doi: 10.1177/1099636216656485 performance improve- ment …nilanjan/patra_and_mitra_2016.pdf ·...

Original Article

Mixed-mode fractureof sandwich composites:Performance improve-ment with multiwalledcarbon nanotubesonicated resin

Alak K Patra and Nilanjan Mitra

Abstract

An experimental investigation on sandwich composite materials composed of glass-

fiber face sheet and polyvinyl-chloride foam core has been carried out. The research

demonstrates improvement in mixed-mode delamination fracture toughness values

of samples under mixed-mode bending condition. The improvement is recorded

with addition of a certain percentage by weight of multiwalled carbon nanotubes

in comparison to conventional samples. An easy and cost-effective methodology

of multiwalled carbon nanotube insertion through sonication of epoxy resin followed

by mixing with hardener and vacuum resin infusion technology for manufacturing

of sandwich composites has been utilized in this study. The study also identifies the

optimum weight percentage of multiwalled carbon nanotube addition in the resin

system for maximum performance gain in mixed-mode fracture toughness. The results

of observations in this study have been supported by field emission scanning electron

microscope studies as well as high-resolution transmission electron microscope

analysis.

Keywords

Foam, glass-epoxy, sandwich composite, carbon nanotube, mixed-mode fracture

Journal of Sandwich Structures and Materials

0(00) 1–17

! The Author(s) 2016

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DOI: 10.1177/1099636216656485

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Department of Civil Engineering, Indian Institute of Technology, Kharagpur, India

Corresponding author:

Nilanjan Mitra, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302 India.

Email: [email protected]

Introduction

In spite of high strength to weight ratio and many other advantages of sandwichcomposite structures, interfacial face-core delamination remains as one of themajor challenges for these types of structures. Sandwich composites are subjectedto mixed-mode delamination failures in almost all practical loading situations dueto significant differences in material properties and thickness of the materials usedfor the face sheet and the core. Interfacial face-core debonding is considered to beone of the major causes for loss of structural integrity and/or even catastrophiccollapse of these structures. Definition of ‘‘interface’’ for a sandwich compositestructure is an important point to be noted here. Since the resin infiltrates into themicropores of foam core and also in between the glass fibers, there is no distinctinterface region as observed in a metal–metal bond. Typically, the interface regionbetween glass fiber-reinforced plastics (GFRP) face sheet and the polyvinyl chlor-ide (PVC) foam core is formed as a result of bonding between resin system of theface sheet and PVC foam core. Thus, the interface in this type of sandwich is asmeared layer of glass fiber strands, the resin system (used for GFRP face sheet)and some regions of PVC foam core. As a result, there is no distinct layer of resinsystem bonding and separating out the face sheet and core which may be separatelytermed as an interface region.

Research literature has been focused on improving the interfacial delaminationresistance of these structures. Examples include use of shear key insertions [1,2],stitching technology [3–6], z-pin fiber insertions [7–12], peel stoppers [13–14] andalso use of multiwalled carbon nanotube (MWCNT) in epoxy resin mix to promotebetter bonding [15–37]. In this research, the effect of MWCNT in the epoxy resinmix has been investigated to determine performance improvement of the mixed-mode response of glass fiber face sheet and PVC foam core sandwich composites.

The use of MWCNTs for toughening resin systems has become a popular prac-tice after the seminal paper by Iijima [17]. The use of several types of CNTsthrough different methodologies for improvement in fracture characteristics is typ-ically limited to laminated composites in literature. In case of composite laminates,improvements of 60% and 17% in interlaminar fracture toughness have beenachieved with aligned CNTs and sprayed MWCNT, respectively [22,23]. Anotherwork on CFRP laminates recorded an improvement of 32% in interlaminar tough-ness by spraying MWCNTs on Teflon coated peel cloth and transferring them towoven prepregs [31]. In laminated composites, improvements of fracture toughnessby 23% and 20% are reported with the use of 0.5wt% and 0.1wt% of functiona-lized CNTs and MWCNTs, respectively [30,29] in comparison to conventionalcases. In hybrid nanocomposite, Karapappas et al. [35] recorded an increase of60% in mode I fracture toughness with 1wt%MWCNT and 75% increase in modeII fracture toughness (GIIC) with 0.5wt% MWCNT of resin in comparison to theneat epoxy reinforced with carbon fibers only. Garcia et al. [36] used verticallyaligned CNTs (VACNTs) at the interface between the plies of carbon compositelaminate and reported an improvement of 50% in mode I fracture toughness andas high as 200% in mode II fracture toughness values. In nanocomposites,

