Composites Science and Technology 101 (2014) 94–101

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Composites Science and Technology

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Interface fracture of sandwich composites: Influence of MWCNTsonicated epoxy resin

http://dx.doi.org/10.1016/j.compscitech.2014.07.0060266-3538/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail addresses: alakpatra19@gmail.com (A. Patra), nilanjan@civil.iitkgp.ernet.

in (N. Mitra).

Alak Patra, Nilanjan Mitra ⇑Department of Civil Engineering, Indian Institute of Technology, Kharagpur, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 January 2014Received in revised form 2 July 2014Accepted 6 July 2014Available online 14 July 2014

Keywords:A. FoamsB. Glass fibersC. Epoxy resinD. Sandwich compositeE. Carbon nanotubesF. Interfacial fracture

An experimental investigation on sandwich composite materials composed of glass-fiber face sheet andpolyvinyl-chloride foam core has been carried out which demonstrates improvement in interface fracturetoughness values of samples with addition of a certain percentage by weight of multi-walled carbonnanotubes (MWCNT) in comparison to conventional samples. An easy and cost-effective methodologyof MWCNT insertion through sonication of epoxy resin followed by mixing with hardener and therebyusing vacuum resin infusion technology for manufacturing of sandwich composites has been prescribedin this study. The study also identifies the optimum weight percentage of MWCNT addition in the resinsystem for maximum performance gain in interfacial fracture toughness. The results of observations inthis study has been supported by high-resolution transmission electron microscope (HRTEM) analysisas well as field emission scanning electron microscope (FESEM) studies.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The primary advantage of sandwich composite structures ishigh strength to weight ratio. However, these structures sufferfrom one major problem: Interfacial debonding between the facesheet and the core, which has limited their applications in differentbranches of engineering in comparison to monolithic structures.Interfacial debonding may arise during manufacture due to poorresin distribution (which can be controlled by proper quality assur-ance measures) or during in-service loadings (which are beyondcontrol). Interfacial debonding has thereby been identified as oneof the major causes for loss of structural integrity and/or even cat-astrophic collapse of these structure.

The methodologies that have been proposed to improve thedebonding characteristics of sandwich composites can typicallybe classified as: structural modifications and constituent modifica-tions. Structural modifications typically refers to changes in orienta-tion of the materials in the structure and/or addition of newstructural materials to strengthen it and thereby improve the deb-onding characteristics whereas constituent modifications typicallyrefers to changes in the constituent materials to improve uponthe debonding characteristics. Even though structural modifica-tions might offer cost-effective and conceptually easy solution to

the problem but there may be issues related to overall weight gainof the component materials (which affects the major advantage ofthese materials/structures having high strength to weight ratio)when utilizing these techniques for large fabrications. The tech-niques that have been proposed till now under the heading ofstructural modifications are

� Z-pinning which involves penetration of metallic or fibrous pinsthrough the thickness of the uncured laminate using an ultra-sonic gun [1–6].� Stitching concept which involves sewing high-tensile-strength

yarn through the laminate using an industrial sewing machine[7–10].� Peel stopper device [11–13] which is based on principle of

inserting specially shaped core insert into the core such thatthe delamination crack front is rerouted along the internalboundary of the peel stopper without further propagation alongthe face core interface.� Shear key concept [14,15] which involves grooving of the core

region and placing an insert (which may or may not be of thesame material as the face-sheet) attached to the face sheetand of the same dimension as that of the foam-core groove.

On the other hand constituent modifications involves changingand/or improving the properties of constituent materials such asthe face-sheet, core and/or the resin-hardener system. The primaryadvantage of this modification over the previous one (structural

A. Patra, N. Mitra / Composites Science and Technology 101 (2014) 94–101 95

modification) is practically no gain in weight. Apart from additionof toughening agents to brittle resin [16,17], primarily the modifi-cation of constituents for sandwich composites have been doneusing single- or multi-walled, functionalized or un-functionalizedcarbon nano-tubes (CNT). Literature survey reveals different tech-niques of addition of CNT’s within the constituents of laminatedand/or sandwich composites. CNT’s have been dispersed in theinterface of prepreg/plies or in the resin system and GFRP/CFRPsheets [18–27] or have been infused within the polymer foam core[28–30]. CNT’s have also been grown and aligned on the inter layerof prepreg/plies [31–33] or on the surface of the glass- or carbonfibers [33] constituting laminated and sandwich composites. Anexcellent treatise on addition of nano particles in CFRP laminatedcomposites can be found in references [34,35].

