Herein, we report the use of Multi Wall Carbon Nano Tubes (MWCNTs) as nano-compatibilizers based on their astonishing mechanical properties and ease of processing. To fabricate laminate samples, pure MWCNTs were homogeneously dispersed in the fiber-reinforced plastic (FRP) composite with 0, 0.5, 1 and 1.5 wt.
Trang 1Fracture analysis and mechanical properties of three phased glass/
epoxy laminates reinforced with multiwalled carbon nanotubes
Rohit Pratyush Beheraa,*, Prashant Rawata,b, K.K Singha, Sung Kyu Hac, Anand Gaurava,
Santosh K Tiwaric
a Department of Mechanical Engineering, Indian Institute of Technology (ISM), Dhanbad, India
b College of Civil Engineering, Hunan University, Changsha, China
c Department of Mechanical Engineering, Hanyang University, Seoul, South Korea
a r t i c l e i n f o
Article history:
Received 4 December 2018
Received in revised form
7 March 2019
Accepted 13 March 2019
Available online 28 March 2019
Keywords:
Mechanical properties
MWCNTs
Fiber-reinforced plastic
Tensile strength
Compressive strength
ILSS properties
Failure modes
a b s t r a c t
Herein, we report the use of Multi Wall Carbon Nano Tubes (MWCNTs) as nano-compatibilizers based on their astonishing mechanical properties and ease of processing To fabricate laminate samples, pure MWCNTs were homogeneously dispersed in thefiber-reinforced plastic (FRP) composite with 0, 0.5, 1 and 1.5 wt % loading The laminates were prepared with eight plies (4.0± 0.1 mm thickness) using the hand layup technique assisted by the compression moulding method It was found that the tensile, compressive and inter-laminar shear strength (ILSS) increase by 103.81%, 139.78% and 36.06%, respec-tively corresponding to 1 wt % loading of MWCNTs as compared to neat GFRP specimen However, a rapid decrease in strength beyond 1 wt % loading of MWCNTs has been noted Interestingly, the maximum of the tensile strength was higher than that of the compressive strength, and the maximum of the tensile modulus was larger than that of the compressive modulus in the case of 1 wt % loading of MWCNTs It was observed that after a certain loading, the mechanical properties of such laminates can only reach the best value with an optimum loading of MWCNTs In addition, the micromechanical failure modes and effect of MWCNTs loading on internal morphologies of the composites were also intensively explored with the help of Field Emission Scanning Electron Microscopic (FESEM) analysis
© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
In the past few decades, nanocomposites have become
inter-esting for different applications, particularly to improve the
me-chanical properties of polymeric materials using nanoparticles (as
nanofillers) These nanofillers may be graphene [1], chopped
carbon fibers [2], nanoclay, CNTs and their derivatives [3]
Depending on the nature, these nanomaterials have capabilities
to influence the mechanical, thermal and electrical properties of
the produced nanocomposites Several recent investigations have
proved the applicability of CNTs based composites in manyfields,
especially for lightweight and high-performance structures in
aerospace industry and as coating materials in the maritime and
chemical industries [4] The astonishing properties of CNTs in
mechanical (axial Young modulus of 1e5 TPa [5], stiffness,
strength [6], flexibility[7], fracture toughness [8]) and physical (high thermal conductivity, electrical conductivity [3,9], semi-conducting behavior[10]) aspects, have been continuously stud-ied and published The large surface area of nanotubes can act as
an interface and bridging agent for uniform load transfer, but an extreme agglomeration of CNTs is caused due to strong attractive forces between CNTs Such aggregation always demises the properties of nanocomposites mainly in the case of polymer composites[11] The specific surface area of the CNTs is depen-dent on the number of sidewalls and the diameter of the tubes themselves, therefore single-wall CNTs (SWCNTs) have the largest surface area as compared to the double-wall CNTs (DWCNTs) and multiwall CNTs (MWCNTs) Their dispersibility, however, is quite low and the load transfer is difficult as already discussed else-where[12]
In this line, we have used the MWCNTs because of their ease
of bulk production, low cost per unit and high thermal, chemical stability Such CNTs based nano-composites have been under intensive study using different matrix materials, including metals
* Corresponding author.
E-mail address: rohit.pratyush@gmail.com (R.P Behera).
