Investigation of nanomechanical properties of multilayered hybrid nanocomposites

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Sanjeev K. KhannaDepartment of Mechanical and Aerospace

Engineering,University of Missouri,Columbia, MO 65211

P. Ranganathan

S. B. Yedla

R. M. Winter

Department of Chemistry andChemical Engineering,

South Dakota School of Mines and Technology,Rapid City, SD 57701

K. ParuchuriDepartment of Mechanical Engineering,

South Dakota School of Mines and Technology,Rapid City, SD 57701

Investigation of NanomechanicalProperties of the Interphase in aGlass Fiber Reinforced PolyesterComposite Using NanoindentationGlass fiber reinforced polymer composites are widely used as structural materials. Ttwo-component materials can be tailored to suit a large variety of applications. A beunderstanding of the properties of the fiber-matrix ‘‘interphase’’ can facilitate optimdesign of the composite structure. The interphase is a microscopic region around theand hence nano-scale investigation using nano-indentation techniques is appropriadetermine mechanical property variations within this region. In this study the atomic fmicroscope adapted with a commercial nanoindenter has been used to determinvariation of the elastic modulus across the interphase for different silane coatedfiber reinforced polyester matrix composites. A comparative study of the elastic movariation in the various interphases is reported. The results are discussed in the ligthe current limitations of the instrumentation and analysis.@DOI: 10.1115/1.1543966#

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IntroductionFiber-reinforced polymer composites are one of the m

widely used composite materials~Lagache et al.@1#!. The additionof fibers, such as glass fibers, to the polymer matrix dramaticincreases the overall mechanical strength of the composite mrial as compared to the neat polymer. Such a composite consistwo components, namely, glass and a specific polymer.boundary where these components meet is inherent to such mrials and is generally called the interface. The region adjointhis interface is generally referred to as the ‘‘interphase.’’ Tinterphase is a region, which extends nanometers to micronswhich the mechanical and physical properties change frombulk properties of one component~e.g., glass! to the bulk proper-ties of the other component~e.g., polymer!. In the context of glassfiber reinforced polyester composites, the interphase regiovery important, as the load transfer from the matrix to the fiboccurs through this region. The properties of the interphasesignificantly affect the load transfer characteristics and the mroscopic mechanical response of the composite under exteloading. In addition, the fracture behavior and energy absorbcharacteristics of the composite are affected. Hence, knowledgthe factors which can affect the properties of the interphase,an accurate characterization of these properties will be usefuunderstanding such things as adhesion effects and interface faphenomena~e.g., mechanical separation, and crack initiation!. Anunderstanding of a materials performance on a nanometer-sprovides insight and understanding into materials performancea macroscopic scale~Graham et al.@2#, Winter and Houston@3#!and will result in the optimum and safer design of structures uscomposite materials.

A common way to affect the properties of the interphase iscoating the glass fibers with silane coupling agents. Silane coglass fibers are used to improve the adhesion between polyresin and glass fibers~Piggot @4#!, and to reduce the destructiveffects of water and other environmental forces on the adhebetween the two components~Agarwal and Broutman@5#!. Silane

Contributed by the Materials Division for publication in the JOURNAL OF ENGI-NEERING MATERIALS AND TECHNOLOGY. Manuscript received by the MaterialDivision September 7, 2002; revision received May 3, 2002. Associate EditorCate Brinson.

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coatings developed for glass fibers typically have an alkoxy mety attached to the silica group that interacts strongly with glproviding a water resistant bond, and a functionalized endgrthat reacts with the polymer forming a covalent bond~Pludde-mann@6#!. The bonding should be of sufficient strength, becaushas the important role of transferring the load from the matrixthe fiber, which ultimately determines the strength of the compite. However, it has been shown that a very high interfacstrength can result in composites with low fracture toughnewhile moderate bond strengths produce composites with higfracture toughness~Khanna and Shukla@7,8#!.

