fatigue_in_lwrs_va inox 304.pdf

FATIGUE in LWRs

International Conference on Plants Materials Degradation

November 18-20 , 2008

J. MENDEZ (LMPM-ENSMA) J.M. STEPHAN (EDF)

18-20 Novembre 2008 EDF / MAI - ENSMA Poitiers2

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PlanPlan

Solicitation and Materials

Codes and Rules

Questions to solve

New Knowledge in Fatigue Behavior of austenitic stainless steel :

High cycle fatigue and fatigue limit

Interaction between Low cycle and High cycle fatigue

Environmental effects


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FatigueFatigue

Fatigue is defined as a term which "applies to changes in properties which can occur in

a metallic material due to repeated application of stresses or strains, although usually this term applies specially to those changes which lead to cracking or failure"

(General Principles for Fatigue testing of Metals, 1964)

Fatigue is an ageing process !


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FatigueFatigue

Fatigue is an important cause of incidents in NPPs :

Vibrations, Thermal stratification, Vortex in dead legs, Thermalmixing

Fatigue is cited in life extension of NPPs :

Residual life

Upper probability of occurrence due to the accumulation of repeated loadings


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Origin of Fatigue in NPPsOrigin of Fatigue in NPPs

Mechanical loadings :

Vibrations (70%)

Thermal loadings

Initially, fatigue incidents were due to large thermal loadings (Thermal Shocks, Stratification)

Nowadays, thermal fatigue problems concern small random thermal cycles due to mixing or vortex


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Solicitations and MaterialsSolicitations and Materials Mechanical Solicitations

Vibrations

Most frequent fatigue type occurrence : ~70%

High cycle fatigue failure

Mechanical or flow induced vibration loads

Short time, during plant startup or soon thereafter, or after aging (wear, clearance) Design configurations, modifications, BWR power uprates

Locations

Small bore pipes

Tube bundle (Alliage 600) Main steam systems nozzles for measurements

Valves steam : when not tight close or opened : Valve relief "chattering"


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Solicitations and MaterialsSolicitations and Materials Thermal Solicitations

Thermal shocks following modification of plant condition

Not frequent but with high amplitude of temperature (270C) Not localized (hitting the whole component) Vessel, pipes

CVCS nozzle, SIS nozzle, Surge line (AISI 304, CF3, CF8)


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Thermal Stratification

Inherent under some operating conditions (low flow rates) Possible high stress ranges during flow rate variations Dependence on

bearing modes

Vessel, pipes : Surge line (AISI 304), Feedwater Flow Control Systems (A42, A48, 16MND5)


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Solicitations and MaterialsSolicitations and Materials

Conclusion on large thermal loads :

Possible measurements on sites (outer wall)Now covered by existing rules and codes


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Vortex

Only damaging, if associated with abnormal conditions (valve leakage) Except in the case of non-insulated small diameter pipe

Pipes : "Dead Legs" : SIS, RHR (AISI 304), small pipes linked to Primary Circuit


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Turbulent mixing

In general, localizations where thermal fluctuations can take place (piping mixing zones, pumps..)

Inherent under functioning conditions of some systems (pipe junctions, pumps) Possible large damages

Pipes : RHR (AISI 304, 316 ), CVCS (AISI 304, CF8, CF3 ), Pumps (AISI 304, 316, AISI 410)


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Solicitations and MaterialsSolicitations and Materials

Conclusion on high cycle thermal loads :

No direct measurements on sites due to high frequencies involved

Large non linear stresses gradient

Complexity : links between thermal-hydraulics, mechanics (initiation and fracture), materials and plant operations

Problem of multi-cracks

Combination with low cycle fatigue : damage accumulation


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Codes and RulesCodes and RulesASME Code Section III and VIII :

Recent proposed developments :

Revision of fatigue strength curves and margins

Explicit inclusion of environmental effects

Possibility to apply an "initiation + crack growth approach"

can have wide implications in industry

KTA = ASME


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Codes and RulesCodes and RulesRCCM :