Peer review under responsibility of Vietnam National University, Hanoi.
https://doi.org/10.1016/j.jsamd.2019.03.003
2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Trang 2[13,14], ceramics [15,16], polymers [17] and so on Moreover,
CNTs are being used as a secondary reinforcement in FRP
com-posites to enhance their properties beyond the natural limit,
even though there are certain challenges for developing an
efficient and considerably tough three-phase glass/epoxy/
MWCNT nanocomposites These challenges are (i) a standardized
and homogeneous distribution of CNTs in the two-phase glass/
epoxy composites, (ii) the proper interfacial bonding between
thefiller and the matrices which affect the uniform load transfer
from the matrix to the reinforcement, and (iii) the tedious
functionalization of CNTs To minimize these three-shortcoming,
numerous efforts have been made and each procedure has its
own advantages and disadvantages [15,16] Furthermore, three
foremost mechanisms have been taken to explain the load
transfer from the matrix to thefiller and vice-versa, including: (i)
mechanical interlocking, (ii) covalent and non-covalent bonding
between the matrix and the CNTs, (iii) Van der Waals interaction
between CNTs/glass and epoxy composites In the case of
MWCNTs, thefirst one is very problematic owing to the smooth
surface of CNTs and the second one cannot be guaranteed due to
the low friction and the only reason left is addressed for the load
transfer[18] To explore these issues, Liu et al.[19] studied the
tensile modulus and the yield strength by dispersing 2 wt % of
MWCNTs in a nylon-6 matrix and found an increase in values
around 214% and 162%, respectively They proved that all
three-mechanisms mentioned above are applicable for MWCNTs to
explain the outstanding mechanical properties of
MWCNT-Nylol-6 composites Grimmer et al.[20] examined and found that the
incorporation of small volume fractions of MWCNTs to the
glass-fiber composites greatly diminishes the cyclic delamination crack
propagation rates Schadler et al.[18]studied the load transfer in
MWCNT-epoxy composites and found that there is a large scatter
in the compression modulus, they also reported exceptional
trend in tensile modulus and the maximum value of compression
modulus is larger than that of the tensile modulus Allaoui et
al.[21] considered the mechanical and electrical properties of
MWCNT/epoxy composites and found that the Young's modulus
is doubled, and the yield strength is quadrupled at 1 and 4 wt %
loading of MWCNTs, respectively
In this work, we tried to modify the brittle nature of the
bi-directionally woven glassfiber composite by adding MWCNTs as
a secondary reinforcement and as a nano-compatibilizers The
tensile, compressive and inter-laminar behaviors are investigated;
the modulus of tension and compression forces are compared; the
fracture analysis is explored; and the morphological properties are
discussed considering different loadings of MWCNTs Nevertheless,
this paper proposed an optimum loading percentage to exploit the
mechanical properties of the GFRP composites using MWCNTs,
which is the novelty of the paper
2 Synthesis and characterization
The arc-discharge method for the bulk production of high
quality MWCNTs developed by Iijima et al.[22]is well known and
herein, we have adopted the same procedure for the MWCNTs
synthesis and their surface modification as reported by Singh et
al.[23] To confirm the functionalization of the as-prepared
MWCNTs, Raman and FTIR spectroscopic techniques were used and
important features are noted below
Raman spectroscopy is one of the best methods to analyze
carbon nanomaterials Raman spectra of the as-prepared
modi-fied MWCNTs was recorded at an excitation wavelength of
532 nm[24,25] The Raman spectrum of the modified MWCNTs is
presented in theFig 1(a), showing the characteristics of -COOH functionalized carbon nanotubes[24] Modified MWCNTs show intense D and G bands at around 1348 and 1580 cm1, respec-tively, which disclose the prominent structural disorder caused
