environmentally assisted fatigue · environmentally assisted fatigue - j. lapeña, ... the...

ENVIRONMENTALLY ASSISTED FATIGUE

Jesús Lapeña Gutiérrez. [email protected] CIEMAT

IAEA Training Workshop on Assessment of Degradation Mechanisms of Primary Components in Water Cooled Nuclear Reactors: Curren t Issues and Future Challenges CIEMAT, 29 September – 2 October 2014

Environmentally Assisted Fatigue - J. Lapeña, IAEA TW Madrid 2014

What is Fatigue?


Definition of Fatigue per American Society of Testing and Materials:

“The process of progressive localized permanent structure change, occurring in a material subjected to fluctuating stresses and strains...which may culminate in cracks or complete fracture after sufficient number of fluctuations.”

Types of Fatigue

• High -cycle Fatigue (HCF)� High number of stress cycles, relatively low stress amplitude

(below yield strength, above fatigue endurance limit of material)

• Low -cycle Fatigue (LCF)� High stress range, less than ~100,000 cycles

� Stress/Strain above yield strength


Some Fatigue Mechanisms

• Mechanical� Vibration

� Pressure

� Moments

• Thermal� Mixing

� Stratification


Characteristics of fatigue fracture surface

• Distinct crack initiation site (usually at concentr ation location)

• Distinct fracture surface (from 1% to 100% of total surface)

• Beach marks indicative of crack growth

• Crack propagates in the plane of the maximum tensil e stress


Initiation and propagation• Stage I, Crack Initiation: It can be caused by surf ace scratches caused by handling, or tooling

of the material; slip bands or dislocations interse ct the surface as a result of cyclic loading, producing extrusions and intrusions as consequence of alternating stresses.

• Stage II, Crack Propagation: The crack continues to grow during this stage as a result of continuously applied stresses. Striations indicate the crack propagation.

• Stage III, Failure: Failure occurs when the materia l that has not been affected by the crack cannot withstand the applied stress. This stage hap pens very quickly


Initiation: extrusions and intrusions intersect the surface of the material

Propagation: striations indicate the advancement of the crack. There may be thousands of striations in a beachmark

Stages of fatigue

Initiation and propagation

• Total life is the addition of time to initiation of crack and the time of crack propagation�For high stress the time to initiation is low compared with the propagation

period

�For low stress the longer time is the time to initiation


• Some materials exhibit a theoretical fatigue limit, or endurance limit, below which continued loading does not lead to structural failure. Ferrous and Ti alloys.

• Other materials does not present a fatigue limit. In these materials is usually used the fatigue strength, indicating the stress at which failure occurs for a given number of cycles. Nonferrous metals and alloys (Al, Cu, Mg)

Fatigue life, Nf, is the number of stress or strain cycles of a specified character that a specimen sustains before failure occurs.

NfNf

In high-cycle fatigue situations, materials performance is commonly characterized by an S-N curve, also known as a Wöhler curve. This is a graph of the magnitude of a cyclic stress against the logarithmic scale of cycles to failure (N).

S-N curves. Fatigue life


Fatigue S-N curves permit to relate the stress (or strain) range to the number of permitted cycles:

• Knowing the number of cycles, the permitted stress (or strain) can be determined .• Knowing the stress/strain, the permitted number of cycles can be determined.




• In metals and alloys, the process starts with dislo cation movements, eventually forming persistent slip bands that nucleate short cracks.

• The greater the applied stress range, the shorter t he life.• Damage is cumulative . Materials do not recover when

rested.• Fatigue life is influenced by a variety of factors, such as

temperature, surface finish, microstructure, presen ce of oxidizing or inert chemicals, residual stresses, con tact (fretting), etc.


