Virtual testing and predictive modeling: for fatigue and fracture mechanics allowables

420 Pages·2009·19.17 MB·English

by  Bahram Farahmand (auth.)

Virtual Testing and Predictive Modeling
Bahram Farahmand
Editor
Virtual Testing and Predictive
Modeling
For Fatigue and Fracture Mechanics
Allowables
123
Editor
BahramFarahmand
TASS–Americas,asubsidiaryofTASSInc.
12016115thAveNESuite100
Kirkland,WA98034
USA
[e-mail]
ISBN978-0-387-95923-8 e-ISBN978-0-387-95924-5
DOI10.1007/978-0-387-95924-5
SpringerDordrechtHeidelbergLondonNewYork
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(cid:2)c SpringerScience+BusinessMedia,LLC2009
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Acknowledgments
Theauthorisgratefultoallco-authorswhocontributedtothisbook.Theirdedication
and effort for submitting their chapters on time are greatly appreciated. This book
will be dedicated to my dear mother Gohartaj and my lovely wife Vida. My
great appreciation goes to my son, Houman, and my daughter, Roxana, for being
extremelyhelpfulwiththeirsupportduringputtingsectionsofthisbooktogether.
v
Preface
Thematerialsusedinmanufacturingtheaerospace,aircraft,automobile,andnuclear
partshaveinherentflawsthatmaygrowunderfluctuatingloadenvironmentsduring
the operational phase of the structural hardware. The design philosophy, material
selection,analysisapproach,testing,qualitycontrol,inspection,andmanufacturing
arekeyelementsthatcancontributetofailurepreventionandassureatrouble-free
structure.Tohavearobuststructure,itmustbedesignedtowithstandtheenviron-
mental load throughout its service life, even when the structure has pre-existing
flawsorwhenapartofthestructurehasalreadyfailed.Ifthedesignphilosophyof
the structure is based on the fail-safe requirements, or multiple load path design,
partial failure of a structural component due to crack propagation is localized and
safely contained or arrested. For that reason, proper inspection technique must be
scheduledforreusablepartstodetecttheamountandrateofcrackgrowth,andthe
possible need for repairing or replacement of the part. An example of a fail-safe-
designedstructurewithcrack-arrestfeature,commontoallaircraftstructuralparts,
istheskin-stiffeneddesignconfiguration. However,inothercases,thedesignphi-
losophy has safe-life or single load path feature, where analysts must demonstrate
that parts have adequate life during their service operation and the possibility of
catastrophicfailureisremote.Forexample,allpressurizedvesselsthathavesingle
load path feature are classified as high-risk parts. During their service operation,
thesetanksmaydevelop cracks,whichwillgrowgraduallyinastablemanner.To
avoid catastrophic failure, a thorough nondestructive inspection, a proof test prior
to service usage, and a comprehensive fracture mechanics analysis (i.e., safe-life
analysis)arerequestedbythecustomer.
To demonstrate that structural failure of single load path component does not
occurandthattheparthasadequatelifeduringitsentireoperation,acomprehensive
fatigueandfracturemechanicsanalysisusinglinearelasticfracturemechanicsmust
beperformed.Inconductingsafe-lifeanalysis,fullfracturemechanicsdataforthe
material must be available. These data are generated based on the ASTM testing
standards. Because fracture toughness is thickness dependent and structures have
componentsthathavedifferentsizesandthicknesses,numerousfracturetoughness
testsmustbeconductedtoincludetheplanestrain,planestress,andthemixed-mode
conditions. In addition to fracture toughness values, the fatigue crack growth rate
data must also be available to analysts in order to conduct a meaningful safe-life
vii
viii Preface
analysis.Thesetestsarecostlyandtimeconsuming,andthecostandtimeoftest-
ing will increase substantially when scatter in fracture allowable, as the result of
material variation, need to be considered. Therefore, any method that can reduce
thenumberoftestswillbeusefultotheindustrytoavoidunnecessarycostswhen
fractureallowablesareneededasaninputtothelife-assessmentanalysis.
The safe-life analysis of high-risk components will demonstrate the ability or
tolerance of parts at the presence of existing crack under the load-varying envi-
ronment. For this reason the fracture mechanics analysis in many cases is called
“damagetolerance”analysis.Inreality,thetotallifeofstructuralcomponentsisthe
sumofcrackinitiationcyclesandcrackpropagation.Inaircraftindustry,thenumber
of cycles to crack initiation must be accounted for when assessing the total life of
the part. Figure 1 shows the process of creating a crack growth/residual strength
analysis with emphasis on the comparison (feedback) of analysis with practice.
Thatis,crackgrowthanalysisshouldbecheckedagainstresultsobtainedfromthe
fieldexperiencethroughtheintervalinspection.Airlinesalreadyimplementedthis
approach through their Reliability Centered Maintenance program to adjust their
interval inspection period of the entire fleet. As indicated in the figure, the total
life analysis is incomplete without having fatigue and fracture allowables through
testing.
Fig.1 Comparison(feedback)ofanalysiswithpractice.Analysisisincompletewithoutfatigue
andfracturedata
Because the induced stresses in aircraft components must be kept in the elastic
range,thehighcyclefatiguedatageneratedthroughthestresstolife(S–N)canbe
usefulinthefatigueassessmentanalysis(alsocalleddurabilityanalysis).Thesedata
arestressratio(R)dependentandrequireconsiderabletimeandcosttogeneratethe
fullrangeoftheS–Ncurve.Anyanalyticaltechniquethatcanbeusedtogeneratethe
S–Ndatawithoutconductingthetraditionallaboratorycoupontestswillbehelpful