by the incorporation of oxygen functional moieties on the carbon skeleton of the nanotubes[24,25] The presence of a broad 2D (2690 cm1) peak in the spectrum is also a signature of the edge disorder in the nanotubes due to the incorporation of theeCOOH functional groups [24,25] To evaluate the nature of the func-tional groups on MWCNTs, FTIR analysis was carried out and the spectrum is presented in Fig 1(b) [26,27] The as-prepared, modified MWCNTs show a clear absorption for eOH, C-O, and -C¼O [26,27] This confirms the successful incorporation of the -COOH groups on the surface of the carbon nanotubes which is in good consistency with the previous investigations[26,27]
3 Fabrication of MWCNTs based nanocomposites Eight layered quasi-isotropic symmetrical GFRP laminates were prepared with different loadings of the pure samples, i.e 0.5, 1 and 1.5 wt % of MWCNTs Mixing of the surface modified MWCNTs in epoxy resins (Bisphenol-A) was completed using a probe-ultrasonicator (Fig 2(a)) This process is one of the most effective paths for the homogeneous dispersion of MWCNTs in polymer matrices as discussed elsewhere [11] The ultrasonication may cause heat generation in the solution and consequently aggregation
of the MWCNTs Therefore to avoid this phenomenon, the beaker was fully covered in an ice blanket [11] Once mixing of the MWCNTs and epoxy was completed, the hardener (K-6) was mixed
in 10:1 (epoxy: hardener) ratio followed by 15 minutes continuous ultrasonication Herein, the glassfiber used was bi-directionally [(0/90) and (þ45/-45)] woven (600GSM) as provided by M.S In-dustries, Kolkata, India The stacking sequence of the proposed design for the laminates is shown inFig 2(b), i.e (00/þ900), (þ45/
45), (þ45/45), (0/90)//(0/þ90), (þ45/45), (þ45/
45), (0/90) To engineer the three-phased composite laminates, thefirst layer was placed over a flat glass surface and the resin solution was applied manually using a soft brush The same pro-cedure was also applied for all other layers as per proposed sym-metrical design To remove extra resins from the edges of the wet laminates, an iron roller was rolled after placing one layer over the other manually Authors have adopted the hand layup technique for the preparation of wet laminates and curing of the laminates was carried out with the help of a press-molding machine under 40 KN pressure at room temperature for 24 hours (Fig 2(c))
4 Experimental testing method 4.1 Tensile test
The prepared laminates with different loadings of MWCNTs (total 20 samples, 5 for each wt %) were cut out in the dimensions
of 125 mm 15 mm 4 mm using a diamond cutter as per ASTM D3039 requirement The aluminum tabs of dimensions
25 mm 15 mm 2 mm were properly cleaned with propyl alcohol and pasted over the sample laminates as shown inFig 3(a)
to avoid alteration during the testing Each specimen was then gripped in the preparedfixture of the Universal Testing Machine (UTM) which prevented any lateral movements and forces were measured using S-type load cells as shown inFig 3(c) Five such tests were conducted for each loading amount of MWCNTs at a strain rate of 0.5 mm/min
R.P Behera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 299e309 300
Trang 34.2 Compression test
To investigate the compressive strength, seven different
specimens (each wt % of dimension 100 mm 10 mm 4 mm)
were prepared as per ASTM D3410 requirement and a diamond
cutter was used for the sample preparation The properly cleaned
aluminum tabs of dimensions of 45 mm 10 mm 2 mm were
pasted over the specimen (with gauge length of 10 mm) as
shown in Fig 3(b) to avoid alteration This test was performed under the displacement control with a strain rate of 0.5 mm/min
at room temperature The load was measured using the S-type load cell attached to the specimen as discussed in the previous section It is notable that the specimens used were short enough
to prevent any buckling and clasping during the compression measurement Further, any effects caused due to the stress con-centration at the grips were considered insignificantly because
Fig 1 (a) Raman spectra of surface modified MWCNTs and (b) FTIR of the as-synthesized and modified MWCNTs revealing the presence of the -COOH and -OH functional groups.
Fig 2 (a) Set up of Probe Ultrasonication process; (b) Schematic representation of stacking sequence; (c) Set up of press molding machine.
Fig 3 (a) Specimen sample for the tensile test (ASTM D3039); (b) Specimen sample for compression test (ASTM D3410); and (c) Test fixture in UTM machine.
