on-line monitoring of structures and fatigue limit

ON-LINE MONITORING OF

STRUCTURES AND

FATIGUE LIMIT Prof. dr. Nenad GUBELJAK

University of Maribor, Slovenia

Goals are: - developed expert system for structure integrity assessment.

- developed software protocol for structure integrity assessment on-suite,- developed material data bank

- developed working study examples data bank for most often cases- Integrate high tech non-destructive equipment and technique for measurement input dat

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Project Project consortiumconsortiumPartners in Slovenia:-University of Maribor, Faculty of Mech. Eng.-(UMFS) coordinator -WALCH d.o.o., Maribor–(WMB) (private company)

Partners in Serbia:- University of Belgrade, Faculty of Mech. Eng-(MFBg)-sub coordinator- Innovation Centre of Faculty of Mech. Eng, Belgrade,(InnvBg) -University of Belgrade, Faculty of Technology & Metallurgy-(TMFBg) U e s ty o e g ade, acu ty o ec o ogy & eta u gy ( g)-UNO-LUX NS, d.o.o., Belgrade,-(UNO) (private company)

Partners in HungaryPartners in Hungary-Bay Zoltan-Logy Institute, Miskolc, Hungary-Metal Elektro, Budapest, (private company)

Partners in Monte Negro (in procedure)-University of Podgorica, Facutly of Mech. Eng. M t N El t i it E t i P d i (I d t i l t )

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-Monte Negro Electricity Enterprice, Podgorica (Industrial partner)

MOSTIS + FATIGUEMOSTIS + FATIGUE

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HighlightsHighlightsDEVELOPMENT:

•Laboratory testing equipment and procedure for characterisation

-Object grating method in nonstanrdard testing for t d d lt CTOD R

y g q p pof fracture behaviour of components and material:

standard results e.g. CTOD-R curves-Aplication of Fracture mechancis paramters (fracture toughenss + fatigue paramters) for S-N (Wohler) curve deisgn

• Sofware as web application for structure integrity assessment d f l f d !and fatigue life time prediction!

• Selected and designed equipment for characterisation of loading • Selected and designed equipment for characterisation of loading behaviour of components including software for measurment analysis.

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ObjectObject gratinggrating methodmethod in in ObjectObject grat nggrat ng methodmethod n n nonstandardnonstandard testingtesting forfor standard standard

resultsresults e CTODe CTOD R R curvescurvesresultsresults e.g. CTODe.g. CTOD--R R curvescurves

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ProblemProblemProblemProblem

CTOD standard CTOD standard techniquetechnique ofof fracturefracture toughnesstoughness measurementmeasurement is is developeddeveloped for for homogeneoushomogeneous metalsmetals ((WelsWels). ). InIn some some cucrumstancescucrumstances it is it is possiblepossible to to applyapply for for weldweld jointsjoints (BS 7488 (BS 7488 PartPart 22 1998)1998)PartPart 22--1998).1998).

TodayToday, it , it doesdoes notnot existexist technoquetechnoque for CTOD for CTOD testingtesting ofof heterogeneosheterogeneosweldweld jointsjoints ((partlypartly weldedwelded withwith overmatchovermatch andand partlypartly withwithweldweld jointsjoints ((partlypartly weldedwelded withwith overmatchovermatch andand partlypartly withwithundermatchundermatch consumableconsumable))

ReasonsReasons:: NoNo uniqueunique yieldyield stressstressReasonsReasons: : --No No uniqueunique yieldyield stressstress, , --InteractionsInteractions ofof propertiesproperties inin inerfaceinerface regionregion

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ObjectObject

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CTODCTOD--R R curvecurve directdirect measurmentmeasurment

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Origin lines parallel to crack path on specimen prior test


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Crack opening displacement on the surface of specimen at the moment ofstable crack initiation load F=11.2 kN


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Crack opening displacement on the surface of specimen at the moment ofmaximum sustain load F=13.72 kN

CTODCTOD--R R curvecurve directdirect measurmentmeasurment16

10

12

14

6

8

10

F, k

N.