Derived from ASME but with modifications :

Ke optimisation for stainless steels

Ke for mechanical loads and possible Keth for thermal loads

K for mixing zones

Crack like defects fatigue analysis method

JSME :

Some dedicated standard for low and high cycles fatigue

WWER PNAE Code


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Fatigue : Main needsFatigue : Main needs

Definition of screening values with simple parameters

Guidelines to account for complex loadings like random loadings (associated or not with low cycle loads)

Improvements in propagation threshold

Material aspects : Improvements in fatigue curves to take into account surface finishes, mean stresses, welds, residual stresses, environment

Probabilistic aspects in fatigue : random loads

Interaction with variations in metal conditions (ageing, irradiation)


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Research ProgramsResearch ProgramsLast decades :

Low cycles fatigue (Thermal shocks, Stratification) in pipes, nozzles, pumps

Now :

Major programs devoted to thermal cycling in mixing areas and vortex in dead legs

Thermal-hydraulics

High cycles mechanical testing

Material studies : T, Surface finishes, mean stres ses, environment

Research at the micro and meso level : Dislocations, Aggregates


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Questions to solve : Thermal HydraulicsQuestions to solve : Thermal Hydraulics

Experimental :

Quality of on site measurements (external wall) or mock-up measurements (fine measurements on the surface)

Transposition methodologies

Numerical :

Quality of numerical simulations :

R.A.N.S. simulation (mean thermal-hydraulic flows) Large Eddy simulations (LES) (temporal description)

Evaluation of heat exchange coefficients


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Questions to solve : MaterialsQuestions to solve : Materials

Material behaviour (plasticity ; strain-stress curves) Initiation Multi axial loading

Initiation - Treatment of surface finishes : roughness and strain hardening (pre loading): How to determine their levels of influence on fatigue

Initiation - Treatment of mean stress effect (in case of large mean stress) Initiation - Damage accumulation with variable amplitude loading

Initiation - Environmental effects

Transposition to on site functional situation

Validation of Fatigue initiation criteria


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Questions to solve : MaterialsQuestions to solve : Materials

Propagation :

Propagation Threshold

Short cracks

Overload - Variable amplitude loadings

Multi axial loadings


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New knowledge acquired in the recent years on some aspects of the fatigue behavior of austenitic stainless

steels (304L, 316L)New open questions

New knowledge acquired in the recent years on some aspects of the fatigue behavior of austenitic stainless

steels (304L, 316L)New open questions

High cycle fatigue and fatigue limit

effects of surface finish

effects of mean stress

Interaction between LC and HC Fatigue

Environmental effects

Crack initiation and crack growth processes Illustrations from 304L or 316L fatigue data

ENSMA studies in the frame of the PhD of S. Petijean (2003) (EDF, AREVA N.P.)

Y. Lehericy (2007) (AREVA N.P.)L. De Baglion (AREVA N.P.)

examples about :


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Even at very low cyclic strain amplitudes in the HCF range, near the fatigue limit, 304L exhibits a significant plasticity at the macroscopic level

J. C. Le Roux, 2004

Main characteristics of 304L austenitic stainless steel behaviour with regard to fatigue behaviour

a high ductility leading to fatigue limit (195-200 MPa) close to the conventional yield stress


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INFLUENCE OF SURFACE FINISH PARAMETERS ON THE HIGH CYCLE FATIGUE BEHAVIOUR OF A 304L AUSTENITIC STAINLESS STEEL

INFLUENCE OF SURFACE FINISH PARAMETERS ON THE HIGH CYCLE FATIGUE BEHAVIOUR OF A 304L AUSTENITIC STAINLESS STEEL

Machining or surface operations lead to microstructural and mechanical modifications:

What are the most significant factors that have to be taken into account? :

Roughness : relevant parameters?Residual stresses: relaxation?

Surface layer microstructure modification: delays or accelerates crack initiation and growth?

Coupled effects: How to identify the predominating factors?

