On the Wood Chipping Process – a Study on Basic Mechanisms in Order to Optimize Chip Properties for Pulping

ȱ ȱ ȱ ThesisȱforȱtheȱdegreeȱofȱDoctorȱofȱTechnology,ȱSundsvallȱ2010ȱ ȱ ȱ ON THE WOOD CHIPPING PROCESS – A STUDY ON BASIC MECHANISMS IN ORDER TO OPTIMIZE CHIP PROPERTIES FOR PULPING

Lisbeth Hellström ȱ ȱ Supervisors:ȱ ProfessorȱTorbjörnȱCarlbergȱ ProfessorȱPerȱEngstrandȱ ȱ ȱ ȱ FSCNȱȬȱFibreȱScienceȱandȱCommunicationȱNetworkȱ DepartmentȱofȱNaturalȱSciences,ȱEngineeringȱandȱMathematicsȱ MidȱSwedenȱUniversity,ȱSEȬ851ȱ70ȱSundsvall,ȱSwedenȱ ȱ ȱ ISSNȱ1652Ȭ893X,ȱȱ MidȱSwedenȱUniversityȱDoctoralȱThesisȱ83ȱ ISBNȱ978Ȭ91Ȭ86073Ȭ72Ȭ5ȱ ȱ FSCN

Fibre Science and Communication Network ett skogsindustriellt forskningscentrum vid Mittuniversitetet - -

- - ȱ

ȱ Akademiskȱ avhandlingȱ somȱ medȱ tillståndȱ avȱ Mittuniversitetetȱ iȱ Sundsvallȱ framläggsȱtillȱoffentligȱgranskningȱförȱavläggandeȱavȱteknologieȱdoktorsexamenȱiȱ tekniskȱ fysikȱ fredagenȱ denȱ 21:aȱ maj,ȱ 2010,ȱ klockanȱ 10.15ȱ iȱ salȱ Oȱ 102,ȱ MittuniversitetetȱSundsvall.ȱȱ Seminarietȱkommerȱattȱhållasȱpåȱsvenska.ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ȱ ON THE WOOD CHIPPING PROCESS – A STUDY ON BASIC MECHANISMS IN ORDER TO OPTIMIZE CHIP PROPERTIES FOR PULPING Lisbeth Hellström ȱ ȱ ©ȱLisbethȱHellström,ȱ2010ȱ ȱ ȱ FSCNȱȬȱFibreȱScienceȱandȱCommunicationȱNetworkȱ DepartmentȱofȱNaturalȱSciences,ȱEngineeringȱandȱMathematicsȱ MidȱSwedenȱUniversity,ȱSEȬ851ȱ70ȱSundsvallȱ Swedenȱ ȱ Telephone:ȱ +46ȱ(0)771Ȭ975ȱ000ȱ ȱ PrintedȱbyȱKopieringenȱMittuniversitetet,ȱSundsvall,ȱSweden,ȱ2010ȱ

iȱ ON THE WOOD CHIPPING PROCESS – A STUDY ON BASIC MECHANISMS IN ORDER TO OPTIMIZE CHIP PROPERTIES FOR PULPING Lisbeth Hellström ȱ FSCNȱȬȱFibreȱScienceȱandȱCommunicationȱNetwork,ȱDepartmentȱofȱNaturalȱ Sciences,ȱEngineeringȱandȱMathematics,ȱMidȱSwedenȱUniversity,ȱSEȬ851ȱ70ȱ Sundsvall,ȱSwedenȱ ISSNȱ1652Ȭ893X,ȱMidȱSwedenȱUniversityȱDoctoralȱThesisȱ83;ȱ ISBNȱ978Ȭ91Ȭ86073Ȭ72Ȭ5ȱ ȱ ȱ ABSTRACT

Inȱ bothȱ theȱ chemicalȱ andȱ mechanicalȱ pulpingȱ process,ȱ theȱ logsȱ areȱ cutȱ intoȱ woodȱchipsȱbyȱaȱdiscȱchipperȱbeforeȱfibreȱseparation.ȱToȱmakeȱtheȱwoodȱchippingȱ processȱ moreȱ efficient,ȱ oneȱ haveȱ toȱ investigateȱ inȱ detailȱ theȱ couplingȱ betweenȱ processȱparametersȱandȱtheȱqualityȱofȱtheȱchips.ȱOneȱobjectiveȱofȱthisȱthesisȱwasȱtoȱ obtainȱ anȱ understandingȱ ofȱ theȱ fundamentalȱ mechanismsȱ behindȱ theȱ creationȱ ofȱ woodȱ chips.ȱ Anotherȱ objectiveȱ withȱ theȱ thesisȱ wasȱ toȱ investigateȱ whetherȱ itȱ wasȱ possibleȱ to,ȱ inȱ aȱ wayȱ tailorȱ theȱ chippingȱ processȱ soȱ asȱ toȱ reduceȱ theȱ energyȱ consumptionȱinȱaȱfollowingȱmechanicalȱrefiningȱprocess.ȱ ȱ Bothȱexperimentalȱandȱanalytical/numericalȱapproachesȱhaveȱbeenȱtakenȱinȱthisȱ work.ȱTheȱfirstȱpartȱofȱtheȱexperimentalȱinvestigations,ȱwereȱperformedȱwithȱanȱinȬ houseȱdevelopedȱchippingȱdeviceȱandȱaȱdigitalȱspeckleȱphotographyȱequipment.ȱ Theȱ resultsȱ fromȱ theȱ experimentalȱ investigationȱ showedȱ thatȱ theȱ frictionȱ betweenȱ theȱ logȱ andȱ chippingȱ toolȱ isȱ probablyȱ oneȱ crucialȱ factorȱ forȱ theȱ chipȱ formation.ȱFurtherȱmoreȱitȱwasȱfoundȱthatȱtheȱindentationȱprocessȱisȱapproximatelyȱ selfȬsimilar,ȱandȱthatȱtheȱstressȱfieldȱoverȱtheȱentireȱcrackȬplaneȱisȱcriticalȱforȱchipȱ creation.ȱȱ ȱ Theȱ developedȱ analyticalȱ modelȱ predictsȱ theȱ normalȱ andȱ shearȱ strainȱ distributionȱ andȱ toȱ beȱ moreȱ specific,ȱ theȱ modelȱ canȱ predictȱ theȱ compressiveȱ stressesȱparallelȱtoȱtheȱfibreȱdirectionȱforȱanȱassumedȱlinearȱelasticȱandȱorthotropicȱ material.ȱ Theȱ analyticalȱ distributionsȱ wereȱ foundȱ toȱ beȱ inȱ reasonableȱ agreementȱ withȱtheȱcorrespondingȱdistributionsȱobtainedȱfromȱaȱfiniteȱelementȱanalysis.ȱ ȱ Toȱbeȱableȱtoȱstudyȱtheȱchippingȱprocessȱunderȱrealisticȱconditions,ȱwhichȱforȱ exampleȱ meansȱ toȱ useȱ chippingȱ ratesȱ representativeȱ forȱ aȱ realȱ woodȱ chipper,ȱ aȱ laboratoryȱ chipperȱ wasȱ developed.ȱ Detailsȱ regardingȱ theȱ chipperȱ andȱ howȱ toȱ

iiȱ evaluateȱtheȱforceȱmeasurementsȱareȱgivenȱtogetherȱwithȱanȱexampleȱofȱhowȱtheȱ forceȱonȱtheȱcuttingȱtoolȱ(theȱknife)ȱvariesȱwithȱtimeȱduringȱcutting.ȱ ȱ Toȱinvestigateȱtheȱinfluenceȱofȱaȱcertainȱchippingȱprocessȱparameter,ȱtheȱchipsȱ wereȱafterȱ productionȱinȱ theȱ laboratoryȱ chipper,ȱ refinedȱ inȱ aȱ pilotȱ refinerȱ duringȱ conditionsȱ optimizedȱ forȱ TMPȱ (thermomechanicalȱ pulp)ȱ andȱ CTMPȱ (chemithermomechanicalȱ pulp)ȱ processes.ȱ Itȱ wasȱ concludedȱ thatȱ theȱ detailsȱ concerningȱtheȱchipȱprocessȱhadȱaȱlargeȱimpactȱonȱe.g.ȱtheȱenergyȱconsumptionȱinȱ bothȱ firstȱstageȱ andȱ secondȱ stageȱ refining.ȱResultsȱshowingȱ thisȱareȱgivenȱinȱthisȱ thesis.ȱ ȱ ȱ Keywords:ȱWoodȱchipping,ȱChipȱformation,ȱDigitalȱSpeckleȱPhotography,ȱFriction,ȱ Fractureȱ Processes,ȱ Analyticalȱ model,ȱ Laboratoryȱ Woodȱ Chipper,ȱ Forceȱ Measurement,ȱVibrationȱSynthesis,ȱEnergyȱEfficiency,ȱChipȱDamage,ȱȱ TMPȱ,ȱCTMPȱ

iiiȱ SAMMANFATTNING

Förȱ bådeȱ kemiskȱ ochȱ mekaniskȱ pappersmassaȱ såȱ tillverkasȱ flisȱ avȱ trädstockarȱ medȱhjälpȱavȱenȱskivhuggȱinnanȱfibrernaȱsepareras.ȱFörȱattȱgöraȱflisningsprocessenȱ merȱeffektiv,ȱmåsteȱkopplingenȱmellanȱprocessparametrarȱochȱfliskvalitetȱstuderas.ȱ Ettȱmålȱmedȱdennaȱavhandlingȱȱärȱattȱgeȱfundamentalȱkunskapȱomȱmekanismernaȱ bakomȱbildandetȱavȱträflis.ȱȱ ȱ Bådeȱ experimentellaȱ ochȱ analytiska/numeriskaȱ metoderȱ harȱ använtsȱ iȱ dettaȱ arbete.ȱ Deȱ experimentellaȱ undersökningarnaȱ harȱ gjortsȱ medȱ hjälpȱ avȱ egenȱ utveckladȱutrustningȱoch.ȱ ȱ Resultatenȱfrånȱdenȱexperimentellaȱundersökningenȱvisarȱattȱfriktionenȱmellanȱ stammenȱ ochȱ flisningsverktygetȱ harȱ betydelseȱ vidȱ flisning.ȱ Vidareȱ observeradesȱ detȱattȱinträngningsȱprocessenȱärȱapproximativtȱsjälvlikȱ(selfȱsimilar)ȱochȱattȱdetȱärȱ spänningsfältetȱöverȱhelaȱsprickplanetȱsomȱärȱkritisktȱförȱbildandetȱavȱenȱflis.ȱȱ ȱ Denȱ utveckladeȱ analytiskaȱ modellenȱ förutsägerȱ normalȬȱ ochȱ skjuvspänningsfördelningenȱöverȱsprickplanetȱochȱkanȱmerȱspecifiktȱförutsägaȱdenȱ kompressivaȱ belastningȱ somȱ verkarȱ parallelltȱ fiberriktningenȱ iȱ ettȱ linjärtȱ elastisktȱ ochȱ ortotroptȱ materialȱ (trä).ȱ Deȱ analytisktȱ bestämdaȱ fördelningarnaȱ stämmerȱ relativtȱ välȱ överensȱ medȱ motsvarandeȱ fördelningarȱ beräknadȱ medȱ finitȱ elementȱ analys.ȱ ȱ Förȱattȱkunnaȱstuderaȱflisningsprocessenȱunderȱrealistiskaȱförhållanden,ȱvilketȱ bl.a.ȱbetyderȱattȱȱskärhastigheterȱsomȱärȱrepresentativaȱförȱenȱverkligȱprocessȱskallȱ användas,ȱ såȱ utveckladesȱ inomȱ ramenȱ förȱ avhandlingsarbetet,ȱ enȱ laboratoriesflishugg.ȱ Detaljerȱ rörandeȱ flishuggenȱ samtȱ hurȱ uppmättaȱ lastsignalerȱ skallȱutvärderasȱgesȱtillsammansȱmedȱettȱexmpelȱpåȱhurȱkraftenȱpåȱskärverktygetȱ (kniven)ȱvarierarȱunderȱettȱskärförlopp.ȱ ȱ Inverkanȱ avȱ enȱ vissȱ flisningsprocessparameterȱ undersöktesȱ genomȱ attȱ flisȱ tillverkadesȱiȱlaboratorieflishuggenȱvarefterȱdeȱraffineradesȱiȱenȱpilotraffinörȱunderȱ förhållandenȱ somȱ varȱ optimeradeȱ förȱ TMPȱ (termomekaniskȱ massa)ȱ ochȱ CTMPȱ (kemitermomekaniskȱ massa)ȱ processerna.ȱ Detȱ konstateradesȱ attȱ detaljerȱ iȱ flisningsprocessenȱhadeȱstorȱinverkanȱpåȱt.ex.ȱenergiåtgångenȱiȱbådeȱförstaȱstegsȱ–ȱ ochȱandrastegsraffinering.ȱResultatȱsomȱverifierarȱdettaȱgesȱiȱavhandlingen.ȱ ȱ

ivȱ TABLE OF CONTENTS ȱ ABSTRACTȱ iiȱ ȱ SAMMANFATTNINGȱ ivȱ ȱ LISTȱOFȱPAPERSȱ viiȱ ȱ CONTRIBUTIONȱREPORTȱ viiiȱ ȱ 1.ȱ INTRODUCTIONȱ 1ȱ ȱ 2.ȱ ANȱINTRODUCTIONȱTOȱWOODȱCHIPPINGȱANDȱȱ ȱ RELATEDȱITEMSȱ 3ȱ 2.1.ȱ Discȱchippingȱprocessȱ 3ȱ 2.2.ȱ Woodȱchippingȱparametersȱ 4ȱ 2.2.1.ȱ Woodȱpropertiesȱ 4ȱ 2.2.2.ȱ Operatingȱparametersȱ 6ȱ 2.3.ȱ Woodȱchipȱqualityȱ 7ȱ 2.3.1.ȱ Chipȱgeometryȱ 7ȱ 2.4.ȱ Precompressionȱ 9ȱ ȱ 3.ȱ METHODSȱ 11ȱ 3.1.ȱ ReviewȱofȱpaperȱIȱ 11ȱ 3.1.1.ȱ Backgroundȱ 11ȱ 3.1.2.ȱ ExperimentalȱSetupȱ 11ȱ 3.2.ȱ ReviewȱofȱpaperȱIIȱ 12ȱ 3.2.1.ȱ AnalyticalȬmodellingȱofȱtheȱwoodȱchippingȱprocessȱ 13ȱ 3.2.2.ȱ AnalyticalȬmodelȱ 13ȱ 3.2.3.ȱ FEȬmodelȱ 16ȱ 3.3.ȱ ReviewȱofȱpaperȱIIIȱ 16ȱ 3.3.1.ȱ Backgroundȱ 16ȱ 3.3.2.ȱ ExperimentalȱSetupȱ 16ȱ 3.3.3.ȱ ForceȱMeasurementsȱ 17ȱ 3.4.ȱ ReviewȱofȱpaperȱIVȱ 18ȱ 3.4.1.ȱ Backgroundȱ 18ȱ 3.4.2.ȱ Refiningȱtrialȱ 18ȱ 3.4.3.ȱ Energyȱconsumptionȱduringȱchippingȱ 18ȱ 3.4.4.ȱ Waterȱadsorptionȱtestȱ 18ȱ 3.5.ȱ ReviewȱofȱpaperȱVȱ 18ȱ

vȱ 3.5.1.ȱ Backgroundȱ 19ȱ 3.5.2.ȱ Refiningȱtrialȱ 19ȱ ȱ 4.ȱ RESULTSȱANDȱDISCUSSIONȱ 20ȱ 4.1.ȱ Resultsȱfromȱtheȱexperimentalȱinvestigationȱ 20ȱ 4.2.ȱ ResultsȱfromȱtheȱAnalyticalȱandȱNumericalȱStudyȱ 23ȱ 4.3.ȱ ResultsȱfromȱtheȱLaboratoryȱchipperȱdevelopmentȱ 27ȱ 4.4.ȱ ResultsȱfromȱtheȱTMPȱStudyȱ 27ȱ 4.5.ȱ ResultsȱfromȱtheȱCTMPȱStudyȱ 29ȱ ȱ 5.ȱ CONCLUSIONSȱ 31ȱ ȱ 6.ȱ ACKNOWLEDGEMENTSȱ 33ȱ ȱ 7.ȱ REFERENCESȱ 34ȱ ȱ

