research article microwave assisted …downloads.hindawi.com/archive/2016/7058649.pdfresearch...

Research ArticleMicrowave Assisted Manufacturing and Repair ofCarbon Reinforced Nanocomposites

Edward D Sosa12 Erica S Worthy3 and Thomas K Darlington4

1Texas State University San Marcos TX USA2Johnson Space Center Engineering Technology and Science (JETS) Contract Jacobs Houston TX USA3National Aeronautics and Space Administration Johnson Space Center Houston TX USA4nanoComposix San Diego CA USA

Correspondence should be addressed to Edward D Sosa edsosa11gmailcom

Received 11 June 2016 Revised 17 August 2016 Accepted 30 August 2016

Academic Editor Yanqing Yang

Copyright copy 2016 Edward D Sosa et alThis is an open access article distributed under the Creative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

We report a composite capable of advanced manufacturing and damage repair Microwave energy is used to induce thermalreversible polymerization of the matrix allowing for microwave assisted composite welding and repair Composites can be bondedtogether in just a few minutes through microwave welding Lap shear testing demonstrates that microwave welded compositesexhibit 40 bond strength relative to composites bonded with epoxy resin Double cantilever beam testing shows 60 recovery indelamination strength after microwave assisted composite repair The interfacial adhesion and composite repair after microwaveexposure are examined by X-ray computed tomographyThemicrowave processing is shown to be reproducible and consistentTheability to perform scalable manufacturing is demonstrated by the construction of a large structure from smaller components

1 Introduction

Composites offer an immense potential for reducing energyexpenditure during the operation of systems composed ofthese materials due to their light weight This is of significantinterest to the aerospace and transportation sectors sincecomposite materials can lead to the production of more fuelefficient aircraft and vehicles Composites are ideal candidatesfor structural components in aircraft automobiles trainsand space exploration systems since their mass savingstranslate directly into lower operational cost Unfortunatelythe realization of operational savings resulting from lowerenergy expenditure has been overshadowed by cost asso-ciated with composite manufacturing and the concerns ofdamage toleranceThermal processing precision machiningbonding and low damage tolerance all result in increasedcost of the manufacturing assembly and maintenance ofcomposite structures

Conventional heatingmethods used for the production ofcomposites can drive up manufacturing cost as a result of theinefficient heat transfer processes involved in the curing of

these materials Autoclave curing especially of large parts isuneconomical since much of the heat is lost to the surround-ing environment and the conduction of heat within the partis very inefficient due to the low thermal conductivities ofpolymeric materials Furthermore improper curing can sig-nificantly affect the composite propertiesMicrowaves whichare widely used for communications have found potentialapplication in the heating and processing of polymer basedmaterials [1ndash3]Microwave heating which is volumetric sinceit occurs at the molecular level can consume 100 times lessenergy than conventional heating [4] thus leading to fasterand more efficient curing Microwave heating thereforehas the advantages of reduced processing times and greaterenergy savings during composite production [5] In additionto economical curing microwaves also offer the possibilityof uniform and complete heating of parts regardless of theirgeometries [6] This can be a tremendous benefit sincelarge thick and nonuniform parts are not very suitablefor autoclave curing because undesirable thermal gradientswithin both oven andmaterials canmake it difficult to ensureuniform and complete curing If the microwave field with

Hindawi Publishing CorporationJournal of CompositesVolume 2016 Article ID 7058649 9 pageshttpdxdoiorg10115520167058649

2 Journal of Composites

the chamber is uniform heating gradients will be minimizedIdeally a rotating platform will provide uniform exposureMicrowave processing can therefore provide more effectiveand efficient curing thereby leading to a reduction in the timeand cost of composite manufacturing

Another promising capability of microwaves is theirpotential use for bonding or welding of polymer materialsPolymer composites are generally joined through the useof adhesives or mechanical fasteners These methods canrequire extensive surface preparation for adhesive bondingand can result in localized stress fields around machinedareas necessary for the placement of fasteners Polymerwelding is seen as an alternative to the aforementionedbonding methods Techniques such as hot plate ultrasonictechnique induction laser friction stir and electromagneticwelding have been proposed [7 8] Microwave welding hasan advantage over the many other forms of welding becausecomplex three-dimensional structures can be configuredsince the entire component can be irradiated simultaneously[9] Unfortunately most polymers have poor microwaveabsorption properties thereby requiring the use ofmicrowaveadsorbent additivesMaterialswith low-to-mediumdielectricloss factors require no microwave absorber materials whilematerials with high dielectric loss factors require absorbingmaterials at the interface [10] Carbon nanotubes (CNTs)are an ideal microwave absorbing material as they readilyabsorb the energy and convert it into heat due to electroniclosses within the CNT structure [11ndash13] Since microwaveabsorption is a bulk process facilitated by the CNTs adequatedispersion is required to reach the percolation thresholdwhich will minimize any thermal gradient within the com-posite Carbon nanotubes can provide a means to locallyheat the interface of two adjoining surfaces allowing formicrowave welding of individual parts Microwave inducedheating occurs only at the interface resulting in lower powerconsumption since the entire material does not need to beheated The exceptional strength of CNTs can also provideadditional reinforcement of the weld joint through bridgingacross the interface Several studies have demonstrated theability to use CNTs for microwave welding of thermoplasticthermoset and nonpolymeric materials both efficiently andeffectively [14ndash18] The use of microwaves for the weldingof composite materials can lead to new developments incomposite technology particularly in the area of additivemanufacturing

