
site materials. Yokoyama et al. [21] investigated cubic specimens of carbon/epoxy laminates behavior in all
three principal material directions under high strain rate compression test and discussed the failure mechanism
of co mposit es. T hey sho w that b y increa sing strain rate, while the compression is alo ng the fiber d irection, ulti-
mate strength increases, and energy absorbed up to failure strain decreases. In a previous investigation by the
authors [22], the energy absorption and dynamic response of different combinations of glass and graphite pul-
truded composites under longitudinal compression loading was studied. It was found that locating glass fibers in
the inner core raises the ultimate compressive strength and results in better dynamic behavior of the composite.
Since there are many applications where loads are applied in transverse direction, optimization of pultruded
composite materials from thi s point of view is necessitated .
The purpose of this study is to analyze the dynamic behavior and energy absorption of pultruded glass-graph-
ite/epoxy hybrid composites by applying transverse compression load at high strain-rate. A modified SHPB test
system has been used for producing dynamic load. Additionally, a high-speed camera was deployed to record
the event s and monito r crack i nitiati on, propaga tion a nd fai lure of samples during the tests.
2. Specimen Details
Samples for SHPB test were cut from long rectangular cross-section composite beams manufactured by the pul-
trusion process (Pulstar 804 machine). Glass and graphite fibers were used for reinforcement, while volume
fraction of epoxy was maintained at 40%. The graphite fibers were AS4W-12K (Hercules), the glass fibers were
E-Glass (PPG 2001, #12), and the epoxy was EPON 862/W/537 (Shell Chemical Company, USA). All the
SHPB test samples were cut precisely in 6.6 mm × 6.6 mm section from 3.3 mm thick long rectangular beams
(Figure 1).
The six fiber combinations with layup sequence for hybrid composite test samples are shown in Table 1.
Reference [23] describes more details of the manufacturing process for these pultruded beam samples. The
measured bulk densities of the specimens are shown in Table 2.
3. Experimental Technique
The high strain-rate testing was carried out using a modified SHPB located at the Blast and Impact Dynamics
Laborator y, University o f Mississippi. A schematic of SHP B set-up is s ho wn in Figure 2. It co nsists of a str iker
bar, a strain gaged incide nt/input bar and a transmitter/output bar. The bars are made of maraging steel. Speci-
mens are sandwiched between the incident and transmission bars. A copper pulse shaper was applied to attain a
triangular i ncident pulse. Petroleum jelly was used to place specimen between the incident and transmitter bars,
and also to avoid friction and shear effects on the samples during testing.
M. Afrough et al.
Figure 1 . Dimensions of test samples cut from rectangular cross-secti on pultru de d be a m s .
Figure 2 . Schematic of a Split Hopkinson Pressure bar [20].
Table 1. Fiber combinations with layup sequence for hybrid composites [23].
Specimen ID Fiber Volume Fraction (%) Resin S ystem (%) Cross
(AS 4W-12)
(PPG 2001, #1 2)
C F MIX0 1 60 0 40
C F MIX0 2 0 60 40
C F MIX0 3
20 + 20
C F MIX0 4 30 15 + 15 40
C F MIX0 5 20 + 20 20 40
C F MIX0 6 15 + 15 30 40
Table 2. Measured average bulk densities of the pultruded hybrid specimens.
Density (kg/m3)
CF M IX0 1 C FMIX0 2 C F MIX03
1569 1924 1807
CF M IX0 4 C FMIX0 5 C F MIX06
1809 1749 1784
A hi gh-speed video camera (HyperVision HPV-2, Shimadzu Corporation, Japan) was employed to record the
event s and mo nitor the c rack ini tiatio n, propagation and failure of samples during the SHPB compression load-
ing tests. One hundred and two frames of each event were recorded at a frame rate of 500,000 fps. Two 1000W
strobes were used to provide the required lighting.
4. Results and Discussion
Six combinations of pultruded glass-graphite/epoxy hybrid samples were tested under transverse compression
M. Afrough et al.
loading. Figure 3 shows typical str ess wave pulse s includi ng incident, reflected and tr ansmitted pulses. For va-
lidating the dynamic equilibrium condition, a test specimen must be in force equilibrium [22]. The stresses in
both interfaces of each specimen were calculated to ensure that equilibrium was attained (Figure 4).
Five samples were tested for each combination at an average strain rate of 800 s1. Figure 5 shows typical
stress-strain behavior for all the combinations. All curves are plotted until shattering point of each specimen,
with hi gh-speed photography utilized for capturing this moment. They are essentially coincident with each other
until ultimate compressive strength with all of them possessing equal initial stiffness. This synchronization ap-
pears to be from matrix dominant response for these pultruded composites, with compression load applied in the
transverse direction.
Figure 6 illustrates the average ultimate compressive strength for six specimens of each combination.
CFMIX01 with 60% graphite possess the highest ultimate compressive strength and CFMIX02 with 60% glass
has the lowest. The compressive strength of hybrid with 40% graphite in the outer layer and 20% glass in the
Figure 3. Typical incident, reflected and transmitted pulses from SHPB test of a pultruded
hybrid composite sample.
Figure 4 . V alidation of stress equilibrium for tested pultruded hybrid composite sample.