The study of several surface preparation conditions permitted to establish the role of surface finish and identify the relative effect of each factor.

(S. Petitjean, ENSMA 2003, EDF and AREVA (Framatome) collaboration)

What are the effects of these modifications?


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Machining conditions of some selected surface preparations(polishing, turning and grinding)

Machining conditions of some selected surface preparations(polishing, turning and grinding)

And two polished sample conditions :after fine turning (T5), a mechanical polishing using abrasive papers of grades 320, 500, 1000, 2400 and 4000 followed by diamond sprays of 3 m and 1 m is performed.

Polished P1: only the surface irregularities are suppressed; the hardness gradient is preservedPolished P2: :the hardened layer is also taken off.

Samples Tool radius (mm)Feed rate (mm/rd)Speed (rd/mn) Depth of cut (mm)Turned T1 0,4 0,2 1600 0,2Turned T2 0,8 0,2 1600 0,2Turned T3 0,8 0,5 1120 0,2Turned T4 0,8 0,7 560 0,2Turned T5 0.8 0.7 560 0.2

SandBlasted Silica particles pres. 4 barsGround G1 - Manual 800 Grindstone tangential


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Typical roughness profiles of some surface preparationsTypical roughness profiles of some surface preparations

roughness of sample Turned T2

-32

-22

-12

-2

8

18

28

0 0,5 1 1,5 2 2,5 3 3,5 4length (mm)

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m

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roughness of sample Polished P1

-32

-22

-12

-2

8

18

28

0 0,5 1 1,5 2 2,5 3 3,5 4length (mm)

r

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roughness of sample Turned T3

-32

-22

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-2

8

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0 0,5 1 1,5 2 2,5 3 3,5 4

length (mm)

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roughness of sample Ground G1

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Typical appearence of the surface of some 304 L cylindrical fatigue specimens

Typical appearence of the surface of some 304 L cylindrical fatigue specimens

2 mm 2 mm

Turning T3 sandblasting

Coarse grinding polishing


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Turned T3rugosit de l'tat tourn 1b

-32

-22

-12

-2

8

18

28

0 0,5 1 1,5 2 2,5 3 3,5 4

longueur (mm)r

u

g

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s

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t

(

m

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Echantillon tourn T3

longueur (mm)r

u

g

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(

m

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With regard to fatigue, a 3D characterization of roughness is required

Sandblasted

- craters distributed on the specimen surface - mean dimensions: 20 m in depth, 100 m in diameter

-32

-22

-12

-2

8

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28

0 0,5 1 1,5 2 2,5 3 3,5 4length (mm)

r

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(

m

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rugosit de l tat sabl

longueur (mm)

r

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m

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roughness of sample Ground G1

-32

-22

-12

-2

8

18

28

0 0,5 1 1,5 2 2,5 3 3,5 4

rugosit de ltat meul cylindrique

longueur (mm)

R

u

g

o

s

i

t

(

m

)

2 mm

sets of facets Lmean = 0.5 / 0.6 mm.

crossed by straight grooves

Ground

samples


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Associated gradient of hardness in the near surface layer for seven selected sample preparations

Evolution of microhardness Vickers HV25

180

200

220

240

260

280

300

320

340

0 100 200 300 400 500 600depth (m)

m

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2

5

Turned T1Turned T2Turned T3Turned T4Polished P1Polished P2Ground G1


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Surfacemicrostructuremodifications:recristallisation, mechanical twinning, high density of

dislocations, martensitic transformations

600 nm

colle

fine recristallisation, dislocations distorded twins and dislocations twins and dislocations(turned, sandblast) (severe turning) (turned, sandblast, polished)

phase transformations ( )


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Residual stress profile

Profil des contraintes rsiduelles en fonction de la profondeur

-400

-300

-200

-100

0

100

200

300

400

0 20 40 60 80 100 120Profondeur (m)

C

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Tourn 1b

Evolution des contraintes rsiduellesen fonction de la profondeur

Tourn T3

- Specimens turned and ground: tensile residual stresses at the specimen surface

- Specimens polished or sandblasted compressive residual stresses

Example of residual stresses profile as a function of depth ( condition Turned T3)