Load_04BPoints for normalization

0

2

4

00 1 2 3 4 5

COD, mm

CMOD vs Load plot

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6

1 0+1.5

+2.5+3.0

+2.0

+3.5af,avr

af,front

2

0

2

-5 0 5 10 15 20CO

D, m

m

0 5

+0.5+1.0

-6

-4

-2 -0.5-1.0

-1.5-2.0

-2.5-3.0

-3.5

3,5

4 af,avr

af,front

distance, mm

2,5

3

3,5

m CTODRigid body line

1,5

2

CO

D, m

m

CTOD-δ5

line

0

0,5

1

ao,front

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0-5 0 5 10 15 20

distance x, mm


16

Maximum load

v1(x=8.705 mm) v2(x=8.414 mm)

8

10

12

F, k

N

Crack initiation

v3(x=8.113 mm)16

COD CTOD-δ5 CTODASTM NM CTOD

2

4

6

F

12

14CODv3,pl

CMODLLD, pl

CTOD-δ5 CTODASTM,NM CTODFITNET

0

2

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

COD mm 6

8

10

Load

, kNCOD, mm

2

4

COD vs Load plot0

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

CODpl, mm

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VV

a

CMOD

W-a

Vp1Vp2

x1

x2

rp(W-a)

CMODpl

W

Δx

W

14

16CODv3,pl

CMODLLD pl

CTOD-δ5 CTODASTM,NM CTODFITNET

8

10

12

oad,

kN

C O LLD, pl

COD vs Load plot2

4

6Lo

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00 0,5 1 1,5 2 2,5 3

CODpl, mm


0,4

0,5

0,3

kN/m

m2

0,1

0,2PN

, k PniPfiPniPni-fitted

b=0.18864c=0 05966

00 0,5 1 1,5 2 2,5 3

c 0.05966d=0.037

0,6

0,8

CMODpl, mm

0,4

CTO

D, m

m

CTOD-R curve 0,2

C

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00 0,2 0,4 0,6 0,8 1 1,2

Δa, mm

Experimental analysis of crack propagationExperimental analysis of crack propagation

Object grating method is advantageously used in measuring of difi d CTOD t t t i ith i iti l k imodified CTOD tests on two specimens with initial crack in

macroscopic heterogeneous welded joint.

Advantages:

Flexibility, non-contact method is possible in high temperaturesy, p g pDynamic measurements can be performed

Disadvantages:Disadvantages:

Sensitivity of the raster on the surface

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ConclusionsConclusions

R lt i ifi tl h th t th f t b h i d dResults significantly show that the fracture behaviour depend on material at the vicinity of crack tip, and interface conditions in the direction of crack propagation

Object grating method-OGM is successfully used for fracture behaviour of macroscopic heterogenous welded jointp g j

OGM to provide an oportuinity for fracture toughess test of components out of laboratorycomponents out of laboratory

Consequently, OGM solves problem of transferibility bettween laboratory specimens and componentslaboratory specimens and components

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2 2 Model for Fatigue Thresshold PredictionModel for Fatigue Thresshold Prediction

The pure fatigue crack propagation threshold ΔKth,R=-1 is equal to the lower value of equations:

3/13 3/131, )120(104 aHVK Rth ⋅+⋅=Δ −

−=

5.150038.01, +⋅−=Δ −= uRthK σ

Where the pure fatigue crack propagation threshold ΔKth as function of crack p g p p g thlength, and ΔKth,R=-1 (a constant value for given tensile strength or hardness) are in MPa·m1/2, the crack length a in mm, the Vicker’s hardness HV in kgf/mm2 and the ultimate tensile strength σu in MPa. g g u

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1 1 Fatigue thresshold predictions:Fatigue thresshold predictions:

100 As Delivered 430 HV

Q+T 590 HV

Murakami 430 HV

.

Murakami 430 HV

Murakami 590 HV

10

MPa

√m

HVΔKth~(area

½)0.33ΔKth,R

Δ Kth , M

HV

Short cracks Long cracksHV

1

1 10 100 10001/2

1919

1 10 100 1000area 1/2, μm

1 1 Fatigue microstructural threshold:Fatigue microstructural threshold:

The initial crack length is given by the position of the strongest microstructuralbarrier if the material were free of cracks or crack like flaws. This intrinsic resistanceis considered to be microstructural threshold for crack propagation as :

dYK ΔΔ dYK eRdR ⋅Δ⋅=Δ πσwhere Y is the geometrical correction factor. In most cases the nucleatedmicrostructurally short surface cracks are considered semicircular, and the value ofY would then be 0 65Y would then be 0.65.Because the plain fatigue limit depends on the stress ratio R, the microstructuralthreshold also does. The value of d is usually given by the microstructuralcharacteristic dimension as grain size e g for steel in as delivered conditioncharacteristic dimension, as grain size, e.g. for steel in as delivered conditiond = 10 μm, and Q+T conditions d = 5 μm.

