FITNET FFS MK7 Section 8 CREEP MODULE. Module Coordinator: RA Ainsworth BRITISH ENERGY, UK

Transcription

1 FITNET FFS M7 Setion 8 CREEP MODULE Module Coodinato: RA Ainswoth BRITISH ENERGY, U

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3 (1 May 26) FITNET M7 Symbols a a a g ak size initial ak size ak size afte gowth a min ak size below whih the ak gowth ate is assumed to be onstant a ak gowth ate A mateial onstant (ee ak gowth) B mateial onstant (yli ak gowth) B n net seimen thikness C mateial onstant (yli ak gowth) C (t) tansient ak ti aamete C C * steady state ak ti aamete * * mean estimate of C duing tansient ealy yles d sufae ee damage aumulated in a yle suf D total sufae ee damage suf E (L J k elasti modulus f ) failue assessment uve initial value of elasti-lasti ak ti aamete fo ombined loading fato fo the weld zones stess intensity fato I id I i stess intensity fato mat ee toughness (TDFAD) maximum stess intensity fato in yle max minimum stess intensity fato in yle min s s stess intensity fato due to imay load fato fo the effet of yli stain hadening o softening stess intensity fato due to seonday loading mateial onstant (yli ak gowth) L load atio P / PL max L ut-off on TDFAD n ee stess exonent P load P L limit load mateial onstant (ee ak gowth) q q o Q fation of total load ange fo whih ak is judged to be oen mateial onstant (yli ak gowth) size of the yli lasti zone ak yli lasti zone size at the ak ti R stess intensity fato atio ( = min / max ) R * length in estimate of C R stess intensity fato atio (2CD) R stess atio (2CD) FITNET 26 All ights eseved 8-1

4 FITNET FFS M7 Cee Module S y t i minimum.2% oof stess initiation time t time to eah steady yli state y t o sevie life to date t g time equied fo the ak to oagate by an amount Δ a g t h hold time at high temeatue t m maximum allowable time at temeatue t utue time t edistibution time t ed s CD desied futue sevie life t time fo ontinuum damage failue T eene temeatue U U U U V Z e T ee aea unde load-dislaement uve elasti aea unde load-dislaement uve lasti aea unde load-dislaement uve total aea unde load-dislaement uve aamete teating inteations between imay and seonday stess elasti follow-u fato α oeffiient of themal exansion β, γ mateial onstants (ee ak initiation) δ i a i itial ak ti oening dislaement (ee ak initiation) Δ ak gowth oesonding to initiation Δ J ange of J-integal Δ ee dislaement Δ Δ e Δ T ε t elasti dislaement lasti dislaement total dislaement Δ total sufae stain ange (yli ak gowth) Δ eff stess intensity fato ange fo whih ak is oen ε ee stain ate at stess ε equivalent ee stain ate ε ee stain ε ε e elasti stain ee stain ate ε, ee stain ate at stess ε, ee stain ate at stess ε f ee dutility e ε elasti stain at stess e+ ε elasti lus lasti stain at stess 8-2 FITNET 26 All ights eseved

5 (1 May 26) FITNET M7 e+ + ε elasti lus lasti lus ee stain at stess ε elasti lus lasti stain at stess η μ ν homogeneous exeimental alibation fato stess exonent in owe law lastiity Poisson's atio shot-tem flow stess initial stess.2.2% ee stength 1. 1.% ee stength d max stess at a small distane ahead of the ak ti eak equivalent welding esidual stess nominal stess n l eene stess initial value of the total eene stess eene stess ate eene stess fo fist yle y= 1 eene stess fo imay loading homogeneous aked body eene stess R y u,hom ee utue stength yield stess ultimate tensile stess 2CD CT DMW TDFAD Two Citeia Diagam Comat Tension seimen Dissimila metal weld Time Deendent Failue Assessment Diagam FITNET 26 All ights eseved 8-3

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7 (1 May 26) FITNET M7 8 Cee module 8 Cee module Intodution Oveall Poedue Establish Cause of Caking Define Sevie Conditions Collet Mateials data Cee Rutue Data Cee Defomation Data Cee Dutility Data Cee Cak Initiation/Inubation Data Cee Cak Gowth Data Cyli Cak Gowth Data Method I Method II Othe Data Elasti and Physial Constants Stess-stain Data Fatue Toughness Data Pefom Basi Calulations Stess Intensity Fatos Refeene Stess C* Paamete Redistibution Time, t ed C(t) Paamete Chek Signifiane of Cee and Fatigue Insignifiant Cee Insignifiant Fatigue Insignifiant Cee-Fatigue Inteations Pefom Assessment Calulations Calulate Rutue Life, t CD Calulate Cak Inubation Time, t i Calulate Cak Size afte Gowth, a g Assess Signifiane of Results Sensitivity Analysis Remedial Ation Reot Results Additional Infomation Teatment of Defets in Weldments Intodution Soe Signifiane of Welding Residual Stesses Simlified Assessment Detailed Assessment Seifi Modes of Caking Teatment of Seonday Loading Failue Assessment Diagam Methods Intodution Cee Cak Initiation Assessment Poedues TDFAD Aoah Two Citeia Diagam Comaison of Paametes FITNET 26 All ights eseved 8-5

8 FITNET FFS M7 Cee Module Comaison of the TDFAD and the Two Citeia Diagam The d Aoah Bibliogahy FITNET 26 All ights eseved

9 (1 May 26) FITNET M7 8.1 Intodution The oedue in this setion seifies methods fo assessing defets in stutues oeating at high temeatues and subjet to ee-fatigue loading onditions. The basi ingedients equied fo an assessment ae: (i) the oeating onditions; (ii) the natue of the defets; (iii) mateials data; and (iv) stutual alulations to oelate mateials data tests with the behaviou of omlex stutues. This infomation is then used to assess whethe a defet of a given size will gow to an unaetable size by ee-fatigue mehanisms in a given sevie life unde a given loading histoy. The oedue an eadily be adated to onside assessments of othe tyes, suh as: (a) the loadings whih give a life equal to a given sevie life; (b) the initial flaw size whih will just gow to the maximum aetable size in a given sevie life (and hene the magin fo a given flaw size); () the ombinations of mateials oeties, geomety and loadings fo whih ak ti behaviou has a negligible effet on lifetime. An altenative oedue in Setion assesses whethe o not a small, defined ak extension will ou in the equied sevie life using a failue assessment diagam aoah simila to that in Setion 6. Anothe oedue in Setion uses the alulation of a stess at a small distane ahead of the ak ti, the d aoah, to assess whethe signifiant ak extension ous in the sevie life. 8.2 Oveall Poedue In this setion, a ste-by-ste oedue is set out fo assessing a omonent ontaining a known o ostulated defet unde ee-fatigue loading. Flowhats fo the oedue ae given in Figue 8.1 to Figue 8.3. These addess a omonent that is equied to oeate fo a futhe eiod, t s, at high temeatue. Continuum damage aumulation and ak gowth ae addessed. The ases of insignifiant ee and insignifiant fatigue ae inluded as seial ases. The oedue may be alied to a omonent that has not yet seen oeation at ee temeatues, o one that has aleady oeated fo a eiod, t o, at high temeatue. In the latte ase, advie is given additionally on the effet of the time at whih the defet is assumed to fom. The stes in the oedue ae listed below. Futhe infomation on efoming the individual stes is given in Setions 8.3 to 8.1. STEP 1. Establish Cause of Caking and Chaateise Initial Defet It is fist neessay to establish the ause of the aking to ensue that the ee oedues ae aliable. This is disussed in Setion 8.3. The defet tye, osition and size should also be identified. Fo defets found in sevie, this oess may equie the advie of mateials and non-destutive testing exets, atiulaly fo the ase of defets in welds. Suitable sensitivity studies should be efomed to addess unetainties. The deteted defet should be haateised by a suitable bounding ofile amenable to analysis. Defets whih ae not of simle Mode I tye should be esolved into Mode I oientation. Note that it may also be ossible late to e-haateise a defet in the ase that the initial assessment leads to an unaetable esult. Advie on defet haateisation is ontained in Setion 5.1. Advie on methods fo deteting and measuing defets is inluded in Annex E and oves omonents oeating at high temeatue. STEP 2. Define Sevie Conditions It is neessay to esolve the load histoy into yle tyes suitable fo analysis. This inludes both histoial oeation and the assumed futue sevie onditions. Advie is ontained in Setion 5.2. The sevie life seen to date and the desied futue sevie life should be defined. FITNET 26 All ights eseved 8-7

10 FITNET FFS M7 Cee Module Fo the ase of a omonent that was known to be defet-fee at the stat of high-temeatue oeation, an estimate of the time at whih the defet fomed should also be detemined. Suitable sensitivity studies should be efomed to addess unetainty in the time of defet fomation. STEP 3. Collet Mateials Data It is fist neessay to define the mateials elevant to the assessed featue inluding, in the ase of weldments, the weld metal and heat-affeted zone stutues. Then it is neessay to ollet the mateial oeties aoiate to the tye of assessment to be efomed (yli, ee, et) ove the aoiate temeatue ange and in the oet ylially-onditioned state. Fo examle, it may be neessay to onside the effets of themal ageing and edued dutility due to intenal oxidation oduts leaking to eed fatue aths. In atie, the equiements ae influened by the outome of the tests fo signifiant ee o fatigue in Ste 6 below. Time-indeendent mateial oeties ae equied fo the stability analyses in Stes 5 and 11, noting that fatue toughness oeties may be equied fo ee-damaged mateial. STEP 4. Pefom Basi Stess Analysis Elasti stess analyses of the unaked featue should be efomed fo the extemes of the sevie yles identified unde Ste 2 assuming homogeneous oeties. The analysis should allow fo any hanges fom the stat of oeation; fo examle, ineased stess due to loss of setion by wall thinning o ineased temeatues due to edution in themal diffusivity as a esult of sufae sale. In the ase of yli loading, a shakedown assessment of the unaked featue should then be efomed. It should be detemined that the featue satisfies stit o global shakedown. If shakedown annot be demonstated, it may be neessay to emloy inelasti analysis methods. If shakedown is demonstated, the ak deth should be suh that the omliane of the stutue is not signifiantly affeted. The extent of the yli lasti zone at the sufaes of the omonent should be evaluated as this may affet the method of alulating ak gowth in Ste 9. STEP 5. Chek Stability unde Time-Indeendent Loads The omonent should be assessed against failue by time-indeendent mehanisms unde fault o oveload onditions at the initial defet size using the fatue at of this oedue. This assessment should use the initial values of any esidual stesses, not those in the shakedown state. If failue is oneded at this stage, the assumtions in the analysis should be evisited o emedial ation taken. Only if suffiient magins an be justified is it emissible to ontinue to Ste 6 to justify futue sevie life. STEP 6. Chek Signifiane of Cee and Fatigue The signifiane of ee should be assessed. If ee is insignifiant then the assessment beomes one of yli loading alone and Stes 7 and 1 below ae omitted. Convesely, if fatigue is judged to be insignifiant, then the assessment beomes one of steady ee loading alone and futhe onsideation of yli loading is not equied. A futhe test detemines if ee-fatigue inteation is signifiant. If it is not, simlified summation ules fo ombining ee and fatigue ak gowth inements may be adoted. Setion 8.7 ontains detailed advie. STEP 7. Calulate Rutue Life based on the Initial Defet Size The time to ontinuum damage failue (ee utue), t CD, is fistly alulated based on the initial ak size fom Ste 1; if this is less than the equied sevie life, then magins ae not aetable and it may not be neessay to efom ak gowth alulations. The estimate of utue life is based on a alulated limit load eene stess and, fo edominately imay loading, the mateial s ee utue data. Fo damage due to yli elaxation and due to the elaxation of welding esidual stesses, dutility exhaustion methods ae moe aoiate. The detailed alulations ae desibed in Setion 8.8. STEP 8. Calulate Initiation Time The initiation time is the time, t i, fom the stat of the assessed eiod of high-temeatue oeation io to whih no signifiant ak gowth ous. Deending on the ause of aking, its loation within a weldment and the tye of loading it may be ossible to alulate a non-zeo initiation time. It is onsevative to ignoe 8-8 FITNET 26 All ights eseved