Self-healing composites [19ndash21] are an area of extensiveresearch as self-repairing materials can help improve thedamage tolerance of composites Extrinsically healable com-posites utilize external stimuli such as heat electromagneticradiation or pressure to facilitate repair Composites thatcan be internally repaired through the use of heat couldutilize microwaves as the heating source Previous workhas shown that microwaves can be used to rapidly heatthermally healable composites consisting of a bismaleimidetetrafuran (2MEP4F) matrix [22 23] Microwave heatingis demonstrated to be an efficient and effective method ofheating An additional advantage of using microwaves as theheat source as opposed to electrical resistance heating is theability to locally heat the material Localized heating provides

healing only of the area to be repaired thereby not subjectingthe entire structure to softening which could jeopardizethe structural integrity if under load The thermal energysupplied by the microwaves allows reversible Diels-Alderchemistry within the bismaleimide tetrafuran (2MEP4F)matrix that facilitates the healing through polymer mobil-ity Since composite welding is also facilitated by polymermobility it only stands to reason that microwave assistedbonding of these composites should be possibleThe 2MEP4Fresin system that constitutes the matrix of these compositesis thematerial that provides themeans for composite weldingand repairThermal reversible crosslinking between the furanand maleimide groups of the polymer allows for mobilitywithin the matrix when heat is applied The mobility resultsin the ability to bond matrix materials across the damage siteor welding interfaces The carbon nanotube additive merelyfacilitates heating through its microwave absorption prop-erty We report preliminary microwave welding experimentson 2MEP4F carbon fiber composites

2 Experimental

Composite panels were fabricated using multiwall carbonnanotubes (MWCNTs) deposited onto carbon fibers followedby infiltration with a bismaleimide tetrafuran (2MEP4F)resin Carbon fibers were spray coated using a dispersedsolution of MWCNTs as previously reported [20] Unidi-rectional carbon fiber composites were then fabricated bycustom resin transfer molding of the 2MEP4F resin [24 25]This procedure was used to fabricate six-ply unidirectionalcomposite panels measuring 10 cm times 10 cm A total of fivepanels were fabricated of which one panel had a Teflon sheetinsert at the midplane of the composite the 5 cm times 10 cmTeflon insert spanned half the area of the panel This particu-lar panel was used for double cantilever beam testing inwhichthe insert provided a simulated crack in order to measuredelamination properties The other four composite panelswere used for lap shear testing All the fabricated panels werethen dryly cut into approximately 1 in times 4 in rectangularpieces using a mill

Microwave processing of composites was accomplishedusing a Microwave Research and Applications Inc BP-111Microwave Processor which operates at 245GHz and hasvariable power up to 1000 W The microwave processor isequipped with the True-To-Power power control system thatallows sample temperature regulation through power adjust-ment of the magnetron source The oven was modified toallow continuousmonitoring of the sample thermal profile AFLIR A35SC infrared camera is mounted on a bored openingon the top of the microwave processor chamber A wire meshgrid is placed on the inside of the opening while a germa-nium window was placed outside that opening to preventmicrowaves from escaping This design allows for real-timethermal imaging of the sample under microwave exposureFixtures fabricated in-house out of Mylar block allowedpressure clamping of composite panels during microwavewelding and repair Two sets of fixtures were fabricated onewith a circle of 1-inch diameter bored at the midpoint of the

Journal of Composites 3

(a) (b)

Figure 1 (a) Lap shear test specimens prior to microwave welding shows metal sleeves used to expose only bonding area to microwaves (b)Clamping fixture with 1-inch-diameter hole bored into Mylar blocks to try to simulate a focused microwave source Clamp set 2 consisted ofMylar blocks without hole in the center and with no metal tape covering of blocks

Pressure PressureTensile load Tensile load

Microwave welding

First lap

Welding of fractured

Second lapsurfaces

shear testshear test

Figure 2 Lap shear testing schematic As-manufactured composites are cut to ASTM defined dimensions Panels are overlapped over a 1-inch-by-1-inch area and held in place by clamps The panels are heated by microwaves to initiate welding and the test After testing weldingis repeated to demonstrate reproducibility

6-inch-long rectangular block with the other being a solid 3-inch block The 6-inch blocks were masked with metal tapeto try to simulate a focused microwave source The pressurebetween the blocks was applied by nylon screws Only thearea to be welded or repaired was exposed to microwavesThe rest of the test specimen was covered with a sleeve madeof metallic tape to prevent heating of that portion of thecomposite (Figure 1(a)) The sleeve had a Kapton tape innerliner to prevent arcing between carbon fiber and metal orfusing of the sleeve to the composite