In a typical glass fiber reinforced polyester matrix composthe fiber diameter is about 10–25mm and the elastic modulus othe fiber and polyester are about 70 GPa and 3.5 GPa, restively. The size of the interphase region is estimated to be 1–3mmin thickness and is an annular region around each fiber~Winterand Houston@3#, Xiaoyan et al.@9#, and Thomason@10#!. Depthsensing nanoindentation~DSI! is one technique, which can possbly measure quantitatively the variations in the elastic properwithin the interphase. It is also possible to measure interphproperties using a unique scanning probe microscope calledinterfacial force microscope~IFM! ~Joyce and Houston@11#!.

Depth Sensing Indenter. The AFM when used in conjunction with a Hysitron TriboScope™ is a depth sensing indentatdevice, which measures the amount of penetration of an indetip into the sample and the corresponding contact load. Aalone historically has been used to obtain topography informatbut in the last decade, it has been applied to qualitatively demine nanomechanical properties,~Vanlandingham et al.@12#,DeVecchio and Bhushan@13#, Cappella and Dietler@14#,Sheiko@15#!.

The depth sensing nanoindenter used in this study~HysitronTriboScope™! couples the nanometer lateral positioning capabties of scanning probe microscopy with the normal force measing capability of traditional indenters. The Hysitron capacitanforce sensor has been coupled with the Digital Instruments pieelectric scanner in the present setup. The Hysitron sensor conof a sharp Berkovich tip~tip radius of the order of 10 nm! locatedon a tip holder fastened to the center of the capacitance fosensor. The sample which sites on top of the piezo electric scais scanned in relationship to the tip using the AFM piezoelecL.

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scanner if a topographical image is desired. When this systeused to indent the sample surface a load versus displacemenis obtained, which is a representation of the mechanical respof the material under investigation. The Hysitron software recothe force curve which is a plot of the force response as a funcof the vertical motion of the scanner. The force-displacement dcollected from this instrument requires careful elastic contanalysis. Such analysis was first pioneered by Bulychev andkhin @16#. Doerner and Nix@17# developed the first convenienmethod to use this theory and later Oliver and Pharr@18# refinedthis method into its most commonly used form. From this analythe contact compliance and material hardness can be extract

The DSI and IFM use indenter tips having a diameter of ten’shundred’s of nanometer. However, accurate measurement oBerkovitch tip shape makes absolute measurements prone to~Doerner and Nix@17#!.

Typical indenter loads for polymer composites are of the orof several micro-Newtons and displacements are of tens to hdreds of nanometers. Hence, the DSI and IFM are suitablemeasuring the mechanical response of a material over the naeter length scale, both lateral and normal to the interrogasurface.

One must however, pay careful attention to the topographynature of the surface being investigated. This aspect of the msurement is especially important when the probing tip diameteon the order of the frequency of the surface roughness and wthe depth of indentation is on the order of the amplitude ofsurface roughness. If the sample has significant surface roughamplitude variation, Greenwood et al.@19#, showed that erroneouresults might be obtained. Lemoine and Laughlin showedatomic force microscopy~AFM! can be used effectively to reveathe surface roughness using tapping mode and contact mode@20#.The AFM is based on a weak cantilever that bends due to atttive or repulsive forces when the silicon nitride tip that is adheto the cantilever is brought into contact, intermittent contactnear contact~dependent upon the mode of operation! with thesample surface. A laser is focused on the cantilever above thand is reflected to a split photodiode detector which then sethe deflection of the cantilever. The AFM can be used in the scning mode to measure the surface topography or in the force mto perform qualitative indentation tests. In this study the AFMused only to obtain topographical information.

In the present study it should be noted that only a very smannular layer surrounding the fiber, of the order of few micro~few thousand nanometers! in thickness, is being investigated. Details for polishing monolithic and glass fiber reinforced polymsamples to obtain surfaces suitable for nano-mechanical stuhave been provided by Paruchuri@21#.

In this paper, the nanometer-length scale interrogation of gfiber reinforced polyester composite samples is presented.chanical property changes across the glass-polyester interpregion are discussed. Specifically, the variation of the elamodulus over the interphase region has been obtained using dsensing nanoindentation for different silane coatings~i.e., cou-pling agents! applied to the glass fiber surface. The resultsdiscussed in the light of the current limitations of such studiMeasurement of the topography of the surface is also a veryportant issue in nano-mechanical investigation, and hence isbriefly presented here.