11 (1 May 26) FITNET M7 this eiod and assume that ak gowth ous on fist loading. Calulation methods ae set out in Setion 8.8. The ause of aking influenes the detemination of an initiation time. Fo examle, a natually-ouing ee defet, suh as a Tye IV weld defet, may not exeiene an initiation eiod io to maosoi ak gowth, unless the initial defet has extended aoss the egion of extensive ee damage leading to its fomation and thus has its ti in essentially undamaged mateial. STEP 9. Calulate Cak Size afte Gowth The ak size at the end of the assessed eiod of oeation is alulated, following the advie in Setion 8.7, by integating the aoiate ee and fatigue ak gowth exessions. This integation is simlified in some ases, deending on the outomes of the signifiane tests in Ste 6. Changes in eene stess due to ak gowth should be inluded in the alulations. STEP 1. Re-Calulate Rutue Life afte Cak Gowth The time to ontinuum damage failue should be e-alulated taking into aount the ineased ak size fom Ste 9. Cak gowth alulations should not be efomed in atie beyond an aetable utue life. It is onsevative to base the estimate of utue life on the final ak size as this neglets slowe aumulation of ee damage when the ak size is smalle duing gowth. STEP 11. Chek Stability unde Time-Indeendent Loads afte Cak Gowth In atie, this ste is aied out in onjuntion with the ak size alulations of Ste 9. The ak gowth alulations of that ste should not be efomed beyond a ak size at whih failue by time-indeendent mehanisms is oneded at fault o oveload load levels using the fatue oedue. STEP 12. Assess Signifiane of Results Magins against failue ae not esibed hee and ae left to the use to set. The sensitivity of the esults of the eeding stes to ealisti vaiations in loads, initial flaw size and loation, and mateial oeties must, howeve, is assessed as at of a sensitivity study. The vaious modelling assumtions made an also be evisited with a view to eduing essimisms in the analysis if unaetable magins ae detemined. If this still fails to esult in an aetable assessment, the otions of eduing futue sevie onditions, o eai o elaement of the defetive omonent should be onsideed. This is disussed in Setion 8.9. STEP 13. Reot Results The esults of the assessment, inluding magins detemined, and the details of the mateials oeties, flaw size, loads, stess analysis alulations, et, used in the assessment should be omehensively eoted. This failitates both veifiation of the atiula assessment and eeatability in futue assessments. FITNET 26 All ights eseved 8-9

12 FITNET FFS M7 Cee Module 8.3 Establish Cause of Caking Befoe efoming alulations, an investigation should be aied out to detemine the most likely ause of aking. At least, this should involve a ombination of non-destutive testing, visual examination and metallugial examination. If it is ossible to identify the mode of aking, this an ovide qualitative infomation on the elative ontibutions of ee and fatigue to the oveall oess; ee ak gowth is geneally integanula wheeas fatigue ak gowth tends to be tansganula. Also, whee ossible, a dimensional hek should be aied out to establish whethe thee has been any signifiant distotion. When a defet has been disoveed in a omonent that has been in sevie, the onsevative assumtion fo the alulation of ontinuum damage is that the ak initiated ealy in life. This should be assumed unless thee is evidene to the ontay. Whee the defet an be eliably justified to have fomed afte the stat of high-temeatue oeation, then it is emissible to use an estimate of the oesonding time in the alulations of ak gowth and ontinuum damage. The eene stess aoiate to this eiod is then detemined on the basis of the unaked body. Suitable sensitivity studies should be efomed to addess the effet of the assumtion on the obustness of the assessment. Patiula aution should be taken if the defet fomed in sevie by ee mehanisms, noting the omments in the next aagah onening the effets of ee damage on ak gowth oeties. Signifiant ee damage, away fom the ak ti, obably indiates that thee has been loal ove-heating o ove-stessing. In these iumstanes, all ak gowth alulations should take aount of the mateial in its damaged state. In addition, whee aks ae disoveed in mateial that has exeiened extensive ee damage, it is essential fo end-of-life assessment that mateial data ae used whih ae fully eesentative of mateial in its damaged state. Data on fatue toughness and emaining ee utue life ae atiulaly sensitive in this eset. Whee thee is evidene of envionmentally assisted aking (multile aking without avitations), the methods equied ae disussed in Setion Define Sevie Conditions This oedue is aliable to omonents whih oeate fo long eiods at steady o steady yli onditions of load (stess), o dislaement, and temeatue. Eah loading and temeatue must be defined fo the loations of inteest. In making an assessment, it is onsevative to assume that all the loading is load-ontolled and ignoe stess elaxation; it may also be assumed that infequent shot-tem oveloads will not modify the ak ti onditions signifiantly. Seifiation of the sevie onditions must define the load, temeatue and life seen to date. In addition to histoial oeation, futue sevie onditions must also be defined. The loading must inlude all time indeendent, tansient and fault loading. In many iumstanes, the sevie load and temeatue histoy an be boken down into a numbe of bloks duing whih stess and temeatue ae sensibly onstant. This will simlify the analysis. Whee tansient loading ous, duing eithe stat-u o shutdown, the numbe of load and temeatue yles and thei magnitude should be established. In defining the futue life that is equied fom lant, onsevative editions have to be made about the likely tansient onditions that will be exeiened. 8.5 Collet Mateials data This setion outlines the mateial oeties data equied to follow the stes in the oedue set out in Setion 8.2. Futhe disussion is ontained in Setion 5.4 and samle mateials data ae esented in Annex M. Some of the mateial oeties may be inte-elated and it is neessay to use onsistent mateial oeties data in diffeent stes of the oedue. This is of atiula imotane when mateial oeties data ae obtained fom a numbe of diffeent soue eenes. Whee ossible, mateials oeties data should be obtained by following testing standads. Howeve, standads ae not available fo measuing all oeties fo efoming a omehensive ee-fatigue ak initiation analysis and Setion 8.11 inludes guidane set out in, fo examle, odes of atie in these ases. 8-1 FITNET 26 All ights eseved

13 (1 May 26) FITNET M Cee Rutue Data Cee utue data ae equied to alulate the utue life of the emaining ligament and to estimate the uent ontinuum damage level in the ligament as the defet gows Cee Defomation Data Cee defomation data ae equied fo steady loadings to estimate the ee ak inubation time and subsequent ee ak gowth ates using eene stess tehniques. Fo ases with steady imay load o lage elasti follow-u, fowad ee data olleted unde onstant load onditions ae aoiate. Fo essentially stain-ontolled onditions, in the absene of follow-u, stess elaxation data may be moe aoiate than fowad ee data. Reliable onstitutive equations ae needed to ovide a smooth tansition between these extemes. Fo ee-fatigue loadings, a desition is equied of the ee defomation of the mateial in the elevant yli ondition in ode to estimate ee ak gowth ates duing the dwell eiods. Cee defomation data may also be equied to alulate the time fo failue by ontinuum damage using a dutility exhaustion aoah o to estimate ee damage at the sufae fo use in a ee-fatigue ak gowth law. Often a simle owe law exession o ( ) n ε ε = (8.1) o is used to desibe ee stain ate. Moe geneally, ee defomation is desibed by thee stages: imay, seonday and tetiay. Equation (8.1) desibes the seonday stage wheeas some othe laws onside only an ineasing stain ate afte the imay stage. Rathe than desibing the ee stain ate, ee defomation data ae often eesented by isohonous stess-stain uves, see Figue 8.4, o tables giving, fo examle, the stess equied to give a total stain of 1% afte a seified time at a seified temeatue. Examles of ee stain ate equations and isohonous data ae ontained in Annex M Cee Dutility Data Cee dutility data may be equied to alulate the time fo failue by ontinuum damage using a dutility exhaustion aoah o to estimate ee damage at the sufae. In addition, ee dutility data may be used to estimate ee ak gowth ates fo situations in whih exliit ak gowth data ae not available Cee Cak Initiation/Inubation Data Fo situations whee fatigue is insignifiant, it may be ossible to take aount of an inubation eiod io to ak extension. Cee ak inubation data may be exessed in tems of a itial ak ti oening dislaement, δ i, o, fo widesead ee onditions, by a elationshi of the fom: * β t(c i ) =γ (8.2) whee t i is the inubation time and β and γ ae mateial onstants. In situations whee exliit inubation data ae not available, it is ossible to estimate the inubation time fo widesead ee onditions using aoximate exessions given late. In addition, two altenative aoahes fo editing inubation times ae given in Setion Cee Cak Gowth Data Cee ak gowth data ae equied to alulate ak gowth unde steady loading onditions o to estimate the ak extension duing dwell eiods fo ee-fatigue onditions. Cee ak gowth data ae geneally esented as a simle elationshi of the fom: * q a= A(C ) (8.3) FITNET 26 All ights eseved 8-11

14 FITNET FFS M7 Cee Module whee A and q ae mateial onstants. Annex M gives some tyial values of these onstants fo a numbe of mateials. The onstants A and q may deend on the test seimen geomety used to obtain ee ak gowth data. Fo a onsevative assessment, it is eommended that omat o deely aked bend seimens ae hosen and that data satisfy validity iteia seified in standads. Howeve, if use of moe seifi data an be justified, to allow fo loss of onstaint fo examle, altenative test seimens an be used as desibed in the CRETE ode of atie [8.18] Cyli Cak Gowth Data The tye of yli ak gowth data equied deends on the size of the defet elative to the yli lasti zone at the sufae of the omonent. Fo small defets embedded in the yli lasti zone, a stain based method fo alulation of ak gowth is set out in Setion (A). This uses the stain based ak gowth law temed Method II that is desibed in Setion Fo aks lage than the yli lasti zone, a Pais law modified fo ak losue is used. This is temed Method I and is desibed in Setion Deending on the aliation, it may not be neessay to ollet both Method I and Method II data Method I The yli omonent of ee-fatigue ak gowth equied fo a Method I ak gowth ate law is desibed by da dn f =CΔ l eff (8.4) whee C and l ae mateial and temeatue deendent onstants. Δ eff is the stess intensity fato ange fo whih the ak is judged to be oen, as evaluated fo omonent aliations by equation (8.6) below. In situations whee yli ak gowth data have been obtained fom tests with signifiant lastiity, it is eable to evaluate Δ fom exeimental estimates of ΔJ. Howeve, it will be essimisti to use data eff whih have been oelated with elastially alulated Δ eff values. Futhe infomation on the use of fatigue ak gowth data is given in Setion 7, whih inludes altenative exessions to equation (8.4) Method II The yli omonent of ee-fatigue ak gowth equied fo a Method II ak gowth ate law is desibed by a high stain fatigue ak gowth law of the fom: da dn f =Ba Q a a (8.5) min a min whee =.2mm is the ak deth below whih the ak gowth ate is assumed to be onstant. B and Q deend on mateial, stain ange and envionment and an be detemined exeimentally. These laws aly fo a total sufae stain ange Δε t, while the defet is embedded in the yli lasti zone of size at the sufae of the omonent. Futhe infomation on stain based methods fo fatigue ak gowth assessment is ontained in Setion Othe Data In addition to the ee data desibed in Setions , it may be neessay to have othe data to efom an assessment. These ae listed below. In addition, fo the altenative methods in Setion 8.1, some seial equiements ae needed when altenative aoahes ae followed and these equiements ae disussed in the aoiate at of Setion FITNET 26 All ights eseved

15 (1 May 26) FITNET M Elasti and Physial Constants Values fo Young s modulus, E, Poisson s atio, ν, and the instantaneous o mean oeffiient of themal exansion, α, ae equied to efom the basi stess analysis and may also be equied if a detailed shakedown analysis has to be efomed Stess-stain Data Values fo the minimum monotoni.2% oof stess ae equied to hek ak stability fo timeindeendent loadings and to efom a shakedown analysis. Cyli stess-stain data ae equied to detemine the stess intensity fato ange when yli lasti defomation ous and to detemine defomation stess-stain loos. Cyli stess-stain data may be desibed by a elationshi between the total stess ange and the total stain ange of the hysteesis loos though the use of the Rambeg-Osgood equation. Fo ylially stable mateials, the stess-stain hysteesis loo an be eonstituted fom this equation but the oess is less suessful fo hadening and softening mateials. Fo situations in whih stit shakedown is ahieved, these data will not be equied as the shottem esonse will be elasti. Howeve, yli stess-stain data will be equied fo moe sevee loading. The fato s is an exeimentally deived fato whih an be alied to the minimum.2% oof stess of the mateial to give a level, S s y, whih is the lagest semi-stess ange fo whih the mateial has stable i.e. exhibits non-athetting, yli stess-stain behaviou. s is theoe equied to efom a shakedown analysis. s with temeatue fo Tye 316 and wought feiti steels is given in R5 Volume 2/3. The vaiation of Additional details on the deivation of Fatue Toughness Data s ae given in Setion A1.3 of R5 Volume 2/3. Values of the fatue toughness ae equied to hek ak stability fo time-indeendent loadings. In geneal, data elate to mateials whih have exeiened no io global (ligament) o loal (ak ti) ee damage. It is theoe neessay to onfim that the fatue toughness values used fo assessing ak stability unde time indeendent loadings ae aoiate fo the mateial ondition ahead of the ak ti. 8.6 Pefom Basi Calulations Stess Intensity Fatos The linea elasti stess intensity fato,, deends on the loading and the ak size and may vay with osition aound a ak font. Fo yli loading, it is neessay to evaluate the stess intensity fato ange and the atio, R, of minimum min to maximum stess intensity fato, max. The value of R should be alulated fom a shakedown analysis athe than a simle elasti analysis. This is beause ee duing a yle tends to lead to a yli stess state whih gives a lowe value of R than the initial elasti esonse. The shakedown analysis only affets the value of R and not the total stess intensity fato ange, as the esidual stess is indeendent of osition in the yle. In the absene of yli lastiity in the unaked body, the effetive stess intensity fato ange defined by: Δ eff is Δ =q Δ (8.6) eff o FITNET 26 All ights eseved 8-13

