Master Thesis Presentation

Prof. Dr. -Ing. Wolfgang Fricke , TUHH

Prof. Dr. -Ing. Uwe Weltin, TUHH

Dipl. -Ing. Olav Feltz, TUHH

Serkan Birinci15 September 2008

Konstruktion und Festigkeit von Schiffen

SERKAN BIRINCI

M.Sc. International Production Management

Prof. Dr. -Ing. Wolfgang Fricke, TUHH



15 September 2008

EXPERIMENTAL AND NUMERICAL FATIGUE STRENGTH INVESTIGATION OF A WELDED

STRUCTURE AND ASSESSMENTWITH DIFFERENT APPROACHES






Main Aim

• To assess the fatigue strength of four different structural details by applying different

fatigue assessment methods

• Comparison of these methods according to the applicability for a specific detail

Introduction Theoretical Background Experimental Setup Results Conclusion

Page 1






Fatigue FailureThe progressive and localized structural damage that occurs when a material is

subjected to a cyclic loading.

Steps of Crack Formation• Crack Initiation

• Crack propagation

• Final Fracture

Major Influences on Fatigue Strength• Local Parameters of Geometry : Toe Radius, Weld Angle and Surface Crack Depth

• Loading : Fluctuating and Repeating

• Material Type

• Fusion Process, Residual Stresses

• Stress Concentration Effects : Key way, Hole and Non-welded Root Gaps


Page 2







Fatigue Assessment Approaches

Page 3

• Global Approaches

- Nominal Stress Method

• Local Approaches

- Structural Hot Spot Stress Method

- Effective Notch Stress Method

• Linear Elastic Fracture Mechanics Approach














Nominal Stress Classes

Number of Cycles (N)

∆σ[M

Pa]

Page 4












- Linear Stress Extrapolation

- According to Xiao and Yamada



Page 5












- Linear Stress Extrapolation

- According to Xiao and Yamada



Page 6














Page 7

RADIUS 1 mm














Page 8

IK = . .a Yσ π

• KIC = Fracture Toughness

• KI = Stress Intensity Factor







Relation Between Stresses

Base Plate

Cover Plate

Strengths and Weaknesses of the Methods

Nominal Stress Method

- Local geometry properties not evaluated

- But included in detail classes and S-N curves

Structural Stress Method

- Omits the detail classes

- Captures the local macro geometric effects

- Excludes the notch effect

Effective Notch Stress Method

- Includes the effect of local weld toe geometry

n

kmax

max

I

= Nominal Stress

= Maximum Notch Stress

= Maximum Structural Stress

K = Stress Intensity Factor

s

σ

σ

σ

Page 9







Specification of Specimens

• 4 cases ( each of 10 Specimens )

- Size : 300 mm × 50 mm × 12 mm

- Weld Throat Length (a) : 3 mm and 7 mm

- Loading Type : Load Carrying and Non-Load Carrying

SECTION A-A

Lc / 2

T

T/2

Lw

a

F

Page 10







Specification of Experiment

• The Applied Repeated Nominal Stress

- 90 MPa – 120 Mpa for 3 mm Load Carrying Fillet Weld Joint

- 120 MPa – 170 Mpa for 3 mm Non-Load Carrying Fillet Weld Joint

- 120 MPa – 210 Mpa for 7 mm Load Carrying Fillet Weld Joint

- 150 MPa – 200 Mpa for 7 mm Non-Load Carrying Fillet Weld Joint

• Frequency : 30 Hz

• Stress Ratio, R = 0

Page 11







Observed Crack Formation

Page 12

TOE

CRACK

ROOT

CRACK

a

ROOT

CRACK

a

TOE

CRACK

a

TOE

CRACK

a

3 mm a-Length

Load Carrying Fillet Weld

3 mm a-LengthNon-Load Carrying

Fillet Weld

7 mm a-Length

Load Carrying

Fillet Weld

7 mm a-Length

Non-Load Carrying Fillet Weld

σ σ

σ σ σ σ

σ σ







Observed Crack Formation

Weld root crack in 3 mm

load carrying fillet weld joint

Page 13







Test Results for Nominal Stress Method

S - N RESULTS OF THE FATIGUE TEST

54.7

10

100

1000

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

Number of Cycle ( N )

Stress Range ( N/m

m2)