Experimental ProceduresThis section provides specific details on the processing and

face preparation of fiber-reinforced polyester samples, for nascale investigation, and the nano-indentation experiments.

Preparation of Glass Fiber-Reinforced Polyester„GFRP…Composites. The unsaturated polyester(-(CH25CHRCO)n2,Ashland Specialty! was cured using 1 percent by weight of methethyl ketone peroxide~MEKP, C8H18O6 , Aldrich Chemical Co.!,an initiator, and 0.035 percent by weight of cobalt octano

Journal of Engineering Materials and Technology


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(CO,@CH3(CH2)3CH(C2H5)CO2#2Co), Aldrich Chemical Co.!,an accelerator. All the above mentioned components were toughly mixed for 2-3 min by hand in a plastic container. Care wtaken that no visible air bubbles were trapped in the mixture. Tmixture begins to gel after about 45 min, hence there is enotime for any visible air bubbles to escape. Cylindrical specimewere prepared by pouring the reactant mixture in plastic cylindhaving an inside diameter of 15 mm. Prior to the introductionthe reacting mixture E-glass fibers pretreated with a silane coawere aligned axially in the center of the plastic cylinder andcured at both ends so that the fibers remained centered incylinder and taut. Two silane coupling agent pretreatmenamely octyl trimethoxy silane~OTS, CH3(CH2)7Si(OCH3)3 and3-~methacryloyloxy! propyl-trimethoxy silane ~MPS,CH2C(CH3)CO2(CH2)3Si(OCH3)3 , were used. OTS couples thpolyester matrix and the glass fibers with weak~;1 kcal/mole!van der Waal secondary bonds, whereas MPS couples theseconstituents with strong~;100 kcal/mole! covalent bonds. Thesamples were allowed to cure at room temperature~;26°C! fortwo days, and then post-cured at 52°C for 4 hours followby 63°C for 5 hrs to ensure complete curing of the polym@Khanna~22!#.

Polishing Techniques for Glass Fiber Reinforced Polyester„GFRP…. Samples of about 3 cm in height were cut from tcured cylindrical sample. The sample was held in a sample hoand polished manually. The cross sectional area of each cylincal sample was first polished on silicon carbide~SiC! 400 gritpaper followed by a 600 grit paper~PSI Testing Systems, Inc!,using a dry polishing technique for 15 minutes each. Thesepapers were used to reduce the gross surface roughness ospecimen. Then each sample was polished on a diamond poliscloth ~Allied High Tech products, Inc! with a 6 mm diamondsuspension~Buehler, Inc! and Metadi fluid as an extender~Bue-hler, Inc! for 30 minutes. From this stage onwards the wet~aque-ous! polishing technique was used. The next polishing step wperformed with a 1mm diamond suspension~Buehler, Inc! andMetadi fluid as an extender on Mastertex polishing cloth~Buehler,Inc! for 30 minutes.

The samples were then polished with sub-micron colloisilica suspensions on Mastertex cloth~Buehler, Inc!. OP-S~Struers Co.! which is a colloidal silica suspension, with a particsize of 0.04mm was used first. It has a pH value of 8, and henis basic in nature. The OP-S solution causes etching of the gfiber. The sample was polished with OP-S suspension on Matex cloth for 75 minutes with distilled water as a lubricant. Aftthis step, the sample was polished with the OP-U suspen~Struers! on Mastertex cloth for 1 hr and 45 minutes with distillewater as a lubricant. OP-U contains colloidal silica particles hing a particle size of 0.04mm. It has a pH value of 10, which isalso basic in nature and etches the glass fiber. The only differebetween OP-S and OP-U solution is in the pH value. The samwere washed in soap and distilled water between polishing sto prevent contamination of the polishing compounds. Finallypolished samples were cut to a thickness of approximately 5using a diamond saw~Buehler!. More details regarding the polishing procedure can be found in the reference, Paruchuri@21#.

