Lecture 10: Fatigue of welds

Transcription

1 Kul Fatigue of Structures Lecture 10: Fatigue of welds Learning outcomes After the lecture, you understand fatigue phenomena in welded structures know the main influencing factors for fatigue strength of welded structural can apply the common methods for fatigue strength assessment of welded structures 2 1

2 Contents Main influencing factors for fatigue strength of welded joints Fatigue assessment of welded structures Fatigue approached for welded structures Reading: Metal Fatigue in Engineering: Chapter 12 3 Complex structural geometry 4 2

3 Variable local geometry Complex structural geometry Undefined micro geometry 5 6 3

4 Sharp undercuts and inclusions 7 Even welds with good profiles may have sharp defects 8 4

5 Welds cause both structural and local stress concentrations F A F m A Membrane stress m m Bending stress b b Nonlinear peak stress nlp nlp 9 Welds have global stress concentrations 10 5

6 Welds have global stress concentrations 11 Stress concentrations reduce fatigue strength welds have both local and global stress concentrations Tensile mean stresses reduce fatigue strength welds have high residual stresses that act as mean stresses STRESS, S Residual Stress = + Sy Effective local stress S loc Material strength affects crack initiation but not crack growth weld fatigue is dominated by crack growth 0 Nominal stress S n due to applied load cycles TIME 12 6

7 Welds have much lower strength that the base materials due to early crack initiation and high tensile welding stresses

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12 Fatigue assessment of welded structures 23 Fatigue assessment of welded structures 24 12

13 Fatigue assessment of welded structures IIW recommended methods for fatigue assessment of welded structures Nominal stress Hot LEFM spot macro-geometric stress effects weld toe radius fabricated structure typical material thickness Effective notch stress cyclic plastic zone size size scale, m 25 Fatigue assessment of welded structures IIW recommended methods for fatigue assessment of welded structures 26 13

14 Fatigue assessment of welded structures crack tip elements / cm elements / cm elements / cm element / cm 3 27 Nominal Stress Approach F My A I F 28 14

15 Nominal Stress Approach 29 Nominal Stress Approach FAT class gives the allowable stress at 2x10 6 cycles to failure For shorter lives use m N C, where C is the fatigue capacity. 6 Therefore, for any class 2x10 3 FAT C FAT C x x x

16 Influence of misalignment F F average misalignment is already includes in the FAT class extra misalignment is include with a stress magnification factor, k m 31 Nominal Stress Approach Fatigue life 32 16

17 Nominal Stress Approach 33 Nominal Stress Approach 34 17

18 Nominal Stress Approach 35 Influence of stress ratio Stress range Normal stress () Shear () 1 slope Endurance, cycles 36 18

19 Influence of stress ratio time time R min max time 37 Influence of thickness 38 19

20 Influence of temperature 39 Structural Stress approach In many structures nominal stress is not possible to define Weld details do not always look like the categorized details Finite Element Analysis is often a basic tool for designers 40 20

21 Structural Stress approach M Large opening M Curved beam M Shear lag p F M Flange curling Discontinuity effect in a shell Eccentric joint 41 Structural Stress approach Welds have stress concentrations due to misalignment Source: Niemi, E., Designers guide for hot spot fatigue analysis, IIW document XIII-WG ,

22 Structural Stress approach Welds have global stress concentrations 43 Structural Stress approach F m A F A Membrane stress m Bending stress b Nonlinear peak stress nlp m b Structural stress method use m + b 44 22

23 Structural Stress approach Two -N lines depending of weld type FAT 90 load carrying FAT 100 non-load carrying 45 Structural Stress approach Cases a e are good weld designs Structural hot spot approach can be used In cases f j cracks grow from the root side of the weld; good design practice normally avoids root cracks a) b) c) d) e) f) g) h) i) j) 46 23

24 Structural Stress approach Hot spot stress: The structural stress at the location of expected fatigue crack initiation Nonlinear stress peak Total stress Structural stress 0.4 t 47 Structural stress Hot spot stress is dependent on plate thickness Linear extrapolation to weld toe In some cases quadratic extrapolation Structural hot spot stress Non-linear stress peak Weld toe Strain gauge A Strain gauge B 48 24

25 Structural stress Two gages at 0.4 t and 1.0 t Three gages at 0.4t, 0.9t and 1.4 For fixed distance gages, a function may be fitted Structural hot spot stress Non-linear stress peak Weld toe Strain gauge A Strain gauge B 49 Use of FEA in structural stress analysis Use of finite elements a) b a b) Solid elements a a Shell elements - Including weld -Excluding weld c) d) l + t/2 l + t/2 Thickness t l = weld leg length 50 25

26 Model examples Element length = 1.0 t Extrapolation points Element width Extrapolation points at nodes Shell elements Solid elements 51 Notch stress approach Notch stress method aims to calculate the fatigue effective stress at notch tip Extremely tedious, but flexible method for fatigue assessment of details such as weld profiles etc

27 Notch stress approach Notch stress method aims to calculate the fatigue effective stress at notch tip Extremely tedious, but flexible method for fatigue assessment of details such as weld profiles etc. Kt1 t t/ 53 Notch stress approach K t 1 t Lawrence equation as decreases, K t increases t = plate thickness = weld toe radius = geometry constant related to structural stress concentration K au f K t 1 1 Petersen equation 1 a as decreases, K f decreases (constant K t ) 54 27

28 Notch stress approach 35 stress concentration Kt Kf toe radius Lawrence - worst case notch theory 55 Notch stress approach Neuber microsupport theory and Radaj notch rounding approach f = effective notch radius * = 0.4 mm material constant (365 MPa) s = 2.5 plane strain multiaxial constant = actual notch radius (default 0.0 mm) stress concentration Kt Kf Effective notch: f = +s* toe radius 56 28

29 Notch stress approach 57 Notch stress approach 58 29

30 Notch stress approach 59 Notch stress approach

31 Notch stress approach 2 61 Notch stress approach 62 31

32 Notch stress approach 63 Notch stress approach 64 32

33 Notch stress approach Effective stress based on FAT 225 Constant slope 3 Fatigue limit at 1x10 7 cycles (constant amplitude) Bi-linear curve for variable amplitude loading 65 Fatigue assessment of welded structures Stress, Nominal stress: Many -N lines depending of weld type Structural stress: Two -N lines depending of weld type Notch stress: One -N line for all welds Cycles to failure 66 33

34 Project work proposal - optional Estimate fatigue resistance i.e. S-N curve for thin deck structure Use one or several approaches Utilize IIW or DNV 30.7 fatigue design recommendation If you are would like to participate, please send to heikki.remes@aalto.fi. The fist meeting will be organized on Thursday workshop at