viȱ LIST OF PAPERS

Thisȱthesisȱisȱmainlyȱbasedȱonȱtheȱfollowingȱfiveȱpapers,ȱhereinȱreferredȱtoȱbyȱtheirȱ Romanȱnumerals:ȱ ȱ PaperȱIȱ AȱMethodȱforȱExperimentalȱInvestigationȱofȱtheȱWoodȱChippingȱ Processȱ ȱ HellströmȱL.M.,ȱGradinȱP.A.ȱandȱCarlbergȱT.ȱ ȱ NordicȱPulpȱandȱPaperȱResearchȱJournalȱ,ȱ23:3,ȱppȱ339Ȭ342ȱ(2008)ȱ ȱ PaperȱII AnȱAnalyticalȱandȱNumericalȱStudyȱofȱsomeȱaspectsȱofȱtheȱWoodȱ ChippingȱProcess HellströmȱL.M.,ȱIsakssonȱP.,ȱGradinȱP.A.,ȱErikssonȱK.ȱ NordicȱPulpȱandȱPaperȱResearchȱJournal,ȱ24:ȱ2,ȱppȱ225Ȭ230,ȱ(2009)ȱ ȱ PaperȱIIIȱ Aȱ Laboratoryȱ Woodȱ Chipperȱ forȱ Chippingȱ Underȱ Realisticȱ Conditionsȱ ȱ Hellströmȱ L.ȱ M.,ȱ Gullikssonȱ M.,ȱ Carlbergȱ T.ȱ Toȱ beȱ submittedȱ toȱ ExperimentalȱMechanicsȱ ȱ PaperȱIVȱ Theȱ Influenceȱ ofȱ theȱ Woodȱ Chippingȱ Processȱ onȱ theȱ Energyȱ EfficiencyȱDuringȱMechanicalȱPulpingȱ ȱ Hellströmȱ L.ȱ M.,ȱ Engstrandȱ P.,ȱ Gregersenȱ Ø.ȱȱToȱ beȱ submittedȱ toȱ Holzforschungȱ ȱ PaperȱVȱ Energyȱ Efficiencyȱ Improvementȱ Potentialȱ inȱ TMPȱandȱ CTMPȱ byȱ AxialȱPrecompressionȱofȱWoodȱDuringȱChippingȱ ȱ Hellströmȱ L.ȱ M.,ȱ Engstrandȱ P.ȱȱToȱ beȱ submittedȱ toȱ J.ȱ ofȱ Pulpȱ anȱ PaperȱScienseȱ ȱ ȱ ȱ ȱ

viiȱ CONTRIBUTION REPORT

Theȱauthorsȱcontributionȱtoȱtheȱpapersȱpresentedȱinȱthisȱthesisȱareȱasȱfollows:ȱ ȱ PaperȱI.ȱ Performingȱ allȱ theȱ experimentalȱ workȱ andȱ writingȱ theȱ article.ȱ Interpretationȱ ofȱ theȱ resultsȱ inȱ coȬoperationȱ withȱ Perȱ Gradinȱ andȱ TorbjörnȱCarlberg.ȱ ȱ PaperȱII.ȱ DiscussingȱdetailsȱwithȱPerȱGradinȱandȱKjellȱEriksson.ȱDoingȱallȱtheȱ numericalȱworkȱandȱwritingȱtheȱarticle.ȱTheȱfiniteȱelementȱanalysisȱ wasȱperformedȱbyȱPerȱIsaksson.ȱ ȱ PaperȱIII.ȱ Performingȱstrengthȱanalysesȱofȱtheȱchipperȱduringȱitsȱdevelopment.ȱ Carryingȱoutȱallȱtheȱtheoreticalȱworkȱandȱwritingȱtheȱarticleȱtogetherȱ withȱ Torbjörnȱ Carlberg.ȱ Theȱ implementationȱ ofȱ theȱ theoreticalȱ modelȱandȱnumericalȱanalysisȱwasȱperformedȱbyȱMårtenȱGulliksson.ȱ ȱ PaperȱIV.ȱ Developingȱ anȱ ideaȱ regardingȱ theȱ chipȱ pretratmentȱ togetherȱ withȱ Øyvindȱ Gregersen.ȱ Performingȱ allȱ theȱ experimentalȱ workȱ (exceptȱ theȱ refining)ȱ andȱ writingȱ theȱ article.ȱ Discussingȱ detailsȱ withȱ Perȱ Engstrand.ȱ ȱ PaperȱV.ȱ Performingȱtheȱexperimentalȱworkȱ(exceptȱtheȱrefining).ȱWritingȱtheȱ article.ȱ Interpretationȱ ofȱ theȱ resultsȱ inȱ coȬoperationȱ withȱ Perȱ Engstrand.ȱ ȱ ȱ ȱ ȱ ȱ

viiiȱ

1. INTRODUCTION Theȱ totalȱ worldwideȱ productionȱ ofȱ virginȱ woodȱ pulpȱ amountsȱ toȱ aboutȱ 180ȱ millionȱtonsȱaȱyear.ȱThisȱincludesȱbothȱchemicalȱandȱmechanicalȱpulp.ȱCommonȱforȱ bothȱ ofȱ theseȱ typesȱ ofȱ pulpȱ isȱ thatȱ theȱ rawȱ materialȱ (theȱ logs)ȱ hasȱ toȱ beȱ cutȱ intoȱ smallȱpartsȱi.e.ȱchipsȱbeforeȱtheȱfibresȱcanȱbeȱseparated.ȱIfȱitȱisȱassumedȱthatȱ70%ȱofȱ theȱ weightȱ ofȱ chipsȱ becomesȱ pulpȱ (inȱ theȱ chemicalȱpulpingȱ processȱ forȱ example,ȱ someȱ ofȱ theȱ materialȱ isȱ dissolvedȱ duringȱ theȱ chemicalȱ treatment)ȱ thenȱ itȱ canȱ beȱ concludedȱthatȱatȱleastȱ260ȱmillionȱtonsȱofȱchipsȱareȱproducedȱannually.ȱFromȱthisȱ itȱfollowsȱthatȱtheȱchippingȱprocessȱisȱanȱimportantȱpartȱofȱtheȱpulpingȱprocess.ȱ ȱ Aȱcommonȱdemandȱfromȱtheȱpulpȱandȱpaperȱindustryȱisȱtheȱsmallestȱpossibleȱ variationȱ ofȱ certainȱ chipȱ populationȱ parameters,ȱ inȱ particularȱ theȱ chipȱ thicknessȱ distribution.ȱ Aȱ narrowȱ distributionȱ promotesȱ uniformȱ productȱ properties,ȱ e.g.ȱ inȱ chemicalȱimpregnationȱprocesses,ȱandȱisȱgenerallyȱrecognizedȱasȱaȱcharacteristicȱofȱ aȱ highȬqualityȱ productȱ byȱ consumers.ȱ Forȱ theȱ mechanicalȱ refiningȱ process,ȱ oneȱ importantȱparameterȱisȱtheȱbulkȱdensityȱofȱtheȱchipsȱsinceȱitȱdeterminesȱtheȱdegreeȱ ofȱpackingȱinȱtheȱfeedingȱscrew.ȱ ȱ Thisȱmeansȱinȱpracticeȱthatȱitȱmightȱbeȱmoreȱimportantȱtoȱproduceȱchipsȱwithȱaȱ narrowȱ thicknessȱ distributionȱ ratherȱ thanȱ aȱ specificȱ thickness,ȱ soȱ thatȱ itȱ isȱ moreȱ important,ȱthatȱtheȱchippingȱtoolȱretainsȱitsȱcharacteristicsȱoverȱtimeȱratherȱthanȱitȱ retainsȱaȱgivenȱsharpness.ȱInȱorderȱtoȱpredictȱtheȱinfluenceȱofȱforȱexampleȱtoolȱwearȱ onȱtheȱchipȱthickness,ȱitȱisȱnecessaryȱtoȱunderstandȱtheȱunderlyingȱmechanismsȱofȱ chipȱformationȱinȱdetail.ȱ ȱ Traditionally,ȱtheȱchippingȱhasȱbeenȱperformedȱinȱsuchȱaȱwayȱsoȱasȱtoȱminimizeȱ theȱ compressiveȱ damageȱ inducedȱ onȱ theȱ chipsȱ byȱ theȱ highȱ compressiveȱ stressesȱ thatȱ mightȱ occurȱ forȱ aȱ certainȱ choiceȱ ofȱ chippingȱ processȱ parameters.ȱ Thisȱ isȱ aȱ demandȱ fromȱ theȱ producersȱ ofȱ chemicalȱ pulpȱ andȱ itȱ hasȱ beenȱ assumedȱ thatȱ thisȱ shouldȱbeȱbeneficialȱalsoȱforȱchipsȱintendedȱforȱtheȱproductionȱofȱmechanicalȱpulp.ȱ ȱ Refiningȱ woodȱ chipsȱ toȱ produceȱ mechanicalȱ pulpȱ isȱ veryȱ energyȬintensiveȱ process.ȱ Oneȱ ofȱ theȱ mainȱ aimsȱ inȱ theȱ developmentȱ ofȱ theȱ mechanicalȱ pulpingȱ processȱ isȱ toȱ achieveȱ theȱ desiredȱ degreeȱ ofȱ refiningȱ withȱ aȱ minimumȱ ofȱ energyȱ expenditure.ȱReducingȱenergyȱconsumptionȱisȱofȱutmostȱimportanceȱtoȱproducersȱ ofȱ mechanicalȱ pulp,ȱ becauseȱ ofȱ bothȱ increasingȱ electricityȱ pricesȱ andȱ theȱ importanceȱofȱenvironmentalȱissues.ȱTheȱinterestȱofȱmechanicalȱchipȱpretreatmentȱ hasȱ arisenȱ dueȱ toȱ anȱ expectedȱ highȱ potentialȱ toȱ achieveȱ energyȱ reductionȱ inȱ refining.ȱ Oneȱ ofȱ theȱ aimsȱ withȱ thisȱ thesisȱ hasȱ thereforeȱ beenȱ toȱ investigateȱ theȱ possibilitiesȱtoȱperformȱmechanicalȱpretreatmentȱofȱchipsȱintendedȱforȱmechanicalȱ pulping,ȱalreadyȱduringȱtheȱchippingȱprocess.ȱ

1 ȱ Toȱbeȱableȱtoȱformulateȱcriteriaȱforȱdeterminingȱtheȱonsetȱofȱcreationȱofȱaȱwoodȱ chipȱoneȱshouldȱbeȱableȱtoȱstudyȱtheȱdeformationȱfieldsȱinȱtheȱvicinityȱofȱtheȱedgeȱ ofȱ theȱ chippingȱ tool.ȱ Anotherȱ aimȱ withȱ thisȱ thesisȱ wasȱ thusȱ toȱ studyȱ theȱ deformationȱ fieldsȱ byȱ utilisingȱ aȱ Digitalȱ Speckleȱ Photographyȱ (DSP)ȱ equipment,ȱ whichȱtogetherȱwithȱimageȱprocessingȱsoftwareȱmakesȱitȱpossibleȱtoȱdetermineȱtheȱ strainȱfieldȱonȱtheȱsurfaceȱofȱtheȱwoodȱspecimenȱduringȱtheȱcuttingȱofȱchips.ȱȱ ȱ TheȱresultsȱfromȱtheȱDSPȱinvestigationsȱgiveȱquantitativeȱinformationȱaboutȱtheȱ fractureȱprocessesȱandȱdeformationȱfieldsȱinȱaȱwoodȱspecimenȱduringȱchipping.ȱ ȱ Onȱ theȱ otherȱ hand,ȱ toȱ getȱ aȱ qualitativeȱ pictureȱ regardingȱ theȱ influenceȱ ofȱ differentȱparametersȱonȱtheȱstressȱandȱstrainȱfieldsȱinȱtheȱchip,ȱitȱshouldȱbeȱofȱvalueȱ toȱhaveȱaccessȱtoȱaȱsimpleȱanalyticalȱmodel.ȱAsȱanotherȱaimȱwithȱthisȱthesis,ȱsuchȱaȱ modelȱwasȱdeveloped,ȱtheȱrelevanceȱofȱwhichȱwasȱcheckedȱagainstȱaȱmoreȱgeneralȱ FiniteȱElementȱ(FE)ȱmodel.ȱOneȱoutcomeȱofȱtheȱmodelȱwasȱthatȱitȱpredictedȱhighȱ compressiveȱ stressesȱ inȱ theȱ fibreȱ directionȱ dueȱ toȱ theȱ wedgingȱ actionȱ ofȱ theȱ chippingȱtoolȱ(theȱknife).ȱThisȱopensȱupȱtheȱpossibilityȱtoȱchooseȱchippingȱprocessȱ parametersȱ toȱ actuallyȱ maximizeȱ theȱ compressiveȱ damageȱ inducedȱ toȱ aȱ chipȱ duringȱchipping.ȱThisȱshouldȱinȱtheȱsequelȱbeȱreferredȱtoȱasȱ“directedȱchipping”ȱ ȱ Toȱevaluateȱtheȱpulpingȱresponseȱonȱdirectedȱchipping,ȱanȱinȱhouseȱdevelopedȱ laboratoryȱ woodȱ chipperȱ locatedȱ atȱ FSCNȱ (Fibreȱ Scienceȱ andȱ Communicationȱ Network)ȱatȱMidȱSwedenȱUniversityȱwasȱusedȱtoȱproduceȱwoodȱchipsȱunderȱsomeȱ differentȱchippingȱconditions.ȱTheȱdevelopmentȱofȱthisȱequipmentȱwasȱyetȱanotherȱ aimȱwithȱtheȱwork,ȱandȱtheȱequipmentȱisȱdescribedȱinȱdetailȱinȱthisȱthesis.ȱToȱcoverȱ allȱpossibleȱchippingȱconditions,ȱtheȱlaboratoryȱwoodȱchipperȱwasȱdesignedȱsoȱasȱ toȱadmitȱtheȱvariationȱofȱcuttingȱanglesȱinȱtwoȱorthogonalȱplanes.ȱFurtherȱon,ȱtheȱ cuttingȱrateȱcanȱbeȱvariedȱinȱtheȱintervalȱ0ȱtoȱ50ȱm/sȱwhichȱisȱwellȱinȱtheȱintervalȱ foundȱforȱcommercialȱwoodȱchippers.ȱAlso,ȱtheȱdesignȱadmitsȱanȱeasyȱchangeȱofȱ chippingȱ toolȱ andȱ theȱ toolȱ holderȱ isȱ equippedȱ withȱ forceȱ sensorsȱ suchȱ thatȱ theȱ forcesȱ inȱ threeȱ orthogonalȱdirectionsȱ canȱ beȱ measured.ȱ Dueȱ toȱinertiaȱeffects,ȱ theȱ forceȱthatȱisȱmeasuredȱandȱtheȱforceȱactingȱonȱtheȱknifeȱisȱnotȱtheȱsameȱsoȱthatȱaȱ twoȱdegreesȱofȱfreedomȱmodelȱhasȱtoȱbeȱutilizedȱinȱorderȱtoȱextractȱtheȱrealȱcuttingȱ forceȱversusȱtimeȱduringȱcutting.ȱ ȱ Toȱ summarise,ȱ thisȱ workȱ isȱ anȱ attemptȱ toȱ developȱ bothȱ experimentalȱ andȱ analyticalȱtoolsȱforȱtheȱstudyȱofȱtheȱwoodȱchippingȱprocessȱandȱitsȱimplicationsȱonȱ theȱwoodȱrefiningȱprocess.ȱȱ

2 2. AN INTRODUCTION TO WOOD CHIPPING AND RELATED ITEMS Theȱfollowingȱsectionȱisȱintendedȱtoȱmakeȱtheȱreaderȱmoreȱacquaintedȱwithȱtheȱ topicsȱ dealtȱ withȱ inȱ thisȱ thesis.ȱ Theȱ sectionȱ startsȱ withȱ aȱ shortȱ descriptionȱ ofȱ theȱ discȱchippingȱprocess,ȱfollowedȱbyȱaȱshortȱpresentationȱofȱrelevantȱwoodȱchippingȱ parametersȱ andȱ aȱ discussionȱ ofȱ woodȱ chipȱ quality.ȱ Mechanicalȱ pretreatmentȱ ofȱ woodȱchipsȱwillȱbeȱtheȱtopicȱofȱtheȱfinalȱpartȱofȱthisȱsectionȱ

