Proceedings of SerbiaTrib '13

2 12(9)(10)Where E MW WCNT and E m are the Young’s moduluss forthe MWCNTs and matrix respectively while v MW WCNTand l/d are the volume fraction andaspect ratioo ofMWCNTs respectively. From Eq. 7 it can be seenthat E NC strongly depends on the geometry of f theMWCNTs such as theiraspect ratio. The lengthh ofthe fibres ranges from 1-25μm while variousdiameterswere measuredas seen in Figure 8. TakingΕ MWCNT = 1GPa whichh is muchgreater thanE m =3.3GPa the predicted values versus the volumefraction of the nanotubes of E NC based on Eq. 7 isshown in Fig. 9.It can be seen that the Halpin–Tsaiformula for d=5nm gives a slight different valuee forE NC compared to the ones measured from thenanoindentation using the calibrationprocedure.Figure 8.Measurementss of the outer diameter of theMWCNTs.Figure 9. Comparison of the experimental modifiednanoindentation results with the Halpin-Tsai anddThostenson-Chou models.Thostenson and Chou [20]modifiedd the HalpinTsai theory towards its applicability tonanotubereinforced composites. Thostensonand Chouconsidered that, in the case ofMWCNT, only theouter shell would w carryy the load as logicallassumption off the relativelylow bonding withinner layers. According A to this assumption, theeffective MWCNT elasticmodulus was evaluatedby considering the application of all loads only totheouter crosssection(outer diameter andgraphite layerr thicknesss which iss taken ast=0.34nm). Eq. 11 has been derived in orderrcalculate the maximumobtainableE NC for acomposite withh aperfect distribution of the CNTsandimpregnation within the epoxy. Predictionscomputed by using Thostenson-Chou modelshow a reduced level of efficiencyfor largediameters while for d=3nm the prediction iscompared welll with the experimentally deriveddmodulus for 1% %wt (0.56%vf) MWCNTS.This occurss despite thee fact that as shown bytheSEM investigations there are locally highernanotube concentrationswithin the composite.Accounting for any errors associated with theexperimentallyy derivedvaluesthe results have tobe interpretedd as a lower boundary of theobtainable moduli.Additionally, the presence ofvoids developed during mixing the hardener withtheMWCNT/ /epoxy-suspension via ultrasonicmixing and mechanicall stirring may haverestrainedtheecompositesfrom their fulllmechanical performancepotential. The highhviscosity disabled a fully adequate degassing ofthenanocomposite with voids remaining in thematrix.The initial failuree had been caused bythese voids and expressedd itself in the t reduceddfracture strain in the tensile tests.It is clear therefore t that despite the fact thattthemodels used in this work are valuable toolstowards the prediction of the elastic modulus ofthenanocomposites they do not totally correctlyrepresent thevarious issues associated with thecontent, morphology and type of f nanotubesincorporating a variety of f diameters and lengths.Also, and most importantly they consider thenanotubes agglomerated-free compared with experimentaldata.whichh may bemisleading when 37013 th International Conference C onn Tribology – Serbiatrib’13