Tensile residual stresses over the first 50 mCompressive below


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Condition Ra Rt HV25at 30 m

depth

Depth of the hardened layer

(m)11111111 ((((Pa) 22 22 22 22 (MPa)

phase

(m) (m) transform.?T1 1,6 7,2 323 250 235 300 -T2 1.94 8.85 316 200 220 260 -T3 9.6 38.1 337 250 395 385 -T4 - 78.3 345 250250 375 630 Yes

Polish.P1 0,34 1,44 318 150 -190 -185 YesPolish.P2 0,34 1,44 212 30 -240 -240 YesGround 6,3 36,4 345 200 320 480 -

S.Blasted 3.65 19,85 343 150 -770 -730 Yes

Summary of the surface characterisations of the selected samples by turning, grunding, blasting and polishing

Several combinations of roughness, residual stresses, hardeness and microstructures

Used to identify the relative role of each factor on the fatigue properties


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- Fatigue tests under constant load amplitudes control

* S-N curves in air at T = 25C under a constant load ratio (R = 0.05) or at constant mean (mean = 0, 60, 125, 195 MPa)

Seven surface preparations investigated:

3 Turned, 1 sandblasted, 1 severe ground, 2 polished


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S-N curves (f = 10 Hz, R = 0.05 and T = 25C) of samplespresenting the selected surface preparations

S-N curves (R = 0.05, T = 25C)

120

130

140

150

160

170

180

190

200

210

220

230

240

1,00E+05 1,00E+06 1,00E+07

Number of cycles

/

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(

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P

a

)

Turned T1

Turned T2

Turned T3

Turned T4

Polished P1

Ground G

Sandblast SB

Polished and sandblast

Turned

Ground


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S-N curves (f = 10 Hz, R = -1 and T = 25C) of samples presenting some selected surface preparations

S-N curves (f = 10 Hz, R = -1 and T = 25C) of samples presenting some selected surface preparations

Whler curves (R = -1, T = 25C)

160170180190200210220230240250260270280

1,00E+04 1,00E+05 1,00E+06 1,00E+07Number of cycles

/

2

(

M

P

a

) Turned T1Turned T4Polished P1Ground G1


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- A strong influence of the surface finish on the fatigue limit at R = 0.05 and T = 25C:Ground samples ( 125 MPa) < Turned samples ( 155 MPa) < Polished samples ( 195 MPa)

-effects of the surface finish are less pronounced at R = -1 Ground samples ( 175MPa) < Polished samples ( 195 MPa)

A non-influence of the mean stress was commonly assumed for austeniticstainless steels.

Are the results influenced by performing fatigue tests underload control?


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INTHERPOL results

100

1000

10000 100000 1000000

Number of cycles

S

a

l

t

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Courbe RCC-Mcourbe moyenneESSAI1 Knu_minEssai4 dlardageEssai4 cote 135 mm (bross)Essai4 cote 150 mm (bross)Essai4 cote 175mm (meul)Essai2 soudureEssai2 PmaxEssai3 soudureEssai3 Pmax

taper

current area

raw

tournsoft grinded

polished

F. CURTIT, EDF R&D / MMC


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Gradual elongation observed at max = 410 MPa

020406080

100120140160180200220240260280300320340360380400420

0 0,009 0,018 0,027 0,036 0,045 0,054 0,063 0,072 0,081 0,09elongation

I

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Gradual elongation observedat R = 0.05 (mean = 215 MPa) and f = 10 Hz

F

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p

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elongation

Allongement des prouvettes cycles m = 215 MPa (max = 410 MPa) f = 10 Hz

Allongement

F

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p

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maximum applied stress(MPa)

elongation (%)after 600

cycles

elongation at failure or at the fatigue limit (%)

420 10.7 11.9 (P1, failure at 289 000 cycles)