2.1. Disc chipping process Inȱ bothȱ theȱ chemicalȱ andȱ mechanicalȱ pulpingȱ processes,ȱ theȱ logsȱ areȱ cutȱ intoȱ smallerȱpartsȱi.e.ȱwoodȱchipsȱbyȱforȱexampleȱaȱdiscȱchipperȱbeforeȱfibreȱseparation.ȱ ȱ Theȱchippingȱisȱnormallyȱperformedȱinȱaȱdiscȱchipper,ȱwhereȱtheȱbasicȱdesignȱ (theȱ Wiggerȱ chipper)ȱ wasȱ developedȱ inȱ 1889ȱ [1].ȱ Theȱ discȱ chippersȱ ofȱ todayȱ areȱ improvementsȱ fromȱ thisȱ Wiggerȱ chipper.ȱ Thereȱ areȱ otherȱ techniquesȱ toȱ produceȱ woodȱchips,ȱbutȱonlyȱdiscȱchippersȱwillȱbeȱconsideredȱinȱthisȱthesis.ȱHowȱaȱdiscȱ chipperȱworksȱisȱshownȱschematicallyȱinȱFigureȱ1.ȱ

ȱ Figure 1. Wood chipping by a disc chipper [2]

Inȱaȱdiscȱchipper,ȱtheȱchipȱlengthȱcanȱbeȱcontrolledȱbyȱtheȱTȬdimensionȱ(Figureȱ 2),ȱi.e.ȱtheȱdistanceȱbetweenȱtheȱknifeȱtipȱandȱtheȱdiscȱwearȱplateȱtogetherȱwithȱtheȱ spoutȱangleȱHȱ(Figureȱ3).ȱTheȱfeedingȱthroughȱtheȱchipperȱcanȱbeȱcontrolledȱbyȱtheȱ clearanceȱangleȱ(Figureȱ3),ȱDȱ[3].ȱ

3 Figure 2. The T-dimension (with the permission Figure 3. The angles (with the of Iggesund Tools AB) permission of Iggesund Tools AB)ȱ

Theȱedgeȱangleȱ(Figureȱ3), E,ȱisȱoftenȱcombinedȱwithȱaȱlargerȱangleȱ(theȱbevelȱ angle,ȱE’)ȱatȱtheȱedgeȱtoȱgiveȱtheȱknifeȱhigherȱstrengthȱandȱdurability.ȱ ȱ Theȱknifesȱareȱmountedȱinȱradialȱdirectionsȱonȱtheȱchipperȱdisc,ȱveryȱmuchȱlikeȱ spokesȱ onȱ aȱ bicycleȱ wheel.ȱ Theȱ commonlyȱ usedȱ cuttingȱ speedsȱ areȱ 20Ȭ40ȱ m/sȱ (varyingȱinȱtheȱradialȱdirection).ȱTheȱchipȱlength,ȱL,ȱisȱnormallyȱsetȱtoȱbeȱaboutȱ20Ȭ 25ȱ mmȱ (inȱ theȱ grainȱ direction).ȱ Inȱ aȱ discȱ chipperȱ thisȱ willȱ giveȱ anȱ averageȱ chipȱ thickness,ȱt,ȱofȱaboutȱ3Ȭ5ȱmm.ȱ ȱ 2.2. Wood chipping parameters Dependingȱonȱwhatȱtypeȱofȱchipperȱthatȱisȱusedȱandȱtheȱwoodȱquality,ȱoneȱwillȱ getȱ aȱ distributionȱ ofȱ chipȱ thicknesses,ȱ whichȱ mayȱ beȱ moreȱ oreȱ lessȱ narrow.ȱ Oneȱ shouldȱ keepȱ inȱ mindȱ thatȱ theȱ woodȱ andȱ fibreȱ propertiesȱ varyȱ notȱ onlyȱ betweenȱ treesȱbutȱalsoȱwithinȱaȱtree.ȱ ȱ 2.2.1. Wood properties TheȱLatinȱwordȱforȱtimberȱisȱ“materia”.ȱWoodȱisȱoftenȱreferredȱtoȱasȱaȱmaterialȱ butȱ sometimesȱ alsoȱ toȱ asȱ aȱ structure.ȱ Onȱ aȱ macroscopicȱ scaleȱ itȱ isȱ logicalȱ toȱ callȱ woodȱaȱmaterialȱbutȱonȱaȱmicroscopicȱscale,ȱwoodȱhasȱaȱpronouncedȱstructure.ȱThisȱ canȱ beȱ seenȱ inȱ Figureȱ 4,ȱ whereȱ differentȱ regionsȱ referredȱ toȱ asȱ earlyȱ wood,ȱ latewoodȱandȱtransitionwoodȱcanȱbeȱidentified.ȱ

4 Earlywood Latewood Transitionwood

Figure 4. Cross-section of spruce wood (approximately one annual growth ring) as observed in a light microscope [4] ȱ InȱNorwayȱspruce,ȱtheȱearlywoodȱcellȱwallȱthicknessȱisȱapproximatelyȱ1Ȭ3ȱPmȱ whileȱ latewoodȱ cellsȱ haveȱ aȱ wallȱ thicknessȱ ofȱ 2Ȭ7ȱ Pmȱ [5].ȱ Theȱ densitiesȱ ofȱ theȱ earlywood,ȱ latewoodȱ andȱ transitionwoodȱ areȱ approximatelyȱ inȱ averageȱ 300,ȱ 900ȱ andȱ450ȱkg/m3ȱrespectivelyȱ[6].ȱ ȱ Woodȱ isȱ anȱ anisotropicȱ materialȱ exhibitingȱ uniqueȱ andȱ varyingȱ mechanicalȱ propertiesȱ inȱ differentȱ directions.ȱ Toȱ beȱ moreȱ specific,ȱ woodȱ isȱ cylindricallyȱ orthotropicȱwithȱreferenceȱtoȱaȱsystemȱofȱaxesȱalignedȱwithȱtheȱradial,ȱtangentialȱ andȱaxialȱdirectionsȱasȱshownȱinȱFigureȱ5.ȱ ȱ Axial Radial

Bark Cross-sectional Tangential Sapwood

Cambium

Heartwood

ȱ Figure 5. The principal anisotropy axes in a tree stem [7] ȱ Woodȱexhibitsȱbothȱviscoelasticȱandȱplasticȱbehaviourȱthatȱisȱhighlyȱdependentȱ onȱ bothȱ moistureȱ andȱ temperature.ȱ Theȱ formationȱ ofȱ smallȬsizeȱ chipsȱ willȱ showȱ variationsȱ because;ȱ dueȱ toȱ seasonalȱ changesȱ theȱ logsȱ willȱ haveȱ varyingȱ temperaturesȱandȱmoisturesȱcontent.ȱAtȱlowȱtemperaturesȱmoreȱslenderȱchipsȱareȱ

5 formedȱ (soȱ calledȱ pinȱ chips).ȱ Pinȱ chipsȱ andȱ sawdustȱ areȱ producedȱ inȱ largerȱ quantitiesȱparticularlyȱwhenȱchippingȱfrozenȱandȱveryȱdryȱwoodȱ[8].ȱ Asȱ forȱ theȱ influenceȱ ofȱ moistureȱ itȱ hasȱ beenȱ observedȱ thatȱ theȱ inputȱ ofȱ mechanicalȱ workȱ decreasesȱ andȱ theȱ chipȱ thicknessȱ increasesȱ withȱ increasingȱ moistureȱcontentȱtoȱaȱfairlyȱsteadyȱvalueȱwhenȱtheȱsaturationȱmoistureȱcontentȱisȱ reachedȱ[9].ȱ ȱ Itȱhasȱbeenȱobservedȱthatȱcuttingȱinȱradialȱdirectionȱofȱtheȱannualȱgrowthȱringȱ gaveȱ aȱ moreȱ (moreȱ regularȱ inȱ shape)ȱ uniformȱ chipȱ thicknessȱ thanȱ cuttingȱ inȱ theȱ tangentialȱ directionȱ [10].ȱ Twaddleȱ showedȱ thatȱ theȱ thereȱ isȱ aȱ relationȱ betweenȱ annualȱgrowthȱringȱorientationȱandȱchipȱthicknessȱ[11].ȱTheȱchipsȱareȱthickerȱwhenȱ cutȱinȱtheȱdirectionȱwhereȱtheȱringȱorientationȱisȱ0qȱtoȱtheȱknifeȬedgeȱthanȱforȱtheȱ situationȱ whereȱ thisȱ orientationȱ isȱ 90qȱ toȱ theȱ knifeȬedge.ȱ Theȱ chipȱ thicknessesȱ versusȱ chipȱ lengthȱ areȱ shownȱ inȱ Figureȱ 6ȱ forȱ Loblollyȱ pine.ȱ Theȱ averageȱ chipȱ thicknessȱincreasesȱalmostȱlinearlyȱwithȱincreasingȱchipȱlength.ȱ ȱ

ȱ Figure 6. The average chip thickness vs. chip length when cut in different ring orientationsȱ

2.2.2. Operating parameters Operatingȱ parametersȱ suchȱ asȱ differentȱ anglesȱ affectȱ theȱ chipȱ thickness.ȱ Theȱ clearanceȱangle,ȱD,ȱcontrolsȱtheȱpullingȱofȱtheȱlogsȱtowardsȱtheȱchipperȱplateȱ[12],ȱ andȱ thereforeȱ theȱ chipȱ sizeȱ distribution.ȱ Theȱ clearanceȱ angleȱ alsoȱ affectsȱ theȱ resultingȱcuttingȱforceȱ[13].ȱ ȱ Theȱ edgeȱ angleȱ used,ȱ E,ȱ isȱ normallyȱ betweenȱ 30˚ȱ andȱ 37˚ȱ [14].ȱ Kivimaaȱ andȱ Murtoȱ[10]ȱandȱBuchananȱandȱDuchnickiȱ[9]ȱshowedȱthatȱaȱdecreaseȱinȱtheȱknifeȱ angleȱfromȱ40˚ȱtoȱ30˚ȱreducesȱtheȱchipȱthickness,ȱtheȱcuttingȱforceȱandȱchipȱdamage.ȱ BuchananȱandȱDuchnickiȱ[9]ȱalsoȱidentifiedȱatȱleastȱtwoȱchipȱformationȱprocessesȱ

6 i.e.ȱ anȱ openingȱ modeȱ andȱ aȱ forwardȱ shearȱ mode.ȱ Theȱ 20qȱ andȱ 30qȱ knifeȱ angleȱ producedȱchipsȱbyȱopening.ȱTheȱ40˚ȱknifeȱangleȱformedȱchipsȱbyȱforwardȱshearingȱ inȱ aȱ smallȱ percentageȱ ofȱ casesȱ andȱ theȱ 50˚ȱ knifeȱ angleȱ formedȱ chipsȱ byȱ forwardȱ shearingȱinȱmostȱinstances.ȱ ȱ Atȱshortȱchipȱlengths,ȱanȱincreaseȱinȱbevelȱangle,ȱE’,ȱgivesȱriseȱtoȱaȱthinnerȱchip,ȱ butȱtheȱeffectȱisȱoppositeȱforȱlongerȱchipsȱ(seeȱFigureȱ8)ȱ[13].ȱ ȱ With an increase in the spout angle, H , the chips become thicker at the same length [15], [16]. It is believed that it is the complementary angle, O equal to O 90 D E H , that controls the chip thickness rather than the spout angle H [15]. ȱ Pinȱchipsȱareȱformedȱinȱlargerȱquantitiesȱparticularlyȱwhenȱtheȱcuttingȱspeedȱofȱ theȱdiscȬchipperȱisȱveryȱhighȱ[8].ȱ ȱ 2.3. Wood chip quality Inȱ mostȱ pulpingȱ processes,ȱ aȱ narrowȱ chipȱ thicknessȱ distributionȱ isȱ normallyȱ desired.ȱ Inȱ orderȱ toȱ improveȱ chipȱ qualityȱ inȱ termsȱ ofȱ chipȱ geometry,ȱ theȱ mechanismsȱofȱchipȱformationȱmustȱbeȱbetterȱunderstood.ȱInȱtheȱfollowingȱsectionsȱ theȱimportanceȱofȱchipȱgeometryȱareȱdiscussed.ȱ ȱ 2.3.1. Chip geometry Theȱ importanceȱ ofȱ chipȱ geometryȱ hasȱ beenȱ aȱ sourceȱ forȱ discussionȱ sinceȱ oneȱ firstȱstartedȱmakingȱpulpȱoutȱofȱchips.ȱChipsȱproducedȱbyȱaȱdiscȱchipperȱhaveȱaȱ typicalȱparallelepipedicalȱshape,ȱandȱtheȱthreeȱdimensions;ȱlength,ȱL,ȱthickness,ȱt,ȱ andȱ width,ȱ wȱ asȱ definedȱ inȱ Figureȱ 7.ȱ Whatȱ isȱ consideredȱ toȱ beȱ theȱ optimalȱ chipȱ geometryȱdiffersȱbetweenȱmills.ȱȱ ȱ

ȱ Figure 7. Chip geometry; length, L, thickness, t and with, w ȱ

7 Itȱ hasȱ beenȱ reportedȱ inȱ theȱ literatureȱ (c.f.ȱ [3],ȱ [10],ȱ [11]ȱ andȱ [13])ȱ thatȱ forȱ theȱ sameȱ processȱ parametersȱ andȱ geometryȱ ofȱ theȱ chippingȱ tool,ȱ theȱ ratioȱ betweenȱ lengthȱandȱthicknessȱofȱtheȱchipȱisȱ(inȱsomeȱaverageȱsense)ȱconstant.ȱTheȱmatterȱisȱ alsoȱdiscussedȱinȱ[17]ȱwhereȱanȱattemptȱtoȱgiveȱanȱexplanationȱtoȱthisȱobservationȱ isȱmade.ȱ ȱ

Figure 8. The relative chip length vs. chip length and bevel angle, Ec [13] ȱ Oneȱ requirementȱ fromȱ theȱ pulpȱ andȱ paperȱ industriesȱ isȱ thatȱ theȱ variationȱ inȱ chipȱsizeȱdistributionȱshouldȱbeȱasȱsmallȱasȱpossibleȱbecauseȱitȱaffectsȱ(amongȱotherȱ things)ȱ forȱ exampleȱ theȱ packingȱ degreeȱ inȱ theȱ compressionȱ screwsȱ usedȱ toȱ transportȱtheȱchips.ȱItȱisȱgenerallyȱacceptedȱthatȱtheȱchipȱqualityȱaffectsȱtheȱwholeȱ pulpȱproductionȱandȱtherebyȱtheȱpropertiesȱofȱtheȱpulp.ȱ ȱ Inȱtheȱcaseȱofȱchemicalȱpulping,ȱtheȱwoodȱisȱimpregnatedȱwithȱchemicals.ȱSmallȱ sizedȱ chipsȱ willȱ beȱ overcookedȱ (i.e.ȱ theȱ ligninȬdissolvingȱ chemicalsȱ wouldȱ haveȱ startedȱtoȱattackȱtheȱcelluloseȱandȱweakenȱtheȱfibres).ȱTheȱpenetrationȱrateȱofȱacidȱ sulphiteȱ liquorsȱ inȱ theȱ grainȱ directionȱ isȱ 50ȱ toȱ 100ȱ timesȱ fasterȱ thanȱ inȱ theȱ crossȬ grainȱ directions.ȱ Otherȱ pulpingȱ liquorsȱ penetrateȱ withȱ nearlyȱ equalȱ ratesȱ inȱ allȱ directionsȱ[18].ȱ ȱ Oversizedȱchipsȱwillȱbeȱundercookedȱ(i.e.ȱwillȱstillȱbeȱaȱchunkȱofȱunseparatedȱ fibres).ȱ Piecesȱ ofȱ woodȱ thatȱ areȱ tooȱ smallȱ (e.g.ȱ ‘shoeȱ pegs’ȱ andȱ sawdust)ȱ tendȱ toȱ agglomerateȱandȱcreateȱplugsȱinȱsomeȱtypesȱofȱchemicalȱpulpingȱequipmentȱ[18].ȱ ȱ