Miner's rule linear damage hypothesis, states that where there are k different stress magnitudes in a spectrum, Si (1 ≤ i ≤ k), each contributing ni(Si) cycles, then if Ni(Si) is the number of cycles to failure of a constant stress reversal Si, failure occurs when:

C is experimentally found to be between 0.7 and 2.2. Usually for design purposes, C is assumed to be 1. This can be thought of as assessing what proportion of life is consumed by stress reversal at each magnitude then forming a linear combination of their aggregate

CN

nk

i i

i =∑=1

Cumulative damage by fatigue


σa

∆σ

Mean stress (σm)

σu

σyσmax

σmin

45º

Alternating stress

Compression Tension

Time

Str

ess σσσσa

σσσσm

∆σσσσ

σσσσmax

σσσσmin

Time

Str

ess σσσσa

σσσσm

∆σσσσ

σσσσmax

σσσσmin

Time

Str

ess σσσσa

σσσσm

∆σσσσ

σσσσmax

σσσσmin

Time

Str

ess σσσσa

σσσσm

∆σσσσ

Time

Str

ess

Time

Str

ess σσσσa

σσσσm

∆σσσσ

σσσσmax

σσσσmin

Goodman diagramNotation in cyclic stresses

R = σσσσmin/σσσσmax

σσσσm = (σσσσmax + σσσσmin)/2

σσσσa = (σσσσmax - σσσσmin)/2

Graphic methods to represent combined effects of mean stress and alternating stress


Corrosion Fatigue is an environmental reduction of fatigue life (crack initiation) and environmental acceleration of fatigue crack growth in a material under the simultaneous and synergistic interaction of cyclic or fluctuating mechanical tensile stress and corrosive environment

Corrosion Fatigue

Fatigue Considerations


Fatigue Considerations

• Environmental Effects�Water environment reduces fatigue life and increases fatigue

crack growth rate

• Crack initiation and crack growth. Aspects to take into account: �Material properties

• Structural factors (surface condition, welds, notches, defects, discontinuities)

�Loading (stress range, mean stress, frequency, strain rate)

�Environment (temperature, dissolved oxygen, conductivity…)


Environmental effects on fatigue life(initiation of cracks)


ASME Code fatigue design

• The ASME Code fatigue design curves, given in Appen dix I of Section III, are based on strain–controlled tests of small polished specimens at room temperature in air. The design curves have bee n developed from the best–fit curves to the experimental fatigue str ain vs. life ( εεεε–N) data

• These best-fit curves have been adjusted for the ef fects of mean stress on fatigue life and then reducing the fatigu e life at each point on the adjusted curve by a factor of 2 on strain (or s tress) or 20 on cycles, whichever is more conservative.

• As described in the Section III criteria document, these factors were intended to account for data scatter (including mat erial variability) and differences in surface condition and size between th e test specimens and actual components.




•The factor of 20 on life was regarded as the product of three subfactors:

•Scatter of data (minimum to mean) 2.0•Size effect 2.5•Surface finish, atmosphere, etc. 4.0

ASME design curves for Carbon Steel and Low Alloy Steel


• ASME Code, Section III contains requirements for p ipes and components to prevent fatigue failures during the operational per iod. The current generation of nuclear power plants (NPPs) has been normally de signed for an operation period of 40 years. Many power plants will extend t his operation period to 60 years.

• Fatigue test data from the US, Japan, and elsewhere show that the LWR environment can have a significant impact on the fa tigue life of carbon and low-alloy steels, austenitic stainless steel, and n ickel-chromium -iron alloys.


NUREG/CR-6815

Fatigue ε-N data for (a) carbon steels and (b) austenitic SS

Environmental effects on fatigue life

• Different approaches for incorporating the effect o f the coolant water environment in crack initiation by fatigue exist. The most common approach is the incorporation of an environmental fatigue corre ction factor (Fen) in the fatigue derivation of the cumulative usage factor. The Fen formulas and the S-N fatigue curves differ but the general equations a re:


Fen = Nair,RT / Nwater CUF = ΣUpartial * Fenpartial

• During last years there has been extensive technica l debate in ASME and NRC, about how to update the fatigue design curves to explicitly incorporate the environmental factors affecting the fatigue lif e, and the computation of the CUF.

• As a result, the ASME Code process has not been abl e to obtain a consensus position on the issue and therefore comprehensive c ode revision has been slow in development.

Regulatory Guide 1.207

• The developers lacked sufficient data to explicitly evaluate and account for the degradation attributable to exposure to aqueous coo lants.