2020

1 1 Fatigue Crack Growth Rate Fatigue Crack Growth Rate

constitutive relationship of general validity be established between the rate of fatiguecrack growth, da/dN, :

mRthappl KKC

dNda )( 1, −=Δ−Δ⋅=

where C and m are Paris range constants obtained from long crack fatigue behaviorand ΔKth R=-1 is the crack growth threshold as lower value of Eq. (3) and (4). The

dN

th,R=-1 g q ( ) ( )fatigue crack propagation life from crack initiation up to critical crack length ac can beobtained by integrating expression (5) and using expression (4) for the threshold ofthe material (ΔKth,R=-1).th,R 1

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2 2 Fatigue Crack Driving Force Fatigue Crack Driving Force

In the case of smooth specimens or spring after shot-peening, the stress can be considered constant for any crack length, equal to the nominal applied stress Δσn. The following general expression can be used to estimate the applied d i i f f ti f k l thdriving force as a function of crack length

aYK nappl ⋅Δ⋅=Δ πσpp

h Δ i th i l li d t Th k t ti f tiwhere Δσn is the nominal applied stress range. The crack aspect ratio as a function of crack length has to be defined for the combination of component geometry and loading conditions, which allows definition of the value of the parameter Y as a f nction of crack length In case of small embedment crack here the crack is onlfunction of crack length. In case of small embedment crack, where the crack is only few inclusion size, we are considering shape function by Y = 0.65

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Fatigue threshold as a function of area1/2 parameter for 51CrV4 and fatigue crack driving force as difference between applied force and threshold

100100

Crack No crack

crack propagation consider for fatigue life time

Pa√m

Crack initiation propagation under Δ K appl,R=‐1 Δ K appl,R=‐1

Δ K for Paris fatigue rangeonly if

10

Δ Kth , M

P

ΔK CRΔ K C

Δ K appl >Δ K th

Δ

Microstructural Threshold

Δ K dR

Δ K thR

1

1 10 100 1000grain size d Inclusion size a i

Critical crack length a c

2323

area 1/2 , μmsize a i


Fatigue crack driving force as difference between applied force ΔKappl and threshold

ΔKth,R obtained by Chapetti's model by applying. It is possible to determine the

b f l f il di diff i l i i d diff li dnumber of cycles to failure regarding to different inclusion size and different applied

fatigue stress magnitude Δσn, by using simple integration of da/dN with

i t ll bt i d t f P i f ti k tiexperimentally obtained parameters of Paris fatigue crack propagation range

(C=8·10-8 and m=3.25)

Fatigue crack propagation occur only if applied crack driving force ΔKappl is higher

than threshold ΔKth,R and if inclusion size as area1/2 is higher than value in

intersection between threshold limit curve ΔKth,R and applied crack driving force line.

F ti k i i t t til iti l k l th Th l f iti lFatigue crack is going to propagate until critical crack length ac. The value of critical

crack length is determined by fracture toughness of material KIC=33.19 MPa·m1/2

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2 2 Fatigue threshold for two conditions of steelFatigue threshold for two conditions of steelA d li d 430 HV100 As delivered 430 HV

Q+T 590 HV

1300MPa, 590 HV

1000MPa, 590 HV 900 MPa

1000 MPa

1300 MPa

a√m .

1000MPa, 590 HV

900MPa, 590 HV

910 MPa, 430 HV

730 MPa, 430 HV730 MPa

910 MPa

ΔK10

Δ Kth, M

Pa ΔKth,R

Long cracksΔKth~(area

½)0.33

Δ

Short cracks

1

1 10 100 10001/2

Fig. shows that as delivered steel (softer) switch to constant threshold valueΔKth=10.76 MPa·m1/2 at larger size of inclusion ai=120 μm than quenched and

area 1/2 , μm

2525

tempered (Q+T) steel, with size of inclusion ai=35 μm and with constant thresholdvalue ΔKth,R=-1=9.154 MPa·m1/2

2 2 Fatigue results in form SFatigue results in form S--N curvesN curvesThe results in form of S N curves obtained by using proposed model for bothThe results in form of S-N curves obtained by using proposed model for bothconditions of 51CrV4 steel. -Q+T steel can be subjected to higher fatigue stressamplitude (Dsn =1300 MPa) than steel in as delivered condition (Dsn=900 MPa) forsame number of cycles to failuresame number of cycles to failure.