410 9.4 -

400 8.8 -

380 5.3 -

350 3.8 -

330 2 2.9 (T3, failure at 572 000 cycles)

310 1.4 -

280 - 1.2 (G, failure at 806 000 cycles)

260 - 0.5 (G, un-failed at 10 millions of cycles)

progressive elongation during the first cycles then accommodation-specimens present different levels of pre-deformation according to the

applied load level

Fatigue tests at R = 0.05 (T= 25C)


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-Elongation is very low at the fatigue limit level for ground specimens.-In this case it can be concluded about the effect of the mean stress (with regard to theresults at R= -1): a detrimental effect of a positive R is revealed.

Influence of the mean stress on the fatigue limit (T= 25C)

Sample preparation

Fatigue limit (MPa) R = -1

Fatigue limit (MPa)

R = 0.05 (elongation)

Polishing 200 185 (5.3%) Sandblasting - 185 (5.3%)

Turning 190 / 200 150 / 155 (2.5%) Grinding 170 125 (0.5%)


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Diagram of Haigh

Evolution of the fatigue limit versus the applied mean stress

100110120130140150160170180190200210

0 25 50 75 100 125 150 175 200mean stress (MPa)

/

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tat politat meulpo

Fatigue limit of ground specimens is decreased by 50 MPa when a high positive mean stress is applied

Fatigue limit of polished specimens isweakly modified by the applicationof a mean stress

In fact :

The detrimental influence of a positive mean stress on polished specimens is

compensated by the hardeninginduced by cycling the materialunder load control

polishedpolishedground

Additional results for other mean values


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- great influence of the surface finish at R = 0.05 and T = 25C :Ground samples ( 125 MPa) < Turned samples ( 155 MPa) < Polished samples ( 195 MPa)

-effects of the surface finish are less pronounced at R = -1 :( 175 MPa/195 MPa)

Validity of the reference curve for polished specimens since establishedunder load control and high mean stresses?


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Such results led to reconsider HCF data of austenitic stainless steels

In particular the role of control parameter in fatigue tests hasbeen reexamined :

In most conditions, strain rather than stress amplitude has to be imposed in order to be more representative of the real mechanical solicitations in components

L. Vincent et al, CEA (DMN SRMA ) confirmed the detrimental effect of a mean positive stress for 304L polished specimens

by performing tests under constant


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160

170

180

190

200

210

220

0 50 100 150 200 250

Nf>5.106 cyclesNf


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Related effect of a monotonic or a cyclic predeformation and the application of a positive mean stress

Related effect of a monotonic or a cyclic predeformation and the application of a positive mean stress

detrimental effect of prehardening in strain

controlled tests particularly with a positive mean stress

beneficial effect under stress control

Courbes de Whler des tats pr-crouis (R = 0.05, T = 25C)

150

160

170

180

190

200

210

220

230

1,00E+05 1,00E+06 1,00E+07Nombre de cycles

(

/

2

)

0

a

p

p

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Etat P1pr-croui

Etat Mpr-croui

Results are mainly affected by the parameter of control used for the fatigue

tests

[V. Doquet, S. Tahiri]

[S. Petitjean, J. Mendez]

Results recently confirmed on monotonically pre strained specimens by EDF/ECP PhDThesis


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Relative role of roughness, surface hardening and residual stresses:

Thermal treatments were applied at different temperature levels to relax residual stresses or even to eliminate the hardened surface layer.

Main results:

- a poor influence of residual stresses which is explained by the relatively high plasticity level reached in 304L even for HCF

- a limited effect of surface hardening .

- a predominant effect of the surface topography:

the acuity, length, depth and orientation of the machining grooves are the important specific features with regard to fatigue damage. Therefore, conventional roughness parameters (Ra, Rt ) are not relevant to predict the fatigue behavior of machined or surface treated samples


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Damage processes and fatigue limit

For polished specimens: no cracks are observed on non-failed specimens after 107 cycles.

Fatigue limit is in this case clearly associated with non initiation conditions.