Lap shear testing was performed to determine the inter-facial bond strength of microwave welded composite relativeto adhesively bonded composites A schematic of the exper-imental test plan for lap shear testing is shown in Figure 2Testing was conducted in accordance with ASTM D5868Test specimens were of the length and width specificationsas defined by the standard however the thickness of 10mmwas below the nominal value of 25mm The thickness ofthe specimens was limited by the mold used to fabricate thecomposite panels All other standard requirements were metLap shear test specimens consisted of two of the rectangularpieces bonded across a 1-inch-by-1-inch overlap area A totalof seven lap shear test specimens were prepared Six weremade by microwave welding of the bond area while thereference test specimen was bonded using a two-part 3MScotch-Weld 2216BA epoxy adhesive This adhesive wasalso used to bond a set of one-inch glass fiber grips on eachend of the test specimens after the microwave welding and

epoxy bondingwas performed Testswere conducted onMTSInsight load frame using a 2000 lb load cell

Double cantilever beam (DCB) tests were conductedto determine the retention in delamination strength aftermicrowave assisted composite repair The panel fabricatedwith a 13-micron-thick Teflon insert was used for these testsSimilar procedures as outlined inASTMD5528were followedwith the exception that the length was below the specified fiveinches The length of the specimens was limited by the moldused to fabricate the composite panels Piano hinges weremounted with epoxy adhesives onto the panel sides wherethe insert was placed to allow pulling apart of the panelPrior to testing the initial crack initiation point for eachsample was marked with silver ink by imaging the side of thesample under a Zeiss Discovery V20 Stereomicroscope Testsamples were pulled apart using the same MTS Insight loadframe Crack propagationwasmonitoredwith a video cameraduring DCB testing and was also measured afterwards underthe stereomicroscope and by X-ray computed tomography

Microwave processed specimens were analyzed to exam-ine the effectiveness of welding and repair as well asany damage induced by microwave exposure Compositeswere characterized after cutting in order to identify anypre-microwave processing damage that could be attributedto composite manufacturing or cutting X-ray computedtomography (CT) was used for inspection of the interfacialbonding at the weld internal composite damage and damagerepair


(a) (b)

025 in

(c)

Figure 3 X-ray computed tomography images of composite panels (a) after cutting of fabricated panels (b) after first microwave weld cyclewith clamp 1 and (c) after second improvedmicrowaveweld cyclewith clamp 2 As-fabricated samples had low internal defect concentrationsThe first weld cycle shows low interfacial bonding between panels as well as high porosity due to internal voids resulting from rapid thermalexpansion and nonuniform pressure Second weld cycle exhibits better interfacial bonding and repair of internal damage however someregions in some regions throughout the thickness still showed incomplete bonding

3 Results

Prior tomicrowavewelding composite panels were inspectedby X-ray CT to detect whether any defects or damage wasinduced by the cutting of panels CT scans provided 3D imag-ing of some regions throughout the thickness of compositefrom each direction allowing visual inspection of any signifi-cant cracks or voids within the samples All composite panelswere paired labeled and color coded (green red yellow andorange) to keep track of the faces and sides that were tobe bonded so that subsequent CT scans always monitoredthe same locations CT scans of the double cantilever beamsamples were used to visually identify crack propagation andrepair therefore pretest analysis was conducted on thesesamples

The first set of lap shear test samples (Samples 1ndash3) wereprepared bymicrowave welding using the clamp fixtures with1-inch diameter cut out (Figure 1(b)) This first set consistedof three samples that were heated to the required temperature(sim90∘C) for the occurrence of bond dissociation betweenthe furan and maleimide groups This temperature was heldfor approximately 3 minutes to allow welding to take placeThermal imaging of the composite welding showed that insome panels the edges heated more rapidly with heat pro-gressing inward until eventually the 1 inch times 1 inch weldingarea reached thermal equilibrium It was also observed thatarcing occurred along the edges of these samples and at timesbetween the samples Severe arcing resulted in damage tothe composite exposing some of the carbon fibers along withdecomposition of the polymer matrix CT analysis of thewelded composite panels showed that internal damage wasinduced by arcing effects or rapid heating resulting in voidformation (Figure 3(b)) It is possible that prestress existswithin the composites duringmicrowaveweldingmost likelyat defect sites associated with manufacturing or machiningof the composite structure Another internal stress that canexist during microwave processing is thermal stress inducedby rapid or nonuniformheatingThese stresses are what likelyresult in the blistering observed in CT data CT analysisalso showed that the interfacial bonding across the weld areawas poor The pressure applied by the clamps will influencethe weld unfortunately due to safety concerns strain gaugeswere not bonded to the test specimens because it was notknown how they would react to microwaves Lap shear tests

supportedCT analysis since themicrowaveweld strengthwasonly 10 of the strength of the epoxy bonded compositeThe value of 10 is established by taking the ratio of theultimate tensile load of the best welded composite to epoxybonded composite It was concluded that the open area of the110158401015840-diameter hole did not provide sufficient pressure acrossthe bonding area to facilitate polymer mobility across theinterface of compositesThe clamping devices were simplisticand a more sophisticate design of clamps that enables con-trolled pressure will provide more reliable welds