The approach considered in this manuscript is to dispersemulti-walled unfunctionalized CNT in epoxy resin using a sonica-tor. Even though a number of works on determination of mode 1and/or mixed mode characteristics have been done in this regardfor laminated composites; to the best of the authors knowledgethere appears to be no work done on influence of sonicated multi-walled CNT dispersed resin system in interface fracture character-ization of GFRP/PVC-core sandwich composites. The current workfocuses on this issue in which vacuum resin infusion methodologyhas been utilized for final manufacture of these samples. Close tothis topic being addressed in this manuscript there exists relatedliterature on using CNT modified resin system to improve fracturecharacteristics of laminated composites: a study [18] reported 32%increase in mode 1 fracture toughness on spraying functionalizedMWCNT solution on interlayer of CFRP laminates; another study[20] revealed 14% improvement in mode 2 fracture toughness val-ues (through End-Notch flexure tests) by spreading solution of eth-anol and MWCNT with an airbrush on surface of prepreg layers; anincrease in mode 1 fracture toughness values by 32% was achievedby spraying MWCNT’s on Teflon coated peel cloth and subse-quently transferring them to the woven prepreg layers [19].

The paper has been arranged in the following manner: Section 2describes the process of manufacture of the samples along withHRTEM investigations for MWCNT dispersion in resin; Section 3describes experimental investigations carried out with the manu-factured samples; Section 4 discusses the results as obtained inexperimental investigation (double-cantilever-beam test) andmicrograph studies (FESEM) for samples with and withoutMWCNT; Section 5 provides discussion and conclusion to the man-uscript including possibilities of future work.

Fig. 1. HRTEM of samples with different weight percentage of multiwalled CNT: (a)0.5 wt% CNT, (b) 1.0 wt% CNT, (c) 1.5 wt% CNT, and (d) 2.0 wt% CNT.

2. Process of manufacture

MWCNT (supplied by CheapTubes of outer diameter less than8 nm, inside diameter 2–5 nm, length 10–30 lm, specific surfacearea 500 m2=g, bulk density 0:27 g=cc) was mixed in epoxy resin(Airstone 780E of Dow Chemicals with 1400 mPa�s viscosity and1.15 g/cc density at 25 �C) using a tip sonicater (500 W VibracellHigh Intensity Liquid processor with 20 kHz probe of Sonics andMaterials) for 2.5 h intermittently at 60% amplitude. Different per-centages of MWCNT by weight of epoxy resin (0.5, 1, 1.5 and 2.0)were utilized in the sonication process. Room temperature (of25 �C) was maintained throughout the process of sonication. Aftersonication, hardener (Airstone 785H of Dow Chemicals with13 mPa�s viscosity and 0.94 g/cc density at 25 �C) was added tothe mixture in 30 wt% of epoxy resin and stirred for 15 min at150 rpm.

HRTEM analysis was done to measure the dispersion of MWCNTin the epoxy resin. Samples of MWCNT sonicated and hardenedepoxy resin system were trimmed with LEICA EM trim followedby cutting to 100 nm thickness samples using LEICA EMFCS