Trang 4the repeated failures were witnessed at the gage section of all the
loaded GFRPs
4.3 Inter-laminar shear strength (ILSS)
The inter-laminar shear strength tests were conducted based on
the short beam shear strength test (SBS) For this test, seven
specimens of dimensions 24 mm 8 mm 4 mm were prepared as
per obligation of ASTM D2344, shown schematicaly inFig 4(a) The
gage length of the specimen was 20 mm The tests were performed
at a strain rate of 0.5 mm/min as shown inFig 4(b) For the ILSS
measurement, the gauge length of the specimens were kept very
small to minimize the effect of bending during the failure of
lam-inates under tension and compression Thus, the main failure
mechanism was dominated by the pure shear phenomenon
The standard equation used for the calculation of inter-laminar
shear strength is noted below:
where,
F*¼ ILSS or Short-beam strength (ILSS) (MPa)
P¼ Max load observed during test (N)
b¼ Specimen width (mm)
h¼ Specimen thickness (mm)
The cut specimens’ samples are shown inFig 5(a)e(c) for the
tensile, compressive and ILSS test, respectively
The UTM (as shown inFigs 3(c) and 4(b)) used for this work was
fully computerized and the machine can be operated at loading
rates varying from 0.01 to 10 mm/min It had maximum load
car-rying capacity of 50 KN Henceforth, using the UTM, the maximum
values of load and stress for different samples under tension,
compression and ILSS were noted and the maximum load carrying
capacity was compared for the same wt.% samples in tension and
compression for the systematic analysis The maximum tensile and
compressive modulus for different wt % were also compared with
neat samples
5 Results and discussion
5.1 Tensile properties
Most of the important mechanical properties of materials,
such as yield strength, elasticity, ultimate tensile strength and
ductility can be obtained by the tensile analysis [21] For the
present investigation,five samples were tested and the obtained
values are presented in Table 1 for better understanding The
graph shown in Fig 6a represents the average values of the
tensile stress and it can be observed that the maximum stress values for the GFRP laminate composites are 122.70 MPa, 144.02 MPa, 250.08 MPa and 163.78 MPa for neat, 0.5, 1 and 1.5 wt % of MWCNTs loading, respectively The Young's modulus
of elasticity (ET) was determined using the stressestrain data based on the tensile tests whose values were 2956.62, 3453.71, 6315.15 and 3937.02 MPa for the neat, 0.5, 1 and 1.5 wt % of MWCNTs/glass/epoxy sample laminates, respectively and all are presented in Table 1 for the sake of simplicity The maximum value for the modulus (ETMax.) is 6315.15 MPa at 1 wt % of MWCNTs loading and the average Young's modulus (ETavg.) for the tensile specimen laminates is found to be 4165.625 MPa It can be observed that there is a continuous increase in the stress value as compared to neat GFRP specimen up to 1 wt % MWCNTs incor-porated laminates as shown inFig 6 This enhancement can be accounted to the delay in the crack generation and propagation owing to the nucleation and bridging effect of MWCNTs[18] The role of MWCNTs to reinforce the mechanical properties is sche-matically presented in Fig 7 Thus, the optimum loading of MWCNTs attributes a strong cross-linkage and enshrouding be-tween the interfaces of resin which ultimately causes a delay in the crack propagation [11] Moreover, with the increase in the loading percentage of MWCNTs, the tensile strength of the FRP laminate increased as fracture behavior shifted from brittle to ductile like hackle for the homogenously dispersed MWCNTs[11] Thus, the strong nanoparticle covalent bonding between the matrixereinforcement interfaces is also responsible for the improved strength of the MWCNTsfilled laminate specimens[18] Further, from the tensile analysis, it can be observed that with the increase of MWCNTs loading, there is a fall in stress value by 34.5% from 1 wt % to 1.5 wt % Such a drastic reduction in the value of stress may be attributed to the aggregation of MWCNTs (as shown inFig 9d), which takes place when MWCNTs loading increases beyond 1 wt %[11] In the present situation, the ag-gregation of MWCNTs is mainly due to the selective distribution and the high degree of entanglement of MWCNTs with matrices
[11,18] Therefore, the optimum stress value is 250.08 MPa at 1 wt
% MWCNTs incorporated GFRP and the average Young's modulus (ETAvg.) for the tensile test specimen is 4165.625 MPa
5.1.1 Failure mechanism and fracture analysis The failure mechanism is one of the most important aspects for the composites and polymeric materials[18] There are so many reasons for the failure in the case of FRPs and similar materials In this particular work, it is assumed that the failure originates from a region where there is the maximum stress concentration or an inherent defect is generated in the composite specimens during the testing as explained by Y Iwahori et al.[28] However, the homo-geneous mixing of nanofillers in case of the three-phase compos-ites is a serious concern and cannot be explained through the logic developed by Y Iwahori et al.[28], because the mixing of nanofillers
fixture setup for SBS test.