In order to provide a more uniform pressure across thewelding area the solid 3-inch clamp was used for subsequentmicrowave processing Samples 1ndash3 were rewelded in similarmanner followed byCT inspection before conducting the sec-ond set of lap shear tests During the microwave processingin situ thermal imaging displayed more uniform heating andthe absence of arcing during exposure CT imaging showedthat the rewelded composites had greater interfacial bond-ing and underwent damage repair (Figure 3(c)) Previousvoids within the composite were eliminated after a secondmicrowave exposure displaying the healable nature of thecomposites A second set of lap shear test specimens (Sam-ples 4ndash6) were welded to establish the reproducibility andconsistency of the welding process Similar observations wereobserved in both welding characteristics and mechanicalproperties

After the rewelding of Samples 1ndash3 the percentage ofstrength relative to the epoxy bonded composite increasedfrom 10 to 40 While the rewelded composites exhibithigher retention in strength at the weld analysis of CT scansacross the entire weld thickness identified regions wheregaps still existed between the panels This indicates thatcomplete bonding between the two welded panels was notfully achieved and translated into a lower-than-desired bondstrength of at least 75 It is expected that greater strength canbe accomplished if complete bonding can be achieved Futurework will investigate the use of a reinforcement agent such asa wire mesh or thin sheet of nanotube ldquoBuckyrdquo paper SinceDiels-Alder chemistry can occur between the CNT side walland the polymermatrix [26] it is believed that direct bondingbetween CNT and matrix may have significant improvementin the weld strength In any case welding was shown to bereproducible with comparable or better weld strengths aftereach welding cycle


0

200

400

600

800

1000

1200

1400

0 01 02 03 04 05 06 07 08 09Extension (in)

Epoxy controlSample 1Sample 2Sample 3

Sample 4Sample 5Sample 6

Load

(ftmiddotlb

)

Figure 4 Lap shear tests of microwave welded composites relativeto an epoxy bonded control sample Microwave welded compositesexhibit higher modulus but lower ultimate strength Welded com-posites Samples 1 4 and 5 exhibit sudden decrease in load followedby continued load increase which is attributed to partial shear of theweld plane

Figure 4 shows the lap shear test results of all six weldedsamples relative to the epoxy controlThe curves show similarbehavior among the samples First the modulus portion ofthe curves indicate that the welded composites exhibit higherstiffness than the epoxy bonded composite Second the lapshear curves show a region of yielding with increased loadfollowed by failure Finally some composites exhibit suddenload decrease followed by load recovery The observation ofhigher stiffness is attributed to the intermolecular bondingbetween the polymers at interface The bonding at interfacein the welded composites occurs because of crosslinkingbetween furan and maleimide groups within the 2MEP4Fthermoset resin It is this reversible crosslinking that allowsthe Diels-Alder chemistry which accounts for the thermallyreversible polymerization and thus welding The interfacialadhesion within the epoxy bonded composite is not directlyknown but could be due to polar forces or direct chemicalbonding between polymer chain groups of the epoxy and2MEP4F resins In either case it is reasonable to expectthe intermolecular bonding between the 2MEP4F interfacesis stronger than the intermolecular bonding between theepoxy and 2MEP4F Table 1 shows the mechanical propertiesfor both 2MEP4F and epoxy resins While there is nodocumented comparison between the shear strengths of thetwo systems the values obtained by lap shear testing of theepoxy bonded composites are comparable to range of valueslisted in Table 1 The values of 62ndash11MPa correspond todifferent plastic materials bonded with the 2216BA epoxyand are in good agreement with the approximately 1300 psiobtained in our test For homogeneous isotropic materialsa simple relation exists between elastic constants namely

Table 1 Mechanical properties of the thermal reversible 2MEP4Fand the epoxy resin used for bonding of lap shear test samples

Mechanical properties of 2MEP4F and epoxy resin2MEP4F 2216BA epoxy

Compressiontesting

Youngrsquos modulus(GPa) 414a

Overlap sheartestingb

Shear strength(MPa) 62ndash110c

Shear modulus(MPa) 1500c

aReference [27] b3M Scotch-Weld technical data sheet and cASTM D1002

119864 = 2119866(1 + V) In this equation 119864 is Youngrsquos modulus119866 is the shear modulus and V is the Poisson Ratio If Vis small then 119864 = 2119866 The 3M technical data sheet doesnot provide any information on Youngrsquos modulus or PoissonRatio of the epoxy Making the simple approximation thatYoungrsquos modulus is twice the shear modulus would indicatethat the 2MEP4F has higher stiffness While this assumptionis not verified by empirical testing in our laboratories theapproximation supports our observation Future tests willmake direct comparison of such mechanical properties