Pü=10%

Pü=50%

Pü=90%

Pü=97.7%

3mm Load Carrying Fillet Weld - Crack at Weld Root

7mm Load Carrying Fillet Weld - Crack at Weld Toe

3mm Non-Load Carrying Fillet Weld - Crack at Weld Root and Weld Toe

7mm Non-Load Carrying Fillet Weld - Crack at Weld Toe

Page 14

Stress Range (N / m

m2)







Investigated Variables3 mm throat length

• Weld size 7 mm throat length

Load carrying weld• Loading type

Non-load carrying weld

• Effect of contact analysis in FEM program (ANSYS)

Assumptions for FEM Model

• Assumption of specimen

- Flank angle of 135 ˚

- Weld Toe Radius 1 mm

- Throat thickness of 3 mm and 7 mm

• Assumption of FEM model

- Plane stress condition

- 90 MPa Nominal Stress

- Restraints applied end of the main plate

Page 15







Structural Hot Spot Stress Method by Linear Extrapolation

AXIAL STRESS

( m m )0 T 0,4 T 1,0 T

0 T

a) Coarse Mesh

c) Coarse Mesh Refined

b) Finer Mesh

d) Finer Mesh Refined

Page 16

• 3 mm and 7 mm throat thickness

are insensitive to non-linear peak

stress







LINEAR STRESS EXTRAPOLATION AT THE WELD TOE

88000

90000

92000

94000

96000

98000

100000

102000

104000

106000

108000

00.20.40.60.81

DISTANCE (T)

AXIAL STRESS

L3_Non-Load Carrying Coarse MeshL7_Non-Load Carrying Coarse MeshL3_Non-Load Carrying Coarse Mesh RefinedL7_Non-Load Carrying Coarse Mesh RefinedL7_Non-Load Carrying Finer MeshL3_Non-Load CarryingL7_Non-Load CarryingL3_Load CarryingL7_Load Carrying

3 mm load carrying

7 mm load carrying

Page 17

Structural Hot Spot Stress Method by Linear Extrapolation






1 mm


Structural Hot Spot Stress Method by Xiao and Yamada

Page 18

Notch Stress Modeled

1 mm mesh size Modeled

1 mm








Page 19

10

100

1000

1.E+05 1.E+06 1.E+07


Structural Stress at 1 m

m Depth ( M

pa )

R = 0

FAT 100

3mm Non-Load Carrying without contact analysis, Ks= 1.21,Weld Toe Crack 3mm Non-Load Carrying with contact analysis, Ks= 1.27,Weld Toe Crack7mm Load Carrying without contact analysis, Ks = 1.34,Weld Toe Crack 7mm Load Carrying with contact analysis, Ks = 1.24,Weld Toe Crack 7mm Non-Load Carrying without contact analysis, Ks = 1.19,Weld Toe Crack 7mm Non-Load Carrying with contact analysis, Ks = 1.16,Weld Toe Crack

WÖHLER S-N CURVEFORMED WITH

AXIAL STRESS σX Notch Stress Modeled








Page 20

10

100

1000

1.E+05 1.E+06 1.E+07



m Depth (Mpa)

R = 0

FAT 100

3mm Non-Load Carrying, without contact analysis, Ks= 1.37, Weld Toe Crack 3mm Non-Load Carrying, with contact analysis, Ks= 1.46, Weld Toe Crack7mm Load Carrying, without contact analysis,Ks = 1.63, Weld Toe Crack7mm Load Carrying, with contact analysis,Ks = 1.42, Weld Toe Crack7mm Non-Load Carrying, without contact analysis, Ks = 1.31, Weld Toe Crack7mm Non-Load Carrying, with contact analysis, Ks = 1.25, Weld Toe Crack


PRINCIPAL STRESS σ1 Notch Stress Modeled








Page 21

10

100

1000

1.E+05 1.E+06 1.E+07



m Depth (Mpa)

R = 0

FAT 100

3mm Non-Load Carrying, without contact analysis, Ks= 1.06, Weld Toe Crack 3mm Non-Load Carrying, with contact analysis, Ks= 1.11, Weld Toe Crack7mm Load Carrying, without contact analysis, Ks= 1.20, Weld Toe Crack7mm Load Carrying, with contact analysis, Ks= 1.12, Weld Toe Crack7mm Non-Load Carrying, without contact analysis, Ks= 1.10, Weld Toe Crack7mm Non-Load Carrying, with contact analysis, Ks= 1.07, Weld Toe Crack