16 FITNET FFS M7 Cee Module whee Δ=max - min and q o is the fation of the total load ange fo whih a ak is judged to be oen. This may be estimated onsevatively fom: q = 1 R q o o = ( 1 -.5R)/(1- R) R < (8.7) whee R = min / max Refeene Stess Fo ee ak gowth evaluation, it is neessay to evaluate the eene stess at the stat of the dwell. The eene stess fo simle imay loading is detemined by the methods of limit analysis and is defined by: = P /P (,a) (8.8) y L y In ases whee yli loading is esent the load P is evaluated fom the stess, odued by the shakedown analysis, at the time in the yle oesonding to the ee dwell. It should be noted that this is not neessaily at the eak stess in the yle. P L is the value of P oesonding to lasti ollase assuming a yield stess y. The effet of the flaw must be inluded in evaluating the lasti ollase load. When ee ak gowth is being onsideed, the elevant flaw size is the size of the oiginal flaw lus the amount of ak gowth. Fo the uoses of initial alulation of the time fo failue by ontinuum damage mehanis and alulation of inubation time, the elevant flaw size is that of the oiginal defet C* Paamete Fo steady state ee, the ak ti stess and stain ate fields (and hene ee ak gowth ates) may be * haateised by the C aamete. This is the ee equivalent of the J-ontou integal used to desibe elasti-lasti fatue. It may be evaluated by finite element analysis but a eene stess based estimate of * C is often used. This is (8.9) C = ε [ (a),ε ]R * Hee, ε is the ee stain ate at the uent eene stess and ee stain, ε, aumulated unde the eene stess histoy u to time t; that is, a stain hadening ule is used to define ee stain ates unde ineasing stess duing the ak inubation and gowth stages. This is aliable to omlex time-vaying ee laws. The haateisti length, R is defined by (8.1) R=( / ) 2 whee is the stess intensity fato due to imay load only. As both and ae dietly ootional to the loading P, the value of R is indeendent of the magnitude of P. Howeve, R does vay with ak size and, when ee ak gowth is being onsideed, both and should be alulated fo the defet size equal to the size of the oiginal ak lus the amount of ee ak gowth. The value of R is also diffeent at the sufae and deeest oints of a semi-ellitial sufae defet due to diffeenes in the values of. Fo othe than simle imay loadings, estimates of * C ae given in Setion FITNET 26 All ights eseved

17 (1 May 26) FITNET M Redistibution Time, t ed This alulation is only equied when yli loading is insignifiant. Time is equied fo stess edistibution due to ee fom the initial elasti state at the stat of a ee dwell. The equiement fo the stess edistibution to be omlete and widesead ee onditions to be established may be exessed in tems of a edistibution time, t ed. This may be exessed onveniently in tems of the eene stess fo ases of imay load only as ε [ (a),t ] = (a) E (8.11) ed whee ε [ (a),t] is the aumulated ee stain at the eene stess fo time, t, and ak length, a, fom uniaxial ee data. Equation (8.11) alies fo steady ee loading unde imay stesses. When alulating ak gowth unde signifiant yli loading (see Setion 8.7), it may be neessay to onside the ealy yles befoe the steady yli state is eahed. Time is equied fo the mateial esonse to the yli loading to eah a steady yli state o shakedown. This time, t y, an be estimated in tems of the eene stess fo the fist yle, y=1, and the eene stess unde steady yli onditions fo ombined imay and seonday loading,, as: y=1 y=1 ( + ) ( - ) ε [,t y]=z 2 E (8.12) whee Z is an elasti follow-u fato, whih ontols the ate of stess elaxation to steady state ee. Futhe details of the teatment of seonday loading ae given in Setion C(t) Paamete Fo times less than the edistibution time, it may be neessay to alulate the tansient ak ti aamete C() t. An inteolation fomula fo C() t duing the tansition between initial elasti loading and steady state seonday ee is C(t) (1 + ε /ε ) C 1/(1-q) e = * (1 + ε 1/(1-q) /ε e ) - 1 (8.13) ε is the aumulated ee stain at time t, ε e is the elasti stain and q is the exonent in the ee whee ak gowth law of equation (8.3) with q-n/(n+1) whee n is the exonent in equation (8.1). Fo times in * exess of the edistibution time, C() t aoahes C. 8.7 Chek Signifiane of Cee and Fatigue In many ases the omlexity of a ee-fatigue ak gowth assessment an be avoided by efoming simle alulations to demonstate the insignifiane of ee and/o fatigue. In the event of both ee and fatigue being shown to be signifiant, simle tests an also be used to demonstate insignifiant ee-fatigue inteations, and thus emove the oneous equiement to geneate mateial fatigue data inooating the effets of ee holds. The test fo insignifiant ee alies when both Method I and Method II data of Setions and ae used. The tests fo insignifiant fatigue and ee-fatigue inteation only aly to Method I. FITNET 26 All ights eseved 8-15

18 FITNET FFS M7 Cee Module Insignifiant Cee The signifiane of ee stains should be detemined fo the assessed loading and temeatue histoy. Cee may be signifiant fo some tyes of loading histoy but not fo othes. The effets of ee may be negleted if the sum of the atios of the hold time t to the maximum allowable time t m, at the eene temeatue, T, fo the total numbe of yles is less than one: N t/t m(t ) < 1 (8.14) j j=1 The values of t m deend on mateial, ak size and temeatue. R5 ontains uves in Figues A6.6 and A6.7 fo austeniti Tye 316 and Tye 34 mateials. Fo othe mateials, guidane on ee exemtion may be taken fom BS 791. In BS 791, t m, fo mateials with ee utue dutilities > 1%, is taken as the time equied to ahieve an aumulated ee stain of.2% at a stess level equal to the eene stess. Fo dutilities < 1%, t m should be detemined on the basis of ee stains with a magnitude equal to 1/5 th of the ee utue dutility. Howeve, it should be noted that fo onsisteny with the insignifiant ee tests fo unaked stutues, values of t m should not be geate than those allowed in design odes Insignifiant Fatigue It should fist be detemined whethe o not ee behaviou is unetubed by yli behaviou. This test should be efomed both fo the oveall stutual esonse and fo stesses loal to the ak ti. Sine Ste 4 of the oedue of Setion 8.2 equies that the ak deth is suh that the omliane of the stutue is not signifiantly affeted, the test fo the oveall stutual esonse may be demonstated by showing that the elasti stess ange does not exeed the sum of the steady state ee stess and the stess to ause yield at the othe exteme of the yle. Futhe infomation is ontained in R5. The test fo stesses loal to the ak ti may be made by demonstating that, fo the most sevee fatigue yle, the yli lasti zone at the ak ti is small. Unde yli loading, the allowable elasti stess ange is 2 y in the absene of yli hadening o softening, and the yli lasti zone size at the ak ti, ak 2 = β(δ/2 y ), whee β is tyially 1/2π in lane stess and 1/6π in lane stain. Moe geneally, the yli lasti zone size at the ak ti should be alulated using the yli yield o.2% offset stess. This yli lasti zone size should be shown to be muh less than the ak size o any othe dimension haateisti of the stutue, suh as setion thikness o emaining ligament ahead of the ak. If the above tests ae satisfied, yli loading effets on ee ak gowth an be negleted. Futhe, fatigue is insignifiant, ovided that estimated fatigue ak gowth does not exeed 1/1 th of the estimated ee ak gowth. At the stat of an assessment only aoximate estimates of gowth ae equied and bestestimate data fo both ee and fatigue ak gowth should be used to alulate this atio. These aoximate alulations an be ined when esults of a detailed assessment beome available Insignifiant Cee-Fatigue Inteations When both ee and yli loading ae shown to be signifiant, the signifiane of ee-fatigue inteation should be detemined. In geneal, the effet of ee damage on fatigue ak gowth ates has little influene on the total ak gowth e yle ovided the latte inludes an exliit alulation of ee ak gowth. Hene, ee-fatigue inteation is insignifiant and mateial data that allow fo inteations, whih lead to enhaned fatigue ak gowth ates, ae not equied. It is adequate, theoe, in Ste 9 of Setion 8.2 to sum ee ak gowth with ontinuous yle fatigue ak gowth estimates. Thee ae two exetions to this geneal ule: 8-16 FITNET 26 All ights eseved

19 (1 May 26) FITNET M7 (i) (ii) In ases whee the tests fo insignifiant yli loading and fatigue indiate that ee is etubed by yli behaviou, but fatigue ak gowth is shown to be only a small fation of the total ak gowth e yle (i.e. fatigue ak gowth does not exeed 1/1 th of the ee ak gowth), the enhanement of fatigue ak gowth by ee damage may be lage. In these iumstanes the onstants in equation (8.4) should be obtained fom tests at hold times elevant to the sevie aliation being assessed. In ases whee aks ae oagated by fatigue though mateial heavily damaged by io ee, oagation ates ae likely to be ineased. In these iumstanes, a fato should be alied to the fatigue data, deending on the amount of io ee damage. BS 791 eommends that this fato should be detemined exeimentally, and elates heavy io ee damage to a value of damage fato D geate than about Pefom Assessment Calulations Calulate Rutue Life, t CD Both stess-based and dutility-based aoahes may be used fo assessing ee damage. Fo loadings whih ae edominantly onstant and imay, the stess is well known and it is aoiate to use stess/time-to-utue elationshis fo assessment. Fo damage due to yli elaxation, the stain aumulated is limited in eah yle and dutility methods ae aoiate. Fo edominately imay loading the time, t CD, fo ee damage to oagate though a stutue and lead to failue is taken as t CD =t [ (a)] (8.15) is the utue time at stess,, fom onventional stess/time-to-utue data and the eene stess is alulated fo the imay loads only fo the uent ak size, a. Pio to ak gowth the utue time is alulated fo the initial defet size, a. If t CD is less than the emaining assessment time then emedial ation must be taken. Fo ombined and yli loading, it may be neessay to evaluate t CD fom a dutility exhaustion aoah; futhe details ae given in Setion 8.1. whee t ( ) Calulate Cak Inubation Time, t i The inubation time, t i, is defined as the time duing whih the initial ak blunts without any signifiant ak extension. Inubation is defined fo engineeing uoses as oesonding to.2mm ak extension. The method fo eesenting inubation data then deends on obseved seimen esonse. Fo steady state ee onditions with an essentially onstant dislaement ate, the inubation time in test seimens is * oelated with exeimental estimates of the ak ti aamete C by equation (8.2). Use of the estimate of fom equation (8.9) fo the initial ak size a, then ovides an estimate of t i. Moe geneally, inubation times an be elated to measuements of a itial ak oening dislaement, δ i, whih an then be used to alulate a itial eene stain as ε [ (a ),t ] = [δ /R (a )] - (a )/E (8.16) n/(n+1) i i If fatigue is signifiant it is onsevative to set the inubation time to zeo. Howeve, a ee-fatigue ak inubation time (o yles) may be alulated using the FAD o sigma-d aoahes outlined in Setion 8.1. Fo ellitial o semi-ellitial defets, the inubation time should be taken as the lowe of the values obtained fom equation (8.16) at oints oesonding to the majo and mino axes of the ellise o semi-ellise. FITNET 26 All ights eseved 8-17