As the load increases yielding occurs due to stresstransfer into the matrix region near the interface Yieldingcontinues with increased load until ultimate failure occursAfter failure close observation of the composite panelsshowed delamination within the panels since carbon fiberscould be seen in areas that appeared to be abraded Oneof the welded composites exhibited cleavage along the fiberdirection a distance away from the weld area It is proposedthat failure occurs within the matrix as opposed to purely atthe interface It is believed that the microwave induced voidsobserved in the CT analysis are the point of failureThe stresstransfer eventually exceeds the strength in these damagedregions resulting in break of the weld It is believed thatthese abraded regions are where the most effective weldingoccurred within the microwave processed composites but arealso an area of higher defect concentrationThis delaminationeffect was observed in both the welded and epoxy bondedcomposite The final feature observed in some of the weldedcomposites is a sharp decrease in load with ensuing loadrecoveryThis is attributed to shear slip that occurs at bondedregions of the interface As noted previously CT scan showedareas that were not bonded As load is applied to the interfacesome of these weld regions shear resulting in the sharpdecline in load but not total failure Therefore the lower-than-desired weld strength can be attributed to incompleteinterfacial bonding and internal defects It is believed thatcomplete bonding across the weld interface and eliminationof microwave induced defects will result in bond strengthcomparable to that of the epoxy adhesive

Double cantilever beam testing is shown in Figure 5Although specimen dimensions were slightly below theASTM D5528 specifications DCB testing can still providea good measure of delamination recovery and thereforethe healing effectiveness of microwave processing With theexception of Sample 2 before microwave healing all curves


0

1

2

3

4

5

6

7

8

9

0 01 02 03 04 05 06 07 08 09Crosshead displacement (in)

Sample 1Sample 1 healedSample 2

Sample 2 healedSample 3Sample 3 healed

Load

(ftmiddotlb

)

Figure 5 Double cantilever beam testing of composite samples before and after microwave processing (healed) Post-microwave processingsamples exhibit incomplete healing as indicated by lower load tolerance before delamination onset The continued increase in load withcrosshead displacement indicates greater tear resistance

(a) (b)

(c) (d)

Figure 6 Inspection of crack propagation length after DCB testing of as-fabricated composites (a) optical microscopy of sample (b) CTanalysis of Sample 3 (c) CT analysis of Sample 2 and (d) CT analysis of Sample 1 Lengths of cracks correspond to crosshead displacementand crack deflection is clearly visible

exhibit similar slopes prior to delamination This is to beexpected since all the samples are fabricated from the samematerials and are cut from the same manufactured paneland should therefore have the same stiffness The differencein Sample 2 before microwave processing is likely to beattributed to the grips or piano hinges Aside from this differ-ence the curves before and after microwave healing displaysimilar features The first feature is that the crosshead exten-sion corresponds directly to the length of crack propagationFigure 6 shows the crack lengths measured after DCB testingby optical microscopy and CT analysis It is clearly evidentthat the crack length tracks identically with the crossheadextensions in theDCB curvesThe explanation for differencesin crack propagation under similar load is that they may beattributed to lower residual stresses higher degree of crackdeflections or crack bridging by CNTs The second featureobserved in the DCB curves is the onset of delamination thatoccurs at lower loads after microwave assisted repair of the

initial cracks that is shown in Figure 7 This is to be expectedif complete healing of the crack is not achieved For eachsample the load values at the onset of delamination beforeand after microwave processing were used to calculate therepair efficiencyThe onset is established as the point at whichthe load no longer increases and begins to plateau followedby a decrease in load Samples 1 and 2 showed approximately30 recovery while Sample 3 showed 60 recoveryThe finalfeature common to the post-microwave processing curves isthe increase in load after delamination onset It is not fullyunderstood what contributes to the steady increase in loadafter delamination onset however this would tend to indicatebetter tear resistance It could be possible that fiber or CNTbridging results after microwave exposure Previous work onour composites has indicated that the possibility exists thatDiels-Alder chemistry could occur between the polymer resinand the CNTs due to localized heating of the CNTs duringmicrowave exposure [22] If bridging or CNT bonding to


(a)

025 in

(b)

(c)

Figure 7 CT scans of composite panels before and after microwave assisted repair of delamination in (a) Sample 1 (b) Sample 2 and (c)Sample 3 It is observed that complete healing of the delaminated surface is not achieved through the entire thickness of the composite panel

∘C 1000

238

Figure 8 Demonstration of potential advanced manufacturing through microwave welding of a larger structure from small compositecomponents In situ thermal imaging during microwave exposure shows rapid localized heating at the weld points

the matrix is occurring it could explain the observed steadyincrease in load after delamination onset Regardless of theexplanation the fact that healing can be achieved and thatdelamination resistance maybe occurring is promising forimproved damage tolerant composites