Results

Surface Roughness of GFRP Samples.The AFM was usedto determine the surface topography of the samples polishedcording to the procedure described previously and elsewhere@21#.Figures 1 and 2 show a 60mm360mm AFM image acquired intapping mode of a polished GFRP sample. All AFM images wnot filtered or otherwise processed in any way. Figure 1 showssurface plot, which is a three-dimensional representation oftopography of the fiber-polymer model composite while Fig.shows the section analysis. The section analysis allows examtion of the topographical data along a line and in particular, atpositions where markers were placed. In Fig. 2 the markersplaced in such a way that one marker is on the edge of the fi

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and another marker is in the interphase region. In this waysurface roughness amplitude for three different locations wanalyzed on the same fiber. Based on the three analyses, themean square~rms! surface roughness amplitude obtained is of torder of 10 nm. The rms roughness is the root mean square overtical deviations of the surface profile from the mean line. Tsection lines were also analyzed for surface roughness frequ

Fig. 1 Surface plot of GFRP sample using the AFM

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by enlarging the section analysis line graph and determiningfrequencies of roughness and using the power spectral defeature of the AFM software. The power spectral density ofrough surface is a statistical function, which provides the spafrequency composition of the surface roughness profile in termthe root mean square roughness value of each frequency conent. The average or the mean square frequency of the pospectral density when inverted gives a measure of the avespatial wavelength of the surface roughness profile. On avethe surface roughness frequency was;10 mm/cycle as deter-mined from the power spectral density feature of the AFM soware. Thus the surface roughness amplitude of less than the 1was ten to twenty times smaller than the typical indentation deand the surface roughness frequency of 10mm/cycle was at leastfifty times larger than the radius of the tip of the Berkovich identer of the DSI.

Measurement of Load-Displacement Characteristics andAnalysis. In a typical depth-sensing indentation test, load is aplied to an indenter, which is in contact with the surface of tspecimen following a user defined load-time sequence underdefined constant loading rates. The depth of penetration ofindenter into the sample and the force are continuously recorfor one complete cycle of loading and unloading. A loadisplacement curve is obtained from which the unloading datathen analyzed according to a model for the deformation ofelastic half space by an elastic punch, which relates the conarea and contact stiffness at the peak load to the elastic mod~Oliver and Pharr@18#!. The indenter used in the present studythree-sided pyramidal Berkovich diamond indenter. A typic

Fig. 2 Section analysis of a GFRP sample surface using the AFM

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Fig. 3 A typical load-penetration curve obtained during nano-indentation of afused quartz sample

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load-penetration curve and the parameters obtained from a desensing system, during the indentation of the standard fuquartz sample to a maximum load of 800mN, is illustrated inFig. 3.

The indentation load-displacement curve obtained by desensing indentation system is analyzed according to the Oand Pharr@18# method. According to this method, the unloadincurve of the load-displacement curve is fit to a power lfunction.

P5A* ~h2hf !m (1)

whereP5 load ~mN!, h5penetration depth~nm!, hf5final depth~nm!, andA and m5power law constants.

The derivative applied at the maximum load point givescontact stiffness and along with other parameters the redumodulus of the sample is obtained. The equations describing tparameters are given by

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Calibration Experiments and Results. A fused quartzsample was indented with loads ranging from 100mN to 3600mN. This range of loads provided a range of contact depths fr10 nm to 125 nm. The elastic moduli obtained for different loais shown in Fig. 4. It should be noted that the elastic moduvalues obtained for loads greater than 800mN ~or a contact depthgreater than 40 nm! are nearly constant at 72 GPa, which is telastic modulus of bulk quartz. For loads less than 800mN there islarge spread in the values of the elastic moduli. Possible reafor the spread at low loads are discussed later. Thus for the

Materials and Technology

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system used in this study and quartz and other hard materialsas glass, an indenter load of 800mN or higher was used to obtainaccurate values of the elastic modulus.