1500 Q+T 0.035 mmAs delivered 0.12 mmQ+T 0.5 mmQ+T 0 25

Effect of Presterssing and shot peening at → R<‐6.8, ai=0.5

1250

.

Q+T 0.25 mmTesting in MBHAExperiment 0.5mm

R= 1 a =0 035 mm

750

1000

σn, M

Pa .

R=‐1, ai=0.12 mm

R=‐1, ai=0.035 mm

500

750Δσ

R=‐1, ai=0.25 mm

250R=‐1, ai=0.5

R=‐1, ai=0.5 mm

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1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

N, cycles

2 2 Fatigue results in form SFatigue results in form S--N curvesN curvesSteel in Q+T condition shows higher fatigue resistance with smallest inclusions sizeSteel in Q+T condition shows higher fatigue resistance with smallest inclusions sizeai=0.035 mm. For same Q+T steel the S-N analysis has been performed for twodifferent size of inclusion 0.25 and 0.5 mm. Fatigue four point bending tests areperformed by cracktronic Romul with loading ration R=-1performed by cracktronic Romul, with loading ration R 1.



1250

.


R= 1 a =0 035 mm

750

1000

σn, M

Pa .

R=‐1, ai=0.12 mm

R=‐1, ai=0.035 mm

500

750Δσ

R=‐1, ai=0.25 mm

250R=‐1, ai=0.5

R=‐1, ai=0.5 mm

2727

1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

N, cycles

2 2 Fatigue results in form SFatigue results in form S--N curvesN curvesTests were conducted also by spring producer on a leaf spring with same inclusionTests were conducted also by spring producer on a leaf spring with same inclusionsize ai = 0.5 mm. The residual stresses on the surface should be σRS = -1100 MPa. Allsix springs after presetting and shot-peening were subjected the same loading regime(720±630 MPa) an applied stress amp Δσ =1260 MPa and effective ratio R = -6 8(720±630 MPa), an applied stress amp. Δσn 1260 MPa and effective ratio R 6.8.



1250

.


R= 1 a =0 035 mm

750

1000

σn, M

Pa .

R=‐1, ai=0.12 mm

R=‐1, ai=0.035 mm

500

750Δσ

R=‐1, ai=0.25 mm

250R=‐1, ai=0.5

R=‐1, ai=0.5 mm

2828

1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

N, cycles

4 4 ConclusionConclusion

Chapetti's model has been used for determination of a threshold ΔKth from short crack of grain size until critical crack length from one or few millimeterscorresponding to applied maximum stress Δσn/(1-R).

Life time of spring material subjected to applied stress amplitude Δσn frommicrostructural threshold up to critical crack length of high strength steel has beenmicrostructural threshold up to critical crack length of high strength steel has beendetermine, by combining fracture mechanics parameters KIC,fatigue propagation Paris range parameters C and m,and considering inclusion size a as crack initiation areaand considering inclusion size ai as crack initiation area.

Model shows very good agreement with experimentally obtained f ti lt f i t l i Q T diti ith i l i ifatigue results for spring steel in Q+T condition with same inclusion size.

Residual stresses change loading ratio to more negative value (e.g. R = -6.8) and additionally extended life time of spring subjected to higher stress amplitude. .

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Software applicationSoftware application

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Software applicationSoftware application4.1 Material # 1: Base

Level Description

Default

valueUnit

0 1 2 3value 0 1 2 3

Young's Modulus of Elasticity (E) 210 GPa

Poisson's ratio (ν) 0,3 -

Plastic constraint factor (m) 1,5 -( ) ,

Rel - MPa

Rp02 - MPa

Rm - MPa

Kmat, Jmat, Δmat - -

Charpy impact energy (Cν) J

or - - - - -

W ki t t (T) °CWorking temperature (T) °C

Temperature at 28J of Charpy impact

energy (T28J)

°C

Probability of failure (Pf) %y ( )

Yield strength at 0.2% plastic strain

(Rp0.2)

MPa

Tensile strength (Rm) MPa

Crack fracture resistance in terms of

K, J or δ (Kmat , Jmat , δmat)

N/mm1.5, N/mm, mm

Engineering stress-deformation curve

from single axis tensile test (R e)

File

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from single axis tensile test (R–e)

Stress-strain curve type - -


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Case studyCase study::A project for an Canadian Arctic harbour requires the design of a steel box beam

structure which dimensions are: width = 2.0 m, height = 1.5 m, thickness 25 mm.