To demonstrate that microwave welding of these com-posites can lead to advanced manufacturing a larger struc-tural architecture was constructed from small componentsSmaller rectangular and square pieces were machined toallow mating of pieces The structure was loosely assembledand placed in the oven and then exposed to microwaveswith as little as 40WattsThermal imaging duringmicrowaveprocessing shows rapid localized heating of the compositeinterfaces (Figure 8) The temperature at the weld joints ishigher because of higher microwave absorption of exposedcarbon nanotubes and fibers It is possible that electroniclosses at the interfaces may also contribute to this rapidlocalized heating Previous work with damaged compositesexposed to microwaves shows similar observations [23]Temperature at the weld joints reached above 90∘C in lessthan 10 seconds and the structure was heated for a one-minute period of time after which it was allowed to cool

for one minute All but one of the contact points fusedtogether even without the application of external forces orclamping The strength of the weld points was not measuredhowever excessive pulling by handdid break theweldsWhilethe welds are not strong (in large part because no forcewas applied during the weld) the concept of 3D weldingthrough the use of microwave is demonstrated Forwardwork will involve improving weld strength of these adjoiningstructures This demonstration shows promise for the abilityto weld larger structures from small components which canlead to an advanced manufacturing technique that can bemore cost and time effective

It is expected that several factors will influence the weld-ing efficiency and strength at the jointsThewelding efficiencymay be influenced by the heating time and pressure appliedat the joint These two factors will influence the degree ofbonding as they directly affect the extent of repolymerizationand polymer mobility across the interfaces The microwavepower and heating temperature have minimal influence onthe welding process The microwave system only has tosupply enough power to reach the target temperature of100∘C to initiate repolymerization The weld strength will


be influenced by several factors including thermal gradientspressure heating time defects and additional reinforcementVariations in temperature or pressure across the weld jointmay lead to inconsistent bonding at the interface Insufficientheating time will cause incomplete repolymerization Defectssuch as voids will lower the weld strength Finally theintroduction of additional reinforcement such as a wire meshor chopped fibers will improve strength

4 Conclusion

Theability to usemicrowave energy toweld and repair carbonfiber composites has been demonstrated The incorporationof carbon nanotubes into the carbon fiber composite pro-vides the mechanism for microwave absorption that allowsheating of the thermal reversible polymer that comprisesthe matrix The reversible chemistry of the polymer resinsystem is utilized to weld and repair composites Lap sheartesting shows higher initial stiffness of composite welds ascompared to an epoxy bonded composite which is attributedto greater intermolecular bonding of the interfaces and tothe high modulus of the polymer itself [25] Microwavewelded composites exhibit 40 of the bond strength of theepoxy welded composite Double cantilever beam testing wasused to measure microwave assisted repair of delaminatedcomposites Tests show up to 60 recovery in delaminationstrength after microwave processing Visual inspection ofsome regions throughout the thickness weld and repair by X-ray computed tomography shows incomplete welding acrossthe bonding interface and partial healing of delaminatedsurfaces It is expected that weld strengths on the orderof epoxy adhesive strengths and full repair of compositedelamination can be achieved with improved microwaveprocessing Furthermore microwave welding allows for theconstruction of a larger structure from smaller componentsdemonstrating the ability to use microwaves for advancedcomposite manufacturing

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this manuscript

References

[1] L Zong S Zhou N Sgriccia et al ldquoA review of microwaveassisted polymer chemistryrdquoThe Journal of Microwave Power ampElectromagnetic Energy vol 38 no 1 pp 49ndash74 2003

[2] TWang and J Liu ldquoA review of microwave curing of polymericmaterialsrdquo Journal of Electronics Manufacturing vol 10 no 3pp 181ndash189 2000

[3] J Wei andM C Hawley ldquoUtilization of microwaves in process-ing of polymer composites- past present and futurerdquo PolymericMaterials Science and Engineering ACS Division of PolymericMaterials Science amp Engineering vol 72 pp 10ndash12 1995

[4] P K D V Yarlagadda and T C Chai ldquoAn investigation intowelding of engineering thermoplastics using focused micro-wave energyrdquo Journal ofMaterials Processing Technology vol 74no 1ndash3 pp 199ndash212 1998

[5] E T Thostenson and T-W Chou ldquoMicrowave processingfundamentals and applicationsrdquo Composites Part A AppliedScience and Manufacturing vol 30 no 9 pp 1055ndash1071 1999

[6] W I Lee and G S Springer ldquoMicrowave curing of compositesrdquoJournal of Composite Materials vol 18 no 4 pp 387ndash409 1984

[7] A L Buxton ldquoWelding technologies for polymers and compos-itesrdquo in Proceedings of the IMechE Seminar 2002

[8] T H North and G Ramarathnam Welding of Plastics ASMHandbook Volume 6 Welding Brazing and Soldering ASMInternational 1993

[9] R J Wise and I D Froment ldquoMicrowave welding of thermo-plasticsrdquo Journal of Materials Science vol 36 no 24 pp 5935ndash5954 2001

[10] A P da Costa E C Botelho M L Costa N E Narita and JR Tarpani ldquoA review of welding technologies for thermoplasticcomposites in aerospace applicationsrdquo Journal of AerospaceTechnology and Management vol 4 no 3 pp 255ndash265 2012

[11] K R Paton and A H Windle ldquoEfficient microwave energyabsorption by carbon nanotubesrdquo Carbon vol 46 no 14 pp1935ndash1941 2008