In ground specimens the number of cycles to form a crack of the size of the groove is very similar to the number of cycles needed to propagate it to failure.

Fatigue limit associated with the effective K threshold obtained for long cracks.


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fissures de fatigue

0

500

1000

1500

2000

2500

0 100000 200000 300000 400000 500000Nombre de cycles

l

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Etat T5 -smax = 330MPa

Etat M - smax= 290 MPa

Amorages multiples et coalescence

Crack growth of natural short cracks in turned or ground specimens (R = 0.05)

Initiation on two connected machiningdefaults (turned sample)

Multiple cracking and coalescence processes (ground specimen)

multi cracking

Evolution of crak length with the number of cycles


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Curves established for max = 290 MPa and max = 330 MPa are superimposed to theda/dN-Keffective curvefor long cracks

R=0,05tTurned T5

cycledunder

330 MPa

R=0,05 -Ground cycled

290 MPa

Crack propagation curves R = 0.1 ( CT samples) and R = 0.05 (cylindrical specimens)

- R = 0.05 (cylindrical specimen turned T3 (max = 330 MPa)- R = 0.05 (cylindrical specimen : ground - max = 290 MPa)

Crack propagation curves of 304L at RT

1,E-08

1,E-07

1,E-06

1,E-05

1,E-04

1,E-031 10 100

K (MPa.rac(m))

d

a

/

d

N

(

m

m

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c

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K (MPa. m )

R = 0,1 -

CT -nominal

R=0,1-

CT-effective

- R = 0.1 (CT specimen) (C. Sarrazin-Baudoux et J. Petit - 2001)

But K is not the accurateparameter when plasticity is

high


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Needs: Control the residual HCF properties of ASS in presence of a fatigue predamage due to the occurrence of solicitations of high amplitude (stops and starts, power increase events)

Evaluate the effect of the surface finish

Objectives and Procedures

Influence of a LCF pre-damage on the 304Lstainless steel fatigue limit

Study of the 304L behavior loadedunder LCF conditions

Cyclic behavior

LCF induced damage

Fatigue limit of pre-damaged samples

Damage accumulation

Influence of surface finishEffect of mean stressRelevant physical or mechanical parameters for the prediction of the fatigue life.typically t/2 = 0,3%

In the HCF range


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1

2

4

3

Test conditions : t/2 = 0,3% R=-1f=0,333 Hz T=25C

NF = 25 000 cycles

3750 cycles 7500 cycles

Crack initiation 4000 cycles

(polished specimens with hardened surface layer)


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N = 7 500 cycles (30%de NR) (three different specimens)

Scattering associated with the crack length atthe initiation

Initiation at twin boundaries :80 m < L0 < 150 m

The hardened layer delays crack initiation but accelerates crack growth


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Crack growth described by a Tomkins et Wareign type law:

dL/dNp = C . (p/2) . L

L = L0 . exp (C . (p/2) . Np)

Material constantset Na ~ 4 000 cycles

with Np=N - Na

Identification of C and from experimental data :

L0 = 80 mp/2 = 0,175 %Lf = 4 500 m

C = 6,5 .10-3 = 2

L = L0 . exp (6,5.10-3 . (p/2)2 . Np)

L

n

(

L

/

L

0

)

Polished specimens with hard surface layer(200 m)

Nombre de cycles en propagation


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Effect of surface grinding on LCF resistanceEffect of surface grinding on LCF resistance

polishedsevere grindingspherical grinding

Polished NF 25 000 cycles

Severe grinding NF 7 500 cycles

Spherical grinding NF 20 000 cycles


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304L surface, ground with a spherical grindstone

The groove orientations are randomly distributed on the surface.The size of grooves, perpendicular to the loading direction in which cracks preferentially initiate, are in this case similar to the microstructure dimensions.