• The staff evaluated two distinct methods for incorp orating LWR environmental effects into the fatigue analysis of ASME Class 1 c omponents. � The first method involves developing new fatigue curves that are applicable to LWR

environments. Given that the fatigue life of ASME Class 1 components in LWR environments is a function of several parameters, this method necessitates the development of several fatigue curves to address potential parameter variations. Alternatively, a single bounding fatigue curve could be developed, but this approach might be overly conservative for most applications.

� The second method involves using an environmental correction factor (Fen) to account for LWR environments by correcting the fatigue usage calculated with the ASME “air” curves. This method affords the designer greater flexibility to calculate the appropriate impacts for specific environmental parameters.


Selected procedure


• As a consensus has not reached, NRC has edited the Regulatory Guide 1.207. The NRC staff has selected the Fen method as an acceptable method to properly incorporate the LWR environmental effects into fatigue analyses of ASME Class 1 components, for new reactors . Its technical base has been developed in ANL (NUR EG/CR-6909)

• NRC report NUREG-1801, Revision 2, “Generic Aging Le ssons Learned (GALL) Report,” identifies acceptable aging management programs for fatigue and cyclic operation for the period of extended operation .


The NRC regulatory guidance can result in difficult ies in demonstrating acceptable fatigue usage for both new plant design and license renewal

approval

Evaluation Procedure: Fen Criteria

Fen is the fatigue life reduction factor and is defined as:

Fen = NA/NW

NA: fatigue life of the component material in a room temperature air environment

NW: fatigue life in LWR coolant at operating temperature.

A generic equation for computing Fen factor is:

S* is a factor based on Sulfur content (only for some materials. For SS: S*=1)T* a factor based on service temperatureO* a factor based on dissolved oxygenε* a factor based on strain rate



Regulatory Guide 1.207 (Stainless Steels)• The staff also reviewed the nonconservatism of the current ASME Code design curve in

respect to the existing fatigue data for austenitic stainless steels. Recent evaluations of stainless steel test data indicate that the ASME cu rve is inconsistent with the appropriate test materials and conduct of the fatig ue test.

• Consequently, through this regulatory guide, the NR C staff endorses a new stainless steel air design curve (from NUREG/CR-6909).


This new air design curve uses margins of 12 for cy clic life and 2 for stress, instead of 20 for cyclic life and 2 for stress.

Regulatory Guide 1.207 (Stainless Steels)• Under high cycle fatigue conditions the new curve p redicts lower fatigue lives

than the previous ASME curve. It is significantly m ore conservative than the former in the region of ~10 4 to 107 cycles, which can present problems for designers.

• Fen defined for stainless steel in NUREG/CR-6909 should be used in conjunction with the new stainless steel air design curve when evaluating the fatigue usage of ASME Class 1 components.


Regulatory Guide 1.207 (Carbon & Low -Alloy Steels )

• For carbon and low alloy steels the curves of fatigue in air in NUREG / CR-6909, ap proved by the NRC in Regulatory Guide 1.207, are less conserv ative than the current design curves in ASME Code. These curves, as in stainless steels, us e margins of 12 for cyclic life and 2 for stress, instead of 20 and 2 respectively.

• Fen value defined for carbon and low alloy steels i n NUREG / CR-6909 can be used with the new design curve in air in the evaluation of the us age factor for ASME Class I components.

• However, since the ASME design curves are more cons ervative, the RG 1.207 also allows use these curves with Fen.


Environmentally Assisted Fatigue Screening: Process and Technical Basis for Identifying EAF Limiting Locations. EPRI

Fen factors


Regulatory Guide 1.207 Fen factors


Comparisson of Fen from NUREG/CR -6909 and Japanese model JNESS -SS-0701 for austenitic SSs

Comparison between estimated fatigue curves from the ANL model and JSME model in BWR &PWR environment for stainless steels.

Ref: Strömbro, 2011

ANL model

NUREG/CR 6909

JNESS-SS-0701

Stainless Steels Stainless Steels

ln(Fen)=0.734-έ*T*O*

έ*= 0 (έ>0.4%/s)

έ*= ln(έ/0.4) (0.0004≤έ≤0.4%/s)

έ*=l n(0.0004/0.4) (έ≤0.0004%/s)

T*= 0 (T<150ºC)

T*= (T-150)/175 (150≤T<325°C)

T*= 1 (T≥325°C)

O*=0.281 (all DO levels)

Fen =1.0 (εa≤0.10%)

[Ref.]