20 FITNET FFS M7 Cee Module Calulate Cak Size afte Gowth, a g The extent to whih ak gowth alulations ae equied deends on the elative magnitudes of the sevie life to date, t o, the desied futue sevie life, t, s and the inubation time, t i ; this may be summaised as follows. If t o+ t s < t i, the ak will not inubate and a g = a. If the ak inubates duing the assessment time, then it is neessay to alulate the ak size, a g, afte gowth in time t o + t s - t. i If the ak has inubated io to the assessment, then it is neessay to alulate the ak size, a g, afte gowth in time t s. The time equied fo the ak to oagate by an amount Δa g is denoted t g. Thee ae a numbe of diffeent egimes fo alulations of ak gowth and these ae set out below. (A) Caks gowing inside the yli lasti zone,, at the sufae of the omonent. In this egime the Method II high stain ee-fatigue ak gowth law should be used. This is eqn (8.5) fo insignifiant ee. When ee is signifiant, the ee-fatigue ak gowth e yle is given by: da da suf = (1 - D ) dn dn f -2 (8.17) suf whee (da/dn) f is the fatigue ak gowth e yle fom eqn (8.5) and D is the total sufae ee damage (taking aount of stess state, if neessay) aumulated u to the uent time fom evey yle and is: suf N D = (d ) (8.18) j=1 suf j suf whee (d ) is the ee damage aumulated in the j th yle and the summation is aied out u to the j suf uent time. The tem (d ) is evaluated at the sufae of the unaked omonent and is given by the dutility exhaustion method as j t h, j suf ε (d ) j= dt ε f(ε ) (8.19) whee t th, j is the j th ee dwell eiod, ε is the instantaneous equivalent ee stain ate duing the dwell and ε f (ε ) is the ee dutility at that stain ate, aounting fo stess state. The stain ate ε is evaluated at the instantaneous stess duing the dwell obtained fom stess elaxation data. The elaxation data should oesond to a stating stess equal to the unaked-body stat-of-dwell stess at the sufae,. In atie, many yles will be of a simila tye, in whih ase the summation in equation (8.16) is simlified. When D 1 suf, equation (8.17) edits an infinite ak gowth ate. Howeve, this should not be inteeted as editing the failue of the omonent. This oesonds to the exhaustion of ee dutility at the sufae of the omonent and the instantaneous ak deth, a, should be set to the deth of the yli 8-18 FITNET 26 All ights eseved

21 (1 May 26) FITNET M7 lasti zone,. If is geate than the ak deth that the stutue an safely toleate unde sevie and oveload onditions then emedial ation should be taken. Caks deee than ae subjeted to nominally yli elasti defomation and the Method I gowth law should be used as set out below. (B) Cak length, a, geate than the yli lasti zone size,, at the sufae of the omonent. In this egime, the Method I ak gowth ate law of equation (8.4) is used and the total ak gowth e yle, da/dn, is obtained as the simle sum of the ontibutions due to yli and ee ak gowth ates: da/dn = (da/dn) +(da/dn) (8.2) f The fatigue ak gowth ate (da/dn) f is given by equation (8.4) with the onstants modified fo hold-time effets only if ee-fatigue inteations ae shown to be signifiant in Setion If fatigue ak gowth has been shown to be insignifiant in Setion 8.7.2, this tem is omitted. The ee ak gowth e yle in equation (8.19) also deends on loading egime as set out in (i) - (iv), below. (i) Steady state ee ak gowth fo times t > t ed, with insignifiant yli loading Fo the load ontolled ase and the attainment of steady state ee onditions the ee ak gowth is obtained fom ee ak gowth data in the fom of equation (8.3). Equation (8.16) is used to estimate The ee ak extension e yle, eiod, t h : * C fo ak sizes between a and a g fo use with equation (8.3). (da/dn), is evaluated as the integal of equation (8.3) ove the dwell t h * q = A(C ) dt da dn (8.21) (ii) Non-steady state ee ak gowth, t < t ed, when yli loading is insignifiant. To allow fo the ineased amlitude of the ak ti fields at shot times, it is assumed that fo times less than the edistibution time (t<t ed ), equation (8.3) may be genealised to (8.22) a= AC [ ( t)] q t +t > t Fo situations whee i g ed, the effets of the edistibution eiod an be allowed fo by using the ak gowth ates of equation (8.3) multilied by a fato of 2 fo t<t ed, i.e. * q a= 2 A( C ) fo ti t < ted * q a = A( C ) fo t ted (8.23) If the total time fo the assessment does not exeed t ed, then this simlified teatment of tansient ee is not adequate and it is neessay to use the aamete C( t ) exliitly, fom equation (8.13), in estimating ee ak gowth. The ee ak extension e yle, eiod, t h, as: (da/dn), inluding tansient effets is then evaluated ove the dwell FITNET 26 All ights eseved 8-19

22 FITNET FFS M7 Cee Module t h da q = A[ C( t)] dt dn (8.24) (iii) Ealy yle ee ak gowth, t < t y, when yli loading is signifiant. Fo a omonent outside stit shakedown a mean estimate of used u to t y of equation (8.1). Whee only elasti analysis is available, * C duing the tansient eiod, * C is defined as: * C, may be C = ( + ) 2 (8.25) * y= 1 εr y= 1 [ whee ε is evaluated as ε ( + ) 2]. As ak gowth is aoximately linealy deendent on the ak gowth duing the time t y is not atiulaly sensitive to the value of t y but deends imaily on the aumulated ee stain, y 1 * C, Z( = ) / E. Fo the ealy yles, io to stutual shakedown, the ee ak extension e yle, ( da / dn), is evaluated ove the dwell eiod, t h, as: t h ( / * q ) [ ] da dn = A C dt (8.26) (iv) Steady yle ee ak gowth, t > t y, when yli loading is signifiant. t t At y, equation (8.26) is elaed by equation (8.21), but yle obtained fom a shakedown analysis. * C is alulated fom the loads in the steady 8.9 Assess Signifiane of Results Aliation of the assessment oedues will lead to one of the following esults: i) The final defet size leads to an aetable end-of-life safety magin. In this ase, a sensitivity analysis should be aied out to ensue that the safety magin is not ovely sensitive to vaiations in the inut aametes of the assessment. ii) Failue o exessive ak gowth is indiated within the equied sevie life. In these iumstanes, the assessment may be evisited with a view to eduing the assumed essimisms. In the event that aetable end-of-life safety magins still annot be demonstated, emedial ation should be taken. These senaios ae both disussed in futhe detail below Sensitivity Analysis The limiting onditions fo a failue assessment ae desibed elsewhee in the FITNET oedue. The limiting state is not nomally aetable fo engineeing uoses and onfidene in detemining safe loading onditions is taditionally gained by alying safety o eseve fatos in design alulations. Howeve, the aliation of atiula numeial fatos in fatue analyses an be misleading beause of the inheent but vaiable inte-deendene of the aametes ontibuting to fatue behaviou. Confidene in assessments is gained in two stages. The use of lowe bound limit load solutions, togethe with ue bound loads, defet sizes and stess intensity fato values, ovides onfidene that the failue assessment is suitably onsevative. This should then be einfoed by investigating the sensitivity of the assessment oint to vaiations of aoiate inut aametes. 8-2 FITNET 26 All ights eseved

23 (1 May 26) FITNET M7 Fo examle, the sensitivity analysis may onside unetainties in the sevie loading onditions, the extaolation of mateials data to sevie onditions, the natue, size and shae of the flaw, and the alulational inuts. Fo defets found in sevie, the sensitivity of the assessment to any assumtion about whethe the ak is aleady gowing may be tested by efoming assessments both with and without the inubation stage. It is eommended that, whee ast-seifi data ae not available, an initial assessment be efomed using best-estimate, mean data. The sensitivity study should inlude the following ombinations: (i) lowe bound ee ak gowth ate with ue bound ee stain data, (ii) ue bound ee ak gowth ate with lowe bound ee stain data. It should be eognised that a sensitivity analysis whih ombines ue bound ee stain data, lowe bound ak initiation data and ue bound ee ak gowth data is likely to be ovely onsevative. Confidene in an assessment is gained when it is ossible to demonstate that ealisti hanges in the inut aametes do not lead to damati edutions in the end-of-life safety magin. Futhe onfidene in the assessment and in any aoiate insetion eiod is gained when onsideation of the end-of-life ak gowth ate shows that thee is not aid ak extension leading to imminent failue. Details of the sensitivity analysis should be eoted with the assessment esults. An altenative to the deteministi aoah is to use obabilisti methods to dietly detemine failue obabilities. These methods make use of the statistial vaiation of the inut aametes, athe than assuming that the aametes ae single-valued, as in the deteministi aoah. Suh assessments equie estimates of the statistial distibutions of the vaiable inut aametes. Advie is ontained in Annex H Remedial Ation Many egulations and design standads equie, o at least eommend, eiodi insetion of omonents unde loads suh as intenal essue and/o high temeatue oeation. Often the egulations equie a essue test to be efomed, veifying that essue omonents ae not leaking. Altenatively, egulations aet the use of some non-destutive testing, demonstating omonent integity. Tyial e-insetion eiods ae evey ten yeas o lowe. Insetions an be efomed using some of the available methods and tehniques desibed in Annex D. Insetion of omonents oeating in the ee ange uses these tehniques to demonstate integity, but othe tehniques ae also used to detemine ee status, inluding: Magnetite laye measuement: the magnetite laye is indiative of the oeational effetive temeatue. The laye is usually measued by ultasoni testing, but sometimes destutive testing of samles extated fom seleted oints, onfims ultasoni testing esults. Combined with tube thikness measuements, this tehnique is used fo ee emaining lifetime detemination. Hadness: hadness is not a lea indiato of emaining lifetime, but it is a hea tehnique and in ombination with elias an ovide useful infomation about ee omonent status. Metallogahi elias; the analysis of elias ovides infomation about hysial and stutual damage of the omonent. As elias equie some sufae eaation, it is usual to omlement these with hadness measuements. The tehnique is also used to analyse deteted aks, heling to detemine aking mehanisms and ak gowth, fo inut into emaining lifetime assessment. Dimensional analysis: ee defomation an be deteted and suveyed by seleted measuements. X-ay diffation: this tehnique ovides infomation on mateial omosition, level of stess and ee status. Mio-seimen destutive testing: some tests ae available, oviding infomation on mateial oeties and ee status. The most fequently used tehniques ae the miniatue seimen ee test, the imession ee test and the small unh test. These tehniques ae desibed in moe detail in Annex D. If failue o exessive ak gowth is indiated within the equied sevie life, then it may be ossible to evisit the assessment with a view to eduing the assumed essimisms. Fo examle, it may be ossible to obtain seifi mateials data that is less essimisti than the use of bounding data. Only in the event that an aetable safety magin still annot be demonstated should emedial ation be taken. FITNET 26 All ights eseved 8-21

24 FITNET FFS M7 Cee Module The most fequent measue, when the alulated lifetime is lowe than exeted is to modify the e-insetion eiod and/o eommend the use of diffeent insetion tehniques. Fo examle, if alulations assoiated with hadness measuements indiate that the emaining lifetime of a omonent is lowe than 2 hous, it is ossible to look fo aks (magneti atiles, dye enetant liquid, ultasoni testing), efom metallogahi elias, dimensional analysis o magnetite laye measuements. Remedial ation may involve hanging the in-sevie aametes (suh as load, temeatue o desied sevie life) and then using the assessment oedue eithe to demonstate aetane o to estimate at what time eai will be neessay. Altenatively, the defetive omonent may be eaied o elaed. The sensitivity analysis is atiulaly useful fo indiating whih mateials oeties may signifiantly influene the assessment. Fo examle, if emedial ation is equied beause the desied sevie life exeeds the utue life thee is little oint in geneating auate ee ak gowth data in an attemt to imove the assessment Reot Results When eoting the esults of a stutual integity assessment, the infomation listed below should be esented. 1. LOADING CONDITIONS - e.g. sevie load and temeatue and sevie life inluding life seen to date; onditions onsideed fo time indeendent loadings, fo examle tansient o fault loadings; ategoisation of loads and stesses. 2. MATERIAL PROPERTIES - mateial seifiation; ee utue data, inubation COD, ee stain data, ee ak gowth data; time-indeendent assessment mateial data (yield stess, ultimate tensile stess and fatue toughness fo mateial with ee damage at ak ti); elasti data (Young's modulus, Poisson's atio); whethe data obtained by diet testing o indiet means; soue and validity of data. 3. DEFINITION OF FLAW - flaw loation, shae and size; allowane fo sizing eos; whethe ehaateisation of flaw undetaken. 4. REFERENCE STRESS - soue of limit load solution; yield iteion; whethe loal and/o global ollase onsideed. 5. STRESS INTENSITY FACTOR SOLUTION - soue of solution (e.g. standadised solution, finite-element analysis). 6. SIGNIFICANCE OF CREEP AND FATIGUE esults of tests fo insignifiant ee, fatigue and ee-fatigue inteations, if aliable. 7. TIME INDEPENDENT ASSESSMENT - fom of assessment, eseve fatos. 8. CYCLE DEPENDENT ASSESSMENT Tye of fatigue ak gowth law; stess intensity fato ange, ak losue aamete, sufae stain ange, ee damage; time to shake down to steady yli state. 9. TIME DEPENDENT ASSESSMENT - esults of analysis; inubation time, edistibution time, defet size afte gowth, ak veloity aamete, time to utue by ontinuum damage. 1. SENSITIVITY ANALYSIS - inut aametes against whih sensitivity studies undetaken (e.g. flaw size, mateial oeties, et); esults of eah individual study. 11. REPORTING - natue of the quality assuane to whih the analysis has been subjeted FITNET 26 All ights eseved