M. Afrough et al.
Figure 5 . Typical stress-strain curve for all hybrids tested by SHPB transverse compression
Figure 6. Ultimate compressive strength (MPa) for all hybrids tested by SHPB transverse
compression loading.
core, CFMIX05, is slightly nearer to graphite/epoxy. The small difference between the highest and lowest ulti-
mate compressive strength demonstrates that using different portions of glass and graphite fibers does not dra-
matically change the compressive strength under transverse compression dynamic loading. This small difference
results from the stronger bond between graphite fiber and epoxy matrix compared to the bond between glass fi-
ber and epoxy matrix, perhaps due to the sizing of graphite fibers for improving adhesion during pultrusion
manufacturing. As can be seen in Figure 7, graphite fibers (Figure 7(a)), have mostly disintegrated after the
compression event while gla ss fibers (Figure 7(b)), remained almost intact.
As can be seen in Figure 5, all the specimens absorbed almost equivalent amounts of energy (integration of
area under stress-strain curve) during the dynamic compression tests, with specimens having greater volume
fraction of graphite fibers absorbing marginally more energy. CF06 samples showed a distinct drop towards the
M. Afrough et al.
(a) (b)
Figure 7 . Micrograph vi ew of speci mens after trans ver se compression event (500×). (a) Graphite fib e rs ; and (b) Glass fibe rs .
Figure 8 . Sp ecific energy absorption for all hybrid specimens tested by SHPB under transverse compression loading.
end of the compressive event, perhaps due to the larger portion of glass fibers located in the central region.
However, the specific energy absorption (energy absorbed per unit mass) for each specimen is distinct due to
their different densi ties (Figure 8). As shown i n Table 2, specimens with more graphite fibers have lower den-
sity, which resulted in higher specific energy absorption. Specimens with 60% graphite (CFMIX01) and 40%
graphite (CFMIX05) show the highest specific energy absorption; while 60% glass (CFMIX02) exhibits the
lowest specific energy absorption.
As men tioned, a high -speed video camera was deployed to monitor the crack initiation, prop agation and fail-
ure of samples during the SHPB compression loading tests. For these samples, the event (from time zero to
crack initiation of specimen) takes 55 μs -75 μs. Four stages of specimen CFMIX05 deformation are shown in
Figure 9. The stress versus time histor y of this specimen is dep icted in Figure 10. Three red dot s show starting
point, ultimate strength point and crack initiation point. The maximum load (ultimate strength) occurs at 51 μs
and crack initiates at 66 μs. The image at 79 μs shows that the fiber glass in the specimen core detaches from
epoxy matrix. The results indicate that the entire sample has not been completely damaged at the peak stress. It
should be noted that the compression load is applied transverse to the fiber direction, i.e., load is perpendicular
to the fiber orientation for each of the tested hybrid composites.
M. Afrough et al.
Figure 9 . Four stages of specimen CFMIX05 deformation under transverse compression at different times.
Figure 10. Stress versus time history of CFMIX05 under transverse compression loading. Three red dots show st.
A comparison between this study and previous investigation conducted by the authors on longitudinal com-
pre ssive load ing [22] shows dramatic difference in the dynamic response of graphite-glass/epoxy pultruded hy-
brids under transverse and longitudinal loading conditions. Figure 11 and Figure 12 show the ultimate com-
pressive strength and average specific energy of cylindrical hybrid combinations where GL60, GR60, GL30 and
0 μs
66 μs
51 μs
79 μs
M. Afrough et al.
Figure 11. Ultimate compressive strength for different combinations of pultruded hybrids
under longitudinal compression loading [22].
Figure 12. Ave r age specific energy absorption for different combinations of pultruded hy-
brids un de r lo ng it u di na l c ompre s sion l oa ding [22].
GR30 have respectively similar hybrid combinations of CF02, CF01, CF06 and CF04 used in this study. It is
apparent that under transverse compression dynamic loading, ultimate compressive strength of graphite-glass/
epoxy pultruded hybrids is about one-third o f tha t und er lo ngi tud inal c ompr e ssion d yna mi c lo ading. On t he ot h-
er hand, specific energy absorption under longitudinal compression loading shows an opposite trend to the re-
sponse under transverse compression loading.
5. Conclusion
In this i nvestigation, six combinations of pultruded glas s-graphite/epoxy hybrids have been experimentally stu-
died und er t rans ver se hi gh s tr ain -rate compression loading, using a modified SHPB. It was observed that failure
of specimens loaded along transverse direction was do minated by matrix failure. It was also observed that 60%
grap hite (CF MIX01 ) has t he highe st ultimate co mpressive strength. The ultimate compressive strength was also
marginally greater with higher percentage of graphite. This marginal difference results from the stronger bond
between graphite fiber and epoxy compared to that between glass fiber and epoxy, perhaps due to the sizing of
graphite fibers for better adhesion. Location of glass or graphite fibers inside the pultruded composites had no
significant effect on the ultimate compressive strength under transverse compression dynamic loading. While all
M. Afrough et al.
specimens absorbed almost equivalent amount of energy, the graphite/epoxy samples demonstrated significantly
higher specific energy absorption compared to the other combinations. This was due to its lo wer density, while
glass/epox y sho wed the lowest specific energy absorption. Moreover, comparison between this study and a pre-
vious investigation conducted by the authors showed significant differences in the response under longitudinal
and trans verse dynamic l oad i ng.
The authors wish to acknowledge funding received from US Army Research Office-DURIP Grant # W911NF-
13-1-0248 for the high-speed digital cameras used in this research. The authors would also like to thank Mr. P.
Matthew Lowe, for his help w ith sample prep a ration.
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