2 Journal of Sandwich Structures and Materials 0(00)

an enhancement in mixed-mode fracture toughness approximately by 31% formode ratio 1 with 0.5wt% of MWCNT has also been reported [37]. The aboveset of literature shows different percentage increase in performance with addition ofMWCNT in nanocomposites or laminated composites only. It should be pointedout here that the percentage gain in fracture performance of the samples dependslargely on the type of resin system used, type of functional group of the MWCNTand also on the material characteristics of the constituent materials which are to bebonded with the resin system. In fact, MWCNTs not only toughens the resinsystem but also infiltrates within the adjacent constituent materials that are to bebonded and thereby improves mechanical performance.

Literature report use of several types of CNTs for improvement in fracturecharacteristics of nano, hybrid or laminated composites only. But to the best ofthe authors’ knowledge, there appears to be no work done on influence ofMWCNTs on mixed-mode fracture toughness of glass-epoxy (G/E) and PVCcore sandwich composites. This manuscript strives to investigate the influence ofMWCNTs on mixed-mode fracture of G/E–PVC core sandwich composite alongwith determination of optimum percentage of MWCNTs. A sandwich compositediffers significantly from laminated composites, since it consists of at least twodifferent materials (having significant differences in material and geometrical prop-erties). Thereby the characteristics of a bi-material interface (present in sandwichcomposites) cannot be directly extrapolated from results obtained from single-material interfaces (as present in laminated composites).

This manuscript demonstrates improvement in fracture performance of sandwichcomposite with different wt%ofMWCNT addition on different modemixities (mea-sured by the ratio of mode II to mode I fracture toughness values) of mixed-modebending (MMB) samples. In addition, themethodology used in this researchwork forMWCNTsaddition in epoxy resin ismuch simpler than that usedbyother researchersfor addition of MWCNT in resin mix for composite laminate manufacture.

Details about the process of manufacturing are presented in Process of manu-facture section along with HRTEM investigations. Experimental investigation sec-tion describes the experimental investigations carried out with the manufacturedsamples; Results and discussions section discusses the results as obtained in experi-mental investigation (MMB test) and micrograph studies (FESEM) for sampleswith and without MWCNT; Conclusions section provides discussion and conclu-sion to the manuscript including possibilities of future work.

Process of manufacture

Epoxy resin (Araldite CY 230-1 IN of Huntsman Advanced Materials with1300–1800 mPa s viscosity and 1.13–1.16 g/cc specific gravity at 25�C) andMWCNTs (Cheap Tubes Inc.) with outer diameter less than 8nm, inside diameter2–5 nm, individual length of 10–30mm, specific surface area 500 m2/g, bulk density0.27g/cc) are used. The above mentioned resin and MWCNTs are thoroughly mixedwith an ultrasonic liquid processor (OSCAR make SONAPROS PR- 500 MP

Patra and Mitra 3

processor with frequency (20� 3) kHz, ultrasonic power 500W and 18mm ØTitanium horn) for 2.5 h intermittently (15 s on and 5 s off for the first hour, 10 son and 5 s off in second hour and 5 s on 5 s off for the last half an hour) at 60%amplitude maintaining room temperature (25�C). The process of ultrasonication ofthe same epoxy resin has been repeated with 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%and 0.5wt%, respectively. Hardener (Aradur HY 951 IN of Huntsman AdvancedMaterials with 10–20 mPa s viscosity and 0.97–0.99 g/cc specific gravity at 25�C) in10wt% of epoxy resin is mixed with the mixture after sonication by stirring at150 for 15min.

Dispersion of MWCNTs at different weight percentages of epoxy resin is studiedthrough HRTEM analyses. Hardened mixtures of sonicated epoxy resin and hard-ener are trimmed with LEICA EM trim and cut to 100 nm thick samples usingLEICA EMFCS Ultra-Microtome machine. These samples are then placed oncopper grids and examined through JEOL JEM2100 HRTEM with 80 k magnifi-cation at an operating voltage of 200 kV. HRTEM images in Figure 1 for resinsamples with MWCNTs (0.2–0.4wt% of epoxy resin) can be observed to showrelatively good dispersion; however, the optimum wt% of MWCNT for the bestvalue of mixed-mode fracture toughness cannot be stated clearly from visual obser-vations. Resin samples with MWCNTs for 0.1wt% and 0.5wt% of resin demon-strate low dispersion and agglomeration effects, respectively. An efficient dispersionor agglomeration of MWCNTs in resin is related to relative van-der Waals forces,curvature, and relative surface energy between MWCNTs and resin. In this research,ultrasonication is done to overcome the attractive forces, but MWCNTs may reag-glomerate on discontinuation of external forces after sonication [21]. It should benoted in here that efficient load transfer between the constituent materials of the facesheet and the core is determined by the better nanotube–matrix interface propertiesand primarily influenced by the optimum dispersion of MWCNTs in the nanocom-posites [22].