AXIAL STRESS σX









Page 22

10

100

1000

1.E+05 1.E+06 1.E+07

Life Cycle ( N )


m Depth (Mpa)

R = 0

FAT 100

3mm Non-Load Carrying, without contact analysis, Ks= 1.20, Weld Toe Crack 3mm Non-Load Carrying, with contact analysis, Ks= 1.26, Weld Toe Crack7mm Load Carrying, without contact analysis, Ks= 1.50, Weld Toe Crack7mm Load Carrying, with contact analysis, Ks= 1.27, Weld Toe Crack7mm Non-Load Carrying, without contact analysis, Ks= 1.23, Weld Toe Crack7mm Non-Load Carrying, with contact analysis, Ks= 1.16, Weld Toe Crack



PRINCIPAL STRESS σ1








Page 23

1 mm mesh

size ModeledNotch Stress

Modeled

• All compared parameters of notch stress modeled mesh give higher values

than 1 mm mesh size model.

• Not having 1 mm fictitious radius causes increased stresses at the plate edge

of 1 mm mesh size model, and correspondingly causes decreased stresses.

• Xiao and Yamada SHS Stress Approach highly sensitive to mesh size at the weld toe.

• All the stress values of contact analysis used 1 mm mesh size model are

higher than without contact analysis used.








Page 24

Notch Stress Modeled Fictitious notch radius rf = 1 mm

Fictitious radius rf = 1 mm

R1

R1• All principal stresses were taken from

the points in where the crack formation

occurred in the real test specimen.








Page 25

10

100

1000

1.E+05 1.E+06 1.E+07


Effective Notch Stress Range (Mpa)

R = 0

3 mm Load Carrying Fillet with contact analysis Kf = 7.50, Weld Root Crack3 mm Load Carrying Fillet without contact analysis Kf = 6.38 , Weld Root Crack3mm Non-Load Carrying with contact analysis, Kf = 3.50, Weld Toe Crack3mm Non-Load Carrying without contact analysis Kf = 3.40, Weld Toe Crack3mm Non-Load Carrying with contact analysis, Kf = 3.73, Weld Root Crack3mm Non-Load Carrying without contact analysis Kf = 2.28, Weld Root Crack7mm Load Carrying wih contact analysis, Kf = 3.27, Weld Toe Crack7mm Load Carrying without contact analysis Kf = 3.96, Weld Toe Crack7mm Non-Load Carrying wih contact analysis, Kf = 2.76, Weld Toe Crack7mm Non-Load Carrying without contact analysis Kf = 2.96, Weld Toe Crack

FAT 225 ( m = 3 )

• Design assign curve

with FAT 225

• Highest Kf value obtained

3 mm Load Carrying Fillet

Weld

• Lowest Kf value obtained

3 mm Non-Load Carrying

Fillet Weld

• The contact analysis result

of 3 mm fillet weld higher

than the without contact

analysis

• The contact analysis

result of 7 mm fillet weld

lower than the without

contact analysis








Page 26

With Contact

Analysis

Without Contact

Analysis

• Maximum Stress at

the Weld root

• Maximum Stress at

the Weld Toe

• 3 mm fillet weld with the contact analysis

conducts bending forces from the weld toe to

weld root

• Contact effect increases the effective notch

stress in the weld toe







Linear Elastic Fracture Mechanics

Page 27

With Contact

Analysis

• FRANC2D crack growth program used to evaluate the fatigue life of 3 mm throat

thickness Load carrying and Non-Load carrying weld model

• Plane stress assumption

• To prove the accuracy of program, a test was done with a very simple specimen with

0.5 mm and 1.0 mesh size. Then,result compared with analytical result.

a0= 0.1 mm C = 5.21·10-13

∆a = 0.1 mm KIC = 2245 [N/mm3/2]

P = 90 Mpa m = 3.0

IK = . .a Yσ π

/ 2 / 2 1 / 2 10

1 1 1

( / 2 1)m m m m me

NC m Y a aσ π − −

= −

⋅ ∆ ⋅ − ⋅ ⋅

/ 2 / 2 1 / 2 1

1 1 1

( / 2 1) ( ) ( )m m m m mc c

NC m Y a a a aσ π − −

∆ = − ⋅ ∆ ⋅ − ⋅ ⋅ + ∆

(1)