25 (1 May 26) FITNET M7 nown essimisms inooated in the assessment oute should be listed. All deatues fom the oedue should be eoted and seaately justified. A seaate statement should be made about the signifiane of otential failue mehanisms emote fom the defetive aeas. 8.1 Additional Infomation Teatment of Defets in Weldments Intodution This setion gives the infomation equied to aly the oedue of Setion 8.2 to defets in weldments. The soe of the setion is fist desibed in Setion A atiula oblem with weldments is the esene of esidual welding stesses. Although these elax as ee stains aumulate, they may edue ee life by initiating ak gowth at shote times and by ineasing ak gowth ates. Often the effet is small, atiulaly fo stess-elieved welded joints made fom ee-dutile mateials. Theoe, a simle hek is made in Setion to assess whethe welding esidual stesses ae signifiant. Then, a simlified, onsevative assessment oedue is desibed in setion In some ases, the level of onsevatism may be exessive and a moe detailed oedue is desibed in Setion to emove some of this ove-onsevatism. It should, howeve, be eognised that validation fo the detailed oedue is limited to seifi weldment tyes. Finally, Setion desibes some seifi modes of aking obseved in high temeatue weldments Soe Defets in austeniti and feiti simila metal welds and in onventional bi-mateial dissimila metal welds (DMWs) an be assessed. Allowane is made fo welding esidual stesses whee neessay as set out in Setion Both ak gowth and ontinuum damage aumulation ae addessed. Defets whih ae haateised as being fully in eithe the feiti o austeniti mateials adjaent to a DMW should be assessed using the oedue of Setion 8.2 and the aoiate homogeneous mateial oeties. Exeiene with the DMW ak gowth oedues of Setion is uently limited to feiti 2.25C1Mo to austeniti Tye 316 ie o tube welds made using eithe austeniti o nikel-based (Inonel) weld metal. The oedue may, howeve, be used fo othe geometies and mateial ombinations ovided that the equied data have been obtained. Seonday stesses ae assumed to elax ove a edistibution eiod. In some ases, the seonday stesses may have omletely elaxed due to ee in sevie at the stat of the assessment eiod. It is, howeve, onsevative to teat all stesses as imay and to neglet elaxation. Allowane fo an inubation time of a e-existing defet io to signifiant ak oagation is made fo simila weldments unde steady loading. No suh eiod is uently assessed, within the oedues of Setion fo DMWs; it is assumed that ak gowth ous immediately uon loading. Fo eefatigue loading, it is geneally assumed that thee is no inubation eiod Signifiane of Welding Residual Stesses Simle tests, as set out below, may be alied to the ase of esidual stess fields with modest elasti followu, Z 3. Outside this ange, the esidual stess should be onsideed signifiant. Welding esidual stess do not signifiantly affet the esults of an assessment if E max δ i <.1 R ( ao) n/( n+ 1) (8.27) FITNET 26 All ights eseved 8-23

26 FITNET FFS M7 Cee Module whee max is the eak equivalent welding esidual stess, and δ i, the inubation ak oening dislaement. In a non-stess-elieved weldment, max may be onsevatively taken as the mateial yield stess, but in atie, inequality (8.27) may be diffiult to satisfy fo aks in non-stess-elieved weldments. Nomalised though-wall as-welded esidual stess ofiles ae given in Annex C fo some geometies. The length, R, is alulated fom equation (8.1) assuming homogeneous mateial oeties. When δi R (a o ), as is usually the ase, it is onsevative to set n/(n+1) equal to unity Simlified Assessment In the simlified estimate, the time fo ee utue failue and the extent of ee ak gowth ae estimated. It is assumed that the ak inubation time is zeo. Bulk ee damage is assessed assuming that the weld is homogeneous with the utue oeties of the weakest egion. The time to failue by ontinuum damage mehanisms, t CD, is alulated using equation (8.15). The alulation is efomed seaately fo eah of the weldment onstituents using the aoiate utue data, but with the eene stess always defined fom equation (8.8) fom the homogeneous limit load. The value of t CD used in the assessment is the smallest of those detemined fo the diffeent egions, i.e. the lowest life obtained fom equation (8.15). In ode to assess ee ak gowth, C* is alulated fom equation (8.16). It is assumed that the weld is homogeneous fo the uoses of alulating the eene stess of equation (8.8) and the length aamete of equation (8.1). C* follows fom equation (8.9) with ee stain ate data of the fastest eeing egion. The extent of ee ak gowth is then obtained fom equation (8.21) with this value of C* and ee ak gowth data fo the egion whee the flaw is loated, unless the flaw an oagate into mateial with a highe ak gowth law. Fo othe than simle imay loadings, estimates of C* ae given in Setion Detailed Assessment Fo the uoses of alulating the initiation time and ee ak gowth, the eene stess, alulated fo the seifi loation of the ak. Fo ontinuum damage alulations, a eene stess is alulated fo eah miostutual zone. These eene stesses ae diffeent, in geneal, fom that alulated using equation (8.8) assuming homogeneous ee oeties aoss the joint, as used in Setion The eene stess fo a given zone is a fato k times the oesonding homogenous aked-body eene stess,, that is,hom, is = k (8.28),hom whee,hom is obtained fom equation (8.8). The fato k is diffeent fo the diffeent weld zones. The soue of diffeent k values may be illustated by onsideing a ie unde essue and end load. Fo some stess states, suh as essue only, the maximum inial stess is aoximately aallel to the fusion bounday ( hoo stess ontol ) and is diffeent in the diffeent weld zones as a esult of stess edistibution; this leads to k values diffeent fom unity. Fo ases of axial-stess ontol, suh as unde high system loads whee the maximum inial stess is tansvese to the fusion bounday, oveall stess edistibution annot ou and k=1 is moe likely FITNET 26 All ights eseved

27 (1 May 26) FITNET M7 The eene stess fo eah onstituent in a weldment is detemined fom a limit load alulation efomed with the assumed yield stess of eah egion ootional to the ee utue stength of the oesonding mateial fo the sevie lifetime. The esulting limit load may diffe fom that of the homogeneous omonent due to the effets of the mismath in lasti (ee) oeties. The eene stess fo eah zone then follows fom equation (8.8) but using the loal yield stess and the mismath limit load. Note that the fato k is then the atio of the esulting eene stess to the homogeneous value. Advie on mismath limit loads is given in Setion 6 and Annex B. In geneal, the fato k is loading, geomety and mateial deendent and aounts fo the elevation in stess in stong mateial within the weldment o, onvesely, the edution in stess in weake mateials in the weldment. This weld edistibution fato an be onveniently exessed as a funtion of the atio of weld metal/aent metal minimum ee ates fo the ase of a ie-ie butt weld unde intenal essue loading. Some illustative k fatos fo weld metal, HAZ and aent mateial fo diffeent feiti.5cmov weldment ombinations ae shown in Table 8.1 fo a ie with the atio of extenal to intenal adius, o / i = 1.52 and tyial ee oeties. Table 8.1 The influene of weld metal/aent metal minimum ee ate atio on k fatos fo.5cmov weldments (*oase miostutue; ** ined). The k fatos wee deived fo an unaked ie-ie butt weld, with atio of oute to inne adius of 1.52, unde intenal essue alone. Weld Metal/Paent Metal Min. Cee Rate Ratio 1 (.5CMoV/.5CMoV) k Fato Paent Weld HAZ (2.25C1Mo/.5CMoV) * 1.** 14 (1C.5Mo/.5CMoV) The time to failue by ontinuum damage mehanisms, t CD, should be alulated using equation (8.15). The alulations should be efomed seaately fo eah of the weldment onstituents using the aoiate utue data, and the oesonding eene stess fom equation (8.28). The value of t CD is the smallest of those detemined fo the diffeent egions. The time to inubate a gowing ee ak, t i, is alulated using equation (8.16). The ee stain, eene stess and inubation ak oening dislaement, δ i, ae seifi to the miostutual egion within whih the ak ti is loated. Howeve, fo the ase of hoo stess ontol, omatibility of hoo stain ate aoss the weld imlies that ε in equation (8.16) an be elaed by the oesonding aent stain at the aent eene stess. When δ / R i (a o ) is small and the ight hand side of equation (8.16) is negative, the inubation time, t i, should be set to zeo. The ee ak gowth aamete, * C, should be estimated fom equation (8.9) using the value of and the ee oeties aoiate to the mateial egion within whih the ak ti is loated, togethe with the value of R detemined fom equation (8.1) but assuming homogeneous mateial oeties. Fo the ase FITNET 26 All ights eseved 8-25

28 FITNET FFS M7 Cee Module of hoo stess ontol, omatibility of hoo stain aoss the weld imlies that ε in equation (8.9) fo * C an be elaed by the oesonding aent stain ate at the aent eene stess. In ode to obtain ak gowth ates fom C*, the ee ak gowth law should be seifi to the miostutual egion within whih the ak is gowing and should aount fo hanges in gowth ate with any hanges in miostutue that the ak enountes. In alying tansient oetions, stess edistibution is omlete and widesead ee onditions ae established when the edistibution time, t ed, defined by equation (8.11) is exeeded fo the egion ontaining the defet. Fo bi-mateial dissimila metal welds (DMWs), some simlifiations of the above aoah ae ossible if it is assumed that the defet ous on the bounday between two mateial states. In atie, ak oagation in these DMWs is geneally assoiated with the feiti base to weld metal intefae egion. This intefae egion is onsideed hee fo simliity as onsisting of the feiti base metal, the weld metal and the heat affeted zone (HAZ).. Fo a aked weldment, the homogeneous limit load eene stess of equation (8.8) is used with oesonding uniaxial oss-weld utue data to assess ee utue of the DMW. To assess ee ak gowth, the stess intensity fato used in the alulation of the length sale R in * equation (8.1) is detemined assuming homogeneous elasti oeties fo the weld onstituents. C is estimated by equation (8.9) using the homogeneous eene stess and the ee stain ate, ε, evaluated fo a eene mateial at the eene stess level. It is onsevative to equate the eene mateial to the feiti HAZ. This assumes that, unde simle uniaxial loading, the HAZ is the egion of lowest ee esistane. A ee ak gowth ate exession of the fom of equation (8.3) is then used fo intefaial ak gowth. A and q should be detemined fom testing ee ak gowth seimens of standad geomety, suh as the omat tension (CT) seimen, using onventional test oedues. The seimen should be suh that the feiti to weld intefae egion lies in the lane of the ak in the test seimen. Cae should be taken that the state noth meets the aoiate intefae miostutue. The seimen should be extated whee ossible fom a full-size weldment so as to eodue auately the details of the feiti HAZ miostutue. It is not neessay, howeve, that the seimen ontains the austeniti base metal as defomation in the CT seimen, fo examle, is dominated by bending about the ligament ahead of the ak ti and hene by the mateials adjaent to the ak. The homogeneous exeimental alibation (η) fato * elating C to the measued load and ee dislaement ate should be used. This has been shown to give * a lose aoximation between exeimentally alulated and finite element values of C fo the bi-mateial CT seimen unde load ontol fo a ange of mismath in ee ates Seifi Modes of Caking Stess-elief aking in feiti weldments. Ciumfeential heat-affeted-zone (HAZ) and tansvese weld metal aking aise duing ost-weld stess elief heat teatment o vey ealy in the lant oeating life. Initiation of aking is vey deendent on mateials omosition, weldment miostutue and esidual stess; the latte two fatos being itially deendent on welding and heat teatment oedues. In tun, ak gowth is also deendent on miostutue and stess, both of whih vay initially as a funtion of osition and hange futhe as a funtion of time and temeatue as the weldment is exosed to lant onditions. The effet of multiaxial stess state on ee utue may need to be onsideed. Tye IV aking in feiti weldments. This mode of failue involves the initiation and gowth of iumfeential ee aks in the low temeatue extemity of the HAZ adjaent to the untansfomed aent mateial. It has been obseved fom times midway though the design life and onwads. Axial loading ove and above the nominal axial stess due to intenal essue is signifiant in omoting this mode of aking. The inteitially tansfomed egion is thin making it diffiult to ollet mateials data elevant to the inteitial egion and to efom multi-mateial stess analysis of the weldment whih inludes the thin zone. The auay of the assessment is often limited by the unetainty of the axial stesses ouing in ie wok. Tansvese weld metal aking in feiti weldments. This is a mode of aking whih has been enounteed in ie to ie weldments subjeted to edominantly intenal essue loading. Unde these onditions the weldments invaiably eah o exeed thei design lives by whih time the hoo stain aumulation initiates axial aks, tansvese to the weldments, in the moe oase-gained olumna egions of the weldments. This geneally involves multile ak initiation leading eventually to exessive defomation, bulging and the fomation of seonday iumfeential aks in the weld metal, whih esult in eventual failue FITNET 26 All ights eseved