Manufactured sandwich composite samples having MWCNTs in different per-centages by weight of epoxy resin are tested in MMB setup for determination ofoptimum wt% of MWCNT in epoxy resin for enhancement of mixed-mode frac-ture toughness of the samples. Semi-rigid closed cell PVC foam with a thickness of30mm, cell size of approximately 400 mm, density 100 kg/m3 and trade name ofDivinycell H100 manufactured by DIAB Inc. is used as the core material in thisresearch work. The PVC foam core is covered with one layer of stitched combin-ation mat followed by a layer of woven roving mat made from glass fibers on bothsides. The assembly is placed inside an air tight vacuum bagging system (face sheetand core covered by flow media, porous Teflon sheet and highly permeablebreather cloth). The resin system prepared through sonication of resin withMWCNTs and mixing with hardener is then allowed to pass through the preformto cast the sandwich composite panel by vacuum resin infusion (VRI) technology.

The input and output infusion lines are blocked after complete part wetting ofthe system which is then cured under vacuum for more than 48 h at room tem-perature (25�C). The process is repeated for manufacturing sandwich panels with


Figure 1. HRTEM images of samples with different weight percentage of multiwalled CNT:

(a) 0.1 wt% CNT, (b) 0.2 wt% CNT, (c) 0.3 wt% CNT, and (d) 0.4 wt% CNT, (e) 0.5 wt%.

Patra and Mitra 5

different wt% of MWCNTs. Initial delamination in the sample in the form of pre-crack required for the delaminated MMB test is introduced in the samples with thehelp of 80 mm thick non-stick impermeable Teflon sheet between the face sheet andthe PVC core during resin infusion process for sample preparation by VRI method.

Experimental investigation

Figure 2 shows a schematic diagram of the test setup along with sample dimen-sions. Samples of dimensions 190mm� 35mm� 35mm (where length¼ 190mmand width¼ 35mm and thickness¼ 35mm as per Figure 2) are cut fromcured sandwich panels. The initial delamination length is kept as 50mm.Samples are pasted with aluminum hinge tabs on its both top and bottom facesheets at initially delaminated part using DP 460 adhesive. Distance betweenthe end point of glued part of hinge and that of delamination (a), as shown inFigure 2, is kept at 25� 1.5mm. It should be noted that there exists previousliterature [38–42] on sandwich composites without MWCNTs to determinemixed-mode fracture toughness values with similar experimental setup(MMB apparatus).

Mixed-mode fracture toughness GMMB of theMMB specimens is calculated usingthe formula provided by Quispitupa et al. [41]. MMB specimens of sandwich com-posite panels can be considered as superposition of cracked sandwich beam (CSB)and DCB specimens. Analytic expressions for the MMB compliance and energyrelease rate for symmetric sandwich specimens (hf1 ¼ hf2 ¼ hf and Ef1 ¼ Ef2 ¼ Ef)have been derived based on load partitioning as

C ¼c

LC1 þ

c� L

2LC2

� �c

L� �1

cþ L

2L

� �þ

cþ L

L

� �2

CCSB ð1Þ

Figure 2. Schematic diagram of MMB test setup with sample.