totheaircraftandaerospaceindustrytoavoidtests.
Preface ix
The sole purpose of this book is to provide the structural engineers with the
present and near-future approaches to the virtual testing techniques, where fatigue
and fracture mechanics data can be generated rapidly with minimum amount of
tests. As mentioned before, fatigue and fracture allowables are needed as an input
tothedamagetoleranceanddurabilityassessmentoffracturecriticalparts.Inmany
instances,analystsdonothavefatiguefracturemechanicsvaluesandthebudgetis
notadequatetogeneratedatathroughtestingapproach.Inotherinstances,thetime
doesnotallowtoconducttests,becauseofdeadlinesthatdesignersmustmeetand
laid down by the customers. Therefore, the virtual testing is the right tool to have
whenboththebudgetandtimedonotallowengineerstoconducttestsfordurability
anddamagetoleranceanalysis.Thevirtualtestingtechniqueforgeneratingfatigue
and fracture allowables will be presented in this book through two unique tech-
niques.Thefirstapproachwillusetheconventionalcontinuummechanicsapproach,
whichwillallowengineerstogeneratetheS–N;fracturetoughness,Kc;andfatigue
crackgrowthratedata(da/dNversusΔK)throughanalyticalapproach.Thesecond
method ofgenerating thesedata isbasedonthefundamental lawsofphysics (i.e.,
the ab initio), where material will be assessed from the bottom-up approach. Both
approaches to the virtual testing will be presented in this book. The latter utilizes
themultiscalemodelingandsimulationtechniquetopredictthematerialproperties.
Forthisreasontheauthorchoosestouse“VirtualTestingandPredictiveModeling”
asthetitleofthisbook.
Chapters1,2,3,4,5,and6ofthisbookwillbededicatedtovirtualtestingusing
the continuum mechanics approach. Both metallic and composite materials will
be addressed with numerous examples related to the aerospace and aircraft parts.
Chapters 7, 8, 9, 10, and 11 will discuss the multiscale modeling and simulation
technique. Both quantum mechanics and molecular dynamics approaches will be
usedtoconductthepredictivemodelinganalysis.
Becauseofoutstandingmechanicalpropertiesofnanoparticles,thereisastrong
futuredemandfortheirapplicationinaerospaceandaircraftstructuralparts.These
particleswhencombinedwithpolymermatrixwillenhancethemechanicalproper-
tiesofpolymer,whichisanimportantfactorinreducingtheweightofthestructural
parts in modern airplanes. The implementation of multiscale modeling and simu-
lation at the interface region between nanoparticles and matrix is challenging and
proper chemistry between nanoparticles and polymer is needed to provide a good
bondattheinterfaceregion.Tomakenanoparticlesmoreeasilydispersibleinpoly-
mer, it is necessary to physically or chemically attach certain molecules, or func-
tionalgroups,totheirsmoothsidewallswithoutsignificantlychangingthenanopar-
ticle’sdesirableproperties.Thisprocessiscalledfunctionalization.Theproduction
of robust composite materials that can allow the transfer of load without causing
localizeddamagerequiresstrongcovalentchemicalbondingbetweenthefillerpar-
ticles and the polymer matrix that can be achieved through the functionalization
process.Chapter12willaddressthemostrecentapproachtothefunctionalization
technique that can be useful to bond dissimilar material with achieving adequate
interfacestrength.
x Preface
Finally,Chapter13willbeallocatedtotheverificationtechniqueusingthestate-
of-the art approach to verify the interface region between nanoparticles and the
matrix by applying the transmitted electron microscope (TEM) and atomic force
microscope (AFM). The experimental flexibility of these techniques will provide
insights into the fundamental structure and deformation processes of nanoscales
materials. The in situ measurement of interface region while material under stress
willbediscussed.
Kirkland,Washington BahramFarahmand
Contents
1 VirtualTestingandItsApplicationinAerospaceStructural
Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
BahramFarahmand
1.1 IntroductiontotheVirtualTesting . . . . . . . . . . . . . . . . 2
1.2 VirtualTestingTheoryandFractureToughness . . . . . . . . . 2
1.3 TheExtendedGriffithTheoryandFractureToughness . . . . . 3
1.4 ExtensionofFarahmand’sTheorytoFatigueCrack
GrowthRateData . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4.1 TheAcceleratedRegionandFractureToughness. . . . 6
1.4.2 TheParisConstants,Candn . . . . . . . . . . . . . . 7
1.4.3 TheThresholdValue(RegionI) . . . . . . . . . . . . 8
1.4.4 Theda/dNVersusΔKfromVirtualTesting
AgainstTestData . . . . . . . . . . . . . . . . . . . . 9
1.5 ApplicationofVirtualTestinginAerospaceIndustry:
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.5.1 Background . . . . . . . . . . . . . . . . . . . . . . . 13
1.5.2 ManufacturingProcessandPlasticDeformation
ofCOPVLiner . . . . . . . . . . . . . . . . . . . . . 14
1.5.3 GeneratingFractureAllowablesofInconel718
ofCOPVLinerThroughVirtualTestingTechnique . . 16
1.5.4 GeneratingFractureAllowablesof6061-T6
AluminumTankThroughVirtualTestingTechnique . . 20
1.6 SummaryandFutureWork . . . . . . . . . . . . . . . . . . . . 22
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2 Tools for Assessing the Damage Tolerance of Primary
StructuralComponents . . . . . . . . . . . . . . . . . . . . . . . . 29
R.JonesandD.Peng
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2 AnEquivalentBlockMethodforPredictingFatigue
CrackGrowth . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3 FatigueCrackGrowthunderVariableAmplitudeLoading . . . 33
2.3.1 FatigueCrackGrowthinanF/A-18AircraftBulkhead 36
xi