Ultra-Microtome; which were then eventually placed on coppergrids. The samples were examined through JEOL JEM2100 HRTEMoperating at 200 kV with 50–500 k magnification. Fig. 1 shows theHRTEM images demonstrating relatively good dispersion for 0.5, 1and 1.5 wt% of MWCNT. Weight % of MWCNT were taken based ontotal weight of the epoxy resin used for sample fabrication.Agglomeration could be observed for 2.0 wt% of MWCNT. It shouldbe noted that both dispersion and agglomeration of MWCNT inresin depends on relative van-der Waals forces, curvature, and rel-ative surface energy of MWCNT vs. that of resin. To overcome theattractive forces ultrasonication had been done in this research,but still reagglomeration is possible after discontinuation of exter-nal forces [21]. Apart from good dispersion at 0.5–1.5 wt% andreagglomeration at 2 wt%, it cannot be stated clearly from visualobservations in Fig. 1 as to which is the optimum wt% of MWCNTin the mix which would give the best values for interfacial fracturetoughness. It should be noted that optimum dispersion of MWCNTin resin matrix system is the key parameter to promote betternanofiber–matrix interface properties to reach an efficient load-transfer between the two constituents of the nanocomposite[22]. Fig. 2 shows magnified view of dispersed CNT for 1.5 wt%MWCNT sonicated resin system. In order to resolve the issue ofdetermination of optimum wt% of MWCNT in mix which wouldgive the best improvement in fracture toughness values, sandwichcomposite samples were manufactured using the resin mix withdifferent wt% of MWCNT and experimentally investigated using amodified double-cantilever-beam test set-up.

Closed cell semi-rigid PVC foam with a density of 100 kg=m3,manufactured by DIAB Inc. and marketed by the trade name ofDivinycell H100, was utilized as core material in this manuscript.The foam-core thickness considered for the experimentalinvestigation is 30 mm and the foam material has a cell size ofapproximately 400 lm. The foam was sandwiched between twoglass-fiber face sheets, each of which is composed of two alternatestitched layers of chopped strand mat (CSM) and woven roving(WR) fiberglass mats. The above prepared resin system was uti-lized to prepare the sandwich composite panel by vacuum resininfusion methodology. The resin-system-flow through the preform(face sheet and core covered by porous Teflon film and highly

Fig. 2. Magnified figure of CNT dispersion at 1.5 wt% CNT.

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permeable breather cloth) is primarily assisted through the vac-uum alone. Upon complete part wetting, the infusion lines areblocked and is vacuum cured at room temperature for at least24 h. It should be noted that the resin-system used in vacuum resininfusion methodology consists of resin, hardener and different wt%of MWCNT.

Induction of initial delamination in the sample, which arerequired for carrying out interfacial fracture toughness tests, wasdone by implanting a 80 lm thick non-stick impermeable Teflonsheet between the face sheet and the core during vacuum resininfusion process for sample fabrication.

3. Experimental investigation

A schematic diagram of the test setup along with sampledimensions are shown in Fig. 3. Samples of dimensions

Fig. 3. Schematic diagram of D

180 mm � 25 mm � 35 mm (where L = 180 mm and b = 25 mm asper Fig. 3) were cut from the initially manufactured panel. Thelength of initially delaminated region for the cut samples was keptas 50 mm. Aluminium hinges were then glued (using DP 460 adhe-sive) on either sides of the face sheet in the initially delaminatedregion. Dimension of a (as shown in Fig. 3) was kept at25 ± 1 mm. Similar experimental investigations on sandwich com-posites to determine fracture toughness values can be found in lit-erature [36–39]. It should be mentioned in this context that atcontinuum level the interface between the glass-fiber face-sheetsand foam-core is a smeared-layer of chopped strand mats withinthe glass-fiber face-sheets, the resin system and the PVC foam-core. Thereby, there does not exist a distinct interface layer of resinsystem separating the face-sheet and the foam-core.

The hinge tabs were fitted to the pneumatic grips of INSTRONElectroPulse 1000. A 1 kN loadcell at constant cross head speedof 0.5 mm/min was then utilized to provide tensile loading to thesamples as shown in Fig. 4 till the point where the crack hasadvanced more than 20 mm from the initial point of delamination.Atleast 10 samples (which yielded results in between mean to 2times the standard deviation) were investigated to arrive at anyconclusion from experimental investigations. The experimentalinvestigation results shown in the manuscript represent mean ofthe results obtained.