GMMB ¼P2

2b2

c

L

c

L� �1

cþ L

2L

� �12

Efh3f

a2 þ 2a�14 þ �

12

h iþ

c� L

2L

c

L� �1

cþ L

2L

� �1

hcGxzþ

a2

D� B2

A

� �" #

þcþ L

L

� �2a2

8

1

Dd�

1

Di

� �� 0BBB@

1CCCAð2Þ

where C is the MMB compliance, c is the lever arm distance, 2L is the span length,�1 is a parameter defined in Quispitupa et al. [41], a is the crack length, � is theelastic foundation modulus [41]. Whereas A, B, D are the extensional, coupling,and bending stiffnesses of DCB sample as defined in Quispitupa et al. [41]. CCSB isthe compliance of the cracked sandwich beam specimen [41], P is the applied load,b the beam width, Dd and Di are the flexural stiffnesses of the debonded and intactregion of the beam defined in Quispitupa et al. [41]. C1 and C2 are the compliancesof the upper and lower legs of the DCB specimen defined in Quispitupa et al. [41].The mode ratio GII/GI is given by

GII

GI¼

PIIa

2PI

� �21

Dd�

1

Di

� �1

hcGxzþ

a2

D� B2

A

� �þ 12

Efh3f

a2 þ 2a�14 þ �

12

h i ð3Þ

where PI and PII are the loads on DCB and CSB specimens as defined inQuispitupa et al. [41].

A minimum of lever arm distance is required to avoid contact between crackfaces is given by

c4/1 L

2� /1ð4Þ

The condition represented in equation (4) is maintained in all the specimensunder MMB tests to maintain the validity of the methodology applied in theseexperiments.

The aluminum hinge tabs are fitted with the attachments of the lever at top andthat of the base at bottom of the steel base of the MMB setup as shown in Figure 3.The lever transfers compressive load to the sample at the upper surface of the topface sheet through a roller attached to the lever. The sandwich beam sample is sup-ported on a clamp and a roller support at bottom which is attached to the base. Thewhole setup is placed on the loading base of Tinius Olsen make UTM. A C-clamplike attachment and a 10 kN load cell at a loading rate of 0.5mm/min are thenutilized to provide compressive load under different mode ratios on the leverthrough the wheels attached to the lever as shown in Figure 3. Samples are

Patra and Mitra 7

continuously loaded for propagation of crack by at least 20mm from the end pointof delamination or the point of crack initiation.

The results of the MMB tests are recorded as per recommendation of ASTMstandard D6671/D6671M-13 [40] supported by visual observations. However, itshould be noted in here that the case for this experiment is different from deter-mination of mixed-mode fracture toughness in ASTM D6671/D6671M-13 [40].In this experiment, the initial delamination present in the sample is orientedtowards one end of the cross-section in between two dissimilar layers of facesheet and core, whereas ASTM D6671/D6671M-13 prescribes a non-adhesiveinsert serving as delamination initiator at the mid plane between two laminatedlayers of similar material properties and geometrical sections. The mean of theresults of investigations on at least 10 samples (yielding the results between mean�two times the standard deviation) is considered for any conclusion derived fromthe experiment. The MMB samples are subjected to an upward directed load at theleft end of the debonded face sheet and a downward directed load at the center,thereby subjecting the specimen to both bending and tensile forces through leveraction under different mode ratios. The initiation of crack after reaching the endpoint of delamination and crack propagation at the face/core interlayer occursslowly with increase in load. The mean of load readings taken from either sideof the sample is taken at the point of initiation of the crack and at other discretepoints at each 2.5mm distance from the end point of delamination with the help ofcamera attachments. Deflected shape of the sample after the experiment is shown inFigure 4. Except for the changes in stiffness, load capacity and ductility, betweenthe samples with and without MWCNTs, no other significant difference in behav-ior is observed between them. The crack kinked into the core at a later stage ofcrack propagation in a large number of samples which agrees with the findings of aprevious study [42] where it is stated that the direction of crack propagationdepends upon the fracture toughness values of the associated components, i.e.the fracture toughness of the face sheet, interface and the core. The mixed-mode

Figure 3. Initially loaded MMB sample in UTM: (a) initial and (b) zoomed in region.


fracture toughness values calculated from the MMB tests of samples with initialdelamination incorporate contribution from both mode I and mode II types offractures.

Results and discussions

Experimental investigations have been carried out for specimens with differentpercentages of MWCNT sonicated resins. But typical plots for load vs. dis-placements of sandwich specimens with MWCNTs (0.3wt%) and without CNTare shown in Figure 5. The plot with 0.3wt% MWCNT has been selected forrepresentation since this percentage provided the best results with regard to per-formance improvements for mixed-mode fracture.