R.P Behera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 299e309 302
Trang 5into three-phase composites leads to the formation of clusters,
cracks and voids owing to the non-uniform stress concentration (as
shown inFig 9 (b)) which can also be the reason for the crack
generation and propagation that ultimately leads to failure in the
composites [28] The mechanism explained in Fig 8 shows the
fracture and cracking of FRP composites during the tensile test
which is an indication of adopted mechanism for the failure[11]
From the FESEM (Fig 8(d)) micrographs, it is clear that cracking at
the interfaces of matrices occurrs at the point of maximum stress,
and then the crack propagates through thefibers and apprears on
the external surface of the specimen [11] In other words, the
debonding between the matrix and the nanofiber/glass is
respon-sible for the fracture which leads to the pull-out phenomenon as
shown inFig 8(a),(b) It is notable that the stress near the tip of the
crack causes the matrix-fiber delamination before the actual bond
breakdown andfinally the crack reaches to the interface between
the two laminates of the composite.Fig 7(a) and the FESEM image
inFig 9(a) show the mechanism of delay in the crack propagation caused by the strong cross-linkage of MWCNTs between the two laminate composites[18]
5.2 Compression properties The compressive strength is often used to state applicability of composite materials as it denotes the ability of a material to withstand load tending to reduce size To further examine the mechanical properties of the fabricated composites, the compres-sive behavior was investigated as per ASTM D3410 and the stress vs strain (Fig 10(a)) curve of the same along with statistical data is presented inTable 2 The compressive results of the studied sam-ples are in good agreement with the tensile properties as mentioned in previous section The maximum compressive stress
Fig 5 Specimen test samples for (a) Tensile (b) Compressive and (c) SBS tests.
Table 1
Statistics of the tensile tests performed for the hybrid MWCNT/glass/epoxy laminate samples.
Stress in MPa
Specimen Number
MWCNT (wt.%).
strength (MPa)
Standard deviation (SD)
Standard deviation (%)
Average Young's modulus (E T ) (MPa)
0 120.57 118.96 125.45 124.23 124.29 122.70 2.782 2.27 2956.62
0.5 145.67 145.98 144.65 142.66 141.14 144.02 2.057 1.43 3453.71
1 247.63 245.97 251.67 253.53 251.16 250.08 3.102 1.24 6315.15
1.5 165.67 161.23 160.78 164.45 166.77 163.78 2.67 1.63 3937.02
Fig 6 (a) Stress vs Strain graph for the tensile test of GFRP composite sample laminate and (b) Statistics of the tests performed according to ASTM D3039 with reference to Table 1
Trang 6(sC) values for the specimens are found to be 10.38, 13.44, 24.89 and
21.32 MPa for the neat, 0.5, 1 and 1.5 wt % of MWCNTs loading in
GFRP composites, respectively Similarly, the compressive modulus
(EC) of the studied specimens were obtained (1069.56, 1383.71,
4194.77 and 2195.31 MPa for the neat, 0.5, 1 and 1.5 wt % of
MWCNTs/epoxy, respectively) from the corresponding graphs and
the data is presented inTable 2for detailed information From the stress vs strain graphs (Fig 10(a)), MWCNTs loading up to 1 wt % is accounted to the uniform load transfer from the matrix to the reinforcement which is further strengthened owing to the intrinsic properties of MWCNTs and their homogenous distribution at the interfaces of the composite Due to the aggregation of MWCNTs at
Fig 7 Schematic diagram of (a) MWCNTs distributed between glass fiber layers which cause an increase in the tensile strength and (b) unmodified GFRP.
Fig 8 (a) mechanism of tensile failure of neat GFRP specimen; (b) nanofiber pull-out and delay in crack propagation caused due to crosslinking of MWCNTs (c) image of the failure mechanism of specimen under tensile load; and (d) FESEM image of debonding followed by fiber pull-out.
R.P Behera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 299e309 304
Trang 7higher loading, the cross-linkage (seeFig 9(a)) at the interfaces get
decreased and therefore abrupt drops in strength can be seen above
1 wt % of loading of nanofillers (Fig 9(d)) In summary, the
maximum value of the compressive modulus (ECmax.) is found as
4194.77 MPa for the 1 wt % of MWCNTs loading and the average
value of the compressive modulus (ECAvg.) for the specimen
lami-nates is found as 2210.837 MPa
5.2.1 Failure mechanism and fracture analysis
Composites and nanocomposites possess different phases with
different elastic properties Hence, there is a high possibility for the
formation of microcracks across various regions of interfaces in the
fabricated composites under the applied load [18] A typical
fracture offibers is shown inFig 11(a),(b) for the specimen under compressive loading The compressive strength of the laminates is controlled by various mechanisms[29] First, it is seen that most of the laminate in the test undergo micro-buckling Buckling is the only area in thefield of structural mechanics where failure is not related to the material strength The collapsing of the material due
to buckling does not deal with the yield of the material [30] Generally, there are two types of micro-buckling modes of failure according to Rosen's model[31](i): out of phase buckling offibers
or extension mode, (ii) in-phase buckling offibers or shear mode as shown inFig 11(e),(f) Their analysis was based on the assumption
of the 2D behavior of composites and on an idealistic approach, so the predicted results in their test were significantly higher than
Fig 9 FESEM images of (a) MWCNTs interlocking; (b) cured sample laminate at 1 wt.% MWCNTs loading containing voids; (c) MWCNTs dispersion; and (d) agglomeration at 1.5 wt.% MWCNTs in GFRP.