(2)

(3)








Page 28

With Contact

Analysis

• Mode I SIF graphs calculated by FRANC2D program and by analytically

• not insensitive to mesh size

• SIF of 1 mm mesh size test model resulted nearly %10 higher value than 0.5 mm

mesh size test modelMODE I SIF HISTORY

-100.000

100.000

300.000

500.000

700.000

900.000

0.000 1.000 2.000 3.000 4.000 5.000

Crack Length

KI

FRANC2D ANALYTICAL








Page 29

With Contact

Analysis

• Fatigue life graphs calculated by FRANC2D program and by analytically

• fatigue life time increased nearly %40, due to 1mm mesh size

NUMBER OF CYCLES vs. CRACK LENGTH

-2.000E+05

2.000E+05

6.000E+05

1.000E+06

1.400E+06

1.800E+06

0.000 1.000 2.000 3.000 4.000 5.000

CRACK LENGTH ( mm )

NUMBER OF CYCLES ( N )........

FRANC2D ANALYTICAL








Page 30

With Contact

Analysis

• Fatigue life and SIF graphs of 3 mm Load and Non-Load carrying Fillet Weld

Non-Load Carrying Weld Load Carrying Weld







Page 31

Conclusions

• When the weld throat thickness of the specimens increases, the crack formation passes from weld

root to weld toe

• The applicability of Structural Hot Spot (SHS) method by Linear Stress Extrapolation

is relatively easier than other methods

• Weld Size variation causes inconsistent results, the SHS method is sensitive to mesh size

• The effects of weld size variation is most clearly seen in Effective Notch Stress Method, and least

seen Xiao and Yamada SHS Stress Method

• The obtained principal stresses in Xiao and Yamada SHS Stress Method are higher than axial

stresses

• All compared parameters of notch stress modeled mesh in Xiao and Yamada SHS Stress Method

result higher stress values than 1 mm mesh size model

• Xiao and Yamada SHS Stress Method is highly sensitive to mesh size at the weld toe

• FRANC2D is sensitive to mesh size, and uses numerical integration to calculate fatigue life.

Therefore, correct mesh size of the model is important.







Page 31

Future Works

• Stress distribution with the variation of the local shape of the weld, the weld toe radius, flank

angle and the mesh size by Xiao and Yamada Method

• Exact mesh size with the calculation of FRANC2D for the 7 mm load carrying and non-load

carrying fillet weld







Page 32

THANK YOU FOR YOUR ATTENTION







Test Results for Nominal Stress Method ( Separated)






S - N RESULTS OF THE FATIGUE TESTLoad Carrying 3mm a-length

Weld Root

10

100

1000

10000

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

Life Cycle ( N )

Stress Range

( Mpa )

3mm Load Carrying Fillet Weld - Crack at Weld Root

Pü=10%

Pü=50%

Pü=90%

Pü=97.7%

R = 0

40.1













S - N RESULTS OF THE FATIGUE TESTNon-Load Carrying 3mm a-length

Weld Toe

10

100

1000

1.E+05 1.E+06 1.E+07

Life Cycle ( N )

Stress Range ( Mpa )


Pü=10%

Pü=50%

Pü=90%

Pü=97.7%

R = 0

75.3













S - N RESULTS OF THE FATIGUE TESTNon-Load Carrying 3mm a-length

Weld Root

10

100

1000

1.E+05 1.E+06 1.E+07

Life Cycle ( N )



Pü=10%

Pü=50%

Pü=90%

Pü=97.7%

R = 0

68.8













S - N RESULTS OF THE FATIGUE TESTLoad Carrying 7 mm a-length

Weld Toe

10

100

1000

1.E+05 1.E+06 1.E+07

Life Cycle ( N )


7 mm Load Carrying Fillet Weld - Crack at Weld Toe

Pü=10%

Pü=50%

Pü=90%

Pü=97.7%

R = 0

82.4













S - N RESULTS OF THE FATIGUE TESTNon-Load Carrying 7 mm a-length

Weld Toe

10

100

1000

1.E+05 1.E+06 1.E+07

Life Cycle ( N )


7 mm Load Carrying Fillet Weld - Crack at Weld Toe

Pü=10%

Pü=50%

Pü=90%

Pü=97.7%

R = 0

75.4

Documents

Master Thesis Presentation