Plots for all the mode ratios show enhancements in peak load-carrying capacitiesof the specimens with 0.3wt% MWCNT of resin over the samples without CNT(neat sample). The quantitative performance improvements are shown in Table 1.Even though there is slight increase in initial stiffness of samples with MWCNT(CNT samples) for all mode ratios indicating higher initial resistance to fracture,the increase is not that significant, and the quantitative estimate has not beenevaluated for this portion. Typically, the pre-peak non-linear portions of theplots demonstrate formation of micro-cracks. Attainment of higher peak strengthsand displacement at the peak strength for the MWCNT doped samples in compari-son to the conventional samples demonstrate improved resistance offered byCNT samples. The improved resistance is induced through prevention of

Figure 4. Final deflection of the MMB sample.

Patra and Mitra 9

Figure 5. Load–displacement plot of MMB Samples with different mode ratio: (a) mode ratio

1.0, (b) mode ratio 0.8, (c) mode ratio 0.5.

Table 1. Increase in peak load-carrying capacity of samples with 0.3 wt% of resin

with respect to the sample without CNT.

Mode ratio

(GII/GI)

Peak load (Pult)

of samples

without CNT (N)

Peak load (Pult)

of samples

with 0.3 wt% CNT (N)

Increase

in peak load-carrying

capacity (%)

1 1282 1725 34.55

0.8 1269 1612 27.03

0.5 1201 1479 23.15


coalescence of the micro-cracks into a macro-crack. This resistance to coalescenceof micro-cracks is typically initiated by the random dispersion of MWCNTs in theresin mix. As the mode ratio is decreased, the mode I component becomes moresignificant (for mode ratio 0.5 in comparison to that of 1), the drop in load imme-diately after attainment of peak-strength of the sample becomes sharper. However,for individual mode ratios, while comparing between samples with and withoutCNT, it can be observed that sharp fall in load after peak is reduced for CNTsamples in comparison to the neat samples. The curve becomes much more flatterindicating that the random dispersion of MWCNT in the CNT samples effectivelyacts to prevent crack propagation in the sample. The displacement ductility of thesample (observed as displacement distance between attainments of peak strength toa drop of 20% from the peak strength) is also observed to increase for samples withMWCNT for all mode ratios in comparison to samples without CNT.

Even without addition of CNT, typically the shear strength is slightly higherthan the normal strength. This fact is corroborated by increase in peak strengthwith increase in mode ratio in Table 1 (mentioned by Quispitupa et al. [42]) forsandwich composites. Since shear resistance offered by the resin mix system inbetween the glass fiber face sheet and the PVC foam is higher than the mode Iresistance to crack, a better regain in strength along with increase in displacementductility is observed with increase in mode ratio. The random dispersion ofMWCNT in the samples is also observed to improve the performance of the sam-ples with increase in mode ratios.

Random dispersion of CNTs in the sample and their modes of failure or pull-outcould be observed from destructive tests on tested samples. The samples in whichexperimentations have been completed are cut into small pieces and then coatedwith gold palladium alloy using coater machine of Quorum series Q150 RES. Thesecoated samples are observed under a Carl Zeiss MERLIN machine at extra hightension voltage of 5 kV for FESEM studies. Figure 6(a) shows random dispersionof MWCNT within the resin mix at the interface observed at 25 k magnification.Figure 6(b) shows small white circular patches which are practically torn fiber ends,observed at 150 k magnification. Figure 6(c) shows MWCNT bridging the resinsystem and glass fiber at 200 k magnification. Figure 6(d) shows bridging ofMWCNT between two resin blocks individually attached to GFRP and foamrespectively at 115 k magnification. Better mechanical interlocking mechanismdeveloped by the MWCNT between the glass fibers, matrix and PVC core resultsin increase in resistance to mixed-mode fracture in MMB samples in comparison tothe samples without MWCNTs.

Figure 7 shows that for all % by weight additions of MWCNT (to the sample),the improvement in performance (in comparison to neat samples) increases till0.3% after which it decreases. This trend of the plots demonstrates that the opti-mum percentage of MWCNT addition to G/E–PVC sandwich containing the spe-cific type of resin system is 0.3wt%. Figure 7 also shows error bars in experimentalobservations. It can also be observed from the figures that values of the maximumfracture toughness at 0.3wt% MWCNT decreases with reduction in mode II

