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Assessment of Partial Penetration and Full Thickness

Welding in Francis runners by Fracture Mechanics Approach

1Assessment of partial penetration weld | Wilhelm Weber | 2012-12-05

Wilhelm Weber, Li Chen, Ulrich Seidel, Jiri Koutnik

Freiburg, 2012-12-05

Outline

1. Overview, nomenclature and objective1. Overview, nomenclature and objective

2. Calculation and verification of stress intensity factors

3. Fatigue crack growth

4. Material resistance

5. Residual stresses

6. Assessment procedure


6. Assessment procedure

7. Summary

Overview, nomenclature and objective


Overview of hydro power plant

Intake, intake gate

Headrace tunnel

Penstock

Powerhouse with turbine and

generator


generator

Draft tube

Tailrace tunnel

Overview power unit

Generator

Bearing

Shaft

Head cover

Spiral case, stay vanes, wicket gates


Spiral case, stay vanes, wicket gates

Turbine runner

Discharge ring, draft tube

Overview turbine runner

Crown

Turbine inlet

Blades

Band

Seal ring


Seal ring

Turbine outlet

Partial penetration welding

Complete joint weld – Full thickness weld – Partial penetration weldComplete joint weld – Full thickness weld – Partial penetration weld

Distribution over blade

crown/band crown/bandcrown/band


Undefined notch geometry inside Treatment as a crack

CJW

CJWFTW PPW

Load scenario

• Simplified stress vs. time scenario • Various load cases• Simplified stress vs. time scenario

Static stress

Dynamic stress

LC A LC B LC C

• Various load cases

• High frequent loading due

to rotor-stator-interaction,

part load vortex, etc.

• Low frequent loading due

to change of load case

• Transient load cases, e.g.


LC A LC B LC C• Transient load cases, e.g.

start, stop, etc.

Objective

• Proof of

relative increase of gap

• Proof of

• Static strength for maximum occurring static loading:

, : fracture toughness

• Infinite fatigue life for high frequent cyclic loading (HFCL) (e.g. RSI):

, : threshold value

• Limited fatigue crack growth for low frequent cyclic loading (LFCL):

(e.g. Start/Stop, load change, etc.)

9

(e.g. Start/Stop, load change, etc.)

, : relative increase of gap

• Evaluation of cutting planes

Assessment of partial penetration weld | Wilhelm Weber | 2012-12-05

welding gap

b

Calculation and verification of stress intensity

factors


Relevant crack model

• Center crack in a plate• Center crack in a plate


+ Fits to orientation of blade

- Ratio of crack length to plate width

depends on connection angle

- Geometric function only for tensile

loading

+ Geometric function for all three

crack opening modes

+ Ratio of crack length to plate

width constant

- Geometric situation only roughly

approximated

• Comparison of crack models

Discussion of different crack models

• Comparison of crack models

Tabulated

data for

SIFs [FKM]

[FKM]

12

• 3 cases of transfer: - Aggressive:

- Moderate:

- Conservative:


Comparison of crack models

• Normalized equivalent SIF:• Normalized equivalent SIF:

conservative

aggressivemoderate

reference

Equivalent SIF according

to FKM-guideline:


Consequences for application

• Conclusion:• Conclusion:

• Good agreement for small cracks

• Moderate transfer model of crack length shows comparable results

• For turbine application aggressive model applicable

Reason: relevant plate width 2t* = 2t/cos(β)


• With crack• Without crack

Verification with 2D

Distribution of v. Mises stress

• With crack• Without crack

LC A


LC B

• Normal and shear stress along path

Behavior of stresses in 2D calculation

• Definition of path• Normal and shear stress along path

through non-welded cross section

• Definition of path


Linearization through

non-welded area

Calculation of stress intensity factors

• Definition of SIFs:• Definition of SIFs:

• Geometric functions [FKM]:

membrane bending

constant 1st order

term


Comparison of analytical SIF with numerical

ones from 2D simulation

Left crack tip Right crack tipLeft crack tip Right crack tip

LC A

Numerically obtained SIF

(reference)

SIF based on stress

at crack tip

SIF based on

linearized stress


LC B

Remarks:

• Good quality of SIFs

based on linearized

stress

• Overestimation of SIF

by using maximum

stress of crack linere

fere

nce

• Distribution of v. Mises stress

Verification with 3D simulation

• Boundary element model • Distribution of v. Mises stress• Boundary element model


• Geometry of analyzed gap

3D simulation for determination of SIFs

• Behavior of SIFs along gap• Geometry of analyzed gap

PPW

• Behavior of SIFs along gap

crack closure


FTW

crack closure

locations of cutting

planes under investigation

Distribution of stresses

• Stress distributions along different paths

Normal stress

Planar shear

stress

Non-planar shear

stress

• Stress distributions along different paths


• Analysis of SIFs• Locations for SIF-calculation

Comparison of analytical with numerical SIFs

from 3D simulation

• Analysis of SIFs• Locations for SIF-calculation

Numerically obtained

SIF (reference)

SIF based on stress

at crack tip

SIF based on

linearized stress

refe

rence

refe

rence


refe

rence

refe

rence

Influence of locally

concave crack front

Fatigue crack growth


Transition of load cases

Limited fatigue crack growth

• Crack propagation rate of transition i:• Crack propagation rate of transition i:

depending on stress ratio

• Crack growth:

; number of occurrence of transition i

forfor

24

; number of occurrence of transition i

• Implicit time integration scheme


Material resistance


Material resistance

• Fracture toughness for static assessment:• Fracture toughness for static assessment:

• VH material database

• Threshold value for crack growth [IIW-recommendations]:

26

• Crack growth rate [IIW-recommendations]:


Residual stresses


Residual stresses

• Based on residual stress measurements• Based on residual stress measurements

• Hole Drilling Method

• Surface and subsurface values available

• Through thickness distribution acc. to FKM-

guideline approximated

• Consequences for assessment:Test specimen

28

• Consequences for assessment:

• Stress intensity factor:

• Cyclic stress intensity factor:

• Stress ratio:


Test specimen

Assessment procedure


• Static stress distribution

Assessment procedure I/II

• Static stress distribution

• Determination of maximum equivalent SIF along relevant area:

Full loadSpeed no load

30

• Determination of maximum equivalent SIF along relevant area:

welding gap plus safety length due to manufacturing tolerances

with


Assessment procedure II/II

• Determination of cyclic equivalent SIF in the same way • Determination of cyclic equivalent SIF in the same way

• Estimation of residual stress and calculation of

• Definition:

• Calculation of maximum possible gap sizes

, : Safety factor on stress

31

• Check of fatigue crack growth: , relative increase of gap


Summary


Summary

• Turbine runners are typically designed as welded structure.• Turbine runners are typically designed as welded structure.

• PPW is used to reduce manufacturing time and cost.

• Through-thickness crack in plate is chosen as crack model.

• Crack model is verified by comparison with 2D and 3D numerical

simulation.

• Fatigue crack growth can be assessed for cyclic loading due to load

33

• Fatigue crack growth can be assessed for cyclic loading due to load

change.

• Residual stresses are approximated for assessment procedure.


Contact:Contact:

Dr.-Ing. Wilhelm Weber

Corporate Technology – R&D, Basic Development

Tel. 07321 37-9576

[email protected]


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