Best Fit Curve for Fatigue life in Air

ln(NA)=6.891-1.920 ln(εa-0.112)

(εa = 36.2NA -0.521 + 0.112)

ln(Fen)=(C-έ*)T*

C= 0.992 (BWR)

C= 3.910 (PWR)

έ*= ln(2.69) (BWR:έ>2.69%/s)

έ*= ln(49.9) (PWR:έ>49.9%/s)

έ*=l n (έ)

(BWR: 0.00004≤έ≤2.69%/s)

(PWR exc. Cast: 0.0004≤έ≤49.9%/s)

(PWR Cast: 0.00004≤έ≤49.9%/s)

έ*= ln(0.0004)

(PWR exc. Cast: έ<0.0004%/s)

έ*= ln(0.00004)

(BWR: έ<0.00004%/s)

(PWR Cast: έ<0.00004%/s)

T*= 0.000969xT (BWR)

T*= 0.000782xT (PWR:T≤325°C)

T*= 0.254 (PWR:T>325°C)

Fen= 1.0

(εa≤0.11% or seismic loading)

[Ref.]

Best Fit Curve for Fatigue life in Air

(εa = 23.0NA -0.457 + 0.11)

BWR PWR

έ=0.0001 %/s έ=0.0001 %/s

έ=0.01 %/s έ=0.01 %/s


Chopra. NUREG/CR-6909

Fen factors - Effect of temperature

• Change in fatigue lives of austenitic SSs with test temperature at two strain amplitudes and two strain rates:� The results suggest a threshold temperature of 150°C, above which the environment decreases

fatigue life in low–DO water if the strain rate is below the threshold of 0.4%/s.

� In the range of 150–325°C, the logarithm of fatigue life decreases linearly with temperature.

� Only a moderate decrease in life occurs in water at temperatures below the threshold value of 150°C.


• In low–DO PWR environment, the fatigue life decreases with decreasing strain rate below ≈0.4%/s; the effect of environment on fatigue life saturates at ≈0.0004%/s.

• Only a moderate decrease in life is observed at strain rates greater than 0.4%/s. A decrease in strain rate from 0.4 to 0.0004%/s decreases the fatigue life by a factor of ≈10.

• For Type 304 SS, strain rate has no effect on fatigue life in high–DO water.

• Life decreases linearly with strain rate in low–DO water

NUREG/CR-6909

Fen factorsEffect of strain rate & DO (aust. SSs )


Fatigue evaluation procedure. NUREG -CR-6909

• The evaluation method uses as its input the partial fatigue usage factors U 1, U2, U3, …Un, determined in Class 1 fatigue evaluations. To inc orporate environmental effects into the Section III fatigue evaluation, th e partial fatigue usage factors for a specific stress cycle or load set pair, based on the current Code fatigue design curves, is multiplied by the environmental fatigue correction factor:

Uen.1 = U1·Fen,1

• The cumulative fatigue usage factor, U en, considering the effects of reactor coolant environments is calculated as the following :

Uen = U1·Fen,1 + U2·Fen,2 + U3·Fen,3 + Ui·Fen,i …+ Un·Fen,n


Extended operation

• The NRC requires license renewal applicants to assess the fatigue usage effects from a reactor water environment and demons trate acceptable fatigue cumulative usage factors (CUF) with the effects of a reactor water environment considered for Class 1 components for the entire pe riod of extended operation.

• NRC report NUREG-1801, Revision 2, the “Generic Agin g Lessons Learned (GALL) Report,” identifies acceptable aging manageme nt programs for fatigue and cyclic operation for the period of extended operation . It describes a process for assessing the impact of the reactor coo lant environment on a set of sample critical components for the plant, exampl es of which are identified in NUREG/CR-6260.� Plants have the option of computing Uen in accordance with guidance from either

NUREG/CR-5704 (for austenitic stainless steels), NUREG/CR-6583 (for carbon and low alloy steels) and NUREG/CR-6909 (for nickel alloy steels), OR NUREG/CR-6909 (for all materials) .