[12] T J Imholt C A Dyke B Hasslacher et al ldquoNanotubes inmicrowave fields light emission intense heat outgassing andreconstructionrdquoChemistry ofMaterials vol 15 no 21 pp 3969ndash3970 2003

[13] P Zhihua P Jingcui P Yanfeng O Yangyu and N YantaoldquoInvestigation of the microwave absorbing mechanisms ofHiPco carbon nanotubesrdquo Physica E Low-Dimensional Systemsand Nanostructures vol 40 no 7 pp 2400ndash2405 2008

[14] P K Bajpai I Singh and J Madaan ldquoJoining of natural fiberreinforced composites using microwave energy experimentaland finite element studyrdquoMaterials and Design vol 35 pp 596ndash602 2012

[15] K B Mani M R Hossan and P Dutta ldquoThermal analysisof microwave assisted bonding of poly(methyl methacrylate)substrates inmicrofluidic devicesrdquo International Journal of Heatand Mass Transfer vol 58 no 1-2 pp 229ndash239 2013

[16] M Kwak P Robinson A Bismarck and R Wise ldquoMicrowavecuring of carbon-epoxy composites penetration depth andmaterial characterisationrdquo Composites Part A Applied Scienceand Manufacturing vol 75 pp 18ndash27 2015

[17] P-C Sung T-H Chiu and S-C Chang ldquoMicrowave curingof carbon nanotubeepoxy adhesivesrdquo Composites Science andTechnology vol 104 pp 97ndash103 2014

[18] U Lopez De Vergara M Sarrionandia K Gondra and J Aur-rekoetxea ldquoPolymerization and curing kinetics of furan resinsunder conventional and microwave heatingrdquo ThermochimicaActa vol 581 pp 92ndash99 2014

[19] D Y Wu S Meure and D Solomon ldquoSelf-healing polymericmaterials a review of recent developmentsrdquo Progress in PolymerScience vol 33 no 5 pp 479ndash522 2008

[20] M R Kessler ldquoSelf-healing a new paradigm in materialsdesignrdquo Proceedings of the Institution of Mechanical EngineersPart G Journal of Aerospace Engineering vol 221 no 4 pp 479ndash495 2007

[21] S D Bergman and F Wudl ldquoMendable polymersrdquo Journal ofMaterials Chemistry vol 18 no 1 pp 41ndash62 2008

[22] E D Sosa T K Darlington B A Hanos andM J E OrsquoRourkeldquoMultifunctional thermally remendable nanocompositesrdquo Jour-nal of Composites vol 2014 Article ID 705687 12 pages 2014


[23] E D Sosa T K Darlington B A Hanos and M J OrsquoRourkeldquoMicrowave assisted healing of thermally mendable compos-itesrdquo Smart Materials Research vol 2015 Article ID 248490 8pages 2015

[24] F Ghezzo D R Smith T N Starr et al ldquoDevelopment andcharacterization of healable carbon fiber composites with areversibly cross linked polymerrdquo Journal of CompositeMaterialsvol 44 no 13 pp 1587ndash1603 2010

[25] J S Park T Darlington A F Starr K Takahashi J RiendeauandHThomasHahn ldquoMultiple healing effect of thermally acti-vated self-healing composites based on Diels-Alder reactionrdquoComposites Science and Technology vol 70 no 15 pp 2154ndash2159 2010

[26] C-M Chang and Y-L Liu ldquoFunctionalization of multi-walledcarbon nanotubes with furan and maleimide compoundsthrough Diels-Alder cycloadditionrdquo Carbon vol 47 no 13 pp3041ndash3049 2009

[27] X Chen F Wudl A K Mal H Shen and S R Nutt ldquoNewthermally remendable highly cross-linked polymericmaterialsrdquoMacromolecules vol 36 no 6 pp 1802ndash1807 2003

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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


the chamber is uniform heating gradients will be minimizedIdeally a rotating platform will provide uniform exposureMicrowave processing can therefore provide more effectiveand efficient curing thereby leading to a reduction in the timeand cost of composite manufacturing

Another promising capability of microwaves is theirpotential use for bonding or welding of polymer materialsPolymer composites are generally joined through the useof adhesives or mechanical fasteners These methods canrequire extensive surface preparation for adhesive bondingand can result in localized stress fields around machinedareas necessary for the placement of fasteners Polymerwelding is seen as an alternative to the aforementionedbonding methods Techniques such as hot plate ultrasonictechnique induction laser friction stir and electromagneticwelding have been proposed [7 8] Microwave welding hasan advantage over the many other forms of welding becausecomplex three-dimensional structures can be configuredsince the entire component can be irradiated simultaneously[9] Unfortunately most polymers have poor microwaveabsorption properties thereby requiring the use ofmicrowaveadsorbent additivesMaterialswith low-to-mediumdielectricloss factors require no microwave absorber materials whilematerials with high dielectric loss factors require absorbingmaterials at the interface [10] Carbon nanotubes (CNTs)are an ideal microwave absorbing material as they readilyabsorb the energy and convert it into heat due to electroniclosses within the CNT structure [11ndash13] Since microwaveabsorption is a bulk process facilitated by the CNTs adequatedispersion is required to reach the percolation thresholdwhich will minimize any thermal gradient within the com-posite Carbon nanotubes can provide a means to locallyheat the interface of two adjoining surfaces allowing formicrowave welding of individual parts Microwave inducedheating occurs only at the interface resulting in lower powerconsumption since the entire material does not need to beheated The exceptional strength of CNTs can also provideadditional reinforcement of the weld joint through bridgingacross the interface Several studies have demonstrated theability to use CNTs for microwave welding of thermoplasticthermoset and nonpolymeric materials both efficiently andeffectively [14ndash18] The use of microwaves for the weldingof composite materials can lead to new developments incomposite technology particularly in the area of additivemanufacturing

