reliable and efficient fatigue design and life prediction

GCRC-SOP 2nd Year International Workshop

Project # 2-1-1

Reliable and Efficient Fatigue Design and Life Prediction Method for Ship and Offshore

Structures

1

Background

Objectives

Research thrust areas

Highlights of results to date

Publication and industry liaison activities

Concluding remarks

Project 2-1-1: Reliable Fatigue Evaluation Method

2

Fatigue is one of critical design and fitness-for-service concerns in ship and offshore structures

Existing methodologies are empirical in nature and difficult to implement in practice, i.e. two critical issues

1. What stress to use? 2. Which S-N curve to use?

Project 2-1-1: Reliable Fatigue Evaluation Method

Project 2-1-1: Reliable Fatigue Evaluation Method 3

A traction-based structural stress method was developed by Dong et al (2001) and supported by two major JIPs

• Mesh-insensitivity in stress concentration determination

• Master S-N curve representation of thousands of large scale fatigue tests

• Adopted by ASME BPV International Code

• BV to publish Guidance Notes shortly

t x (y)

t

t

Structural Stress: Equilibrium Equivalent

Notch Stress: Self-Equilibrating

m b

Project 2-1-1: Reliable Fatigue Evaluation Method 4

2''

t

6

t

fxy

bms

m Normal:

2

''

t

6

t

f yxbms

m In-Plane Shear:

Two dominant traction components:

Nodal Force Vector

Line Force Vector

n

nn

nnn

n f

f

f

f

ll

lll

llll

llll

ll

F

F

F

F

3

2

1

11

112

3322

2211

11

3

2

1

3600

63

000

00636

0

00636

00063

y’x’

N1

N2

N3Nn

E1

E2

E3

En

WeldNodes at Weld

Shell element model

Traction-Based Structural Stress Method

Background - Continued

5Project 2-1-1: Reliable Fatigue Evaluation Method

Focus on rat hole end

Bracket

Web Frame

Side Shell

Longitudinal Stiffener Web

Web Frame Stiffener Web

A Side-Shell Connection

Example: A Full Scale Side-Shell Connection (B. Healy/Technip)

0

1000

2000

3000

4000

5000

abaqus-8r abaqus-4 abaqus-4r nas tran-8r nastran-4

2t

t

0.5t

0.25t

0.125t

0

1000

2000

3000

4000

5000

abaqus-8r abaqus-4 abaqus-4r nas tran-8r nastran-4

2t

t

0.5t

0.25t

0.125t

HSS (.5t/1.5t)

0

1000

2000

3000

4000

5000

6000

abaqus-8r abaqus-4 abaqus-4r nastran-8r nastran-4

2t

t

0.5t

0.25t

0.125t

HSS (.4t/1t)

0

1000

2000

3000

4000

5000


2t

t

0.5t

0.25t

0.125t

0

1000

2000

3000

4000

5000


2t

t

0.5t

0.25t

0.125t

Current Structural Stress MethodTraction-Based Structural Stress Method

Background - Continued


1.E+01

1.E+02

1.E+03

1.E+04

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

Life

Equiv

ale

nt Struct

ura

l Stress

Range, M

Pa

Equivalent Traction Stress Range

mm

ms

s

rIt

S1

2

2

)(

1.E+01

1.E+02

1.E+03

1.E+04

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

Life

Nor

min

al S

tres

s R

ange

, MPa

ASME Mean

ASME III Design

Markl’s Equation(Mean Line for i =1)

BS5500 Design(Smooth ground butt welds)

Nominal Stress Range

mm

ms

s

rIt

S1

2

2

)(

“Thickness Effect” “Loading Mode Effect”

“Stress Concentration Effect”

mm

ms

s

rIt

S1

2

2

)(

mm

ms

s

rIt

S1

2

2

)(

“Thickness Effect” “Loading Mode Effect”

“Stress Concentration Effect”

Master S-N Curve Representation of Test Data

Objectives and Major Thrust Areas


Objectives:

• Develop a reliable, efficient, comprehensive fatigue design and evaluation procedure for ship and offshore structures

Major thrust areas

• Low cycle fatigue (e.g., cycles to failure less than 104-105)

• Theoretical basis for performing counting for arbitrary variable amplitude multi-axial loading

• Very coarse FE model based fatigue screening procedure

Results of Topic 1: Structural Strain Method


Through-thickness stress

t t

Linear Elastic Elastic-Plastic

Y

b

Y

m

SStc

3

213

2

Y

m

S

te

2

e

tc

22

t

O

O

mb

e

YS

YS

oo ,'

ii ,'

c

2

1

2

1

te

R

te

R

i

o

R

Et

RS

tE

b

Y

mm

2'

'

σm+σb

cE

S

RY21

Validation by TWI’s Test Data


100

1000

10000

100 1000 10000 100000

Equivalent Structural Stress Range, M

Pa

Cycle to Failure

Pseudo‐Elastic, Sy=544Mpa Pseudo‐Elastic, Sy=436Mpa

Pseudo Elastic, Sy=726Mpa Elastic only, Sy=436Mpa

Elastic only, Sy=544Mpa Elastic only, Sy=726Mpa

Lines: 2007 ASME master S-N curve:

mean ± 2σmean ± 3σ

Validation by DSME’s Test Data


100

1000

10000

100 1000 10000 100000

Equivalent Stress

Cycles

Mean

+2STD

‐sSTD

+3STD

‐3STD

DSME

2007 ASME’s Mean±3σ

2007 ASME’s Mean±2σ

Results of Topic 2: Multiaxial Fatigue – A Cycle Counting Law


A path-dependent cycle counting procedure was first proposed by Dong et al (2010), but without providing its mechanics basis