Extended operation

• In the case of austenitic Stainless Steels:

�Using the NUREG/CR-5704, the ASME Code fatigue curve is used, and the Fen factors are applied to the ASME Code fatigue usage values.

�When using NUREG/CR-6909 rules, the special fatigue curve provided in Appendix A must be used for austenitic stainless steel


Nakamura

Comparison of evaluation condition between design and PLM


Nakamura

Example of environmental fatigue evaluation in the PLM of typical PWR plant


NUREG/CR-6909, Rev. 1

• NRC has edited the document NUREG/CR -6909, Rev. 1 (“Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials”), as draft report for comments, in March 2014

• This revision modifies some parameters of the Fen equations


BUT….


Several reports indicate a potentially great influence of the environment, leading to the proposal of developing entirely new fatigue analysis procedures. In this sense, a number of fatigue laboratory test data have been obtained under different boundary conditions, performed by diverse international organizations, but the results have not completely solved the assessment of fatigue aging.

The NRC regulatory guidance can result in difficult ies in demonstrating acceptable fatigue usage for both new plant design and license renewal approval

The lack of correlation between the laboratory test data and the in-plant operating experience compromises somewhat the confidence in corrosion fatigue assessment in light water reactor (LWR) environments, thus impeding total safety management of the NPPs from being developed


However, international experience feedback to date is quite favorable, and systematic damage or failures of structural components due to EAF did not usually occur worldwide (within > last 40 years)

Some possible reasons for the apparent discrepancy between lab and field results

• Different loading signals:�simple artificial signals are applied in laboratory tests (sinusoidal,

triangular, trapezoidal, etc.)�during plant operation more complex transients occur.�direct mechanical load (used in lab tests) vs thermally induced

mechanical load (in thermal fatigue)

• Long hold -times at steady-state conditions: �in laboratory tests there is a continuous and simultaneous effect

of mechanical and chemical loading.�during operation of NPPs there are long periods of transient-free

steady-state conditions.�laboratory tests on the effect of hold-times is not always plant

relevant


Some possible reasons for the apparent discrepancy between lab and field results

Effects of LWR environments are not explicitly covered by initial fatigue design. Consequently, systematic degradation or failures due to EAF could have been expected in service.

However, systematic damage or failures of structural components due to EAF did not occur worldwide (within > last 40 years)

The current design curves apparently cover to some extent the LWR environmental effects, although they were not t aken into account


INCEFA projectINcreasing Safety in NPPs by Covering gaps in Environmental Fatigue Assessment

Fatigue life vs. Hold time for SS in BWR waterRef. Higuchi, 2007

More research is necessary to incorporate the effect of some parameters not usually considered in lab testing, as:

•mean stress•hold time•more complex transients•surface roughness


INCEFA project

• This project is submitted in response to the HORIZO N2020 call for proposals

• The project brings together national programmes fro m The United Kingdom, France, Germany, Spain, The EU (Netherlands), The C zech Republic, Switzerland, Belgium, Finland and Lithuania.

• The project is endorsed formally by NUGENIA for imp ortance as a European coordinated research topic.

• The methodology created by the project could be app licable for assessment of safety of the existing EU nuclear reactor fleet, an d for optimising safety characteristics in the design of future reactors. F or both applications safety is better assured through more reliable assessment of fatigue endurance limits than is presently possible.

• A significant experimental programme is a major com ponent of the project, as is the development of environmental fatigue enduran ce assessment methodology. The focus is on increasing the knowled ge basis on reactor life-time management relevant to integrity of structural components.


Environmental effects on fatigue crack growth(propagation of cracks)


Fatigue Crack Growth Curves in Nuclear Codes

Environmental effects on fatigue crack growth

• The ASME BPV Code, Section XI, Appendix A, Article A-4300 contains reference fatigue CGR ( ∆∆∆∆a/∆∆∆∆N) curves for carbon and LAS in air ("air" curves) or in LWR reactor coolant environment ("wet" curves).