As tension is applied to the grips, the initial face-core delamina-tion length increases and crack slowly propagates in the face/coreinterlayer regions. The load vs. deflection plot of the cross-headswere obtained from the computer whereas the crack lengths wereobserved through camera attachments and were taken at discreteload points along the entire span of load vs. displacement plots.Fig. 5 shows final deflected shape of the sample. Apart fromchanges in stiffness, load capacity and ductility, no significantbehavioral difference was observed between samples with andwithout MWCNT. At a later stage of crack propagation, the crackkinked downward into the core in a large number of samples. Typ-ically the explanation to this behavior can be derived from previ-ous analytical study [40] in which it was demonstrated that the

CB test setup with sample.

Fig. 4. Initially loaded DCB sample in UTM: (a) Initial sample and (b) zoomed in region.

Fig. 5. Final deflection of the DCB sample.

Fig. 6. Load–displacement plot of the sample.

Fig. 7. Variation of interfacial fracture toughness with percentage of CNTsonication.

A. Patra, N. Mitra / Composites Science and Technology 101 (2014) 94–101 97

direction of crack propagation depends upon the fracture tough-ness of the associated components (interface region as well asthe two materials on either side of the crack).

The interface fracture toughness calculated from modified dou-ble cantilever beam tests (as done in this manuscript) typically

involves contributions from both mode I and II. Infact there arestandard specifications (ASTM D5528-01) for carrying out DCBtests for laminated composites. However in this case where theproperties of material above and below the initial delaminationare significantly different, it is considered as a bimaterial interfaceregion. For similar homogeneous material on both sides of theinterface, it has been demonstrated analytically [41,42] that ifthe crack travels in a plane without kinking up or down then itreveals a pure mode I type of failure mechanism. However no suchsimilar analogous analytical/numerical conclusions exist whichshows pure mode I behavior for interfacial crack between two dis-similar materials. Conclusions by various previous researches [40]suggest that the kinking of a crack does not demonstrate obser-vance of initiation of mixed mode crack for bimaterial interfaces;infact a crack can be perfectly straight within the interface regiondemonstrating mixed mode behavior. It has been identified thatobservance of kinking in bimaterial interfaces typically dependsupon stiffness and strength of the associated materials at the inter-face as well as the interface itself [40,43]. Thereby it can beexpected in this experimental investigation that both modes con-tribute to interfacial fracture propagation even though the contri-bution of mode I might be higher typically when the crack initiates.

4. Results and discussions

Load–displacement plots for conventional and MWCNT soni-cated samples (1.5 wt% MWCNT) are shown in Fig. 6. Even though

Fig. 8. Micrographs of CNT dispersed in the resin system at the interface of sandwich composite: (a) Interface region between foam-core and GFRP sheet for a sample, (b)interface region at 65 k magnification showing CNT, (c) zoomed in region at 229 k magnification, (d) CNT dispersed in resin system and GFRP surface at 27 k magnification, (e)CNT bridging two resin systems at 119 k magnification, and (f) vertical pullout of CNT at 186 k magnification.

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load–displacement was obtained for all samples with different per-centages of MWCNT; only samples with 1.5 wt% MWCNT havebeen shown in the figure since the samples provides the bestresults. It can be observed that typical MWCNT sonicated samplesimproves the stiffness as well as the peak-load carrying capacity byapproximately 13.06% and 14.4% respectively in comparison withsamples without MWCNT. It can also be observed from the figurethat a drop of around 20% from the peak-load carrying capacityin the post-peak regime was also delayed in the MWCNT sonicated

samples in comparison to conventional samples without MWCNTthereby indicating an increase in ductility of the samples. The rea-son for the above mentioned differences in results between sam-ples with and without MWCNT can be attributed due to presenceof MWCNT dispersed in the epoxy resin mix which resists the for-mation and/or propagation of cracks.

Interface fracture toughness, GC , of the DCB sandwich samples iscalculated using the compliance calibration (CC) method [44] con-sidering the modification due to shear [45]. Similar methodology

Fig. 9. Fracture energy with crack length.