8 Inȱtheȱcaseȱofȱmechanicalȱpulping,ȱtheȱrefinerȱwillȱonlyȱacceptȱaȱpieceȱofȱwoodȱ smallȱenoughȱtoȱenterȱbetweenȱtheȱrefinerȱdiscsȱ[18].ȱ ȱ 2.4. Precompression Oneȱofȱtheȱmainȱaimsȱinȱtheȱdevelopmentȱofȱtheȱmechanicalȱpulpingȱprocessȱisȱ toȱ achieveȱ theȱ desiredȱ degreeȱ ofȱ refiningȱ withȱ minimumȱ ofȱ energyȱ expenditure.ȱ Reducingȱenergyȱconsumptionȱisȱofȱutmostȱimportanceȱtoȱproducersȱofȱmechanicalȱ pulp,ȱ becauseȱ ofȱ bothȱ increasingȱ electricityȱ pricesȱ andȱ theȱ importanceȱ ofȱ environmentalȱ issues.ȱ Theȱ interestȱ ofȱ mechanicalȱ pretreatmentȱ hasȱ arisenȱ dueȱ toȱ theȱpossibilityȱtoȱachieveȱenergyȱreductionȱinȱrefining.ȱ ȱ Toȱ induceȱ damageȱ inȱ woodȱ chip,ȱ axialȱ precompressionȱ hasȱ shownȱ toȱ beȱ theȱ mostȱ efficientȱ methodȱ [19].ȱ Itȱ wasȱ alsoȱ shownȱ thatȱ itȱ wasȱ possibleȱ toȱ achieveȱ aȱ reductionȱinȱrefiningȱenergyȱofȱaxiallyȱprecompressedȱblocksȱofȱwesternȱhemlockȱ byȱ 9%ȱ forȱ TMPȱ (thermomechanicalȱ pulp)ȱ andȱ 40%ȱ forȱ CTMPȱ (chemithermomechanicalȱpulp).ȱ ȱ Precompressionȱofȱwoodȱchipsȱopensȱupȱtheȱwoodȱstructure,ȱwhichȱincreasesȱ theȱ specificȱ surfaceȱ [19],ȱ [20]ȱ andȱ facilitatesȱ theȱ penetrationȱ andȱ uptakeȱ ofȱ chemicalsȱ andȱ water.ȱ Dinewoodieȱ [21],ȱ [22]ȱ observedȱ thatȱ smallȱ dislocationsȱ ofȱ microfibrilsȱinȱtheȱcellȱwallȱprogressedȱtoȱgrowȱtoȱmicroscopicȱcreases.ȱTheȱsmallȱ creasesȱgrowȱsuccessivelyȱtoȱlargerȱcreasesȱupȱtoȱfailure,ȱwithȱincreasingȱloadȱwhenȱ compressingȱwoodȱparallelȱtoȱtheȱgrain.ȱ ȱ Thereȱareȱaȱnumberȱofȱdevicesȱonȱtheȱmarket,ȱwhichȱcanȱbeȱclaimedȱtoȱgiveȱaȱ compressiveȱ mechanicalȱ pretreatment.ȱ Theȱ mostȱ wellȱ knownȱ equipmentsȱ are:ȱ Impressafinerȱ[23],ȱ[24],ȱPlugȱscrewȱfeederȱ[25],ȱPREXȱimpregnatorȱ[26]ȱandȱBiȬVisȱ Extruderȱ[27].ȱTheȱdrawbackȱwithȱthoseȱequipmentsȱisȱthat;ȱtheȱpretreatmentȱisȱnotȱ onlyȱinȱtheȱaxialȱdirectionȱofȱtheȱwoodȱfibre,ȱalsoȱitȱhaveȱtoȱbeȱspaceȱinȱtheȱmillȱinȱ orderȱtoȱinstallȱthisȱextraȱequipmentȱandȱinȱmanyȱcasesȱitȱwillȱbecomeȱaȱbottleneck.ȱ ȱ Aȱ newȱ possibilityȱ toȱ achieveȱ precompressionȱ inȱ woodȱ chipsȱ byȱ adjustingȱ theȱ woodȱ chippingȱ process,ȱ isȱ presentedȱ inȱ [16],ȱ [28].ȱ Duringȱ chipping,ȱ largeȱ compressiveȱ stressesȱ inȱ theȱ fibreȱ directionȱ ofȱ theȱ chips,ȱ mightȱ occurȱ inȱ certainȱ situations.ȱInȱ[17]ȱisȱshownȱthatȱoneȱprocessȱparameterȱthatȱdeterminesȱtheȱsizeȱofȱ thisȱcompressiveȱstressȱisȱtheȱspoutȱangle.ȱThisȱinvestigationȱisȱlimitedȱtoȱanȱelastic,ȱ orthotropicȱmaterial.ȱHowever,ȱinȱ[29]ȱisȱshownȱthatȱtheȱirreversibleȱdeformationsȱ areȱ largeȱ whenȱ plasticityȱ effectsȱ areȱ allowed,ȱ indicatingȱ thatȱ theȱ compressiveȱ damageȱ(irreversibleȱdeformations)ȱcanȱbeȱmadeȱlargeȱbyȱchoosingȱappropriateȱe.g.ȱ spoutȱangles.ȱȱ ȱ

9 Inȱbothȱ[16]ȱandȱ[28]ȱthisȱeffectȱhasȱbeenȱutilisedȱtoȱaccomplishȱaȱmoreȱdirectedȱ wayȱofȱinducingȱcompressiveȱdamageȱtoȱtheȱwoodȱchips.ȱDirectedȱinȱthisȱcontextȱ meansȱ thatȱ differentȱ fromȱ theȱ devicesȱ referredȱ toȱ above,ȱ theȱ largeȱ compressiveȱ stressesȱthatȱoccursȱduringȱchippingȱ(forȱcertainȱprocessȱparameters)ȱwillȱactȱinȱtheȱ mostȱbeneficialȱdirectionȱi.e.ȱparallelȱtoȱtheȱfibres.ȱ

10 3. METHODS Inȱtheȱfollowingȱsectionȱbothȱexperimentalȱandȱanalytical/numericalȱtechniquesȱ areȱdescribed.ȱTheȱmethodsȱareȱpresentedȱinȱtheȱcontextȱofȱaȱreviewȱofȱeachȱofȱtheȱ papersȱI,ȱII,ȱIII,ȱIVȱandȱV.ȱ ȱ 3.1. Review of paper I Theȱ generalȱ aimȱ withȱ paperȱ Iȱ wasȱ toȱ developȱ aȱ deviceȱ andȱ anȱ experimentalȱ methodȱbyȱwhichȱitȱwasȱpossibleȱtoȱdetermineȱtheȱstrainȱfieldȱandȱtoȱobserveȱtheȱ fractureȱprocessesȱinȱwoodȱduringȱchipping.ȱ ȱ 3.1.1. Background Toȱbeȱableȱtoȱformulateȱcriteria’sȱforȱdeterminingȱtheȱonsetȱofȱtheȱcreationȱofȱaȱ woodȱchip;ȱitȱisȱdesirableȱtoȱbeȱableȱtoȱstudyȱtheȱdeformationȱfieldsȱinȱaȱvicinityȱofȱ theȱedgeȱofȱtheȱchippingȱtool.ȱToȱthatȱend,ȱanȱexperimentalȱsetup;ȱWC05ȱhasȱbeenȱ developedȱ whereȱ itȱ wasȱ possibleȱ toȱ controlȱ theȱ cuttingȱ rateȱ andȱ toȱ measureȱ theȱ forceȱ onȱ theȱ tool.ȱ Theȱ setupȱ alsoȱ admittedȱ thatȱ theȱ angleȱ ofȱ theȱ woodȱ specimenȱ withȱrespectȱtoȱtheȱcuttingȱplaneȱcouldȱbeȱvariedȱinȱbothȱaȱhorizontalȱandȱaȱverticalȱ plane.ȱ ȱ Toȱ determineȱ theȱ deformations,ȱ aȱ Digitalȱ Speckleȱ Photographyȱ (DSP)ȱ equipmentȱ wasȱ used,ȱ whichȱ togetherȱ withȱ imageȱ processingȱ softwareȱ madeȱ itȱ possibleȱ toȱ studyȱ theȱ localȱ deformationȱ fieldsȱ i.e.ȱ theȱ strainȱ distributionsȱ onȱ theȱ surfaceȱofȱtheȱwoodȱspecimenȱinȱaȱvicinityȱofȱtheȱchippingȱtoolȱi.e.ȱtheȱknifeȬedge.ȱ ȱ DSPȱhasȱfoundȱlargeȱapplicabilityȱinȱaȱnumberȱofȱinterestingȱapplications.ȱTheȱ methodȱhasȱbeenȱusedȱamongȱotherȱthingsȱe.g.ȱforȱestimatingȱhygroȬexpansionȱofȱ paperȱ [30],ȱ viscoȬplasticȱ propertiesȱ ofȱ metalsȱ [31]ȱ andȱ elasticȱ propertiesȱ ofȱ woodȱ fibreȱwallsȱ[32].ȱUsingȱthisȱtechniqueȱtoȱstudyȱlocalȱparametersȱinȱconnectionȱwithȱ theȱwoodȱchippingȱprocessȱisȱaȱnovelȱapproach.ȱ ȱ 3.1.2. Experimental Setup ToȱbeȱableȱtoȱperformȱtheȱchippingȱinȱaȱwellȬdefinedȱway,ȱaȱspecialȱfixtureȱwasȱ designed.ȱAȱphotoȱofȱtheȱexperimentalȱsetupȱisȱshownȱinȱFigureȱ9.ȱTheȱARAMISȱ measuringȱ systemȱ [33]ȱ wasȱ usedȱ forȱ theȱ deformationȱ analysisȱ ofȱ theȱ specimenȱ surface.ȱAȱhydraulicȱtestingȱmachineȱ(MTS)ȱwasȱusedȱtoȱloadȱtheȱchippingȱtool.ȱToȱ fixȱtheȱwoodȱsample,ȱaȱspecimenȱholderȱwasȱusedȱ(Figureȱ9).ȱTheȱwholeȱchippingȱ deviceȱisȱshownȱinȱFigureȱ10.ȱ ȱ

11 Figure 9. The experimental setup Figure 10. The chipping device; WC05ȱ ȱ Theȱ loadingȱ ofȱ theȱ chippingȱ toolȱ wasȱ accomplishedȱ byȱ mountingȱ theȱ wholeȱ fixtureȱinȱaȱservoȱhydraulicȱtestingȱmachineȱ(MTS),ȱandȱaȱ50ȱkNȱloadȱcellȱwasȱusedȱ toȱ measureȱ theȱ appliedȱ force.ȱ Inȱ thisȱ way,ȱ theȱ cuttingȱ rateȱ canȱ beȱ controlledȱ accurately.ȱ ȱ Inȱ Figureȱ 9ȱ isȱ alsoȱ shownȱ theȱ cameraȱ belongingȱ toȱ theȱ DSPȱ equipment.ȱ Aȱ necessaryȱ conditionȱ forȱ applicationȱ ofȱ theȱ techniqueȱ isȱ however,ȱ thatȱ theȱ surfaceȱ hasȱaȱclearlyȱidentifiableȱstructure.ȱToȱthatȱend,ȱtheȱsurfaceȱofȱtheȱwoodȱspecimenȱ wasȱfirstȱpaintedȱwhiteȱandȱthenȱsprayedȱwithȱblackȱcolourȱtoȱgetȱaȱrandomȱpatternȱ ofȱ blackȱ dots.ȱ Theȱ techniqueȱ reliesȱ onȱ thatȱ theȱ motionȱ ofȱ thisȱ patternȱ canȱ beȱ detectedȱbetweenȱframes;ȱthereforeȱtheȱqualityȱofȱtheȱpatternȱisȱcrucial.ȱ ȱ Asȱ soonȱ asȱ atȱ leastȱ twoȱ imagesȱ ofȱ theȱ testȱ surfaceȱ areȱ captured,ȱ theȱ relativeȱ displacementȱ betweenȱ themȱ canȱ beȱ calculated.ȱ Theȱ softwareȱ includedȱ inȱ theȱ equipmentȱ calculatesȱ theȱ displacementȱ fieldȱ andȱ afterȱ that,ȱ theȱ strainȱ fieldȱ isȱ obtainedȱ byȱ numericalȱ differentiationȱ ofȱ theȱ displacementȱ field.ȱ Forȱ bestȱ results,ȱ theȱ sampleȱ hasȱ toȱ beȱ perpendicularȱ toȱ theȱ camera.ȱ Forȱ thisȱ study,ȱ freshȱ Norwayȱ Spruceȱ(PiceaȱAbies)ȱinȱformȱofȱplanksȱwithȱcrossȱsectionȱ35ȱXȱ82ȱmm2ȱwasȱused.ȱ ȱ 3.2. Review of paper II Theȱ generalȱ aimȱ withȱ paperȱ IIȱ wasȱ toȱ investigateȱ someȱ aspectsȱ ofȱ theȱ woodȱ chippingȱprocess.ȱToȱperformȱthat,ȱaȱsimpleȱanalyticalȱmodelȱwasȱdeveloped.ȱToȱ getȱ someȱ ideaȱ ofȱ theȱ accuracyȱ ofȱ theȱ analyticalȱ model,ȱ aȱ Finiteȱ Elementȱ (FE)ȱ analysisȱwasȱalsoȱperformed.ȱ

12 3.2.1. Analytical-modelling of the wood chipping process Toȱjudgeȱtheȱinfluenceȱofȱdifferentȱparametersȱonȱtheȱwoodȱchippingȱprocess,ȱaȱ simpleȱanalyticalȱmodelȱwasȱdeveloped.ȱItȱmightȱseemȱaȱstrangeȱthingȱtoȱdoȱsinceȱ muchȱ moreȱ reliableȱ resultsȱ canȱ beȱ obtainedȱ byȱ usingȱ theȱ Finiteȱ Elementȱ (FE)ȱ method.ȱThisȱisȱtrue,ȱhowever,ȱanȱanalyticalȱmodelȱisȱmuchȱmoreȱtransparentȱwhenȱ itȱ comesȱ toȱ judgingȱ theȱ influenceȱ ofȱ specificȱ parameters.ȱ Theȱ mostȱ severeȱ limitationsȱ withȱ bothȱ theȱ analyticalȱ andȱ theȱ FEȱ modelȱ areȱ thatȱ theȱ materialȱ isȱ assumedȱtoȱbeȱlinearȱelastic.ȱOnȱtheȱotherȱhandȱitȱisȱfeltȱthatȱexistingȱmodelsȱforȱ anisotropicȱplasticityȱinȱmetalsȱlacksȱsoȱmuchȱofȱphysicalȱrelevanceȱtoȱbeȱappliedȱtoȱ woodȱwithȱsomeȱconfidence.ȱ ȱ 3.2.2. Analytical-model Theȱmodelȱassumesȱslidingȱfrictionȱandȱisȱbasedȱonȱanȱassumedȱdisplacementȱ fieldȱtogetherȱwithȱtheȱtheoremȱofȱminimumȱpotentialȱenergy.ȱSmallȱdeformations,ȱ aȱplaneȱstrainȱsituation,ȱandȱaȱlinearȱelasticȱorthotropicȱmaterial,ȱareȱassumed.ȱ ȱ InȱFigureȱ11ȱisȱshownȱaȱsingleȱwoodȱchip,ȱassumedȱtoȱbeȱclampedȱatȱtheȱlowerȱ boundary.ȱTheȱcuttingȬplaneȱisȱatȱanȱangleȱEȱtoȱtheȱhorizontalȱplaneȱandȱtheȱknifeȱ tipȱ isȱ occupyingȱ anȱ angleȱ D.ȱ Theȱ lengthȱ andȱ thicknessȱ ofȱ theȱ chipȱ isȱ Lȱ andȱ tȱ respectively.ȱ ȱ

Figure 11. An idealised situation ȱ Onȱtheȱleftȱboundary,ȱaȱshearȱloadȱWȱisȱassumedȱtoȱbeȱacting.ȱConsistentȱwithȱtheȱ assumptionȱofȱsmallȱdeformationsȱisȱthatȱDȱisȱaȱsmallȱangle.ȱToȱsimplifyȱmatters,ȱ coordinatesȱ[ȱandȱKȱareȱusedȱasȱshownȱinȱFigureȱ12:ȱ ȱ