• The current ASME XI wet reference fatigue CGR curve s are based on lab data obtained prior to 1980. They depend on ∆∆∆∆K and load ratio R, but not on other variables that are known to be important, such as:� loading frequency� ECP/DO� steel sulphur content.

• The same curves are used for different types of car bon and LAS and for:� BWR/NWC� BWR/HWC/NMCA (Noble Metal Chemical Addition)� PWR primary or secondary side conditions.

• System conditions where environmental effects on fa tigue crack growth can be neglected or excluded are not defined in the presen t ASME Section XI Code.


Corrosion Fatigue data for A533-B and A-508-2 steels and weldments in PWR primary water

Bamford

Many experimental values show higher Crack Growth Rates than considered in the ASME Code





Effect of rise time, tr

The ASME time-based crack growth rate in air is obtained by:

The crack growth rate in environment is obtained by experiments:

ASME mean air equation (austenitic SS)

Tice

Time domain plots (da/dt)



CGR correlation. Experimental data

• Shack and Kassner (NUREG/CR-6176, 1994) developed re lationships indicating effect of PWR environment was relatively small

• But most of this data where obtained at relatively high frequency

• Very limited data at somewhat lower frequency sugge sted a possibly greater effect (Kawakubo data)


• More recently, environmental enhancement of fatigue crack

• propagation has been reported in high temperature water environments�High levels of enhancement of crack propagation in austenitic

stainless steels observed in PWR coolant environment�Environmental enhancement increases with increasing rise time of

the loading cycle� At very long rise times environmental enhancements have been

observed of up to 80X ASME XI mean air CGR


CGR correlation. Experimental data



CGR correlation for stainless stels obtained from testing in Deaerated Pressurized Water (DPW)

Mills



CGR correlation. Experimental data in DPW

Mills

Environmental effects on fatigue crack growth• Experimental data, showing enhancement of CGR in h igh temperature water

environment.• The enhancement is more evident for low ∆∆∆∆K and very long rising time (t r) of

the fatigue cycle• Up to 80X compared with CGR in air has been observe d


Thermal fatigueMixing of hot and cold flows

Stratification


Thermal fatigue

• Branch lines are categorized into three basic configurations depending on attachment to Reactor Cooling System (RCS) piping:�Up-horizontal (UH) configuration – branches attached to the top of the

RCS piping

�Horizontal (H) configuration – branches attached to the side of the RCS piping

�Down-horizontal (DH) configuration – branches attached to the bottom of the RCS piping


Line configurations

Thermal fatigue -


Deardorff

Normally Stagnant RCS Branch Lines

Thermal fatigue


Nakamura

Flow patterns and vortex structures in branch pipes

Thermal fatigue


Velocity fluctuations of the spiral flow in the region 3 in straight pipefor non-uniform temperature condition

Nakamura

Thermal fatigue


Mixing of hot and cold flow in a pipe junction

Temperature distribution

10ºC

1 sec.

Tmain flow =343ºKTsec.flow =278ºK

Ertem-Müller

Thermal fatigue


Example of turbulent mixing (HCF) conducting to cracks

Robert

Thermal fatigue


Example of BWR feedwater nozzle cracking, associated to thermal fatigue

Nozzle Bore Crack Flow Hole Cracks

FeedwaterFlow

ThermalSleeve

Nozzle Blend Cracks

Feedwater Nozzle

Sparger VibrationAnd Cracking

Slip FitJoint

Vessel Wall

Nozzle

Safe End

ThermalSleeve

ReactorDowncomerFlow

LeakageFlow

SpargerRegionof highfrequencythermalcycling andcracking

• Crack initiation: high frequency thermal fatigue• Turbulent mixing: cold leakage flow (220ºC) and hot

downcomer flow (280ºC)• Thermal cycling: 0.01 to 1 Hz• No significant environmental effects• Near surface region

• Crack growth: low frequency CF• Thermal and mechanical loads by start-

up/shut-down, scrams, turbine rolls….• Good correlations with number of plant

transients

Thermal fatigue


Ranking of PWR components with respect to thermal fatigue

Kohlpaintner

Thank you for your attention


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