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for determination of interface fracture toughness of sandwichspecimens using the DCB method has been utilized by previousresearchers [36,38,46]. The compliance calibration method typi-cally depends on change in compliance with crack length and isgiven as

GC ¼P2

2bdCda

ð1Þ

where P is the load applied, b the beam width, C ¼ dP is the compli-

ance, a is the crack length and d the load point displacement. Theexperimental compliance vs. the crack length data of the sandwichDCB specimen is fitted to a power function of crack length as

C ¼ C0an ð2Þ

Based on mean of the values utilized for this work, the value of theexponent n was obtained as 2.22; which is different from the valueof 3 as prescribed in ASTM D5528-01 for calculating mode 1 frac-ture toughness of laminate composites (and is the basis for modi-fied compliance calibration method). Differentiating the value of Cwith respect to a and substituting it in the expression for GC pro-vides the value for interface fracture toughness. Mean (and stan-dard deviation) of the interface fracture toughness values forsamples without MWCNT was obtained as 0.99 N/mm (11.9%)whereas the corresponding values obtained for samples with1.5 wt% MWCNT was 1.33 N/mm (8.3%) thereby demonstrating anincrease of 34.34% for samples with 1.5 wt% of MWCNT in compar-ison to samples without MWCNT addition.

It should be mentioned here that values were specified for only1.5 wt% MWCNT addition, even though calculations were made forother wt% of MWCNT addition as well, since that particular wt%gave the best gain in performance. Fig. 7 shows interface fracturetoughness, GC (calculated using CC method) of the DCB sandwichsamples increases from 0.5 wt% to 1 wt% and attains a maximumvalue at 1.5 wt% prior to decreasing at 2 wt%. Obviously betterthe dispersion of MWCNT in the resin, the better is the improve-ment in fracture toughness values. Fracture toughness values for2 wt% MWCNT addition were observed to be lower than 1.5 wt%addition primarily because of agglomeration of MWCNT (referFig. 1) which eventually turns out as zones of localized stress con-centration thereby acting as regions of crack initiators. The resulton increase of fracture toughness values with addition of CNThas also been demonstrated by previous researchers [18,19] whoobserved an increase of 32% on using CNT for laminated compos-ites. Apparently in a sandwich composite material (due to presenceof two different materials at the interface) it may appear that per-formance would not be great on addition of MWCNT in comparison

to laminated composites (using similar material on the two sides ofthe interface). However, it was observed from this study on sand-wich composites that performance improvement (of 34%) wasquite significant compared to earlier studies on laminated compos-ites. Moreover the methodology of addition of MWCNT for perfor-mance improvement (as prescribed in this study) wascomparatively very simple in comparison to methodologies pre-scribed by previous researchers. The reason for this improved per-formance using a very simple methodology is primarily due torandom dispersion of MWCNTs across the mid plane of interfacebetween the face and core (methodology process contradictory tocomplex methodology used by Garcia et al. [31] to achieveimprovement of 50% in mode 1 fracture toughness values for lam-inated composites).

Field emission scanning electron microscope (FESEM) studieswere also done for the cracked interfacial surface between thePVC foam core and the GFRP face sheet. After completion ofthe experiment, the interface between the PVC foam-core andthe GFRP face-sheet were separated; small samples were cut at dif-ferent regions as shown in Fig. 8a. These samples were gold coatedin Sputter Cooter machine to prevent charge buildup by electronabsorbed by the specimen. A 15 kV accelerating voltage wasapplied to achieve desired magnification. FESEM of the preparedsamples were done using Carl Zeiss SUPRA 40 machine. At 65 kmagnification (refer Fig. 8b) MWCNT filaments were observed tobe dispersed at the surface of the resin system. This feature maylead to enhanced void reduction in the fabricated samples. Zoomedin region (Fig. 8c) shows a single MWCNT in the resin system at amagnification of 229 k. Fig. 8d obtained at a magnification of 27 kshows MWCNT dispersed in the resin system between two glassfibers as well as on the surface of the glass fiber. This demonstratesa better fiber–matrix bond, achieved as a result of high aspect ratioof the MWCNT, in comparison to the conventional glass-fibermatrix bond in samples without MWCNT. The MWCNT act asmechanical interlocking mechanism between glass fibers and thematrix (between the glass fibers and the PVC foam core) whicheventually results in high friction coefficients. Bridging betweentwo resin-system blocks by a MWCNT is shown in Fig. 8e at amagnification of 119 k. This bridging effect prevents crackgeneration and increases crack propagation resistance. The abovefigures may indicate that the pre-peak nonlinear behavior for theMWCNT system (observed in Fig. 6) is due to delamination of theMWCNT from the resin system. Fig. 8f shows pulled out ends ofthe MWCNT at a magnification of 186 k. Typically when load isapplied to the composite system, the matrix (epoxy resin system)starts to crack first and stress is transferred from the lower modu-lus matrix (epoxy resin system) to the MWCNT by bridging effect.MWCNT anchored in the resin system matrix prevents propagationof these cracks. Apart from that, the MWCNT anchored to bothresin system and glass fiber promotes better interfacial bondingbetween the resin system matrix and glass fiber which eventuallyenhances the interface fracture toughness as well as load carryingcapacity of the composite.

