OVERVIEW OF FATIGUE EVALUATION INCORPORATING WELD ... · • Hobbacher, A.F., “Recommendations...

SIMULIA fe-safe User Group Meetings 20161

Jeong K. Hong, Ph.D.

Technical LeadCenter for Welded Structures Research

Battelle

http://www.battelle.org/verity

OVERVIEW OF FATIGUE EVALUATION INCORPORATING

WELD IMPROVEMENT TECHNIQUES USING BATTELLE

STRUCTURAL STRESS METHOD

Outline• Battelle Structural Stress Method (BSSM)

• Weld Improvement techniques

� Defect removal

� Stress improvement

• Observation of fatigue improvement data

� Comparison with Stress Relieved (SR) data

� Characteristics of S-N behavior

− Effect of Load Ratio

− Slope of S-N curve

• Construction of Design S-N Curves for Weld improvement Techniques

• Concluding remarks


Battelle Structural Stress Method


+

bms σσσ +=

Actual Stress state

at a Joint

Equilibrium-Based

Decomposition

(Nodal Force/Moment)

Master S-N Curve Based

Data Correlation or Life

Estimation

mm

m

ss

rIt

S 1

2

2

)(⋅

∆=∆−

∗

σWeld

t σx (y)τ (y)

σm σbσm σb

Weld

t τm

Structural Stress: Equilibrium Equivalent

Weld

t

Notch Stress: Self-Equilibrating

Stress Concentration

Thickness Loading Mode

ref

*

bm

b

t

tt,

σσ

σrwhere =

+=

I(r) –by integrating two stage crack

growth law expressed by SIF

solutions.

m - slope of crack growth rate data


10

100

1000

10000

100000

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

Life

Eq

. SS

Ran

ge,

MP

a

Mean +3*STD -3*STD

+2*STD -2*STD S-N data

σ= 0.247

~1000 fatigue tests

hrange NCS /1−⋅=∆

11577.9

34308.1

13875.8

28626.5

19930.2Mean

hC

3.125

+2σ

-2σ

+3σ

-3σ

Statistical Basis

Master S-N Curve Parameters

Different Joint Types, Loading Modes, Sizes

1

10

100

1000

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Ap

pli

ed

Str

ess

Ra

ng

e, M

Pa

Cycles to Failure

Conventional Representation Using Structural Stress

For Steel Welds

•ASME Div 2, API 579/ASME FFS-1 (2007)

•BV NT3199 (2013)

Master S-N Curve Approach Adopted by Codes

& Standards

Generalized Procedures

for Linear Shell/Plate Element Models


+

+=

nn f

f

f

f

llll

llll

ll

F

F

F

F

.

.

......0063

)(

60

063

)(

6

0063

.

.3

2

1

3322

2211

11

3

2

1

Coordinate rotations and solving

simultaneous equations:

�� Normal (∆σs) Structural Stress (Mode I)

In-plane shear (∆τs) Structural Stress (Mode III) �� f�� 6m��t�

How to Improve Weld Fatigue Strength (I)

• Main Ideas

� Yield Stress Level at Weld >> Lower Residual Stress Level (Stress

Improvement)

� Weld Defect >> Improve Weld Quality (Defect Removal)

� High Stress Concentration >> Reducing Stress Concentration

• Applications

� Traditional Techniques

− Toe Grinding, TIG Dressing, Hammer Peening, Profiling, Shot Peeing,

etc.

� Recent Techniques

− High Frequency Mechanical Impact, etc.


How to Improve Weld Fatigue Strength (II)

• Improvement Techniques

� Improvement of Weld Profile

− Toe Grinding

− TIG Dressing

− Profiling

� Improvement of Residual Stress Conditions

− Peening (Hammer, Shot)

− High Frequency Mechanical Impact


Typical Toe Profiles for Improvement Techniques

compare to AW condition (Pedersen, 2009)


TG (Toe Grinding)

TD(TIG Dressing)

HFMI

Comparison of AW Fatigue Test Data with SR:

Different R effect (Maddox, 1982)

As-Welded vs. Stress Relieved

R= -1

R= 0., 0.5, 0.67

Stress relieving effect of welded joints is

apparent when the applied loading introduces

stress fluctuations in compressive loading.


R = -1, 0, 0.5, 0.67

t=12.7

Mild Steel, BS 4360,

G50B, S70, QT445A

∆��∗� ∆��∗�!�� ∙ # $ %� ∙ &∗ ��'()$)&∗ � * 1 � &'(),& - 01 � &'(),& / 0

0102 ∝ 4567�∗ 89 4∆:98�

An Equivalent ΔK Definition Incorporating Load

Ratio EffectsUsing ∆∆∆∆K* definition & two-stage crack growth model:

Equivalent SS range incorporating R:

σ

Time (t)

maxσ

minσ meanσ

σ

Time (t)

maxσ

minσ

meanσ

0 ≥R

0<R

+∆=∆ σσ

+∆σ

σ∆


∆:∗ � ∆:;:�1<'()$) =∆:; �∆:9, :�1< � ∆:91 � & '(),& - 0∆:; �:�1< � ∆:91 � & '(),& / 0

Two-stage crack growth model

Comparison of Fatigue Test Data Considering R

effect (Maddox, 1982)

As-Welded vs. Stress Relieved

AW - ∆SS

SR - ∆SS (R≥0), ∆SS/R* (R<0)