29 (1 May 26) FITNET M7 Austeniti weldment aking. Solidifiation aking o stess-oosion aking mehanisms an give ise to defets ealy in life o at vaious times thoughout life, esetively, whih may oagate by ee in these weldments. In the absene of suh defets, the antiiated failue mode is tansvese weld metal aking. In addition stess-elief o eheat aking simila to that desibed fo feiti steels above an also ou in austeniti steels. This bittle integanula aking ous in the HAZ lose to the fusion line as a esult of the onentation onto gain boundaies of elaxation stains assoiated with stess elief, o the onentation of ee stains duing extended sevie. This stain onentation is due to stengthening within the matix of the gains esulting fom fine eiitate disesions on disloation netwoks. The mehanism aeas able to oeate in sevie at temeatues as low as 5 C, given suffiiently long times. The oensity to this tye of aking is geatest in the Nb stabilised Tye 347 steel, but it is also enounteed in the Ti stabilised Tye 321 steel. Deendent uon oeating temeatue and the level of esidual o alied stess a simila mehanism may also ou in Tye 316 steel, atiulaly high abon vaieties. It is howeve less likely to ou in nitogen stengthened low abon Tye 316 vaieties. Dissimila metal weldments. Failue of these weldments ous almost exlusively by iumfeential aking along the fusion bounday on the feiti mateial side of the weldment. The stesses aising due to the diffeenes in oeffiients of themal exansion lay an imotant ole in this mehanism of aking, as do oesses of ageing that give ise to eiitates foming along the feiti-austeniti intefae Teatment of Seonday Loading Fo ombined imay and seonday loading, if the initial esonse on loading is elasti, a total eene stess,, is used instead of that defined by equation (8.8) and is s = ( + )/ (8.29) whee, s ae the stess intensity fatos fo the imay loading and the seonday loading, esetively. This total eene stess may elax due to both ee staining and ak gowth and the ate of hange is given by s ( ) ( ) / ( a/ w) s / a/ w / a/ w a Ε ε + s s + = ( + ) w Ζ (8.3) fo a ak of deth a in setion width w, whee Z is the elasti follow-u fato and the ee stain ate is alulated at the total eene stess. Fo elasti-lasti esonse on initial loading, equation (8.13) needs to allow fo lasti stains and must also be genealized fo ombined imay and seonday loading. If it is assumed that lastiity and ee ae both desibed by owe-law equations, with ee stain given by equation (8.1) and lasti stain given μ by ε = β withμ = n, then an estimate of C(t) is n+ 1 n+ 1 () ( ε / ε ) = * n+ 1 ( ε / ε ) ( / Εε ) C t C (8.31) whee ε is the total stain at, * C es to the value evaluated fo the imay loading only, and the initial value of the total eene stess is, defined by equation (8.29) fo elasti esonse on initial loading. In ode to use equation (8.31), it is neessay to estimate ε, whih is the total elasti-lasti stain oesonding to. Fo elasti behaviou on initial loading, this is simly / E. Equation (8.31) may also be exessed as FITNET 26 All ights eseved 8-27

30 FITNET FFS M7 Cee Module 1/(1 q) () ( / ε ε, ε ) = * 1/(1 q) ε, ( ε / ε ) ( / Εε ) C t C (8.32) whee ε, and ε, ae the ee stain ates at equation (8.13) to ombined loading. and esetively, whih genealizes Fo ue imay loading, equation (8.31) an be witten e+ + e+ n+ 1 () ( ε / ε ) = * e+ + e+ n+ 1 e e+ ( ε / ε ) ( ε / ε ) C t C (8.33) whee suesits e, e+ and e++ denote elasti, elasti-lasti and elasti-lasti lus ee, esetively. This genealizes equation (8.13) to the ase when lastiity ous on initial loading. Fo ue imay loading, it is staightfowad to evaluate the stain tems in equation (8.33) as the eene stess is well defined fom the limit load exession of equation (8.8). Fo moe geneal loading, the initial stain tem may be obtained fom an estimate of the initial value of J, J. By analogy with equation (8.9) this is given by J = ε R (8.34) The oedues of Setion 6 may be used to estimate J and give + V ΕJ s = f ( L ) (8.35) whee V is the aamete teating inteations between imay and seonday stess and f ( L ) is defined by the failue assessment diagam. Then, J = s ( + V ) Εf 2 ( L ) 2 (8.36) Fom equation (8.34), P ( / ) ε 2 = s ( + V ) Εf 2 ( L ) 2 (8.37) o ε = ( ) ( 1 s / + V ) 2 2 Ε f 2 ( L ) (8.38) This may be used to define ε if the shae of the stess-stain uve is known FITNET 26 All ights eseved

31 (1 May 26) FITNET M Failue Assessment Diagam Methods Intodution The methods set out in Setion 8.2 fo assessing inubation and the ealy stages of ee ak gowth ae based on the evaluation of aametes inluding ak oening dislaement, δ, and the ak ti aametes C * and C(t) togethe with exeimental data desibing ee ak inubation o gowth. Howeve, fo low temeatue fatue assessment, the onet of a Failue Assessment Diagam (FAD) is used in Setion 6 to avoid detailed alulations of ak ti aametes. In eent yeas, FAD aoahes have been extended to the ee egime. The high temeatue Time Deendent Failue Assessment Diagam (TDFAD) method has been inooated into R5. A key equiement of TDFAD aoahes is the evaluation of a time deendent ee toughness, denoted. mat In Gemany, a simila Two Citeia Diagam (2CD) Aoah has been indeendently develoed to assess ee ak inubation in feiti steels. This aoah uses ak ti and ligament damage aametes, R and R, esetively, whih ae simila to the aametes and L used in the TDFAD. The itial stess intensity fato, Ii, is used as a measue of ak initiation esistane athe than the ee toughness,, used in the TDFAD aoah. Duing the ast two deades elevant mateials data have been obtained fo vaious oto and ast steels used in owe lant tehnology and the method has been alied to the assessment of defets in omonents suh as ast steel omonents. The TDFAD and 2CD methods ae desibed in this setion. Moe detailed infomation is ontained in the Bibliogahy, in atiula in R5 [8.1] and in Ewald et al. [8.29] Cee Cak Initiation Assessment Poedues TDFAD Aoah The TDFAD is based on the FAD seified in Setion 6 and involves a failue assessment uve elating the max two aametes and L, whih ae defined in equations (8.39) and (8.4) below, and a ut-off L. Fo the simlest ase of a single imay load ating alone = / (8.39) Iid mat mat whee Iid is the stess intensity fato and mat is the aoiate ee toughness value, and L = / (8.4).2 whee is the eene stess of equation (8.8) and.2 is the stess oesonding to.2% inelasti (lasti lus ee) stain fom the aveage isohonous stess-stain uve fo the temeatue and assessment time of inteest, see Figue 8.4. The failue assessment diagam is then defined by the equations E ε L = + L E ε 1/2 L (8.41) max L = L > (8.42) max L In equation (8.41), E is Young s modulus and ε is the total stain fom the aveage isohonous stess-stain uve at the eene stess = L.2, fo the aoiate time and temeatue. Fo modeate stesses and times, valid isohonous data should be available to evaluate equation (8.41) fo FITNET 26 All ights eseved 8-29

32 FITNET FFS M7 Cee Module a ange of stess levels (i.e. L values). At low stesses whee ee stains ae negligible, equation 2 1/ 2 (8.41) edues to = (1 +.5L ), whih is indeendent of time. Even at highe stesses the shae of the TDFAD is elatively insensitive to time and this with the low stess limit enables the TDFAD to be onstuted without a need fo auate data extaolation. Thus, equation (8.41) enables the TDFAD to max be lotted with as a funtion of L, as shown shematially in Figue 8.5. The ut-off, L, is defined as L = / (8.43) max R.2 whee is the utue stess fo the time and temeatue of inteest. Howeve, fo onsisteny with the R methods in Setion 6, the value of max L should not exeed /.2 whee is the shot-tem flow stess and.2 is the onventional.2% oof stess. As in Setion 6, may be taken as (.2 + u )/2 whee u is the ultimate tensile stength. A ental featue of the TDFAD aoah is the definition of an aoiate ee toughness whih, when used in onjuntion with the failue assessment diagam, ensues that ak gowth in the assessment eiod is less than a value Δa. Cee toughness values may be estimated indietly fom onventional ee ak inubation and gowth data o evaluated dietly fom exeimental load vesus dislaement infomation. This setion desibes the latte diet aoah fo evaluating ee toughness values. Diet aoahes fo detemining ee toughness ae based on exeimental load-dislaement data. Conside a load-ontolled ee ak gowth test onduted on a standad omat tension (CT) seimen. It is assumed that the amount of ak gowth in the test, Δa, is small, so that the total dislaement, Δ T, may be onveniently atitioned into elasti, lasti and ee omonents, denoted Δ e, Δ and Δ, esetively, whee Δ T =Δ e +Δ +Δ (8.44) Similaly, the total aea unde the load-dislaement uve, UT, may be atitioned into elasti, lasti and ee omonents, denoted U e, U and U, esetively, whee UT = Ue + U + U (8.45) Testing standads then give an exession fo the exeimental total J value and this may be used to give the ee toughness as = + E' η U + n U 2 mat Bn ( W a ) n+ 1 1/2 (8.46) whee W is the seimen width, a is the initial ak length, B n is the net seimen thikness, 2 lane stess and E = E/(1 ν ) fo lane stain onditions, and fo E = E η= (1 a / W) (8.47) fo CT seimens. Values of ee toughness, of ak gowth inement, Δa. mat, ae deived fom ee ak gowth tests as a funtion 8-3 FITNET 26 All ights eseved

33 (1 May 26) FITNET M Two Citeia Diagam In the Two Citeia Diagam (2CD) fo ee ak initiation the nominal stess nl desibes the stess situation in the ligament, i.e. in the fa-field of the ee ak and the elasti aamete Iid at time zeo haateizes the ak ti situation. These loading aametes ae nomalised in a 2CD (Figue 8.6) by the esetive time and temeatue deendent values, whih indiate the mateial esistane against ak initiation. The nomalised aametes ae the stess atio R / = (8.48) n l R fo the fa-field and the stess intensity fato atio R / = (8.49) I id Ii fo the ak ti. The value R is the ee utue stength of the mateial and the aamete haateizes the ee ak initiation of the mateial. This aamete has to be detemined fom seimens with a high atio l id/ n l, eably using CT25-seimens. The 2CD distinguishes thee fields of damage mode seaated by lines of onstant atio R / R. Above R / R = 2 ligament damage is exeted, below R / R =.5 ak ti damage is exeted and between these lines a mixed damage mode is obseved. Cak initiation is only exeted above a bounday line Comaison of Paametes A omaison an be made between the aametes in the TDFAD and 2CD. L an be omaed with R and an be omaed with R. The fist notieable diffeene is that L lies on the absissa of the TDFAD and R lies on the odinate of the 2CD. The eene stess,, an be omaed with the nominal stess, n l. The eene stess an be defined fo the aoiate stess state (lane stess o lane stain) and eithe Tesa o von Mises yield sufaes. Non-dimensional values of eene stess fo a CT-seimen fo two ases ae given below. Based on Tesa fo lane stess onditions li Bn ( W a) W a = P W + a W a W 2 [ 2 2 ( / ) 1 ( / )]. (8.5) Based on von Mises fo lane stain onditions Bn ( W a) W a = P W + a W a W 2 (2 / 3)[ ( / ) ( / )] (8.51) In a simila way, the nominal stess n l [12] fo a CT-seimen an be exessed non dimensionally as nl B n ( W a) Bn W a = P B + W a. (8.52) The eene stesses detemined aoding to lane stess using the Tesa iteion and aoding to lane stain using the von Mises iteion ae omaed with the nominal stess in Figue 8.7. The esults of lane stess (Tesa) ae losest to the values of nominal stess. n l FITNET 26 All ights eseved 8-31