Fig 10 (a) The Stress vs Strain graphs for the compressive test of GFRP composite laminate samples (b) Statistics of the tests performed according to ASTM D3410 with reference to
Table 2
Table 2
The statistics of compressive tests performed for hybrid MWCNT/glass/epoxy laminate samples.
Stress in MPa
Specimen Number
MWCNT (wt.%).
1 2 3 4 5 6 7 Average Compressive
strength (MPa)
Standard deviation (SD)
Standard deviation (%)
Average compression modulus (E C ) (MPa)
0 10.56 10.65 9.87 11.23 10.02 10.88 9.43 10.38 0.626 6.03 1069.56
0.5 14.56 14.89 13.65 12.63 13.23 12.02 13.10 13.44 1.018 7.57 1383.71
1 24.23 25.17 25.36 23.67 25.87 25.61 24.32 24.89 0.820 3.29 4194.77
1.5 22.56 22.21 20.97 21.25 20.56 20.23 21.46 21.32 0.842 3.95 2195.31
Trang 8experimentally observed ones In their model, the authors
pre-dicted the value of the shear mode failure to be lower than that of
the extension mode of failures which was also in agreement with
the experimentally observed results Further, the failure modes are
highly dependent on thefiber volume fraction (Vf) For Vf< 30%, the
extension mode is dominant as with application of the sudden load,
the adjacentfibers tend to deform sinusoidally with the
deforma-tion patterns being 180out of phase, whereas for Vf> 30%, the
fibers adjacent to each other deform transversely in phase
indi-cating the shear mode failure [30] In our experiment, since the
prepared composite is high in fiber content with fiber volume
fraction (Vf~ 55%), therefore, in the coherence to the Rosen's model,
it is feasible to assume the micro-buckling to be in-phase as shown
inFig 11 Also, researchers like Tadjbaksh and Wang in their model
took into consideration the inter-ply micro-buckling of the
cross-ply laminates [32] They modelled the laminate as a single ply
inhomogeneous continuum In this analysis, they observed an
in-crease in resistance of buckling due to adjacent plies Similarly, in
the present study, most of the samples were resisting buckling
owing to the use of cross-ply laminates[33]
Secondly, there is kinking, which is also perceived to be the most
common failure mechanism Kinking in FRPs can be said to be the
consequence of the combination of plastic micro-buckling and the
low strain rate subjected to the fibers A typical kinking failure
mechanism is shown in theFig 11c These initiative mechanisms
like kinking and micro-buckling at micro-structural level lead to
the global instability of the composite materials[30] Lee and Waas
[33] in their investigation on the compressive strength of
uni-directional GFRP laminates found that the kink-bands were
formed at different fiber-volume fractions of the tested GFRP
samples Also, for higherfiber-volume fractions, it remained one of
the major modes of failure Similarly, in this study, it was observed
that the kink bands were formed But, as thefiber volume fraction
of the prepared GFRP was high (Vf~ 55%), no significant stress drop
was observed during testing of the loaded GFRP laminates Thus, it
can be concluded that the chances of formation of kink-bands
de-creases at highfiber volume fractions of loaded bi-directionally
woven GFRP laminates A typical kink band formation is shown in
Fig 11(c) of the loaded GFRP samples
Thirdly, we address the splitting or delamination as another
failure mechanism for the rupture of the composites[33] This kind
of failure modes is supposed to occur when a pre-existingflaw inside the specimen starts growing under compressive loading conditions In multi-directional composites, where thefibers are continuous, cracks with an opening under compressive loading are easily liable to delamination in a macroscopic level andfiber-matrix debonding in a microscopic level The micro-cracks that grow un-der compressive loading tend to form transverse cracks These micro-cracks with a gradual increase in the compressive load grow and their domain of circumferential size increases, and they form interfacial de-bonds[33] Debonding is the transition mechanism followed by the micro-cracking which eventually leads to trans-verse cracking[34] Although, it is assumed that transverse crack originates from the defects or voids present in the matrices owing