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496621000 cycles

785311000 cycles

267951000 cycles

425101000 cycles

depth (m)Surface length (m)Pre cycling

multi-cracking initiation along one straigth groove

N = 1 000 cycles

Weak scattering on themain crack length on four specimens

The main crack already reaches 662 m surface length and 49 m in depth

Effect of a severe grinding on crack initiation and growth


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Crack initiation under LCF

Decrease of the fatigue limit

Even if the initiation of a crack is not achieved, precycling can produce a smalldecrease in the fatigue limit of the 304L due to induced cyclic softening

precycling


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Initial crack depth m

F

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m

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t

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P

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Kyh, eff = 2,1 0,4 MPa.m-1/2

K=Y..(pia)1/2

aYK effth

Dpi

,

=

Fatigue limit predicted by the K effective threshold approach


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Plasticity threshold 180 MPa

/2 > 190 MPa Crack growth controlled by p/2

/2 < 180 MPa Crack growth controlled by Keff

K effective approch40m


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Other subjects

LCF behavior and damage mechanisms in LWR environment

Role of temperatureRole of environment

Always in relation with acceptable surface preparation for industrial components


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Environmental effects on fatigue behavior of Reactor Materials in LWR environment.

Many studies have shown that fatigue lives of reactors materials are reduced in LWR environment when compared to those obtained in air at high temperature.

For austenitic stainless steels, fatigue lives in LWR environment depend on 3 key parameters:- Strain rate- Dissolved Oxygen content- Temperature

These parameters are taken into account to evaluate fatigue life correction factor Fen for austenitic stainless steels in reactor coolant environment (expression based on ANL model):

*Fen = Nair(RT) / Neau(LWR) =

*[Chopra et al.; 2007; NUREG/CR-6909]


LMPMEffects of temperature

The LWR environment effect is enhanced at high temperature.LCF test results obtained on a 304L SS in PWR environment and in Air, at two temperatures 150 C and 300C :fatigue life in water, is strongly reduced at 300C and not so much at 150C even for some LCF tests performed at high strain rates

[Solomon, Amzallag et al.; 2004; PVP Seville]


LMPM

Effects of environment

For Austenitic Stainless Steel, comparison of LCF test results obtained in air at 300C and in deaerated water at high temperature (sa me strain rate and strain amplitude) :

- Same cyclic stress behavior.- Decreasing fatigue life in water environment.

[Cho et al.; 2008; Materials Science and Engineering (248-256)]


LMPMEffects of strain rate

[Chopra et al.; 2007; NUREG/CR-6909][Cho et al.; 2008; Materials Science and Engineering (248-256)]

Detrimental effect of decreasing strain rate in deaerated water at high temperature or in air at 300C :

- Increases cyclic stress behavior.- Decreases fatigue life.

Austenitic stainless steels exhibit Dynamic Strain Aging in the temperature range of 200 to 800C.


LMPMEffects of strain rate

For any strain amplitude, in high temperature deaerated water, reduction of fatigue life increases with decreasing strain rate.

So, decreasing fatigue life in LWR environment is enhanced by low strain rate.

Need for a better consideration of the material behavior in air at high temperature

[2]

[Chopra et al.; 2007; NUREG/CR-6909]

In air, steels, and particularly, austenitic stainless steels are sensitive to Dynamic Strain Aging in the temperature range of 200 to 800C.

*Saturation of environmental effect in High temperature water at :

- 0.0004%/s for 304L SS.- 0.004%/s for 316L SS.

*[Chopra et al.; 2007; NUREG/CR-6909]

**304L, Air 300C0.4%/s & 0.01%/s

xx

[Cho et al.; 2008; Materials Science and Engineering (248-256)]**[De Baglion, Mendez et al.; 2008; PhD AREVA NP in progress]


LMPM

Tests with a more representative signal : SISVariable strain rates, as close as possible to transients applied to Safety Injection nozzles. The strain history corresponds to a cold shock followed by a hot thermal shock. There are 2 types of signals :

Short SIS (840s) & Long SIS (2400s)

In PWR water, for a representative SIS loading signal and ground specimens, experimental Fen are

much lower than expected Fen penalty factors.