34 FITNET FFS M7 Cee Module eesents the odinate of the R5 TDFAD and R eesents the absissa of the 2CD. These atios ae alulated by dividing the linea elasti stess intensity fato by the mateial inubation aametes mat and li, esetively. Hene, the ee toughness aamete itial stess intensity fato l id mat has to be omaed with the li. A omaison between the inubation aametes is shown in Figue 8.8 and Figue 8.9 fo a 1CMoV-steel at 55 C assuming a ee ak initiation length Δa i =.5 mm. In Figue 8.8 (lin-log-diagam) the ee toughness tends to beome equal to the itial stess intensity fato fo long times. Howeve this tend does not beome obvious in Figue 8.9 (log-log-diagam). The aamete.2 in the R5 TDFAD aoah is the stess to give.2% inelasti (lasti lus ee) stain at the assessment time, denoted the.2% inelasti stength. Figue 8.1 shows the vaiation of.2 and the utue stess R with time fo a 1CMoV-steel at 55 C. Fo omleteness, this figue also shows the vaiation in the stess to give 1% inelasti stain. Figue 8.11 shows the.2% and 1% inelasti stength values nomalised by the utue stess R. It an be seen that the atio /.2 R vaies fom.6 at 1 hous to.4 at 1 hous. The atio.1 / R tends to.75 fo long times. This value is the basis of the value of inteet on the odinate of the 2CD Comaison of the TDFAD and the Two Citeia Diagam In this setion a omaison of the R5 TDFAD and 2CD is desibed. As an examle data fo 1CMoV-steel at 55 C ae analysed. Data fom exeiments with diffeent seimen tyes (Comat Tension seimens, Double-Edge Nothed Tension seimens, DENT) wee used. The width of the seimens was u to 2 mm (CT) and 5 mm (DENT) with thikness of u to 1 mm (CT) and 6 mm (DENT). As the ak initiation iteion, a onstant ak length of ai =.5 mm was used. Figue 8.12 shows an examle fo the edition of ee ak initiation time with the TDFAD, a ut-off R/.2 on the L - axis was used. The edition of ee ak initiation time using the 2CD is shown in Figue Both aoahes have been shown to give onsevative editions of ee ak initiation ( a i =.5 mm) fo 1CMoV-seimens tested at 55 C, Figue Both methods tend to be most auate fo longe times with the level of onsevatism of 2CD eduing with test duation. A omaison of ee ak initiation times of both aoahes is shown in Figue In ode to omae the TDFAD and 2CD esults dietly it is neessay to tansfom R to using the following equations L and R to L R = = R.2.2 n l and (8.53) Iid Iid Ii = = R mat mat I id. (8.54) To tansfom L to R and to R the following equations may be used R R nl nl.2 = = L R R Iid Iid mat = = Ii Ii I id and (8.55) (8.56) 8-32 FITNET 26 All ights eseved

35 (1 May 26) FITNET M7 Equations (8.53) and (8.54) allow the TDFAD to be lotted on the 2CD in R - R sae as shown in Figue L sae. Both 8.16 fo 1CMoV-steel at 55 C. Figue 8.17 shows the 2CD lotted on the TDFAD in examles show that the TDFAD is a funtion of time. The omaison is based on lane stess Tesa eene stess solutions. The ut-off values fo the TDFAD ae based on / fo 1 to 1 hous. R.2 Futhe wok is equied to analyse the esults of the diffeent aoahes in detail and then aly both aoahes to assessment of inubation in stutual geometies The d Aoah The d aoah to editing ak inubation is based uon the methodology oosed by Moulin et al. [8.1] and inooated in the Fenh RCC-MR oedues (Aendix A16 [8.6]). It elies on editing the stess stain esonse of a defetive stutue at a defined distane, d, ahead of the ak ti. The method onedes that, fo all but bittle mateials, the stesses and stains obtained fom a onventional elasti analysis do not take into aount loal lastiity at the ak ti that would otentially ou if tue mateial esonse wee modeled. Fo ee, based uon the monotoni tensile stess-stain esonse fo a mateial, the method evaluates the loal stain amlifiation due to lastiity and ee at a distane d ahead of the ak ti. Clealy esonse at a oint d ahead of a ak will deend on mateial oeties as well as the imosed loading onditions. A key featue of the d aoah, theoe, is to define the itial distane, d, to be adoted duing analysis. This is invaiably ahieved exeimentally. Fo Tye 316 mateial Aendix A16 defines the distane d at 5 μm. Fo ee only loading, the method may be alied as follows: 1. Calulate the stess intensity fato assoiated with the ee load, P. This may be aomlished using omendia of stess intensity fato solutions o using detailed numeial models. 2. In the lane of the ak alulate the Rankine equivalent stess, de, that is the geatest elasti inial stess at a distane d ahead of the ak ti using, fo examle, Ceage s simlified exession: de = (8.57) 2πd 3. Using the Neube oedue, alied togethe with the aveage mateial monotoni tensile uve at the assessment temeatue, alulate the effetive elasti-lasti stess dell. Calulation of the elasti-lasti stess elies on maintaining enegy equivaleny between the idealised elasti stess stain esonse and that unde elasti-lasti onditions. Additional onsevatism may be intodued into the alulation by enhaning the total elasti stain ( ε 1 ) by additional lasti stain ( ε 2 ) aumulated unde the imosed imay eene stess : ε e = ε 1 + ε 2 = E de + (a) A 1/ β Theeafte, the effetive elasti-lasti stess ahead of the ak ti, ε T, ae detemined fom the enegy balane: (8.58) dell, and assoiated total stain, E de + ε 2. de = ε T. dell (8.59) FITNET 26 All ights eseved 8-33

36 FITNET FFS M7 Cee Module whee ε T and dell ae elated via the monotoni tensile uve. ε T = dell E + A edll 1/ β (8.6) 4. Calulate the stess utue lifetime, t CD, fo the mateial at the assessment temeatue and stess dell based uon best estimate (mean) mateial oeties. The utue time t CD is deemed to eesent the ak inubation time t i unde ee loading onditions. 5. Unde vaiable loading onditions, eah elevated temeatue dwell ( t h ) eiod may be onsideed based uon the above aoah with ak inubation infeed based uon the summation ove all dwell eiods as: t hi t =1 (8.61) CDi Futhe advie on aliation of the d oedues, inluding advie fo ee-fatigue loading is ontained in R5 [8.1], but it should be eognized that this is still a develoing aea, atiulaly fo aliation to mateials othe than austeniti stainless steels Bibliogahy [8.1] R5, Assessment Poedue fo the High Temeatue Resonse of Stutues, Poedue R5 Issue 3, Bitish Enegy, Gloueste, U (23). [8.2] BS791:1999, Guide on methods fo assessing the aetability of flaws in metalli stutues, inooating Amendment No. 1, BSi, London (2). [8.3] G A Webste and R A Ainswoth, High Temeatue Comonent Life Assessment, Chaman & Hall, London (1994). [8.4] R A Ainswoth, G G Chell, M C Coleman, I W Goodall, D J Gooh, J R Haigh, S T immins and G J Neate, CEGB assessment oedue fo defets in lant oeating in the ee ange, Fatigue Fat Engng Mate Stut 1, (1987). [8.5] R A Ainswoth, M B Ruggles and Y Takahashi, Flaw assessment oedue fo high temeatue eato omonents, ASME J Pes Ves Teh 114, (1992). [8.6] B Dubay, S Chauliot, M-H Laie and S Maie, A16: Guide fo defet assessment and leak befoe beak analysis, CEA Reot DM2S, SEMT/LISN/RT/1-43/A (21). [8.7] Ameian Petoleum Institute, Reommended Patie fo Fitness-fo-Sevie, API579 Daft Issue 12,1999; see T L Andeson and D A Osage, API 579: a omehensive fitness-fo-sevie guide, Int J Pes Ves Piing 77, (2). [8.8] D W Dean, R A Ainswoth and S E Booth, Develoment and use of the R5 oedues fo the assessment of defets in high temeatue lant, Int J Pes Ves Piing, 78, (21). [8.9] R A Ainswoth, D G Hooton and D Geen, Failue assessment diagams fo high temeatue defet assessment, Engng Fat Meh, 62, (1999). [8.1] D Moulin, B Dubay and D Ake, A atial method based on stess evaluation to initiation of aks unde ee and ee-fatigue onditions, Po ASME PVP 223, Pessue Vessel Fatue, Fatigue and Life Management, (1992) FITNET 26 All ights eseved

37 (1 May 26) FITNET M7 [8.11] R A Ainswoth and M C Coleman, Examle of an aliation of an assessment oedue fo defets in lant oeating in the ee ange, Fatigue Fat Engng Mate Stut 1, (1987). [8.12] B Dogan and R A Ainswoth, Defet assessment oedue fo low to high temeatue ange, ASME PVP Confeene, Cleveland, PVP Volume 463, (23). [8.13] ASTM E 1457-: Standad Test Method fo Measuement of Cee Cak Gowth Rates in Metals, Annual Book of ASTM Standads, 3.1, Philadelhia, PA 1913, Ameian Soiety of Testing and Mateials, (2). [8.14] ASTM E 139-: Standad Test Methods fo Conduting Cee, Cee-Rutue, and Stess-Rutue Tests of Metalli Mateials, Annual Book of ASTM Standads, 3.1, Philadelhia, PA 1913, Ameian Soiety of Testing and Mateials, (2). [8.15] ECCC Reommendations Volume 5 Pat I [Issue 4], Guideline fo the Exhange and Colletion of Cee Rutue, Cee Stain-Time and Stess Relaxation Data fo Assessment Puose, Aendix B1, ECCC-Doument, Ed.: S.R. Holdswoth, May (21). [8.16] High Temeatue Mehanial Testing Committee ESIS TC11 Issue 2a: A Code of Patie fo Conduting Nothed Ba Cee Rutue Tests and fo Inteeting the Data, by G.A. Webste, S.R. Holdswoth, M.S. Loveday, I.J. Pein and H. Pue, Novembe (21). [8.17] CEN Woksho Ageement: Codes of Patie fo the Detemination of Unetainties in Mehanial Tests on Metalli Mateials Pat 3: The Detemination of Unetainties in Cee Testing. [8.18] B. Dogan, B. Petovski,. Nikbin, U Ceyhan and DW Dean, Code of Patie fo Euoean Cee Cak Gowth Testing of Industial Seimens, Final Reot of CRETE Pojet, GRD2-CT-2-321, GSS:Geesthaht, Gemany, (25). [8.19] ESIS TC11 W.G. on High Temeatue Testing Welds: Euoean Code of Patie fo High Temeatue Cak Gowth Testing of Welds, ESIS TC11 WG: HTTW, by B. Dogan et al., Vesion 1 (25). [8.2] R6 Revision 4, Assessment of the Integity of Stutues Containing Defets, BEGL Poedue, 21. [8.21] R A Ainswoth, The Use of a Failue Assessment Diagam fo Initiation and Poagation of Defets at High Temeatues, Fatigue Fat. Engng. Mate. Stut., 16, , [8.22] D G Hooton, D Geen and RA Ainswoth, An R6 Tye Aoah fo the Assessment of Cee Cak Gowth Initiation in 316L Stainless Steel Test Seimens, Po. ASME PVP Conf., Minneaolis, 287, , [8.23] R A Ainswoth, DG Hooton and D Geen, Futhe Develoments of an R6 Tye Aoah fo the Assessment of Cee Cak Inubation, Po. ASME PVP Conf., Honolulu, 315, 39-44, [8.24] R A Ainswoth, DG Hooton and D Geen, Failue Assessment Diagams fo High Temeatue Defet Assessment, Engng. Fat. Meh., 62, 95-19, [8.25] D G Hooton and D Geen, The Detemination of Fatue Toughness Values fo Use with Time- Deendent Failue Assessment Diagams, AEA Tehnology Reot SPD/D(96)/579, [8.26] D W Dean and DG Hooton, A Review of Cee Toughness Data fo Austeniti Tye 316 Steels, BEGL Reot E/REP/GEN/24/, 23. [8.27] J Ewald and H eienbug, A Two-Citeia-Diagam fo Cee Cak Initiation, Po. Int. Conf on Cee, Tokyo, , [8.28] J Ewald and S Sheng, The Two Citeia Diagam fo Cee Cak Initiation and its Aliation to an IP-Tubine, Mateials at High Temeatues, 15, , FITNET 26 All ights eseved 8-35