This investigation is aimed at providing a theoretical basis on cycle counting requirements for both uniaxial and multiaxialloading conditions

Cycle counting law:

Among all possible fatigue cycle definitions, the correct one should compute cycles in such a way that it yields the maximum damage

out of all possible paths or an assemblage of paths in space (stress or strain) in a shortest time in a given fatigue loading history

Supported byo The 2nd law of thermodynamics: Entropy reaches maximum at an equilibrium stateo The law of maximum entropy production: A system will select the path or assemblage

of paths out of available paths that minimizes the potential or maximizes the entropy at the fastest rate given the constraints

Implementation of Cycle Counting Law - Four Typical Load Path Patterns


e**

b

c

d

e

b

b b

c

d

c c

d d

e e

e c*

d*

e*

d*

c*

b* b** b* b**

e*

Four basic load-path patterns in general loading histories

Loading pattern Half-cycle 1 Half-cycle 2 Half-cycle 3

(a) b-e c-d d-c*

(b) b-c c-d d-e

(c) d-e c-d b-c

(d) c-d b-c d-e

Cycle Counting Results

Implementation of Cycle Counting Law - 2-D, 3-D, n-D Cycle Counting


P

Q R

R*

x

t 0r0r

s

'zII

sIII

222

12

)()( IIIIIIIIIIe

m

e

m

e

ddddS

dSadSda

2D

3D

Implementation of Cycle Counting Law - Validation by Experimental Data


Sonsino-Kueppers

1.E+02

1.E+03

1.E+04

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

Lo

ca

l eq

. str

es

s a

mp

litu

de

, MP

a

Bending-V Torsion-V

in-phase-V out-of-phase-VPure Bending Pure Torsion

In-plane (Bending+Torsion) Out-of-plane (Bending+Torsion)

EEHS

Sonsino et al (2008) (Path-Dependent Maximum Range Cycling Counting)

PDMR Method

1.E+02

1.E+03

1.E+04

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

Eff

. S

S R

an

ge

, M

Pa

Bending-V Torsion-V

in-phase-V out-of-phase-V

Pure Bending (Sonsino) Pure Torsion (Sonsino)

In-Phase (Sonsino) Out-of-Phase (Sonsino)

PDMR

Results of Topic 3 - Very Coarse Model Fatigue Screening Procedure


The traction-based structural stress method showed mesh insensitivity using models with element size of multiple t’s

For global fatigue screening, it is desirable to have element size in the order of stiffener spacing

This project: a new virtual node method has been developed and demonstrated its promise in doing just that.

f1’

f1

f2

L

l

(1)

(2)f2’

f2’

Old (Dong et al, 04) New (This project)

Coarse Model Fatigue Screening Procedure - Example 1


Element size=50t

Element size=25t

Element =12.5t

Element size=100t)

t=5mm

Geometry

Reference Model

Coarse Models

Example 1 - Results


1

2

3

4

5

6

7

SC

F

VNM - Old

VNM - New

3D Solid Model

Coarse Model Fatigue Screening Procedure - Example 2


Geometry

Reference Model

Element size=50t

Element size=25t

Element =12.5t

Element size=100t)

Coarse Models

Example 2 - Results


1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

SC

F

VNM - Old

VNM - New

3D Soid Model


A 20% Underestimate in SCF Seems Consistent in Various Examples Analyzed

Example 1 Results After Multiplying 1.2

Example 2 Results After Multiplying 1.2

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5S

CF

VNM - Old

VNM - New (x1.2)

3D Soid Model

1

2

3

4

5

6

7

SC

F

VNM - Old

VNM- New (x1.2)

3D Solid Model

21

International Journal Publications Three journal papers prepared

One submitted in Nov. 2012 to Int. J. of Fatigue

Two are going through internal reviews and to be submitted next month

Education 1 Post-Doc (20%)

1 Ph.D. Candidate (100%)

Enter Project No: Project Title in Slide Master

22

Industry-University Liaison Conducted a half-day Workshop on “Advanced Analysis Methodologies

for Offshore and Marine Structures”

Hosted by PNU in April 2012

Over a dozen delegates from major shipyards attended the workshop

Intellectual Properties N/A

Enter Project No: Project Title in Slide Master

Concluding Remarks


A significant progress has been made in each of the three topical areas identified:

• The structural strain method for low cycle fatigue

• The cycle counting law and its physics/mechanics based rationales for cycle counting of arbitrary variable-amplitude loading histories

• A global coarse model (in order of stiffener spacing) fatigue screening procedure

Further validation and refinement will be performed in 2013• More structural details and test cases for demonstrating procedures’

robustness

• Selected fatigue testing in collaboration with Prof. M.H. Kim at PNU

• Documentation and publications

• Liaison with major shipyards on their input and interests

reliable and efficient fatigue design and life prediction

Documents