[Le Duff, Lefranois, Vernot et al.; AREVA NP; 2008; PVP Chicago]

Combined effects of PWR environment and surface finish for LCF test performed using a complex representative loading signal

[Le Duff, Lefranois, Vernot et al.; AREVA NP; 2008; PVP Chicago]


LMPM

Current research work

Determining the mechanisms of crack initiation and growthin vacuum, air and PWR environment

taking account for representative loading signals and temperature effects

Current research AREVA NP ENSMAL. de Baglion PhD Thesis


LMPM

Other new challenges for the next years

Determine crack initiation and crack growth at the micro and meso scales (scale of microstructure) for austenitic 304L or 316L stainless steels taking account for surface finishes

Flat part

At a macroscopic scaleGlobal texture of 316L :

-RX analysis- EBSD analysis

23% 16% 6% 55% (essentially )

Ex: role of the crystallographic texture

200 m


LMPM

316L -20C - p/2=2.10-3 - 5000 cycles - Air

D4 (1

-11)[-101]

D4

= 53 DG4

= 47

A6 (-111)[110]

A6

= 104

DG6

= 90

D6

(1-11)[1

10]

D6 =

64 DG6

=

42

[110]

[110]

[-101]

Identification of active slip systems and crack initiation conditions

through EBSD analysis

Schmid factor:

{111} = (nPG . ) ( dg . ) {111} = cos . cos

nPG

)

)

surface

Plan de

g

lissemen

t

) s urface

)direction deglissement

ns urf.

10 m

orientation PGOf slip systems with regard to

the free surface :

cos PG = nPG . nsurf.

At a microscopic (one grain) or mesoscopic scale (several grains)


LMPM

First stages of crack propagationTransgranular crack initiation

transgranular propagation

- =0.461 > 0.41- =52 et =42( )]6045[],5540[

Sitedamorage

joint de macle

=55 [324]

D4 =0,395

A2=0,341

A2/D4=0,86

- =55- Slip activity in both sides

of the grain boundary

B2=0,461=52=42

Sitedamorage

transgranulaire

propagation

Crack initiation in twin intergranular propagation

The local conditions that favor crack initiation are also favorable to their propagation through the surrounding grains


LMPM

ANR program : AFGRAPFatigue crack initiation in a grain of a polycrystalline aggregate

and propagation in surrounding grains

- Crack initiation criterion : at the scale of the grain taking into account slip activity- Crack propagation criterion : local configuration of grain boundary and of surrounding grains

Modeling of the fatigue crack initiation and of the first stages of propagation in the 316L (influence

of surface properties)Objectives:

Developments based on numerical and experimental tools :- Discrete Dislocation Dynamics simulation- Crystalline Plasticity theory and calculation applied to polycrystalline aggregates- Experimental identification by micro structural analysis at different scales

Industrial and academic partners :EDF R&D, AREVA NP, CEA (SRMA), ARMINES (ENSMP-ParisTech),LMSSMat of ECP (Paris), SIMAP and SYMME of INPG (Grenoble), LMPM of ENSMA (Poitiers)

coupling


LMPM

Example of numerical simulation of stress localisation induced by

roughness

Virgin material Pre-strained polished material

Pre-strained and rough

surface


LMPM

THANK YOU FOR YOUR ATTENTIONTHANK YOU FOR YOUR ATTENTIONTHANK YOU FOR YOUR ATTENTIONTHANK YOU FOR YOUR ATTENTION


LMPM

Questions to solveQuestions to solve Multi-frequential

Variable amplitude Loading

Welds(Geometric singularities,

Residual StressesHAZ)

Industrial surfacefinish

Bi-axialStress State

Mean static loading (pressure and piping constraints)

Crack network(interaction between cracks)

Wall thicknessStress gradient

Multi disciplinary


LMPM

fatigue_in_lwrs_va inox 304.pdf

Documents

thermal fatigue problems

fatigue incidents

nppsorigin of fatigue

fatigue limit interaction

fatigue testing of metals

large thermal loads

mendez lmpmensma

small random thermal