38 FITNET FFS M7 Cee Module [8.29] J Ewald, S Sheng, A lenk and G Shellenbeg, Engineeing Guide to Assessment of Cee Cak Initiation on Comonents by Two-Citeia-Diagam, Int. J. Pes. Ves. Piing, 78, , 21. [8.3] ussmaul, Maile, J Baeiss, H loos, J Ganahe and R Tsheushne: Cee ak investigation of tubine steels with seimens of diffeent size, Po. of the ASME-Confeene, Pessue Vessel and Piing, Denve, USA (July 25-29, 1993) PVP-Vol. 266, Ed. Gaud, Y.S., (1993),. 119/26. [8.31] Maile, A lenk, J Ganahe, G Shellenbeg, M Tame: Cee and Cee Fatigue Cak Behavio of 1C and 9C-steels, Po. of CFEMS8, Tsukuba, Jaan, 1-5 Novembe, 1999, ey Engineeing Mateials, Vols (2). [8.32] Maile, H Theofel, C Weihet, H Maye, C Gedes, S Sheng, Assessment of hot teas in ast steel omonents, Advanes in Defet Assessment in High Temeatue Plant, 2nd HIDA Confeene, 4-6 Otobe 2, Stuttgat, ae S3-6. [8.33] Bitish Standads Institution, Fatue Mehanis Toughness Tests. Pat 4. Method fo Detemination of Fatue Resistane Cuves and Initiation Values fo Stable Cak Extension in Metalli Mateials, BS 7448: Pat 4: 1997, [8.34] Ameian Soiety fo Testing and Mateials, Standad Test Method fo J-Integal Chaateization of Fatue Toughness, ASTM E , FITNET 26 All ights eseved

39 (1 May 26) FITNET M7 ESTABLISH CAUSE OF CRACING. CHARACTERISE INITIAL DEFECT IS THERE EVIDENCE OF STRESS CORROSION CRACING, ENVIRONMENTALLY ASSISTED CRACING OR BUL CREEP DAMAGE? YES SPECIAL CONSIDERATIONS Ste 1 DEFINE PLANT HISTORY AND FUTURE OPERATIONAL REQUIREMENTS: STEADY SERVICE LOADS, TEMPERATURES ; OTHER LOADINGS ; LIFE TO DATE t o ; FUTURE LIFE REQUIRED, t s Ste 2 COLLECT MATERIALS DATA Ste 3 PERFORM BASIC STRESS ANALYSIS Ste 4 CALCULATE MARGIN AGAINST TIME-INDEPENDENT FRACTURE FOR INITIAL DEFECT SIZE Ste 5 MARGIN ACCEPTABLE TAE REMEDIAL ACTION YES IS CREEP OR FATIGUE SIGNIFICANT? FUTURE SERVICE ACCEPTABLE Ste 6 YES PERFORM DEFECT ASSESSMENT FLOWCHART OF FIGURE 2 o 3 Stes 7-11 PERFORM SENSITIVITY STUDIES Ste 12 ARE MARGINS SATISFACTORY? YES FUTURE SERVICE ACCEPTABLE FOR TIME, t s YES YES CAN MORE PRECISE CALCULATIONS BE PERFORMED? CAN MORE PRECISE MATERIALS DATA BE OBTAINED? REPORT RESULTS Ste 13 YES CAN SERVICE PARAMETERS BE DEFINED MORE ACCURATELY? TAE REMEDIAL ACTION Figue 8.1 Flowhat fo Oveall Cee Assessment Poedue FITNET 26 All ights eseved 8-37

40 FITNET FFS M7 Cee Module CALCULATE RUPTURE LIFE, t CD FOR INITIAL DEFECT SIZE Ste 7 t CD > t o + t s? YES CALCULATE INCUBATION TIME, t i MAE MODIFICATIONS FOR N-STEADY STATE CREEP HAVE MODIFICATIONS FOR N-STEADY STATE CREEP BEEN MADE? YES HAVE STEADY STATE CREEP CONDITIONS BEEN ESTABLISHED? YES Ste 8 t i > t o? YES t i > t o + t s? MAE MODIFICATIONS FOR N-STEADY STATE CREEP HAVE STEADY STATE CREEP CONDITIONS BEEN ESTABLISHED? CRAC WILL GROW IN SERVICE, CALCULATE CRAC SIZE AFTER GROWTH IN TIME t s o t o + t s - t i HAVE MODIFICATIONS FOR N-STEADY STATE CREEP BEEN MADE? YES CRAC GROWTH IN SERVICE Ste 9 YES YES RECALCULATE RUPTURE LIFE, t CD FOR FINAL DEFECT SIZE Ste 1 RECALCULATE MARGIN AGAINST TIME-INDEPENDENT FRACTURE FOR FINAL DEFECT SIZE Ste 11 PROCEED TO NEXT STEP IN FLOWCHART OF FIGURE 1 Figue 8.2 Defet Assessment Flowhat fo Insignifiant Fatigue 8-38 FITNET 26 All ights eseved

41 (1 May 26) FITNET M7 YES IS CREEP SIGNIFICANT? Ste 6 CALCULATE RUPTURE LIFE, t CD, FOR INITIAL DEFECT SIZE j= t CD > t o + t s? YES j= YES a j < fo yle j (1 j N)? CALCULATE SURFACE STRAIN RANGE, Δε t CALCULATE STRESS INTENSITY FACTOR RANGE, Δ eff CALCULATE ( da / dn ) FROM f Δε t CALCULATE ( da/dn) f FROM Δ eff IS CREEP SIGNIFICANT? IS CREEP SIGNIFICANT? Ste 9 YES YES ARE CREEP-FATIGUE INTERACTIONS SIGNIFICANT? HAVE ALLOWANCES BEEN MADE FOR EARLY CYCLES? YES YES SPECIAL CONSIDERATIONS, MAE MODIFICATIONS, CALCULATE D CALCULATE ( da/dn) FROM C* da/ dn = ( da / dn ) f (1-D ) -2 da da = dn dn f da/dn = (da/dn) f + ( da / dn ) j =j+1 j = j+1 j = j+1 CALCULATE INCREMENT OF CRAC GROWTH Δa j a j = a j-1 + Δa j j = total numbe of yles? YES YES CALCULATE RUPTURE LIFE FOR FINAL DEFECT SIZE IS CREEP SIGNIFICANT? CALCULATE MARGIN AGAINST TIME-INDEPENDENT FRACTURE FOR FINAL DEFECT SIZE Ste 1 Ste 11 NEXT STEP IN FLOWCHART OF FIGURE 1 Figue 8.3 Defet Assessment Flowhat fo Signifiant Fatigue FITNET 26 All ights eseved 8-39

42 FITNET FFS M7 Cee Module Stess Stess-Stain Cuve (t = ) ε e Isohonous Cuve (t > ).2 ε e.2.2 ε Total Stain Figue 8.4 Shemati Isohonous Stess-Stain Cuves Failue Assessment Line (FAL) UNSAFE 3CMoNiV4-11/AMA T = 55 C t = hs t = 1 hs t = 1 hs t = 1 hs.4 SAFE L 2.5 Figue 8.5 Shemati Time Deendent Failue Assessment Diagam based on Data fom a 1CMoVsteel at 55 C 8-4 FITNET 26 All ights eseved

43 (1 May 26) FITNET M7 1.4 R = 1.2 n l / R 1..8 ligament damage mixed damage CRAC T = onst..6.4 CRAC R / R = 2 ak ti damage.2 R / R =.5 bounday line fo B < 5 mm bounday line fo B 5 mm R 2.5 = I id / Ii 3. Figue 8.6 Two Citeia Diagam fo Cee Cak Initiation fo Cee-Dutile Steels 12 NDF 1 n l n TRESCA_stess n MISES_stain NDF... Non-Dimensional Fato a / W.7 Figue 8.7 Comaison of Refeene Stess and Nominal Stess fo CT-seimens FITNET 26 All ights eseved 8-41

44 FITNET FFS M7 Cee Module 1 Ii, mat 9 (MPa m 1/2 ) 8 7 3CMoNiV4-11/AMA T = 55 C Cs25-seimens a /W =.55 Δa i =.5 mm 6 mat = log(t i ) Ii = log(t i ) t i (h) 1 5 Figue 8.8 Cee Cak Initiation Time fo Cs25-seimens, 1CMoV-steel at 55 C 1 9 Ii, 8 mat (MPa 7 m 1/2 ) 6 mat = 3. t -.24 i Ii = 142. t i CMoNiV4-11/AMA T = 55 C Cs25-seimens a /W =.55 Δa i =.5 mm t i (h) 1 5 Figue 8.9 Cee Cak Initiation Time fo Cs25-seimens, 1CMoV-steel at 55 C 8-42 FITNET 26 All ights eseved

45 (1 May 26) FITNET M (MPa) CMoNiV4-11/AMA T = 55 C = A log(t)+b A B R t (h) 1 5 Figue 8.1 Comaison of ee stength with utue data fo 1CMoV-steel at 55 C 1. SRR CMoNiV4-11/AMA T = 55 C.1 SRR 1. = 1. / R SRR.2 =.2 / R t (h) 1 5 Figue 8.11 Stength/Rutue Ratios (SRR) fo 1CMoV-steel at 55 C FITNET 26 All ights eseved 8-43

46 FITNET FFS M7 Cee Module 1,4 = 1,2 I / mat 3CMoNiV4-11/AMA T = 55 C 1,,8,6 D6-seimen a /W =.4 I = 16.3 MPa m 1/2 n = 96 MPa n l = 96 MPa Tesa = 96 MPa Mises = 96 MPa = 96 MPa,4,2 Δa i = onst. =.5 mm t i measue 127 hs t i TDFAD 56 hs,,,5 1, 1,5 L 2, = /.2 2,5 Figue 8.12 Pedition of ee ak initiation time fo Δa i =.5 mm by using the TDFAD, 1CMoVsteel at 55 C 1.4 R = n 1.2 l /R u/t/t 1..8 ligament damage CRAC Δa i = onst. =.5 mm t i measue 127 hs t i 2CD 115 hs mixed damage 3CMoNiV4-11/AMA T = 55 C D6-seimen a /W =.2 I = 16.3 MPa m 1/2 n l = 96 MPa.6 R /R = 2 t = t i 2CD.4.2 t = t CRAC R /R =.5 ak ti damage bounday line fo B < 5 mm bounday line fo B 5 mm R 2.5 = I id / Ii 3. Figue 8.13 Pedition of ee ak initiation time fo Δa i =.5 mm by using the 2CD, 1CMoV-steel at 55 C 8-44 FITNET 26 All ights eseved

47 (1 May 26) FITNET M7 1 5 t i TDFAD (h) 1 4 3CMoNiV4-11/AMA T = 55 C seimens: Cs25, Cs5, CT1 D15, D3, D6 Δa i = onst. =.5 mm 1 5 t i 2CD (h) 1 4 3CMoNiV4-11/AMA T = 55 C seimens: Cs25, Cs5, CT1 D15, D3, D6 Δa i = onst. =.5 mm t i TDFAD.5 t i fato of satte band: t i (h) 1 5 t i 2CD.4 t i fato of satte band: t i (h) 1 5 Figue 8.14 Comaison between exeimental and edited ee ak initiation times by using TDFAD (a) and 2CD (b) on diffeent seimen tyes fo Δa i =.5 mm, 1CMoV-steel at 55 C 1 5 t i 2CD (h) 1 4 3CMoNiV4-11/AMA T = 55 C seimens: Cs25, Cs5, CT1 D15, D3, D6 Δa i = onst. =.5 mm t i 2CD.9 t i TDFAD fato of satte band: t 1 4 i TDFAD (h) 1 5 Figue 8.15 Comaison between edited ee ak initiation times by using TDFAD and 2CD on diffeent seimen tyes fo Δa i =.5 mm, 1CMoV-steel at 55 C FITNET 26 All ights eseved 8-45

48 FITNET FFS M7 Cee Module 1.4 R = 1.2 n l / R 1..8 ligament damage T = onst. mixed damage CRAC 2CD bounday line fo B < 5 mm bounday line fo B 5 mm TD-2C-FAD t = 1 hs t = 1 hs t = 1 hs CRAC R / R = 2 R / R =.5 ak ti damage R 2.5 = I id / Ii 3. Figue 8.16 TDFAD in tems of 2CD, shemati fo 1CMoV-steel at 55 C TDFAD t = 1 hs t = 1 hs t = 1 hs TDFA-2CD t = 1 hs t = 1 hs t = 1 hs.6 UNSAFE.4.2 SAFE L 2.5 Figue CD in tems of TDFAD, shemati fo 1CMoV-steel at 55 C 8-46 FITNET 26 All ights eseved