Similar calibration experiments were done using the actglass fiber reinforced polyester samples. Nano-indents wereformed in the polyester matrix region with two different loads athe results are shown in Fig. 5. It was found that a load of 300mNresults in an elastic modulus value of approximately 5.74 G60.11 GPa and at 200mN in an elastic modulus of 7.4561.5 GPa. However, these values of the polyester bulk moduare higher than that measured using tensile testbars of thepolyester, which is 1.860.15 GPa. Other investigators~Randalland Julia-Schmutz@23#, Pharr et al.@24#! have also found a simi-lar deviation in the elastic modulus measurements on polymand this behavior is further discussed later. The indent size atmN of 1.3 mm was considerably larger than the indent size of 0mm at a peak load of 200mN. The lower load was used to gainimprovement in lateral spacing of the indents in an effort to dtermine the elastic modulus profile within the interphase regio

Fig. 4 Elastic modulus of fused quartz as a function of contactdepth for different indenter loads

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Effect of Silane Coatings on Elastic Modulus Variation inthe Interphase. Glass fibers coated with two different silaneand uncoated fibers were embedded in the polyester matrix,four different samples were investigated to determine the variain the elastic modulus across the fiber-matrix interphase regThe samples interrogated with the DSI were:~i! P16f ~with nosilane coupling agent on the fiber!, ~ii ! P17MPSf~with MPS si-lane coupling agent on the fiber!, ~iii ! P17OTSf~with OTS silanecoupling agent on the fiber!, and ~iv! 2P17OTSf~the same asP17OTSF, indented for second time around a different glass fib!.The results of the elastic modulus variation as a function ofdistance from the embedded fiber surface are shown in Fig. 6~a!.For a distance of less than 0.75mm from the fiber surface themeasured elastic modulus values are much higher than thapolyester~see Fig. 6~b!!. This is due to the interaction of the strefield created by the indenter with the glass fiber. The elastic molus values tend to converge to one value at a distance of appmately 4mm from the fiber surface for both the coated and ucoated fibers. Hence, for these samples, it may be inferred thasize of the interphase is approximately 4mm. The samples withthe OTS coating show~Fig. 6~a!! a minimum in the elastic modulus values around 1mm distance and then the modulus increasasymptotically to the bulk polyester modulus of approximately

Fig. 5 Elastic modulus versus contact depth in the polyestermatrix

Fig. 6 Variation of Elastic modulus with the distance from thefiber surface for various silane coated glass fiber polyestersamples for „a… distance greater than 0.75 mm and „b… distanceless than 0.75 mm

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GPa. However, for glass fibers coated with the MPS coating sa trend is not noticed and the modulus drops monotonically beachieving a constant elastic modulus value of approximatelGPa at a distance of 4mm from the fiber surface. One explanatiofor this behavior is that the MPS coating creates a relatively hstrength bond and a more brittle interphase, while the OTS cing results in a softer interphase region closer to the fiber surfComparison of the results for P17OTSF and 2P17OTSF samshow the good repeatability of the nano-indentation tests.sample with no coupling agent was loaded to 300mN. A constantvalue of the elastic modulus, within experimental error, is oserved as a function of distance from the fiber surface.

DiscussionApplication of depth sensing indenters to polymeric materials

currently a vigorous area of research. Often the values obtafor the elastic modulus using DSI’s are found to be significanhigher than moduli determined from more traditional techniquLucas et al.@25# measured the elastic modulus of polytetrafluorethylene ~PTFE! reporting a value of 1.2 GPa when usingnanoindenter. However, when the elastic modulus values wmeasured using a traditional tensile test and with dynamicchanical analysis, they obtained values of 0.4 GPa and 0.5 Grespectively. Elastic modulus values determined from nanoinders generally show an increase as the depth of penetration iscreased. It is not clear whether this ‘‘indentation size effect’’real or an artifact of indenter tip defects or other experimenparameters. Vanlandingham et al.@26# have reviewed the use onanoindenters for measuring elastic modulus of polymeric mrials. Their review indicates that there are still several challenthat must be overcome before reliable quantitative results caobtained. Major hurdles include the use of suitable referenceterials for tip shape characterization~Dongmo et al.@27#! are re-quired for data analysis per Oliver and Pharr@18# and appropriatehandling of visco-elasticity, which is not accounted for in thcurrent analysis procedure. Recent efforts in AC modulus mping of polymeric materials employing nano indenters~Asif et al.@28#, Houston and Winter@7#! hold promise to provide dynamicmoduli but require additional refinement.