to improper layering and selective distribution of MWCNTs (see
Fig 9(b)) However, the consequence of the same may enhance the cracking and therefore thefiber-matrix interfacial weakening in the case of an uniaxial transverse loading[35] Such crack initiation and propagation cannot afford applied loads and result in fracture via the kink band formation A typical debonding or shear failure mechanism is shown inFig 12(a),(b) followed as by a transverse cracking in the compression test samples
As described in previous sections, there are three major mech-anisms of failure for the compressive laminate samples, but it is observed that no particular failure is alone responsible for the breakdown of the test samples The failure is governed majorly by a combination of kink-band formation and splitting But there can be other failure modes like splitting or shear failure alone However, this is the rare case, i.e., one among seven specimen samples So, for highfiber volume fraction bi-directional GFRP samples we desig-nate the failure mode by kink-band formation followed by splitting
or shear failure as shown inFig 11(d)
5.3 Inter-laminar shear strength (ILSS) properties The tests were performed for the short beam shear (SBS) strength and the observed strength values are summarized in
Table 3 Also, the statistics of the obtained force values are pre-sented in Fig 13(b) The force vs deflection graph for the test samples (seeFig 13(a)) represents the average values of force for the samples put to test in the Hounsfield UTM
Fig 11 (a) FESEM image of fractured fibers and (b) cracking due to compressive loading; (c) FESEM image of the kink band formation in the test sample specimen; (d) FESEM image failure mechanism of compressive test specimen (Kink-band formation and Splitting); (e) Extension mode or out of phase buckling; (f) Shear mode or in-phase buckling.
R.P Behera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 299e309 306
Trang 9When the shear load in the transverse direction experienced by
a composite laminate exceeds the inter-laminar shear strength
(ILSS), a failure will occur between thefiber layers (which are the
reinforcing materials in FRPs) and its was known as delamination
failure[36] In order to measure the ILSS, a pattern of the pure shear
stress should be generated which will make the composite undergo
an interlaminar shear failure This failure will occur between the
glass woven plies, which act as the reinforcing material In the case
of a load applied perpendicular to thefiber layers, an additional
failure is possible and, therefore, an accurate ILSS estimation is not
possible In this testing, we used the SBS test, where the failure was
caused by both the shear failure and the failure due to bending
(caused by both tension and compression) The failure is not due to
shear alone, so there can be anomaly in the results obtained[36] To
avoid the bending effects of failure, we reduced the gage length, so
that the moment is reduced which ultimately diminishes the fail-ure due to the bending effects[36] In this case, the gage length as per ASTM D2344 is 20 mm, so it can be assumed that the pure shear failure will dominate the bending failure at a greater scale, there-fore, we can neglect the effects of bending
From the graph of Fig 13(a) and from the calculations using equation (1), the Inter-laminar Shear Strength values are 10.909 MPa, 12.388 MPa, 14.843 MPa and 11.663 MPa for MWCNT loading values of 0, 0.5, 1 and 1.5 wt %, respectively, which are noted down inTable 3 The reason for the increase in ILSS of the fiber reinforced composites up to 1 wt % can be addressed to the MWCNTs/epoxy suspension between thefiber layers, which create
a strong cross-linking bond between the layers as shown in
Figs 9(a) and 14(b) Although, the CNTs in this case are not pref-erentially oriented, they are randomly dispersed, it is seen that
Fig 12 (a) debonding mechanism leading to transverse crack and (b) hinderance to the transverse crack caused by loading MWCNTs.
Table 3
The statistics of the SBS test performed for hybrid MWCNT/glass/epoxy laminate samples.