After the initiation of the crack, the crack typically tends topropagate along the interface. The experimental investigationwas stopped momentarily and values of crack length (expressiona in Eq. (1)) were recorded using a travelling microscope alongwith the load (P) and load point displacement (d). Fig. 9 shows thatas the crack length increases the fracture energy values (G)decreases. The justification for this plot can be obtained from stud-ies made by Cantwell et al. [37]. Using a different experimental set-up it was demonstrated by Cantwell et al. [37] that as crack lengthincreases GI

GIIdecreases. It was also stated by them that typically for

sandwich composites with glass fiber face-sheet and balsa woodcore (having relatively similar stiffness and strength as that ofpvc foam core used in this study) fracture toughness of mode 1

100 A. Patra, N. Mitra / Composites Science and Technology 101 (2014) 94–101

is almost 10 times that of the fracture toughness of mode 2. Soobviously it can be expected that as crack length increase interfa-cial fracture toughness (containing components of both fracturemodes) will decrease, as has been observed from experimentalinvestigations carried out in this study.

It should be pointed out that the rate of decrease (measured bythe tangent to the curves shown in Fig. 9) of fracture energy valuesfor samples without MWCNT are higher than those with MWCNT.Out of the different weight % of MWCNT samples investigated,1.5 wt% gave the lowest rate of decrease of fracture energy valuesthereby demonstrating increase in ductility of the samples withaddition of MWCNT. Even though it has been mentioned that nano-tube doping can speed up the damage especially in pure shearloaded laminates [47], no such effect was observed in this studydealing with interface fracture involving mode mixity for sandwichcomposites.

5. Conclusions

The manuscript highlights differences in experimentallyobserved response of interface fracture toughness of samples withand without MWCNT. Out of the different process of addition ofMWCNT in the PVC-core/glass-fiber/epoxy-resin sandwich (ascan be obtained from literature), a simple method of sonicatingthe MWCNT in epoxy resin has been chosen in this research todemonstrate significant improvement in mechanical performanceof samples against interfacial delamination. The research demon-strates an improvement of 14.4% in peak load carrying capacityfor samples with 1.5 wt% MWCNT (weight % being taken withrespect to amount of epoxy resin used for sample fabrication) incomparison to samples without MWCNT. Good dispersion ofMWCNT was also observed with 1.5 wt% MWCNT addition in com-parison to other percentages of addition of MWCNT by weight.Samples with 1.5 wt% MWCNT recorded an increase in fracturetoughness values of 34.34% in comparison to samples without.Fracture micrographs also demonstrated good dispersion and fiberbridging of MWCNT in the epoxy resin system. Ductility was alsoobserved to be higher for samples with MWCNT in comparisonto samples without from fracture energy vs. crack length plots ofthe samples.

Acknowledgements

This work was supported by Department of Science and Tech-nology, India under Award No. SR/S3/MERC-035/2010. The firstauthor would like to thank many students and technicians whohad helped him for carrying out the experiments. Any opinions,findings and conclusions or recommendations expressed in thismanuscript are those of the writers and do not necessarily reflectthose of the Department of Science and Technology, India.

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