Quantitative mechanical property measurements are difficuachieve with the AFM and DSI for numerous reasons~Cappellaand Dietler@14#, Sheiko@15#!. First of all for AFM, the mechani-cal response of the cantilever and sample require deconvoluThe mechanical response of the cantilever must be determinpriori if the mechanical properties of the sample are to betained, which requires considerable effort and uncertainty. Indition, for AFM and DSI the tip shape and the contact area tharequired for mechanical property analysis is difficult to ascertwith the needed precision. Figure 7 shows the pile up behavioP17OTSF samples. The higher magnification of Fig. 7~b! showsthe pile up of the polymer around the indentation very clearly. Teffect of pile-up is that it causes an overestimation of the moduvalues of the sample due to an underestimation of the true conarea by the nanoindentation technique.

Additional uncertainties include hysteresis and creep ofscanning piezo, thermal and mechanical drift of the AFM and Dsystems, lateral motion of the AFM tip during indentation, nolinearity of the AFM deflection sensor, and the appropriate hdling of the visco-elasticity of polymeric materials during mchanical property analysis~e.g., Hertzian contact mechanics!,which rely on the mechanical response being elastic in na~Cappella and Dietler@14#, Sheiko@15#, Mereanukroh et al.@29#!.Vanlandingham et al.@30,31# have applied the AFM and deptsensing nanoindentation to polymeric materials, both thermoptics and thermosets. They have demonstrated that, although qtitative results are difficult to obtain~Vanlandingham @32#!,the AFM and depth sensing nanoindentation are very usefuobtaining comparative results~Vanlandingham et al. @33#,Bogetti et al.@34#!.



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In summary, nano-indenting instruments can provide uniqmechanical property profiling across layers of fractions of microto tens of microns thick. These techniques hold promise forvealing critical chemical and mechanical property relationshipsinterfaces and within interphases. Quantitative results are diffito achieve given the challenges of accurate tip shape measment, the appropriate modeling of sensor-sample mechanicateraction and visco-elasticity. Despite these limitations we hashown within the interphase striking differences between differcoupling agents and between having coupling agents andWith refinement, these measurements can be used to advannite element codes that are designed to model stress-strain beior of the systems studied, thus providing opportunity to numecally test tailored and designed interphases for specific structapplications.

AcknowledgmentThe support of the National Science Foundation under gr

CMS 0196428 is gratefully acknowledged.

Fig. 7 Surface plot showing pile-up of polymer in P17OTSFsample during indentation with a Berkovich tip „a… low magni-fication view, and „b… high magnification view

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tephase on the Elastic Behavior of Composite Materials: Theoretical andperimental Analysis,’’ J. Compos. Mater.,28~12!, p. 1140.

@2# Graham, J. F., McCague, C., Warren, O. L., and Norton, P. R., 2000, ‘‘SpatResolved Nanomechanical Properties of Kevlar® Fibers,’’ Polymer,41, p.4761.

@3# Winter, R. M., and Houston, J. E., 1998, ‘‘Interphase Mechanical PropertieEpoxy-Glass Fiber Composites as Measured by Interfacial Microscopy,’’Pro-ceedings of the SEM Spring Conference on Experimental and Applied Mecics, Society of Experimental Mechanics, Bethel, CT, pp. 355–358.

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@5# Agarwal, Bhagwan D., and Broutman, L. J., 1990,Analysis and Performanceof Fiber Composites, John Wiley & Sons, Inc., New York, Chap. 2.

@6# Plueddemann, E. P., 1982Silane Coupling Agents, Plenum Press, New York,Chap. 2.

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@8# Khanna, S. K., and Shukla, A., 1993, ‘‘Energy Absorption Mechanisms DurDynamic Fracturing of Fibre-Reinforced Composites,’’ J. Mater. Sci.,28,p. 3722.

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Investigation of nanomechanical properties of multilayered hybrid nanocomposites

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