13 Figure 12. Coordinate transformation ȱ Itȱisȱassumedȱforȱux([,ȱK)ȱandȱuy([,ȱK)ȱthat:ȱ ȱ

ux ([,K) Kf ([) and uy ([,K) Kg([)ȱ (1)ȱ ȱ whereȱfȱandȱgȱareȱfunctionsȱtoȱbeȱdetermined.ȱNoteȱthatȱtheȱdisplacementsȱgivenȱinȱ 1ȱsatisfiesȱtheȱrequirementȱthatȱtheȱboundaryȱKȱ=ȱ0ȱisȱclamped.ȱ ȱ Theȱboundaryȱconditionsȱthatȱhaveȱtoȱbeȱsatisfiedȱisȱthatȱtheȱdisplacementsȱinȱ theȱ xȬȱ andȱ yȬȱ directionsȱ i.e.ȱ theȱ displacementsȱ uxȱ andȱ uyȱ equalsȱ zeroȱ onȱ theȱ boundaryȱKȱ=ȱ0,ȱandȱthatȱpointsȱonȱtheȱcuttingȱplaneȱ[ȱ=ȱ0,ȱareȱconfinedȱtoȱmoveȱonȱ aȱplaneȱmakingȱtheȱangleȱDȱ+ȱEȱȱwithȱtheȱxȬaxisȱ(Figureȱ13).ȱ ȱ

Figure 13. Sliding condition ȱ Sinceȱ theȱ onlyȱ externalȱ loadȱ isȱ theȱ shearȱ loadȱ Ĳ(K)ȱ onȱ theȱ boundaryȱ [ȱ =ȱ 0,ȱ theȱ potentialȱenergyȱUȱisȱgivenȱby:ȱ 14 ȱ 1 U (V H V H W J )dS W cu su d* ȱ (2)ȱ ³ 2 x x y y xy xy ³ x y S * ȱ whereȱ Vx, Vyȱ andȱ Wxyȱ areȱ theȱ normalȱ andȱ shearȱ stressesȱ respectivelyȱ andȱ Sȱ isȱ theȱ domainȱinȱtheȱxȬȱyȬȱplane,ȱoccupiedȱbyȱtheȱchipȱandȱīȱdenotesȱtheȱboundaryȱ[ =ȱ0.ȱ Forȱbrevity,ȱcosȱE=ȱcȱandȱsinȱE=ȱsȱhasȱbeenȱused.ȱ ȱ Theȱ stressesȱ areȱ obtainedȱ fromȱ theȱ strainsȱ (derivedȱ fromȱ (1))ȱ throughȱ theȱ constitutiveȱ relationsȱ relevantȱ forȱ anȱ orthotropicȱ material.ȱ Theȱ differentialȱ equationsȱforȱfȱandȱgȱandȱnaturalȱboundaryȱconditionsȱareȱobtainedȱfromȱGU =ȱ0ȱi.e.ȱ fromȱtheȱrequirementȱthatȱtheȱfirstȱvariationȱofȱtheȱpotentialȱenergyȱequalsȱzero.ȱ ȱ Sinceȱ theȱ ambitionȱ isȱ toȱ includeȱ slidingȱ frictionȱ inȱ aȱ consistentȱ wayȱ andȱ theȱ stressesȱ andȱ henceȱ theȱ contactȱ pressureȱ onȱ theȱ boundaryȱ [ =ȱ 0ȱ willȱbeȱlinearȱinȱK,ȱitȱisȱassumedȱforȱĲ(K): ȱ

W (K) kW K mW ȱ (3)ȱ ȱ whereȱ kWȱ andȱ mWȱ areȱ constantsȱ toȱ beȱ determined.ȱ Withȱ W(K)ȱ givenȱ byȱ (3)ȱ theȱ lineȱ integralȱinȱ(2)ȱcanȱbeȱevaluatedȱtoȱread:ȱ ȱ ³ W (cGux sGuy )d* K1Gg(0) K2Gf (0) ȱ (4)ȱ * ȱ whereȱK1ȱandȱK2ȱbothȱdependsȱlinearlyȱonȱkWȱȱandȱmWȱ ȱ Havingȱobtainedȱfȱandȱg,ȱtheseȱareȱinsertedȱintoȱ(2)ȱtoȱobtainȱtheȱstrains,ȱwhichȱ togetherȱwithȱelasticȱconstantsȱwillȱgiveȱtheȱstressesȱandȱinȱparticular,ȱtheȱcontactȱ pressureȱpȱ onȱ theȱ boundaryȱ [ȱ =ȱ 0ȱ whichȱ isȱgivenȱinȱ termsȱ ofȱ theȱ stressesȱ onȱ thisȱ boundaryȱby:ȱ ȱ 2 2 p (V yc V xs 2W xycs) ȱ (5)ȱ ȱ AssumingȱCoulombȱslidingȱfrictionȱoneȱwillȱhave:ȱ ȱ W Ppȱ (6)ȱ ȱ whereȱȝ 0ȱisȱtheȱcoefficientȱofȱfriction.ȱ ȱ

15 Sinceȱ linearȱ conditionsȱ areȱ assumed,ȱ theȱ contactȱ pressureȱ mustȱ dependȱ inȱ aȱ linearȱwayȱonȱtheȱloadingȱparametersȱD,ȱkWȱandȱmWȱsoȱthatȱkWȱandȱmWȱ(forȱaȱgivenȱD)ȱ canȱ beȱ determinedȱ suchȱ thatȱ W = P pȱ isȱ satisfied.ȱ Withȱ kWȱ andȱ mWȱ anȱ approximateȱ solutionȱforȱgȱandȱfȱforȱaȱcaseȱofȱslidingȱfrictionȱcanȱbeȱobtained.ȱ ȱ 3.2.3 FE- model AȱFEȱanalysisȱwasȱalsoȱperformedȱwhereȱslidingȱfrictionȱisȱassumed.ȱTheȱsameȱ assumptionsȱ asȱ forȱ theȱ analyticalȱ modelȱ areȱ madeȱ forȱ theȱ FEȱ analysisȱ i.e.ȱ smallȱ deformations,ȱaȱplaneȱstrainȱsituationȱandȱaȱlinearȱelasticȱorthotropicȱmaterial.ȱ ȱ Theȱ problemȱ wasȱ analyzedȱ usingȱ theȱ finiteȱ elementȱ method,ȱ implementedȱ inȱ theȱ Matlabȱ [34]ȱ code.ȱ Conventionalȱ fourȬnodeȱ isoparametricȱ elementsȱ withȱ twoȱ degreesȱofȱfreedomȱi.e.,ȱtranslationȱinȱtheȱxȬ,ȱandȱyȬdirectionsȱhaveȱbeenȱutilized.ȱAȱ thoroughȱ descriptionȱ ofȱ theȱ elementȱ andȱ itsȱ implementationȱ procedureȱ canȱ beȱ foundȱinȱBatheȱ[35].ȱ ȱ ȱ 3.3. Review of paper III TheȱgeneralȱaimȱwithȱpaperȱIIIȱwasȱtoȱdevelopȱaȱlaboratoryȱwoodȱchipperȱbyȱ whichȱitȱwasȱpossibleȱtoȱproduceȱchipsȱunderȱrealisticȱconditions.ȱ ȱ 3.3.1. Background Inȱ orderȱ toȱ beȱ ableȱ toȱ studyȱ theȱ damageȱ mechanismsȱ andȱ inȱ generalȱ theȱ mechanismsȱ activeȱ whenȱ aȱ woodȱ chipȱ isȱ createdȱ duringȱ theȱ woodȱ chippingȱ process,ȱ itȱ isȱ crucialȱ toȱ haveȱ anȱ experimentalȱ equipmentȱ inȱ whichȱ chipsȱ canȱ beȱ producedȱunderȱrealisticȱconditions.ȱ ȱ Previousȱ studiesȱ c.f.ȱ [9],ȱ [36],ȱ [10],ȱ [11]ȱ andȱ [13]ȱ haveȱ beenȱ restrictedȱ toȱ lowȱ cuttingȱ ratesȱ ifȱ forceȱ onȱ theȱ cuttingȱ toolȱ wereȱ measured.ȱ Inȱ casesȱ whereȱ realisticȱ cuttingȱratesȱwereȱconsidered,ȱnoȱforcesȱwereȱmeasuredȱ[15].ȱ ȱ 3.3.2. Experimental Setup Theȱwoodȱchipper;ȱWC05/HSSȱconsistsȱinȱprincipleȱofȱanȱelectricallyȱpoweredȱ motor,ȱaȱcircularȱdiscȱandȱaȱshaftȱconnectingȱthem.ȱTheȱcircularȱdiscȱisȱequippedȱ withȱaȱchippingȱtoolȱi.e.ȱaȱknifeȱandȱanȱinstrumentedȱholder.ȱȱ Theȱwoodȱspecimenȱholderȱadmitsȱaȱvariationȱofȱtheȱcuttingȱangleȱ(spoutȱangleȱ andȱsideȱangel)ȱi.e.ȱtheȱanglesȱinȱtwoȱplanes.ȱAlso,ȱtheȱwoodȱchipperȱisȱequippedȱ withȱ aȱ windowȱ whereȱ theȱ cuttingȱ processȱ canȱ beȱ recordedȱ throughȱ highȱ speedȱ filming.ȱAnȱoverviewȱofȱtheȱwoodȱchipperȱisȱshownȱinȱFigureȱ14.ȱ ȱ

16 Figure 14. An overview of the wood chipper; WC05/HSS ȱ 3.3.3. Force Measurements Byȱ measuringȱ theȱ resultingȱ forceȱ onȱ theȱ woodȱ chippingȱ knifeȱ inȱ differentȱ situationsȱoneȱcanȱobtainȱvaluableȱinformationȱregardingȱtheȱchippingȱprocessȱandȱ theȱknifeȱholderȱisȱtoȱthatȱendȱinstrumentedȱbyȱforceȱsensorsȱ(seeȱFigureȱ15).ȱ ȱ

ȱ Figure 15. Here is a part of the disc housing removed in order to show a pert of the front side of the disc. ȱ

17 3.4. Review of paper IV Theȱ generalȱ aimȱ withȱ paperȱ IVȱ wasȱ toȱ investigateȱ whetherȱ itȱ wasȱ possibleȱ toȱ modifyȱ theȱ woodȱ chippingȱ processȱ soȱ asȱ toȱ induceȱ crackingȱ inȱ theȱ chips,ȱ whichȱ wouldȱ beȱ beneficialȱ forȱ theȱ chipȱ pretreatmentȱ andȱ refiningȱ processȱ inȱ termsȱ ofȱ energyȱconsumption.ȱ ȱ 3.4.1. Background Refiningȱ woodȱ chipsȱ toȱ produceȱ mechanicalȱ pulpȱ isȱ aȱ veryȱ energyȬintensiveȱ unitȱ operation.ȱ Previousȱ investigationsȱ haveȱ shownȱ thatȱ itȱ isȱ byȱ meansȱ ofȱ mechanicalȱ pretreatmentȱ ofȱ woodȱ chips,ȱ isȱ possibleȱ toȱ quiteȱ substantiallyȱ reduceȱ theȱelectricalȱenergyȱconsumptionȱinȱtheȱsubsequentȱrefiningȱc.f.ȱ[19]Ȭ[27].ȱ ȱ 3.4.2. Refining trial WoodȱchipsȱfromȱspruceȱwereȱproducedȱunderȱwellȬcontrolledȱconditionsȱinȱaȱ laboratoryȱwoodȱchipperȱatȱthreeȱdifferentȱspoutȱanglesȱ(30q,ȱ40qȱandȱ50q)ȱandȱatȱ constantȱ chipȱ length.ȱ Theȱchipsȱ wereȱ refinedȱ inȱaȱ TMPȱ (thermomechanicalȱ pulp)ȱ pilotȱ refinerȱ plant,ȱ andȱ theȱ pulpȱ propertiesȱ freenessȱ andȱ shivesȱ contentȱ wereȱ determinedȱandȱevaluatedȱasȱaȱfunctionȱofȱspecificȱenergyȱconsumption.ȱ ȱ 3.4.3. Energy consumption during chipping Toȱmotivateȱtheȱbenefitȱofȱusingȱaȱspoutȱangleȱofȱ50qȱwhenȱproducingȱchipsȱforȱ mechanicalȱrefining,ȱitȱmustȱbeȱmadeȱclearȱthatȱtheȱincreaseȱinȱenergyȱconsumptionȱ whenȱ chippingȱ atȱ theȱ spoutȱ angleȱ 50qȱ isȱ extremelyȱ smallȱ comparedȱ toȱ whatȱ isȱ gainedȱ inȱ theȱ refiningȱ process.ȱ Toȱ thatȱ end,ȱ twoȱ testsȱ regardingȱ theȱ energyȱ consumptionȱduringȱchippingȱwereȱperformed.ȱ ȱ 3.4.4. Water adsorption test Inȱ orderȱ toȱ quantifyȱ theȱ degreeȱ ofȱ crackingȱ inducedȱ duringȱ chipping,ȱ anȱ experimentȱwasȱperformed.ȱWoodȱchipsȱfromȱeachȱspoutȱangleȱi.e.ȱ30q,ȱ40qȱandȱ50qȱ wereȱcollectedȱandȱimmersedȱinȱwater,ȱandȱtheȱweightȱasȱaȱfunctionȱofȱtimeȱwasȱ recorded.ȱ ȱ 3.5. Review of paper V Theȱ generalȱ aimȱ withȱ paperȱ Vȱ wasȱ toȱ investigateȱ whetherȱ itȱ wasȱ possibleȱ toȱ achieveȱ anȱ improvementȱ ofȱ someȱ ofȱ theȱ propertiesȱ whichȱ areȱ usedȱ toȱ defineȱ theȱ qualityȱofȱprintingȱgrades,ȱi.e.ȱtensileȱindexȱandȱspecificȱlightȱscatteringȱcoefficientȱ byȱ combiningȱ refiningȱ ofȱ chipsȱ producedȱ usingȱ aȱ spoutȱ angleȱ ofȱ 50qȱ withȱ theȱ additionȱofȱbisulphiteȱtoȱtheȱdilutionȱwaterȱduringȱrefining.ȱ ȱ

18 3.5.1. Background Itȱ hasȱ recentlyȱ beenȱ shownȱ (seeȱ previousȱ paperȱ IV)ȱ thatȱ itȱ wasȱ possibleȱ toȱ improveȱtheȱenergyȱefficiencyȱduringȱfirstȱstageȱTMPȱrefiningȱbyȱperformingȱtheȱ woodȱchippingȱsoȱasȱtoȱincreaseȱtheȱcompressiveȱdamageȱinȱtheȱchipsȱandȱhenceȱ breakingȱ upȱ theȱ woodȱ structure.ȱ Theȱ parameterȱ thatȱ wasȱ variedȱ wasȱ theȱ spoutȱ angle,ȱ i.e.ȱ theȱ angleȱ betweenȱ theȱ fibreȱ directionȱ ofȱ theȱ woodȱ specimenȱ andȱ theȱ cuttingȱ plane.ȱ Itȱ haveȱ beenȱ shownȱ earlierȱ thatȱ byȱ utilizingȱ somewhatȱ acidicȱ conditionsȱinȱCTMPȱproductionȱitȱisȱpossibleȱtoȱimproveȱtheȱstrengthȱpropertiesȱatȱ theȱsameȱenergyȱconsumptionȱwithoutȱloosingȱtheȱforȱprintingȱpaperȱsoȱimportantȱ lightȱscatteringȱcoefficientȱ[37].ȱ ȱ 3.5.2. Refining trial Thisȱstudyȱdescribesȱwhetherȱitȱisȱpossibleȱtoȱachieveȱanȱimprovementȱofȱsomeȱ ofȱ theȱ mainȱ propertiesȱ usedȱ toȱ defineȱ theȱ qualityȱ ofȱ printingȱ grades,ȱ i.e.ȱ tensileȱ indexȱ andȱ specificȱ lightȱ scatteringȱ coefficientȱ byȱ combiningȱ refiningȱ ofȱ chipsȱ producedȱusingȱspoutȱangleȱofȱ50qȱ(moreȱproneȱtoȱinduceȱdirectedȱcracks)ȱwithȱtheȱ additionȱ ofȱ bisulphiteȱ solutionȱ (NaHSO3)ȱ toȱ theȱ dilutionȱ waterȱ duringȱ chipȱ refining.ȱ ȱ

