FatigueFatigue lifelife predictionprediction for spheroidal for spheroidal graphitegraphitecastcast ironiron basedbased onon crackcrack growthgrowth from from defectsdefects

detecteddetected bybyXX--rayray tomographytomography

Mehdi ShiraniDept. of Engineering Design and Materials

NTNU, Trondheim

2

SGCI ENSGCI EN--GJSGJS--400400--1818--LT is used inLT is used inwindwind--turbine hubs (Vestas Castings, Kristiansand)turbine hubs (Vestas Castings, Kristiansand)

3

To establish methodology and improve data to optimize large wind turbine components (by reducing the weight and increasing fatigue life)

FeVIND Main GoalFeVIND Main Goal

4

MicroMicro--graph of gas pores in ductile cast iron graph of gas pores in ductile cast iron (Anders (Anders BjBjöörkbladrkblad, KTH, 2008), KTH, 2008)

5

FeVIND FeVIND activitiesactivities

6

Random DefectSingle DefectExplicit FCG analysisda/dn = f(Ds, a; R),

Weakest LinkLocal StressImplicit FCG analysisS-N-curve (a > 1 mm),

‘crack initiation’

ProbabilisticDeterministicMaterial properties

Crack Growth

PP••FAT FAT Probabilistic Fatigue Assessment Tool Probabilistic Fatigue Assessment Tool developed at NTNU/IPM 2003developed at NTNU/IPM 2003--20072007

7

cross-sectional image of a Motorbike Casting obtained by Computed Tomography

XX--ray Computed Tomographyray Computed Tomography

8

SchematicSchematic ofof a CT systema CT system

9

By post processing CT results, it is By post processing CT results, it is possible to access size and location of possible to access size and location of

defectsdefects

10

Using previous data, P Using previous data, P •• FAT draws the defects FAT draws the defects and calculates fatigue life and calculates fatigue life

11

Is it possible to do CT for large parts such as wind turbine Hub?Due to the limited volume of steel that can be examined by conventional inspection methods, the number and size of defects in a large volume have to be estimated by statistical analysis

What is the limitation of this method?What is the limitation of this method?

12

P P •• FAT Random Defect ApproachFAT Random Defect ApproachFor every finite element of a component, random defects are For every finite element of a component, random defects are generated based on the underlying statistical distributions generated based on the underlying statistical distributions

(number, location, size).(number, location, size).

13

Example based on data obtained Example based on data obtained with a standard with a standard optical microscope for ENoptical microscope for EN--GJSGJS--400400--1818--LTLT

Material Properties

0.05number of defects per unit volume

0.05289 mmDefect size scale parameter

0.05973 mmDefect size location parameter

0Defect size shape parameter

8.56 (MPa, M)Threshold stress intensity range (R=0.1)

1.878E-13C (R=0.1)Fatigue Crack Growth data

4.4mFatigue Crack Growth data

245 MPaYield strength

403 MPaTensile strength

150 MPaFatigue limit (R=0.1)

14

Stress range 243 Stress range 243 MPaMPa, R=0.1, Probability of failure is 80%, R=0.1, Probability of failure is 80%**********************************************************************PFAT - Main Result FileDATE: 27/ 1/ 2009TIME: 22: 22: 7**********************************************************************FATIGUE ASSESSMENT MODULE: RANDOM DEFECT**********************************************************************Operating Finite Element File:C:\PFAT\FEA\SAMPLE12.vtfResidual Finite Element File:NoneCrack Growth Model: Short Crack Growth ModelNumber of Analysed Components: 10**********************************************************************ParametersReference Load Amplitude: 1.21500000000Reference Mean Load: 1.48500000000Reference Residual Load: 0.00000000000Fatigue Limit (amplitude) [MPa]: 150.000Stress Ratio for the Fatigue Limit: 0.100000Critical Crack Depth [mm]: 6.00000Crack Growth Exponent: 4.40000Crack Growth Constant [MPa,m]: 1.878000E-13Stress Ratio for the Crack Growth Constant: 0.100000Threshold Stress Intensity Factor Range [MPa,m]: 8.56000Stress Ratio for the Threshold Stress Intensity Factor Range: 0.100000Walker Exponent: 0.780000Expected Number of Defects Per Unit Volume: 5.000000E-02Defect Size Distribution: Generalised Extreme\GumbleDefect Size Location Parameter [mm]: 5.973000E-02Defect Size Scale Parameter [mm]: 5.289000E-02**********************************************************************col1: Component Numbercol2: Fatigue Life**********************************************************************

1 4732862 2124093 3015484 7876285 No Critical Defects6 No Critical Defects7 5374418 5525159 563543

10 434078

15

Probability of failure vs. stress range obtained by Probability of failure vs. stress range obtained by P P •• FATFAT

Probability of failure vs stress range

0

20

40

60

80

100

120

210 220 230 240 250 260

stress range (MPa)

%

16