Self-healing composites [19ndash21] are an area of extensiveresearch as self-repairing materials can help improve thedamage tolerance of composites Extrinsically healable com-posites utilize external stimuli such as heat electromagneticradiation or pressure to facilitate repair Composites thatcan be internally repaired through the use of heat couldutilize microwaves as the heating source Previous workhas shown that microwaves can be used to rapidly heatthermally healable composites consisting of a bismaleimidetetrafuran (2MEP4F) matrix [22 23] Microwave heatingis demonstrated to be an efficient and effective method ofheating An additional advantage of using microwaves as theheat source as opposed to electrical resistance heating is theability to locally heat the material Localized heating provides

healing only of the area to be repaired thereby not subjectingthe entire structure to softening which could jeopardizethe structural integrity if under load The thermal energysupplied by the microwaves allows reversible Diels-Alderchemistry within the bismaleimide tetrafuran (2MEP4F)matrix that facilitates the healing through polymer mobil-ity Since composite welding is also facilitated by polymermobility it only stands to reason that microwave assistedbonding of these composites should be possibleThe 2MEP4Fresin system that constitutes the matrix of these compositesis thematerial that provides themeans for composite weldingand repairThermal reversible crosslinking between the furanand maleimide groups of the polymer allows for mobilitywithin the matrix when heat is applied The mobility resultsin the ability to bond matrix materials across the damage siteor welding interfaces The carbon nanotube additive merelyfacilitates heating through its microwave absorption prop-erty We report preliminary microwave welding experimentson 2MEP4F carbon fiber composites

2 Experimental

Composite panels were fabricated using multiwall carbonnanotubes (MWCNTs) deposited onto carbon fibers followedby infiltration with a bismaleimide tetrafuran (2MEP4F)resin Carbon fibers were spray coated using a dispersedsolution of MWCNTs as previously reported [20] Unidi-rectional carbon fiber composites were then fabricated bycustom resin transfer molding of the 2MEP4F resin [24 25]This procedure was used to fabricate six-ply unidirectionalcomposite panels measuring 10 cm times 10 cm A total of fivepanels were fabricated of which one panel had a Teflon sheetinsert at the midplane of the composite the 5 cm times 10 cmTeflon insert spanned half the area of the panel This particu-lar panel was used for double cantilever beam testing inwhichthe insert provided a simulated crack in order to measuredelamination properties The other four composite panelswere used for lap shear testing All the fabricated panels werethen dryly cut into approximately 1 in times 4 in rectangularpieces using a mill

Microwave processing of composites was accomplishedusing a Microwave Research and Applications Inc BP-111Microwave Processor which operates at 245GHz and hasvariable power up to 1000 W The microwave processor isequipped with the True-To-Power power control system thatallows sample temperature regulation through power adjust-ment of the magnetron source The oven was modified toallow continuousmonitoring of the sample thermal profile AFLIR A35SC infrared camera is mounted on a bored openingon the top of the microwave processor chamber A wire meshgrid is placed on the inside of the opening while a germa-nium window was placed outside that opening to preventmicrowaves from escaping This design allows for real-timethermal imaging of the sample under microwave exposureFixtures fabricated in-house out of Mylar block allowedpressure clamping of composite panels during microwavewelding and repair Two sets of fixtures were fabricated onewith a circle of 1-inch diameter bored at the midpoint of the


(a) (b)



Microwave welding

First lap


Second lapsurfaces









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025 in

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3 Results







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4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)



Microwave welding

First lap


Second lapsurfaces









(a) (b)

025 in

(c)


3 Results







0

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1400

0 01 02 03 04 05 06 07 08 09Extension (in)



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(c) (d)





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025 in

(b)

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4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

025 in

(c)


3 Results







0

200

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1400

0 01 02 03 04 05 06 07 08 09Extension (in)



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(a)

025 in

(b)

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4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0

200

400

600

800

1000

1200

1400

0 01 02 03 04 05 06 07 08 09Extension (in)



Load

(ftmiddotlb

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Compressiontesting










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Load

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(a)

025 in

(b)

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∘C 1000

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4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0

1

2

3

4

5

6

7

8

9




Load

(ftmiddotlb

)


(a) (b)

(c) (d)





(a)

025 in

(b)

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4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a)

025 in

(b)

(c)


∘C 1000

238








4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





4 Conclusion


Competing Interests


References




































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



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