Patra and Mitra 11

components (or decrease in mixed-mode ratio values) which has also been previ-ously demonstrated by Quispitupa et al. [42]. Primarily the reason for decrease inperformance with increased wt% of CNT (>0.3 wt%) is due to agglomerationeffect which subsequently leads to localized stress concentration and zones of crackinitiation. The figure also shows that over a wide range of mode ratios selected forthis work, 0.3% gives the best percentage gain uniformly. However, it should bekept in mind that this 0.3% which gives the best gain in performance typicallydepends on the constituents of the sandwich construction as well as the epoxy resinsystem used in this process. It should be highlighted at this point that authors havereported an improvement of 34.34% in interface fracture toughness value of sand-wich composite sample using MWCNT for predominantly mode I loading [24] with1.5wt.% addition of MWCNT. This significant difference in percentages ofMWCNT (with that reported in this manuscript) is primarily because of theresin system used for the two works and also on type of loading applied.Moreover, the current article focuses on MMB showing different mode ratios offracture, whereas the previous document focused on double cantilever beam experi-ments demonstrating primarily mode I behavior. This behavioral trend in results asobserved in this manuscript (as well as the previous one [24]) with the addition ofMWCNTs has also been previously demonstrated by Fiedler et al. [27] and Zhou el

Figure 6. Micrographs of CNT dispersed in the resin system at the interface of sandwich

composite: (a) CNT dispersed in resin system at interface region at 25 k magnification

(b) numerous CNT pull-out zoomed in 150 k magnification, (c) Zoomed in CNT bridging and at

200 k magnification and (d) CNT bridging crack at 115 k magnification.


al. [28] for mode I stress intensity factor (KIC) of carbon/epoxy composites mixedwith MWCNT. Wichmann et al. [32] also reported 0.3wt% as the upper limit ofCNTs mixed in the resin for maximum percentage gain in nano-reinforced lami-nated composites with VARTM.

Figure 8 shows increment in mixed-mode fracture energy (GMMB) with differentmode ratios for different wt% of MWCNT specimen. Figure demonstrates that theincrease in GMMB values is highest for 0.3wt% MWCNT, and least for 0.5wt% ofMWCNT. The justification of above fact can be sought from HRTEM andFESEM analyses. The larger volume of cylindrical resin system aroundMWCNTs provides reinforcement effect against pull-out mechanism [43].Obviously, better dispersion of MWCNT within the resin system leads to bettermechanical performance against interfacial delamination. It should also be notedthat there is an optimum quantity and addition of MWCNT above the optimumquantity results in agglomeration and thereby do not lead to better performance.

Conclusions

The manuscript highlights the differences in experimentally observed responseof interface fracture toughness of samples with and without MWCNT. Out of

Figure 7. Variation of mixed-mode fracture toughness of MMB sample with percentage of

CNT sonication: (a) mode ratio 1.0, (b) mode ratio 0.8, (c) mode ratio 0.5 and (d) superposed

plots for different mode ratios.

Patra and Mitra 13

the different process of addition of MWCNT in composites (as can be obtainedfrom literature), a simple method of sonicating the MWCNT in epoxy resin hasbeen chosen in this research for the PVC-core/glass-fiber/epoxy-resin sandwich todemonstrate significant improvement in mechanical performance of samplesagainst interfacial delamination subjected to mixed-mode loading condition.The research demonstrates an improvement of 34.55%, 27.03% and 23.15% inpeak load-carrying capacity for samples with 0.3 wt% MWCNT (weight % beingtaken with respect to amount of epoxy resin used for sample fabrication) incomparison to samples without MWCNT for mode ratios 1, 0.8 and 0.5, respect-ively. Good dispersion of MWCNT is also observed with 0.3 wt% MWCNTaddition in comparison to other percentages of addition of MWCNT byweight. Samples with 0.3 wt% MWCNT recorded an increase in fracture tough-ness values of 54.79%, 49.27% and 46.76% in comparison to samples without formode ratio 1, 0.8 and 0.5, respectively. HRTEM images and FESEM analysesdemonstrate good dispersion and fiber bridging of MWCNT in the epoxy resinsystem, respectively.

Acknowledgements

The authors acknowledge the contribution of Mr. Subhajit Das in manufacturing of themixed-mode bending apparatus used in the experiment. The first author would like to thank

many students and technicians who had helped him for carrying out the experiments. Anyopinions, findings and conclusions or recommendations expressed in this manuscript arethose of the writers and do not necessarily reflect those of the Space Research Organization,

India.

Figure 8. Increment in mixed-mode fracture energy (GMMB) with mode ratio for different

wt% of MWCNT specimen.


Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, author-ship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, author-

ship, and/or publication of this article: This work is supported by Indian Space ResearchOrganization, India under Award No. IIT/KCSTC/Chair./New:Appr./13-14/70.

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