19 4. RESULTS AND DISCUSSIONS Inȱ theȱ followingȱ sectionȱ resultsȱ fromȱ anȱ experimentalȱ investigationȱ ofȱ theȱ deformationȱ fieldȱ inȱ aȱ woodȱ specimenȱ duringȱ chipping,ȱ andȱ fromȱ anȱ analytical/numericalȱ modelȱ ofȱ theȱ chippingȱ processȱ areȱ presented.ȱ Furtherȱ on,ȱ aȱ laboratoryȱchipperȱandȱtheȱresultsȱfromȱtwoȱrefiningȱtrialsȱareȱpresented.ȱ ȱ 4.1. Results from the experimental investigation (Paper I) InȱpaperȱIȱtheȱloadingȱofȱtheȱchippingȱtoolȱwasȱaccomplishedȱbyȱmountingȱtheȱ wholeȱfixtureȱinȱaȱservoȱhydraulicȱtestingȱmachineȱ(MTS),ȱandȱaȱ50ȱkNȱloadȱcellȱ wasȱ usedȱ toȱ measureȱ theȱ appliedȱ force.ȱ DisplacementȬcontrolledȱ testingȱ wasȱ performedȱwithȱaȱcrossheadȱspeedȱofȱ1.0ȱmm/sȱandȱtheȱforceȱonȱtheȱchippingȱtoolȱ wasȱmeasured.ȱ ȱ TheȱchargeȬcoupledȬdeviceȱ(CCD)ȱcameraȱfocusedȱonȱtheȱpartȱofȱtheȱspecimenȱ closestȱtoȱtheȱknifeȬedgeȱandȱtheȱsoftwareȱwasȱprogrammedȱtoȱtakeȱ12ȱphotographsȱ perȱsecondȱforȱaȱ2Dȱanalysis.ȱTheȱmaterialȱusedȱinȱtheȱchippingȱtestȱwasȱNorwayȱ spruceȱ(Piceaȱabies)ȱwithȱaȱmoistureȱcontentȱcorrespondingȱtoȱgreenȱwood.ȱForȱtheȱ test,ȱ aȱ specimenȱ withȱ crossȱ sectionȱ dimensionsȱ ofȱ 35ȱ xȱ 82ȱ mm2ȱ wasȱ used.ȱ Theȱ followingȱcuttingȱanglesȱwereȱchosen:ȱsharpnessȱangleȱEȱ=ȱ34q,ȱclearanceȱangleȱDȱ=ȱ 3q,ȱspoutȱangleȱHȱ=ȱ30qȱ(Figureȱ14).ȱ

Figure 16. The cutting angles D, E and H, and the arrow indicates the cutting directionȱ ȱ

20 DueȱtoȱlimitationsȱofȱtheȱcameraȱinȱtheȱDSPȱsystem,ȱtheȱcuttingȱrateȱhadȱtoȱbeȱ keptȱasȱlowȱasȱ1ȱmm/s,ȱwhichȱisȱfarȱbelowȱtheȱrateȱusedȱinȱtheȱchippingȱprocess.ȱ However,ȱ evenȱ thoughȱ itȱ isȱ wellȱ knownȱ thatȱ woodȱ inȱ generalȱ showsȱ aȱ rateȱ dependency,ȱitȱisȱbelievedȱthatȱstudiesȱofȱthisȱkindȱwillȱshedȱsomeȱlightȱoverȱtheȱ basicȱmechanismsȱinvolvedȱinȱcreatingȱaȱwoodȱchip.ȱ AnȱexampleȱofȱtheȱoutputȱfromȱaȱDSPȱstudyȱcanȱbeȱseenȱinȱFigureȱ17,ȱwhereȱtheȱ normalȱstrainȱinȱtheȱradialȱ(R)ȱdirectionȱisȱshown.ȱ ȱ

Figure 17. Normal strain distribution in the Figure 18. Force vs. time R-directionȱ ȱ InȱFigureȱ17,ȱtheȱreadȱcolourȱindicatesȱaȱthinȱregionȱwithȱhighȱnormalȱstrainsȱ justȱpriorȱtoȱchipȱformation.ȱInȱFigureȱ18,ȱtheȱforceȱvs.ȱtimeȱ(inȱessenceȱtheȱknifeȱ edgeȱposition,ȱsinceȱtheȱcuttingȱrateȱisȱconstant)ȱisȱshown.ȱTheȱred,ȱdottedȱverticalȱ lineȱrefersȱtoȱtheȱinstantȱforȱwhichȱFigureȱ17ȱisȱrelevant.ȱItȱcanȱbeȱobservedȱthatȱtheȱ forceȱcurveȱisȱcomposedȱofȱlargeȱamplitudeȱvariationsȱonȱwhichȱsmallerȱvariationsȱ areȱ superimposed.ȱ Theȱ largeȱ variationsȱ correspondȱ toȱ chipȱ formationȱ whileȱ theȱ smallerȱonesȱcorrespondȱtoȱtheȱinitiationȱofȱsmallerȱsubȬcriticalȱcracks.ȱȱ ȱ Duringȱ theȱ experimentsȱ mainlyȱ threeȱ differentȱ typesȱ ofȱ chipȱ formationȱ processesȱhaveȱbeenȱidentifiedȱi.e.ȱanȱopeningȬmodusȱ(Figureȱ19),ȱaȱforwardȱshearȱ modusȱ(Figureȱ20)ȱandȱaȱmodeȱaccordingȱtoȱFigureȱ21ȱwhichȱinȱlackȱofȱbetterȱmightȱ beȱreferredȱtoȱasȱaȱremoteȱopeningȱmode.ȱ ȱ

21 Figure 19. Opening modeȱ Figure 20. Forward shear modeȱ

Figure 21. Remote opening mode ȱ Whichȱoneȱofȱtheȱprocessesȱthatȱwillȱbeȱtheȱmostȱfrequent,ȱisȱlargelyȱdependentȱ onȱtheȱfrictionȱbetweenȱtheȱwoodȱmaterialȱandȱtheȱchippingȱtool.ȱFiguresȱ19,ȱ20ȱandȱ 21ȱ showȱ theȱ influenceȱ ofȱ frictionȱ rangingȱ fromȱ lowȱ frictionȱ inȱ Figureȱ 19ȱ toȱ highȱ frictionȱinȱFigureȱ21.ȱToȱgetȱanȱasȱnarrowȱasȱpossibleȱchipȱthicknessȱdistribution,ȱaȱ godȱ startȱ wouldȱ beȱ toȱ ascertainȱ thatȱ onlyȱ oneȱ fractureȱ processȱ isȱ activeȱ andȱ thisȱ meansȱthatȱtheȱsurfaceȱofȱtheȱchippingȱtoolȱshouldȱbeȱsuchȱthatȱtheȱfrictionȱisȱasȱ lowȱasȱpossible.ȱ

Apartȱ fromȱ e.g.ȱ theȱ friction,ȱ theȱ processȱ ofȱ chipȱ formationȱ isȱ alsoȱ greatlyȱ influencedȱ byȱ theȱ mechanicalȱ propertiesȱ ofȱ theȱ woodȱ andȱ itȱ isȱ obviousȱ thatȱ itȱ isȱ impossibleȱtoȱgetȱaȱsmallerȱvariationȱinȱtheȱchipȱthicknessȱthanȱwhatȱisȱdictatedȱbyȱ theȱinherentȱvariationsȱinȱtheȱwoodȱmaterial.ȱ

Anotherȱ interestingȱ observationȱ made,ȱ isȱ thatȱ beforeȱ aȱ chipȱ orȱ aȱ smallȱ subȱ criticalȱcrackȱhasȱformed,ȱtheȱforceȱvs.ȱtimeȱrelationȱisȱalwaysȱalmostȱlinearȱdespiteȱ theȱ factȱ thatȱ theȱ materialȱ behaviourȱ ofȱ woodȱ isȱ highlyȱ nonȬlinear.ȱ Consideringȱ aȱ hypotheticalȱsituationȱwithȱaȱhomogenousȱsemiȬinfiniteȱstructureȱwithȱnoȱintrinsicȱ

22 lengthȱscale,ȱitȱcanȱbeȱshownȱthatȱirrespectiveȱofȱtheȱmaterialȱbehaviour,ȱtheȱloadȱ vs.ȱpenetrationȱdepthȱisȱalwaysȱaȱlinearȱrelationȱ(selfȱsimilarity)ȱprovidedȱthatȱtheȱ chippingȱtoolȱhasȱstraightȱedges.ȱ

Inȱ wood,ȱ theȱ assumptionȱ regardingȱ theȱ intrinsicȱ lengthȱ scaleȱ isȱ obviouslyȱ notȱ trueȱsinceȱoneȱlengthȱscaleȱisȱdeterminedȱbyȱtheȱannualȱgrowthȱringȱstructure.ȱAlso,ȱ theȱassumptionȱregardingȱtheȱsemiȬinfiniteȱnatureȱofȱtheȱproblemȱdoesȱnotȱholdȱinȱ aȱrealȱsituation.ȱ ȱ InȱFiguresȱ22ȱandȱ23ȱisȱshownȱtheȱforceȱvs.ȱdisplacementȱ(penetrationȱdepth)ȱforȱ twoȱdifferentȱvaluesȱofȱtheȱdistance,ȱd,ȱfromȱtheȱcuttingȱplaneȱtoȱtheȱfreeȱend.ȱInȱ eachȱfigureȱisȱshownȱtheȱresultȱfromȱtwoȱtests.ȱ ȱ

Figure 22. Force vs. displacement for d=10 mm Figure 23. Force vs. displacement for d=20 mm ȱ Itȱ canȱ beȱ observedȱ thatȱ evenȱ thoughȱ theȱ assumptionsȱ regardingȱ homogeneityȱ andȱsemiȬinfinityȱareȱviolated,ȱtheȱcuttingȱprocessȱisȱapproximatelyȱselfȬsimilar.ȱ ȱ 4.2 Results from the Analytical and Numerical Study (Paper II) Inȱ paperȱ IIȱ elasticȱ dataȱ forȱ wetȱ spruceȱ [29]ȱ wereȱ usedȱ inȱ theȱ calculationsȱ i.e.ȱ with:ȱ ȱ Table 1. Material properties

EL ER GLR QLR QRL [MPa]ȱ [MPa]ȱ [MPa]ȱ ȱ ȱ 10000 820 660 0,4 0,033 ȱ Inȱ Tableȱ 1,ȱ E,ȱ Gȱ andȱ Qȱ areȱ theȱ Young’sȱ modulus,ȱ theȱ shearȱ modulusȱ andȱ theȱ Poisson’sȱ ratioȱ respectively.ȱ Theȱ subscriptsȱ L,ȱ Rȱ denoteȱ theȱ principalȱ materialȱ directions,ȱnamelyȱtheȱlongitudinalȱandȱradialȱdirectionsȱrelativeȱtheȱoriginalȱlog.ȱ ȱ

23 Withȱ elasticȱ dataȱ takenȱ fromȱ Tableȱ 1,ȱ theȱ stressesȱ Vyȱ andȱ Wxyȱ alongȱ theȱ crackȬ planeȱareȱcalculatedȱforȱDȱ=ȱ10º,ȱLȱ=25ȱmm,ȱtȱ=ȱ5ȱmm,ȱPȱ=ȱ0,2,ȱEȱ=ȱ60º.ȱTheȱcalculatedȱ stressesȱandȱareȱshownȱinȱFigureȱ24ȱversusȱ[/tȱandȱnormalizedȱwithȱrespectȱtoȱER.ȱ ȱ

Figure 24. Normalized stresses Vy and Wxy along the crack-plane for the case E = 60q and P = 0,2

ItȱwasȱobservedȱthatȱtheȱmodelȱindicatesȱaȱlargeȱinfluenceȱofȱEȱonȱtheȱmagnitudeȱ ofȱWxyȱi.e.ȱaȱsmallȱvalueȱofȱEȱwillȱgiveȱaȱmoreȱpronouncedȱopeningȱmodeȱcomparedȱ toȱaȱlargeȱvalueȱofȱE.ȱ ȱ InȱFigureȱ25ȱareȱshownȱtheȱcontactȱstressȱdistributionsȱforȱtheȱcasesȱconsideredȱ inȱFigureȱ24,ȱandȱforȱtheȱcaseȱEȱ=ȱ30º.ȱ ȱ

24 Figure 25. The contact stress distribution on the chip for E = 60q and E = 30q along the chip-end ȱ Itȱwasȱalsoȱobservedȱthatȱtheȱmodelȱindicatesȱcontactȱstressesȱbeingȱtensileȱinȱaȱ regionȱ closeȱ toȱ theȱ tipȱ ofȱ theȱ chippingȱ tool.ȱ Thisȱ isȱ dueȱ toȱ thatȱ theȱ assumedȱ displacementsȱareȱtooȱsimple.ȱHoweverȱandȱinȱspiteȱofȱthis,ȱtheȱmodelȱpredictsȱaȱ decreasingȱcontactȱpressureȱwithȱaȱdecreasingȱE.ȱ ȱ Comparison of results ResultsȱfromȱtheȱanalyticalȱmodelȱwereȱcomparedȱwithȱresultsȱfromȱFEȱanalysisȱ

(Figureȱ 24).ȱ Theȱ theoreticalȱ calculatedȱ stressesȱ Vyȱ andȱ Wxyȱ alongȱ theȱ crackȬplaneȱ agreeȱreasonablyȱwellȱwithȱtheȱFEȱcalculatedȱstressesȱforȱEȱ=ȱ60º.ȱForȱsmallerȱvaluesȱ ofȱE,ȱtheȱassumptionȱmadeȱregardingȱtheȱdisplacementȱfieldȱbecomesȱinsufficient.ȱ

Constant length-to-thickness ratio Itȱhasȱbeenȱreportedȱinȱtheȱliteratureȱ(c.f.ȱ[3],ȱ[10],ȱ[11]ȱandȱ[13])ȱthatȱforȱtheȱsameȱ processȱ parametersȱ andȱ geometryȱ ofȱ theȱ chippingȱ tool,ȱ theȱ ratioȱ betweenȱ lengthȱ andȱthicknessȱofȱtheȱchipȱisȱ(inȱsomeȱaverageȱsense)ȱconstant.ȱSomeȱconsequencesȱ ofȱthisȱobservationȱwillȱnowȱbeȱdiscussed.ȱConsiderȱFigureȱ26ȱbelow:ȱ ȱ

25 Figure 26. Quarter infinite geometries

Ifȱ itȱ isȱ assumedȱ thatȱ thereȱ areȱ noȱ intrinsicȱ lengthȱ scalesȱ associatedȱ withȱ theȱ material,ȱthenȱtheȱstressȱandȱstrainȱfieldsȱinȱtheȱleftȱgeometryȱi.e.ȱVij(x,ȱy)ȱandȱHij(x,ȱy)ȱ willȱbeȱrelatedȱtoȱtheȱsameȱfieldsȱinȱtheȱscaledȱgeometryȱaccordingȱto:ȱ ȱ * * V ij (x, y) V ij (Ix,Iy) and Hij(x, y) Hij (Ix,Iy) (7)ȱ ȱ ThisȱisȱoftenȱreferredȱtoȱasȱselfȬsimilarity.ȱObviously,ȱtheȱassumptionȱthatȱthereȱ areȱnoȱintrinsicȱlengthȱscalesȱassociatedȱwithȱwoodȱisȱnotȱtrue,ȱsinceȱwoodȱhaveȱaȱ structure.ȱOnȱoneȱlengthȱscaleȱanȱannualȱringȱstructureȱcanȱbeȱidentifiedȱandȱonȱaȱ smallerȱlengthȱscale,ȱaȱfibreȱstructureȱcanȱbeȱseenȱetc.ȱHowever,ȱinȱspiteȱofȱthis,ȱitȱisȱ shownȱ inȱ [36] thatȱ theȱ woodȱ chippingȱ processȱ isȱ approximatelyȱ (atȱ leastȱ forȱ theȱ casesȱconsidered)ȱselfȬsimilar.ȱ