Force in N
Specimen Number
MWCNT (wt.%)
Force (N)
Standard deviation (SD)
Standard deviation (%)
Inter-laminar shear strength (ILSS) (MPa)
0 469.23 468.12 472.27 460.57 457.32 460.73 470.12 465.48 5.80 1.25 10.909
0.5 532.48 536.65 540.45 521.27 526.34 525.25 517.55 528.57 16.42 3.11 12.388
1 637.55 634.23 632.12 621.98 620.45 628.34 658.64 633.33 12.78 2.02 14.843
1.5 503.12 501.67 491.73 487.33 495.51 493.34 510.64 497.62 7.95 1.60 11.663
Fig 13 (a) Force vs displacement graph of GFRP laminate samples obtained from SBS test; and (b) statistics of the tests performed according to ASTM D2344 with reference to
Table 3
Trang 10some of the MWCNTs tend to orient along the thickness direction
which transfers the load from the matrix to the MWCNTs Fan et
al.[36]used oxidized multi-wall nanotubes (OMWCNTs) with the
vacuum assisted resin transfer molding method (VARTM) and the
newly developed injection and double vacuum assisted resin
transfer molding method (IDVARTM) to preferentially orient the
MWCNTs along the thickness direction, which ultimately leads to
increased ILSS due to the effective and uniform load transfer from
the matrix to the CNTs
The decrease in ILSS at 1.5 wt % of MWCNTs loading from 14.843
to 11.663 MPa indicates that the CNTs after 1 wt % loading tend to
re-agglomerate (Fig 9(d)) Thus, it can be assumed that the 1 wt.%
loading is an optimum value of reinforcement, and beyond the
1 wt.% loading, owing to the saturation effect and the attractive Van
der Waal forces among the nanoparticles, the agglomeration effect
tends to dominate and results in major aggregation, indicating that
the ILSS is decreased by 21.42% from 1 to 1.5 wt % of MWCNTs
loading
5.3.1 Failure mechanisms and fracture analysis
The failure mechanism of the ILSS test samples for the SBS test is
relatively easy Since, the load is applied on an overhanging test
sample with a very short gage length, the crack propagation takes
somewhat close to the axial direction[37] Since bending also exists
in the failure mechanism, the crack propagation is not along the
pure axial direction Therefore, there are two mechanisms which
can be accounted for the failure: (i) the shear failure and (ii) the
bending failure The shear failure is a result of the delamination
caused due to the matrix cracking followed by breaking of the bond
between thefiber and the matrix which eventually leads to the
delamination Since the matrix is considered to be the weakest
material in the composite structure, so it is assumed that the crack
initiation takes place from the defects or voids present in the matrix
(Fig 9b), and it is expected to be eventually present after the curing
process[35] This becomes the point of the maximum stress
con-centration, so a load up to its maximum bearing capability along
the axial direction is considered the maximum ILSS Further
application of the load after this point leads to the crack propaga-tion from the defect which causes matrix breaking and the load is transferred from the matrix to the secondary reinforcement, i.e the MWCNTs as shown inFig 14b The matrix breaking and the crack propagation along the near axial direction leads to the delamina-tion of thefiber and the matrix, which upon further spreading conquers a lot of circumferential area under its domain ultimately leading to the shear failure[37] In this testing process, the most dominant failure mode is the inter-laminar shear failure A typical failure mechanism of the SBSS test is shown inFig 14(c)
6 Conclusion
In summary, surface modified MWCNTs were synthesized and unvaryingly dispersed in the glass fiber-reinforced plastic com-posites with 0, 0.5, 1 and 1.5 wt % loading The mechanical prop-erties of specimens were inspected mainly in the context of tensile, compressive and ILSS test In the present study, it has been estab-lished that an optimum tensile stress value of 250.08 MPa was at
1 wt % of MWCNTs loading along with Young's modulus (ETmax) of 6315.15 MPa and the composites failed owing to the debonding followed by thefiber pullout The optimum compressive stress for the composite in the case of 1 wt % of MWCNTs loading was found
of ~24.89 MPa corresponding to 4194.77 MPa compressive modulus Interestingly, the maximum strength value for the ten-sion was found greater than that for the compresten-sion which implies the role of the nanotubes as nano-compatibilizers and as rein-forcement on the properties of the fabricated composites For the studied samples, the tensile modulus was greater than the compressive one indicating that there was less elastic deformation
of the material under tension as compared to compression More-over, owing to the proper dispersion of MWCNTs in the fabricated composites, the increment in the tensile stress and compressive stress were 103.81% and 139.78%, respectively Also, for the 1 wt.%
of MWCNTs loading, the optimum ILSS was found to be 14.843 MPa with an increment of 36.06% as compared to the neat sample Thus, the optimum MWCNTs loaded GFRP can be used in many structural
Fig 14 (a) The crack propagation mechanism in th neat GFRP laminate; (b) Hinderance in the crack propagation due to MWCNTs aligned along the thickness direction; (c) FESEM image of failure mechanism of short beam shear strength (SBSS) test sample specimen (delamination).
R.P Behera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 299e309 308