The designer selects a HS steel with the following characteristics:Yield strength 410 MPaUltimate strength 630 MPaUltimate strength 630 MPa

The operational environment conditions in wintercorrespond to a minimum air temperature of –50°C.

l b l l l l t i 310 MPglobal plus local tension: 310 MPalocal plate bending 20 MPashape factor is a/c = 0.2 weld

F

F

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Case studyCase study-- Default option:Default option:

The Default option assessment requires only to know the steel yield strength and the service temperature and allows only to calculate C or T28J temperature for a given crack size.

For the application we decided to determine the necessary T28J value.

To start a crack size is fixed, equal to a = 10 mm and a T28J = -60°C (10°C below the service temperature. Considering the industrial practices, the Master curve risk level is fixed to 2.5%.

After introducing the stress distribution, linear within the plate thicknessthe software provides a critical crack length equal to 3 7 mmthe software provides a critical crack length equal to 3.7 mm..

This value does not correspond to a fail safe condition,the crack length is smaller than the plate thickness,the crack length is smaller than the plate thickness,so brittle fracture will occur before leakage

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Case studyCase study-- default option assessmentdefault option assessment::

Using the "Repeat calculation" function, the T28J value will be changed until obtaining by iteration a failure point on the limit state curve. The FAD is given in the figure:g g

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The T28J is found equal to -88°C without brittle fracture and with a critical crack length of 19.1 mm.

Case studyCase study-- basic option assessmentbasic option assessment::The Basic option assessment requires to know the steel yield and ultimate

strength, the service temperature and allows to determine Kmat or J or the CTOD.

To start a crack size is fixed, equal to a = 5 mm and a CTOD value is given equal to 0.1.a CTOD value is given equal to 0.1.

A first calculation provides a critical crack length of 11.8 mm.

This value does not correspond to a fail safe condition,the crack length is smaller than the plate thickness,so brittle fracture will occur before leakageso brittle fracture will occur before leakage.

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Case studyCase study-- default option assessmentdefault option assessment::

Using the "Repeat calculation" function, the CTOD value will be changed until obtaining by iteration a failure point on the limit state curve. The FAD is given in the figure:g

Th CTOD i f d l t 0 75 ith t b ittl f t

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The CTOD is found equal to 0.75 without brittle fracture and with a critical crack length of 16.5 mm.

assessmentassessment::The Advanced option, as the Basic option assessment, allows to determine Kmat

or J or the CTOD, requires the service temperature but also to know the steel stress-strain curve.

The stress-strain data are entered, but due to their small number, the "Approximate engineering R-e curve" function is applied.the Approximate engineering R e curve function is applied.

To start a crack size is fixed, equal to a = 5 mm and a CTOD value is given equal to 0 1a CTOD value is given equal to 0.1..

With the same stress distribution data than previously,a first calculation provides a critical crack length of 14.1 mm..

This value does not correspond to a fail safe condition,the crack length is smaller than the plate thickness,so brittle fracture will occur before leakage

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assessmentassessment::

Using the "Repeat calculation" function, the CTOD value will be changed until obtaining by iteration a failure point on the limit state curve. The FAD is given in the figure:g

Th CTOD i f d l t 0 25 ith t b ittl f t d

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The CTOD is found equal to 0.25 without brittle fracture andwith a critical crack length of 16.5 mm.

Case studyCase study-- results summaryresults summary::The application of the 3 FITNET´s option provides the following requirements for

the steel brittleness specification:

Default Optionminimum Kv = 28 J at –88°Cminimum Kv 28 J at 88 C

Basic Optionminimum CTOD = 0 75 mm at-50°Cminimum CTOD 0.75 mm at 50 C

Advenced Optionminimum CTOD = 0.25 mm at –50°C

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ConclusionConclusion

The application of the software of the Fracture module of the FITNET FSS Procedure to real case gives simple single but important information about: materials properties requirements. p p q

The software is possible to use in design stage (choice of right material) and using state.

Software is established with module structure which incorporates each componentas modul with known limit load solutions and stress intensity factor solution.y

The FITNET procedure represents with its sequential approach an ideal basis forcomputer manipulation.

Software check valid conditions for solutions and provide possiblity to changestress intensity factor with new solution or add new component.

computer manipulation.

Material modul includes rutine for input mechanical properties and fracture toughness.

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ConclusionConclusion

Thank you for your attention!Thank you for your attention!

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on-line monitoring of structures and fatigue limit

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