AssumingȱthatȱselfȬsimilarityȱholds,ȱthenȱtheȱstressesȱalongȱtheȱhorizontalȱplaneȱ indicatedȱinȱFigureȱ26,ȱwillȱbeȱidenticalȱinȱtheȱnormalisedȱxȬȱcoordinateȱ\ = x/(IL),ȱ forȱallȱvaluesȱofȱI.ȱInȱtheȱsameȱway,ȱtheȱstressesȱalongȱtheȱleftȱinclinedȱplaneȱwillȱbeȱ theȱ sameȱ inȱ theȱ normalisedȱ coordinatesȱ Q = y/(It).ȱ Fromȱ experiments,ȱ itȱ canȱ beȱ observedȱ thatȱ shortȱ cracksȱ appearȱ whenȱ theȱ chippingȱ toolȱ penetratesȱ theȱ woodȱ pieceȱ toȱ beȱ chipped.ȱ However,ȱ itȱ isȱ notȱ untilȱ theȱ toolȱ hasȱ penetratedȱ aȱ criticalȱ distance,ȱthatȱsuchȱaȱsmallȱcrackȱbecomesȱcriticalȱandȱaȱchipȱisȱformed.ȱȱ ȱ Theȱ conclusionȱ isȱ thatȱ itȱ isȱ notȱ theȱ stressȱ fieldȱ closeȱ toȱ theȱ tipȱ ofȱ theȱ toolȱ thatȱ determinesȱ theȱ creationȱ ofȱ aȱ chip,ȱ butȱ itȱ isȱ theȱ stressȱ fieldȱ overȱ theȱ entireȱ crackȬ planeȱthatȱisȱcritical.ȱ ȱ

26 Thisȱ isȱ veryȱ muchȱ unlikeȱ forȱ instanceȱ whatȱ isȱ seenȱ inȱ e.g.ȱ fractureȱ mechanicsȱ whereȱtheȱcrackȬlengthȱhasȱanȱinfluenceȱonȱtheȱstrength.ȱAnotherȱexampleȱisȱtheȱ strengthȱ ofȱ anȱ infiniteȱ plateȱ withȱ aȱ circularȱ holeȱ whereȱ theȱ diameterȱ ofȱ theȱ holeȱ influencesȱtheȱstrength.ȱ ȱ 4.3 Results from the Laboratory chipper development (Paper III) Inȱ paperȱ IIIȱ aȱ laboratoryȱ chipperȱ wasȱ developedȱ thatȱ admitȱ chippingȱ atȱ ratesȱ thatȱcanȱbeȱvariedȱinȱaȱlargeȱinterval,ȱi.e.ȱatȱratesȱrangingȱfromȱzeroȱtoȱ50ȱm/s.ȱTheȱ woodȱspecimenȱholderȱadmitsȱaȱvariationȱofȱtheȱcuttingȱanglesȱ(spoutȱangleȱandȱ sideȱangle).ȱ ȱ Theȱ knifeȱ usedȱ toȱ cutȱ theȱ chipsȱ isȱ mountedȱ inȱ aȱ knifeȱ holder,ȱ whichȱ isȱ instrumentedȱ inȱ suchȱ aȱ wayȱ thatȱ forcesȱ inȱ threeȱ orthogonalȱ directionsȱ canȱ beȱ measured.ȱSinceȱtheȱactualȱforceȱandȱmeasuredȱforceȱdifferȱdueȱtoȱinertiaȱeffects,ȱaȱ simpleȱmathematicalȱmodelȱisȱdevelopedȱandȱusedȱtoȱevaluateȱtheȱforcesȱactingȱonȱ theȱ knife.ȱ InȱFigureȱ 27ȱaȱ typicalȱ resultȱfromȱ aȱ singleȱ cut,ȱ performedȱ atȱ theȱ spoutȱ angleȱ Hȱ =ȱ 30qȱ andȱ theȱ cuttingȱ rateȱ vcȱ =ȱ 15ȱ m/s.ȱ Onȱ theȱ verticalȱ axisȱ isȱ shownȱ theȱ cuttingȱ forceȱ Pyȱ asȱ anȱ electricalȱ voltageȱ andȱ onȱ theȱ horizontalȱ axisȱ isȱ shownȱ theȱ time.ȱ ȱ

Figure 27. Py (V) vs. Time (s)ȱ ȱ ȱ 4.4 Results from the TMP study (Paper IV) Inȱ paperȱ IVȱ aȱ TMPȱ trialȱ wasȱ carriedȱ outȱ toȱ evaluateȱ ifȱ theȱ woodȱ chippingȱ processȱcanȱbeȱusedȱasȱaȱpossibleȱmechanicalȱchipȱpretreatmentȱmethod.ȱȱ ȱ

27 Inȱ Figureȱ 28ȱ isȱ shownȱ theȱ outcomeȱ ofȱ theȱ refiningȱ trails.ȱ Theȱ circlesȱ andȱ theȱ squaresȱ denoteȱ chipsȱ producedȱ atȱ spoutȱ angleȱ 30qȱ andȱ 40qȱ respectively.ȱ Theȱ unfilledȱandȱfilledȱtrianglesȱdenoteȱtheȱchipsȱproducedȱatȱspoutȱangleȱ50qȱbutȱtheyȱ representȱ twoȱ productionȱ ratesȱ suchȱ thatȱ theȱ unfilledȱ trianglesȱ areȱ theȱ outcomeȱ fromȱ aȱ higherȱ productionȱ rateȱ whileȱ theȱ filledȱ trianglesȱ areȱ theȱ outcomeȱ fromȱ aȱ lowerȱproductionȱrate.ȱ ȱ

Figure 28. Freeness vs. Specific energy ȱ Forȱ aȱ specificȱ energyȱ belowȱ aboutȱ 1000ȱ kWh/tonȱ theȱ curveȱ forȱ theȱ pulpȱ fromȱ chipsȱcutȱatȱtheȱspoutȱangleȱ50qȱisȱaboveȱtheȱotherȱcurves.ȱOneȱexplanationȱmightȱ beȱthatȱsinceȱchippingȱatȱaȱspoutȱangleȱofȱ50qȱresultedȱinȱaȱlargeȱamountȱofȱclustersȱ ofȱ chips,ȱi.e.ȱchipsȱ whichȱwereȱ notȱ entirelyȱ disintegrated,ȱ muchȱ energyȱ mightȱ beȱ consumedȱatȱanȱearlyȱstageȱjustȱtoȱbrakeȱupȱtheȱclusters.ȱForȱspecificȱenergiesȱaboveȱ 1000ȱ kWh/ton,ȱ theȱ processȱ canȱ benefitȱ fromȱ theȱ largerȱ compressiveȱ damageȱ inducedȱbyȱusingȱtheȱspoutȱangleȱ50q.ȱByȱextrapolatingȱtheȱ30qȱspoutȱangleȱcurveȱtoȱ CSFȱ350ȱmlȱitȱcanȱbeȱseenȱthatȱtheȱdifferenceȱinȱenergyȱcomparedȱtoȱtheȱspoutȱangleȱ 50qȱisȱapproximatelyȱ20%.ȱ ȱ Energy consumption during chipping Twoȱ simpleȱ experimentsȱ shows,ȱ omittingȱ possibleȱ effectsȱ ofȱ thatȱ theȱ cuttingȱ energyȱmightȱbeȱrateȱdependent,ȱthatȱtheȱincreaseȱinȱenergyȱconsumptionȱperȱtonȱ dryȱ chipsȱ duringȱ chippingȱ betweenȱ spoutȱ anglesȱ 30qȱ andȱ 50qȱ isȱ onlyȱ marginal,ȱ comparedȱtoȱtheȱspecificȱenergyȱconsumptionȱduringȱfirstȱstageȱrefiningȱ(seeȱFigureȱ 28)ȱwhichȱisȱinȱtheȱorderȱofȱ1000ȱkWhȱperȱtonȱdryȱpulp.ȱ ȱ

28 Water adsorption test Theȱ resultsȱ fromȱ theȱ waterȱ adsorptionȱ testȱ areȱ shownȱ inȱ Figureȱ 29.ȱ Onȱ theȱ verticalȱ axisȱ isȱ theȱ specificȱ waterȱ adsorptionȱ i.e.ȱ theȱ differenceȱ betweenȱ currentȱ weightsȱminusȱtheȱdryȱweightȱdividedȱbyȱtheȱdryȱweight.ȱOnȱtheȱhorizontalȱaxisȱisȱ theȱlogarithmicȱtime.ȱTheȱresultsȱindicateȱthatȱforȱchipsȱproducedȱatȱaȱspoutȱangleȱ ofȱ50qȱtheȱspecificȱwaterȱadsorptionȱisȱclearlyȱhigher,ȱwhichȱimpliesȱaȱlargerȱspecificȱ surfaceȱthanȱforȱchipsȱwithȱtheȱspoutȱanglesȱ30qȱandȱ40q.ȱ ȱ

Figure 29. Specific water adsorption vs. logarithmic time ȱ 4.5 Results from the CTMP study (Paper V) InȱpaperȱVȱitȱwasȱinvestigatedȱhowȱtheȱrefiningȱofȱchipsȱproducedȱatȱtwoȱspoutȱ angles,ȱi.e.ȱ 30qȱ andȱ50q,ȱinfluencedȱ onȱ theȱ propertiesȱ ofȱ CTMPȱ(chemimechanicalȱ pulp).ȱ ȱ InȱFiguresȱ30Ȭ32ȱisȱshownȱtheȱoutcomeȱofȱtheȱrefiningȱtrials.ȱTheȱcirclesȱdenoteȱ chipsȱ producedȱ atȱ spoutȱ angleȱ 30qȱ andȱ trianglesȱ 50qȱ (directedȱ chipping).ȱ Theȱ unfilledȱ markersȱ denoteȱ TMPȱ pulpȱ productionȱ andȱ filledȱ markersȱ denoteȱ CTMPȱ pulpȱproduction.ȱ ȱ ȱItȱ wasȱ foundȱ thatȱ theȱ specificȱ energyȱ inputȱ forȱ aȱ certainȱ CSFȱ (Canadianȱ StandardȱFreeness)ȱshowedȱaȱreductionȱinȱrefiningȱenergyȱinȱtheȱorderȱofȱ15%ȱforȱ pulpȱ fromȱ refiningȱ ofȱ chipsȱ byȱ directedȱ chipping.ȱȱItȱ wasȱ alsoȱ foundȱ thatȱ theȱ additionȱ ofȱ theȱ chemicalȱ NaHSO3ȱ toȱ theȱ dilutionȱ waterȱ hadȱ noȱ influenceȱ onȱ theȱ specificȱenergyȱvalueȱforȱaȱgivenȱCSFȱvalueȱ(seeȱFigureȱ30).ȱ ȱ

29 Figure 30. Freeness vs. Specific energy ȱ Forȱprintingȱgrades,ȱtheȱtensileȱindexȱandȱspecificȱlightȱscatteringȱcoefficientȱareȱ importantȱparameters.ȱFromȱtheȱFiguresȱ31Ȭ32ȱoneȱcanȱseeȱthatȱforȱtheȱsameȱenergyȱ consumptionȱ thereȱ isȱ aȱ substantialȱ improvementȱ forȱ bothȱ tensileȱ indexȱ andȱ lightȱ scatteringȱ coefficientȱ forȱ pulpȱ madeȱ fromȱ chipsȱ producedȱ throughȱ directedȱ chippingȱ andȱ withȱ bisulphiteȱ addedȱ toȱ theȱ dilutionȱ waterȱ asȱ comparedȱ toȱ conventionalȱTMP.ȱ ȱ

Figure 31. Tensile index vs. specific energy Figure 32. Specific light scattering coefficient vs. specific energy ȱ

30 5. CONCLUSIONS To study details regarding the chipping process, an experimental method was developed and was found to be a versatile tool in particular when it comes to studying the local strain fields in the vicinity of the cutting edge of the chipping tool.

One outcome from studies using this experimental method is that it is observed that there exist different types of fracture processes, each giving different chip thicknesses.ȱ Itȱ isȱ concludedȱ thatȱ theȱ frictionȱ betweenȱ theȱ woodȱandȱ theȱ chippingȱ toolȱisȱprobablyȱoneȱcrucialȱfactorȱforȱtheȱchipȱformationȱprocess.ȱ ȱ Anotherȱ outcomeȱ fromȱ theȱ aboveȱ experimentȱ isȱ thatȱ theȱ indentationȱ processȱ isȱ approximatelyȱselfȬsimilar.ȱ ȱ Alsoȱ itȱ isȱ concludedȱ thatȱ justȱ priorȱ toȱ theȱ formationȱ ofȱ aȱ chip,ȱ thereȱ isȱ aȱ concentrationȱofȱstrainsȱinȱaȱnarrowȱzoneȱinȱaȱthinȱregionȱstartingȱfromȱtheȱedgeȱofȱ theȱtoolȱandȱdirectedȱparallelȱtoȱtheȱgrain.ȱ ȱ FinallyȱitȱisȱsuggestedȱthatȱtheȱstressȱfieldȱoverȱtheȱentireȱcrackȬplane,ȱi.e.ȱnotȱonlyȱ theȱ stressȱ fieldȱ closeȱ toȱ theȱ tipȱ ofȱ theȱ chippingȱ tool,ȱ isȱ criticalȱ forȱ chipȱ creationȱ ratherȱthanȱjustȱtheȱlatter.ȱ ȱ Inȱ orderȱ toȱ getȱ aȱ moreȱ qualitativeȱ understandingȱ ofȱ theȱ chippingȱ processȱ anȱ analyticalȱ modelȱ wasȱ developed.ȱ Theȱ modelȱ predictsȱ amongȱ otherȱ thingsȱ theȱ normalȱandȱshearȱstrainȱdistributionȱinȱtheȱcrackȬplaneȱpriorȱtoȱcrackȱinitiationȱandȱ theȱ analyticalȱ distributionsȱ wereȱ foundȱ toȱ beȱ inȱ reasonableȱ agreementȱ withȱ theȱ correspondingȱdistributionsȱobtainedȱfromȱaȱFEȱanalysis.ȱ ȱ Oneȱdrawbackȱwithȱtheȱexperimentalȱmethodȱdiscussedȱaboveȱisȱthatȱtheȱcuttingȱ rateȱ isȱ veryȱ lowȱ comparedȱ toȱ aȱ realȱ situation.ȱ Henceȱ aȱ laboratoryȱ chipperȱ wasȱ developedȱ andȱ foundȱ toȱ beȱ aȱ versatileȱ toolȱ inȱ theȱ processȱ ofȱ increasingȱ theȱ understandingȱ ofȱ theȱ woodȱ chippingȱ process,ȱ i.e.ȱ inȱ particularȱ whenȱ itȱ comesȱ toȱ studyingȱtheȱinfluenceȱofȱcuttingȱangles,ȱcuttingȱrate,ȱknifeȱwearȱandȱmodificationsȱ ofȱtheȱgeometryȱofȱtheȱknife.ȱ ȱ Combiningȱtheȱuseȱofȱtheȱlaboratoryȱchipperȱwithȱrefiningȱtrialsȱitȱwasȱconcludedȱ thatȱ modifyingȱ theȱ woodȱ chippingȱ process,ȱ i.e.ȱ increasingȱ theȱ spoutȱ angle,ȱ wasȱ beneficialȱregardingȱtheȱenergyȱconsumptionȱduringȱrefining.ȱ ȱ Itȱwasȱobservedȱthatȱtheȱspecificȱwaterȱadsorptionȱwasȱlargerȱforȱchipsȱproducedȱatȱ theȱspoutȱangleȱ50q,ȱasȱcomparedȱtoȱchipsȱproducedȱatȱspoutȱanglesȱ30oȱandȱ40o,ȱ

31 whichȱimpliesȱthatȱchippingȱatȱspoutȱangleȱ50oȱwillȱgiveȱaȱlargerȱspecificȱsurfaceȱ areaȱthanȱdoesȱchippingȱatȱlowerȱspoutȱangles.ȱ ȱ Itȱwasȱalsoȱconcludedȱthatȱcombiningȱrefiningȱofȱwoodȱchipsȱproducedȱatȱaȱspoutȱ angleȱ ofȱ 50qȱ andȱ addingȱ bisulphiteȱ toȱ theȱ dilutionȱ waterȱ duringȱ chipȱ refiningȱ improvedȱ theȱ tensileȱ indexȱ andȱ lightȱ scatteringȱ coefficientȱ atȱ theȱ sameȱ energyȱ consumption.ȱ

32 6. ACKNOWLEDGMENTS Iȱ wantȱ toȱ thankȱ peopleȱ workingȱ atȱ Fibreȱ Scienceȱ andȱ Communicationȱ Networkȱ (FSCN)ȱ atȱ Midȱ Swedenȱ Universityȱ andȱ theȱ Departmentȱ ofȱ Naturalȱ Sciences,ȱ engineeringȱ andȱ mathematicsȱ forȱ contributingȱ toȱ theȱ progressiveȱ andȱ inspiringȱ workingȱatmosphere.ȱ