Master

S-N Curve


R = -1, 0, 0.5, 0.67

Inverse Slope of SR =

Inverse Slope of AW

t=12.5mm. Load Carrying Cruciform Joint

As-Welded

Hammer

Peened

Toe

Ground

As-Welded

R=0, -1, 0.5

Insensitive to R

Insensitive to R

Sensitive to R

As-Welded

Weld Fatigue Data Applying Weld Improvement Techniques:Various Load Ratio (R = σσσσmin /σσσσmax) Effects


S-N Behavior of Weld Fatigue Improvement

Technique Data

10

100

1000

10000

1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

Eq

. SS

Ran

ge,

MP

a

Life

Mean+3*STD-3*STD+2*STD-2*STDGrindingHammer PeeningTIG Dressing

As-Welded Master Curve

Inverse Slope = 4

Toe Ground, Hammer Peened, TIG Dressed

Master

S-N Curve


Weld Improvement Recommendations

- Industry Codes and Standards

• Inverse slope (m) = 3 (Same m for AW) in the weld

improvement guideline for Burr Grinding, TIG Dressing,

Hammer Peening, Needle Peening

� 2nd inverse slope (m2=5 for VA, m2=∞ for CA) at N=1x107 cycles

• Hobbacher, A.F., “Recommendations for Fatigue Design of Welded Joints and

Components,” 2nd ed., Springer, 2016

• Recommended Practice DNVGL-RP-0005:2014-06, “RP-C203: Fatigue Design of

Offshore Steel Structures,” DNV GL AS, 2014


Comparison of Toe Grinding and TIG Dressing

Data

Reference Weld Improvement Techniques

Toe Grinding (TG) TIG Dressing (TD)

IIW (m=3) Mean-2*STD 1.3 1.3

DNV-RP-203

(m=3)

Mean-2*STD1.52 1.52

SSM

(m=3.13)

Mean Curve 1.44 1.47

Mean-2*STD 1.38 1.41

# of Data 333 82

Standard

Deviation (σ)0.274 0.273


Factor on Stress Range

Comparison of Hammer Peening & HFMI Data

Reference Weld Improvement Techniques

Hammer

Peening

Hammer

Peening

(R effect)

HFMIHFMI

(R effect)

IIW (m=3) Mean-2*STD 1.3*, 1.5**

DNV-RP-203

(m=3)Mean-2*STD 1.59

SSM

(m=3.13)

Mean Curve 1.52 (All) 1.54 1.25(All) 1.43

Mean-2*STD 1.35 (All) 1.42 1.05(All) 1.31

# of Data65 (All)

36*, 29**65

209(All)

20*, 189**209

Standard

Deviation

.328 (All)

.302*, .352**

.303,

.295*, .315**

.367(All)

.401*, .362**

.306

.408*, .295**


Factor on Stress Range

*Sy≤355 MPa

**Sy>355 MPa

HP, HFMI– Strong R effect is

observed.

FE Analysis Procedure

from References

1. Actual weld shapes (AW and

improvement techniques) were

modeled.

2. Stress Concentrations were

calculated from the FE model (1).

Togasaki, et al. (2010)

Kim, et al (2006)

Toe ground

Mori, et al. (2012)


Battelle’s FE Modeling Procedure

• One representative model is used

for among AW and Weld

Improvement ones.

� Nominal weld shape for AW (3D

model)

• Stress concentration is calculated

from the representative model

using SSM.

• Life prediction is calculated using

the SS based design master S-N

curve constructed for different

weld improvement techniques.


One FE model regardless of weld

conditionNominal

weld shape

Construction of Design Master S-N Behavior of

Toe Ground


Inverse slope (-1/h) =4.0

# of data: 333

σ = 0.300

R: =-1 ~ 0.5

No R effect is observed


TIG Dressed



# of data: 82

σ = 0.337 No R effect is observed

R: =0 ~ 0.1


Hammer Peened (Considering R)



# of data: 65

σ = 0. 306

R: =-0.5 ~ 0.5


HFMI (Considering R)



# of data: 209

σ = 0. 330

R: =-1 ~ 0.827

Fatigue Design S-N Curve comparison (I)

• AW vs. Toe Grinding (TG) & TIG Dressing (TD)


AW

Mean: TD ≅ TG

Mean-2*STD: TG ≅ TD

Fatigue Design S-N Curve comparison (II)


AW

(No R effect)

Mean: HP > HFMI

Mean-2*STD: HP > HFMI

• AW vs. Hammer Peeing (HP) & HFMI

Concluding Remarks (I)

• BSSM based design S-N curves for Toe Grinding (TG), and TIG

Dressing (TD), Hammer Peening(HP), and HFMI have been

constructed.

� Inverse slope of the design curve = 4 is used regardless of the

specifics of the weld improvement techniques.

� The improvement techniques becomes effective when N >

10,000 cycles.

� Toe Ground, TIG Dressed data do not show Load Ratio (R)

effect.

� However, Hammer Peened, and HFMI data show strong Load

ratio (R) effect.


Concluding Remarks (II)

• Comparison of Weld Improvement Techniques

� For Toe Ground, TIG Dressed data

− Fatigue improvement: TD ≅ TG : Mean Curve & Mean-2σCurve

� For Hammer Peened, and HFMI data

− Fatigue improvement: HP > HFMI : Mean Curve & Mean-

2σ Curve


Acknowledgment


The authors gratefully acknowledge the valuable discussions and

partial funding for this study provided by the Battelle Structural

Stress JIP sponsors.

For further informationContact:J. K. Hong [email protected]

OVERVIEW OF FATIGUE EVALUATION INCORPORATING WELD ... · • Hobbacher, A.F., “Recommendations...

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