Theȱ Swedishȱ KKȬFoundationȱandȱ theȱEuropeanȱRegionalȱ Developmentȱ Fundȱ areȱ acknowledgedȱforȱfinancialȱsupport.ȱ

Iggesundȱ Toolsȱ AB,ȱ Holmenȱ Paperȱ ABȱ andȱ SaintȬGobainȱ Abrasivesȱ areȱ greatlyȱ acknowledgedȱforȱsupporting,ȱinȱdifferentȱways,ȱtheȱproject.ȱ

Iggesundȱ Toolsȱ AB,ȱ GRWȱ PRODUKTERȱ AB,ȱ andȱ Staffanȱ Nyströmȱ atȱ theȱ Midȱ Swedenȱ Universityȱ areȱ greatlyȱ acknowledgedȱ forȱ theirȱ supportȱ inȱ developingȱ ofȱ theȱexperimentalȱequipments.ȱ

ThanksȱtoȱallȱpersonsȱatȱSCAȱR&Dȱcentreȱthatȱhaveȱhelpedȱme.ȱ

Iȱ wouldȱ likeȱ toȱ thankȱ myȱ supervisorsȱ Prof.ȱ Torbjörnȱ Carlbergȱ andȱ Prof.ȱ Perȱ EngstrandȱforȱbeingȱeverȬhelpfulȱandȱsupportive.ȱIȱwouldȱalsoȱlikeȱtoȱthankȱProf.ȱ PerȱGradinȱforȱvaluableȱcommentsȱonȱtheȱthesis.ȱ

Allȱ myȱ colleaguesȱ andȱ friendsȱ atȱ theȱ Midȱ Swedenȱ Universityȱ areȱ alsoȱ acknowledged.ȱ

Annaȱ Haeggströmȱ isȱ acknowledgedȱ forȱ hereȱ outstandingȱ workȱ takingȱ careȱ ofȱ administrationalȱtasksȱandȱbeingȱeverȬhelpful.ȱȱ

ManyȱthanksȱtoȱIMT,ȱandȱallȱtheȱpeopleȱthatȱcomesȱeveryȱnowȱandȱthenȱforȱcoffeeȱ atȱ“Torken”.ȱ

Last,ȱ butȱ notȱ least,ȱ thanksȱ toȱ myȱ family,ȱ myȱ childrenȱ Sandra,ȱ Erika,ȱ Mattiasȱ andȱ Emil.ȱAlsoȱthanksȱtoȱmyȱextraȱchildrenȱandȱmyȱfriendsȱforȱsupportingȱmeȱinȱthis.ȱ

33ȱ 7. REFERENCES [1]ȱ KeaysȱJ.L.ȱ(1979);ȱHistoryȱofȱwoodȱchipping;ȱChipȱQualityȱMonograph,ȱHattonȱ J.V.ȱ(Editor),ȱPulpȱandȱPaperȱTechnologyȱSeriesȱNoȱ5ȱ(TAPPI)ȱ [2]ȱ OriginalȱpictureȱfromȱMetsoȱPaper.comȱ [3]ȱ HartlerȱNȱ(1986);ȱChipperȱdesignȱandȱoperationȱforȱoptimumȱchipȱquality;ȱTappiȱJ.ȱ Vol.ȱ69,ȱNo.ȱ10:ȱ62Ȭ66ȱ [4]ȱ SvenssonȱB.A.ȱ(2007);ȱFrictionalȱStudiesȱandȱHighȱStrainȱRateȱTestingȱofȱWoodȱ underȱ Refiningȱ Conditions;ȱ Doctoralȱ thesis,ȱ Midȱ Swedenȱ University,ȱ DepartmentȱofȱNaturalȱscience,ȱSundsvallȱ [5]ȱ Fengelȱ D.,ȱ andȱ Stollȱ M.ȱ (1973);ȱ Variationȱ inȱ cellȱ crossȬsectionalȱ area,ȱ cellȬwallȱ thicknessȱandȱwallȱlayerȱofȱspruceȱtracheidsȱwithinȱanȱannualȱring;ȱHolzforschungȱ 27:ȱ1Ȭ7ȱ [6]ȱ Perssonȱ Kentȱ (1997);ȱ Modellingȱ ofȱ Woodȱ Propertiesȱ byȱ aȱ Micromechanicalȱ Approach;ȱLicentiateȱofȱtechnologyȱthesis,ȱLundȱUniversity:ȱISSNȱ0281Ȭ6679ȱ [7]ȱ Zmitrowiczȱ A.ȱ (2006);ȱ Modelsȱ ofȱ kinematicsȱ dependentȱ anisotropicȱ andȱ heterogeneousȱfriction;ȱInternationalȱJournalȱofȱSolidsȱandȱStructures,ȱVol.ȱ43,ȱ Issuesȱ14Ȭ15:ȱ4407Ȭ4451ȱ [8]ȱ HartlerȱN.ȱandȱStadeȱY.ȱ(1979);ȱChipȱspecificationsȱforȱvariousȱpulpingȱprocesses;ȱ ChipȱQualityȱMonograph,ȱHatton,ȱJ.V.ȱ(Editor),ȱPulpȱandȱPaperȱTechnologyȱ SeriesȱNo.ȱ5ȱ(TAPPI)ȱ [9]ȱ Buchananȱ J.G.ȱ andȱ Duchnickiȱ T.S.ȱ (1963);ȱ Someȱ Experimentsȱ inȱ LowȬSpeedȱ Chipping;ȱPulpȱandȱPaperȱMagazineȱCan.ȱMayȱ1963:ȱT235ȬT245ȱ [10]ȱ KivimaaȱE.ȱandȱMurtoȱJ.O.ȱ(1949);ȱInvestigationsȱonȱfactorsȱaffectingȱchippingȱofȱ pulpȱwood;ȱStatensȱTekniskaȱForskningsanstalt,ȱFinlandȱPubl.9ȱ [11]ȱ TwaddleȱA.ȱ(1997);ȱTheȱinfluenceȱofȱspecies,ȱchipȱlength,ȱandȱringȱorientationȱonȱ chipȱthickness;ȱTappiȱJȱVol.ȱ80,ȱNo.ȱ6:ȱ123Ȭ131ȱ [12]ȱ Hartlerȱ N.ȱ (1962ȱ c);ȱ Studiesȱ onȱ theȱ Suctionȱ Feedingȱ ofȱ Chippers;ȱ Svenskȱ PapperstidningȱVol.ȱ65,ȱNoȱ12:ȱ475Ȭ487ȱ [13]ȱ UhmeierȱA.ȱ(1995);ȱSomeȱfundamentalȱaspectsȱofȱwoodȱchipping;ȱTappiȱJȱVol.ȱ78,ȱ No.ȱ10:ȱ79Ȭ86ȱ [14]ȱ McLauchlanȱT.A.ȱandȱLapionteȱJ.A.ȱ(1979);ȱProductionȱofȱchipsȱbyȱdiscȱchippers;ȱ ChipȱQualityȱMonograph,ȱHatton,ȱJ.V.ȱ(Editor),ȱPulpȱandȱPaperȱTechnologyȱ SeriesȱNoȱ5ȱ(TAPPI)ȱ [15]ȱ Hartlerȱ N.ȱ (1962a);ȱ Theȱ Effectȱ ofȱ Spoutȱ Angleȱ asȱ Studiedȱ inȱ anȱ Experimentalȱ Chipper;ȱSvenskȱPapperstidningȱVol.ȱ65,ȱNo.ȱ9:ȱ351Ȭ362ȱ

34ȱ [16]ȱ PaperȱIVȱinȱ“OnȱtheȱWoodȱChippingȱProcessȱ–ȱAȱstudyȱonȱbasicȱmechanismsȱ inȱ orderȱ toȱ optimizeȱ chipȱ propertiesȱ forȱ pulping”,ȱ Hellströmȱ L.M.,ȱ Dissertation,ȱMidȱSwedenȱUniversityȱ2010ȱ [17]ȱ Hellströmȱ L.M.,ȱ Isaksson,ȱ P.ȱ Gradinȱ P.A.ȱ andȱ Erikssonȱ K.ȱ (2009);ȱ Anȱ Analyticalȱ andȱ Numericalȱ Studyȱ ofȱ someȱ Aspectsȱ ofȱ theȱ Woodȱ Chippingȱ Process;ȱ Nord.ȱPulpȱPaperȱRes.ȱJ.,ȱVol.ȱ24,ȱNo.ȱ2:ȱ225Ȭ230ȱ [18]ȱ Fullerȱ W.S.ȱ (1983);ȱ Chipping,ȱ Screeningȱ andȱ cleaning;ȱ Pulpȱ andȱ Paperȱ Manufactureȱ Vol.ȱ 1ȱ Propertiesȱ ofȱ Fibrousȱ Rawȱ Materialsȱ theirȱ Preparationȱ forȱPulping,ȱM.JȱKockrekȱandȱC.F.B.ȱSterens,ȱTappi;ȱISBN:ȱ0Ȭ919893Ȭ07Ȭ4ȱ [19]ȱ Frazierȱ W.C.ȱ andȱ Williamsȱ G.J.ȱ (1982);ȱ Reductionȱ ofȱ specificȱ energyȱ inȱ mechanicalȱ pulpingȱ byȱ axialȱ precompressionȱ ofȱ wood;ȱ Pulpȱ andȱ Paperȱ Canada:ȱ Vol.ȱ83,ȱNo.ȱ6:ȱT162ȬT167ȱ [20]ȱ BergȱJȬE.ȱandȱGradinȱP.A.ȱ(2000);ȱEffectȱofȱTemperatureȱonȱFractureȱofȱSpruceȱinȱ Compression,ȱ Investigatedȱ byȱ Useȱ ofȱ Acousticȱ Emissionȱ Monitoring;ȱ Journalȱ ofȱ PulpȱandȱPaperȱScience:ȱVol.ȱ26,ȱNo.ȱ8:ȱ294Ȭ299ȱ [21]ȱ DinwoodieȱJ.M.ȱ(1968);ȱFailureȱinȱTimber.ȱPartȱ1.ȱMicroscopicȱChangesȱinȱCellȱ WallȱStructureȱAssociatedȱwithȱCompressionȱFailure;ȱJ.ȱIntl.ȱWoodȱSci.:ȱVol.ȱ21,ȱ 37Ȭ53ȱ [22]ȱ DinwoodieȱJ.M.ȱ(1974);ȱFailureȱinȱTimberȱPartȱ2:ȱTheȱAngleȱofȱShearȱThroughȱtheȱ CellȱWallȱDuringȱLongitudinalȱCompressionȱStressing;ȱWoodȱSci.ȱTechnol.:ȱVol.ȱ 8,ȱ56Ȭ67ȱ [23]ȱ SabourinȱM.J.ȱ(2000);ȱEvaluationȱofȱaȱCompressiveȱPretreatmentȱProcessȱonȱTMPȱ Propertiesȱ andȱ Energyȱ Requirements;ȱ Pulpȱ andȱ Paperȱ Canada:ȱ Vol.ȱ 101,ȱ T54Ȭ T60ȱ [24]ȱ Hillȱ J.,ȱ Sabourinȱ M.,ȱ Johanssonȱ L.ȱ andȱ Aichingerȱ J.ȱ (2009);ȱ Enhancingȱ Fibreȱ Developmentȱ atȱ Reducedȱ Energyȱ Consumptionȱ Usingȱ TMPȱ SubȬProcessesȱ andȱ Targetedȱ Chemicalȱ Applicationȱ andȱ Commercialȱ Scaleȱ Results;ȱ Proceedingsȱ InternationalȱMechanicalȱPulpingȱConferenceȱ2009ȱ [25]ȱ LawȱK.ȬN.ȱandȱLanouetteȱR.ȱ(2000);ȱEffectȱofȱMechanicalȱConditioningȱofȱChipsȱ onȱ theȱ Qualityȱ ofȱ Softwoodȱ TMP;ȱ Pulpȱ andȱ Paperȱ Canada:ȱ Vol.ȱ 101,ȱ No.ȱ 7,ȱ T189ȬT193ȱ [26]ȱ PengȱF.ȱandȱGranfeldtȱT.ȱ(1996);ȱChangesȱinȱtheȱMicrostructureȱofȱSpruceȱWoodȱ ChipsȱAfterȱScrewȱPressȱTreatment;ȱJournalȱofȱPulpȱandȱPaperȱScience:ȱVol.ȱ22,ȱ No.ȱ4,ȱJ140ȬJ144ȱ [27]ȱ Deȱ Choudensȱ C.,ȱ Lombardoȱ G.,ȱ Michalowiczȱ G.ȱ andȱ Robertȱ A.ȱ (1985);ȱ DestructurationȱofȱWoodȱChipsȱinȱaȱCylinderȱPress;ȱPaperiȱjaȱPuu:ȱNo.ȱ3,ȱ108Ȭ116ȱ

35ȱ [28]ȱ PaperȱVȱinȱ“OnȱtheȱWoodȱChippingȱProcessȱ–ȱAȱstudyȱonȱbasicȱmechanismsȱ inȱ orderȱ toȱ optimizeȱ chipȱ propertiesȱ forȱ pulping”,ȱ Hellströmȱ L.M.,ȱ Dissertation,ȱMidȱSwedenȱUniversityȱ2010ȱ [29]ȱ Uhmeierȱ A.ȱ andȱ Perssonȱ K.ȱ (1997);ȱ Numericalȱ Analysisȱ ofȱ Woodȱ Chipping;ȱ HolzforschungȱVol.ȱ51,ȱNo.ȱ1:ȱ83Ȭ90ȱ [30]ȱ LifȱJ.O.,ȱFellersȱC.,ȱSöremarkȱC.ȱandȱSjödahl,ȱM.ȱ(1995);ȱCharacterizingȱtheȱInȬ Planeȱ Hygroexpansivityȱ ofȱ Paperȱ byȱ Electronicȱ Speckleȱ Photography;ȱ Journalȱ ofȱ PulpȱandȱPaperȱScience:ȱVol.ȱ21,ȱNo.ȱ9:ȱJ302ȬJ309ȱ [31]ȱ Kajbergȱ J.ȱ andȱ Wikmanȱ B.ȱ (2007);ȱ Viscoplasticȱ parameterȱ estimationȱ byȱ highȱ strainȬrateȱ experimentsȱ andȱ inverseȱ modellingȱ –ȱ Speckleȱ measurementsȱ andȱ highȬ speedȱphotography;ȱInternationalȱJournalȱofȱSolidsȱandȱStructuresȱ44:ȱ145Ȭ164ȱ [32]ȱ Berganderȱ A.ȱ andȱ Salménȱ L.ȱ (2000);ȱ Theȱ Transverseȱ Elasticȱ Modulusȱ ofȱ theȱ NativeȱWoodȱFibreȱWall;ȱJournalȱofȱPulpȱandȱPaperȱScience:ȱvol.26,ȱNo.ȱ6:ȱ234Ȭ 238ȱ [33]ȱ GOMȱmbHȱ(20040820);ȱARAMISȱuserȱmanual,ȱenȱRev;ȱGermanyȱ [34]ȱ Matlabȱ(2007);ȱVersionȱ7.4.ȱTheȱMathWorksȱInc.,ȱNatick,ȱMAȱUSAȱ [35]ȱ BatheȱK.J.ȱ(1982);ȱFiniteȱelementȱprocedureȱinȱengineeringȱanalysis;ȱPrenticeȬHall,ȱ USAȱ [36]ȱ Hellströmȱ L.M.,ȱ Gradinȱ P.A.ȱ andȱ Carlbergȱ T.ȱ (2008);ȱ Aȱ methodȱ forȱ ExperimentalȱInvestigationȱofȱtheȱWoodȱChippingȱProcess;ȱNord.ȱPulpȱPaperȱRes.ȱ J.,ȱVol.ȱ23,ȱNo.ȱ3:ȱ339Ȭ342ȱ [37]ȱ SvenssonȱE.,ȱEngstrandȱP.,ȱHtunȱM.ȱandȱSvenssonȱB.ȱ(1994);ȱAȱBetterȱBalanceȱ Betweenȱ Shivesȱ Contentȱ andȱ LightȬScatterȱPropertiesȱ ofȱ TMP/CTMPȱbyȱ Sulphurȱ dioxideȱGasȬPhaseȱImpregnationȱPriorȱtoȱDefibration;ȱNord.ȱPulpȱPaperȱRes.ȱJ.,ȱ Vol.ȱ9,ȱNo.ȱ3:ȱ167Ȭ171ȱ ȱ

36ȱ