Fatigue life calculation of welded joints based on fracture mechanics

Ahmed M. Al-Mukhtar*MSc. Materials and Manufacturing Engineering

BSc. Aeronatical Engineering

Supervisors: Prof. Dr.- Ing. habil. H. Biermann* Prof. Dr.- Ing. P. Hübner**

*Institute of Materials Engineering, Technische Universität Bergakademie Freiberg, Germany **Fachhochschule Mittweida, Mittweida, Germany

Institut of Materials Engineering

PHD Programme

Outlines

1) Aim of reaserch (Objectives) - Procedure

2) Introduction (Why fracture mechanics?)

3) Finite Element Analysis (FEA)

- Modelling- Crack path direction

4) Standard and fatigue design recommendations

5) Results (benchmarking)

- SIF calculation and comparison for some welded joi nts- Fatigue life calculation and comparison for some w elded joints- Case study not presented in the recommendation

6) Conclusions

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Aim

- Accurate SIF calculation

- Moreover, calculate the fatigue life, and the fatigue strength (FAT) of the welded joints with different geometries that not presented in recommendations

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Procedures

1) FE modeling which give accurate stress intensity factor.

2) With knowledge of the stress intensity factors KI at different crack depths (a), it was possible to make curve fits for KI(a) for the different loading and use Paris’law to integrate numerically and calculate the expected fatigue life for the specimens.

3) Case studies can be consulted, and the need for well thought out benchmarking of analytical results against experimental databases-IIW

1. Ziel – und Aufgabenstellung1. Aim and Proecdures

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1. Ziel – und Aufgabenstellung2. Introdcution - Why Fracure Mechanics?

- One of the most important deterioration mechanisms of Aircrafts, ships and steel bridge structures which occur in service condition is Fatigue .

- Fatigue Failure , accounts for at least 75% of all machines and structures failures

Goal: Calculate ai, which already existed and crack growth parameters

Fatigue life calculation and FAT

Safe life and lower cost consuming for experiments tests

- Crack normally existed! But may be accelerated due to environment and loading condition-A.A. Griffith (1893-1963)

Aloha Airlines.1988

De Havilland First commercial jet aircraftfatigue cracks initiated at square windows, 1954

• Seven of the Liberty ships built during the world war II has broken completely in two as a result of brittle fractures. (1943)

• However, larger safety factor !!

• Crack normally existed !! How long?

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2. Introdcution - Why Fracure Mechanics?

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2. Introdcution - Why Fracure Mechanics?

Ref.Ingraffea A R. Case Studies of Simulation of Fr acture in Concrete Dams. Eng. Fracture Mech., 35, 1 /2/3, 1990, 553-564.

Fontana Dam, North Carolina, USA

• FRANC2D is a FE based simulator for curvilinear crack propagtion planar structures. CASCA which used for the creating the meshes, is a preprocessor for genrating initial input file for FRANC2D.

• Applicatons• Civil Engineering (Dams)• Mechanical Engineering (Welded joints)

• Output• Stress intensity fcator,SIF(KIC)

• Compare with other FEM sofwareThe results have been shown that FRANC2D has advantages over other simulations to calculate SIF because there are:

• No effect of, mesh size, densitry distributon and those save time and effort.

• Also no problem with badly shaped meshes elements have been disapperad.

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3. FRacture ANalysis Code (Finite Element Analysis FEA ) Two DimensionFRANC2D & CASCA (*)

*-Cornell Fracture Group, http://www.cfg.cornell.edu

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3. FRacture ANalysis Code (Finite Element Analysis FEA ) Two Dimension

Simulation and prediction crack growth in welded an d complex structures

- International insitute of Welding-IIW

- British standards- BS

- Japanese Society of Steel construction-JSSC

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4. Standard and fatigue design recommendations

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5. Results

SIF calculation and comparison

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6Crack length (mm)

SIF

(M

pa.m

^0.5

)FRANC2D

New approach

0

100

200

300

400

500

600

700

800

0 2 4 6 8 10

a (mm)

K (M

pa.m

m1/

2)

Frank,Fisher,ModifiedBS.B=T=15,h=6mm

FRANC2D.B=T=15.h=6

-Non load carrying joint-Cruciform joint load carrying

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5. Results

SIF calculation and comparison

-Butt weld (V-Butt,incomplete and complete penetration-machined and non machined flash)

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8

Crack length (mm)

SIF

(M

pa.m

1/2)

FRANC2D

Empirical solution.Ref.19

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5. Results

Fatigue life calculation and comparison

-Non-load carrying cruciform joint.-Comparison with IIW. FAT 63 Mpa.

-Butt weld. -Comparison with IIW. FAT 80 Mpa.

1

10

100

1000

1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08 1,E+09 1,E+10

Log N (Cycle) S

tres

s R

ange

(M

pa.)

FAT80.IIW

FRANC2D

1

10

100

1000

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

Log N (Cycle)

Str

ess

Ran

g (M

pa.)

FAT63.IIW

FRANC2D

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-New Geometry not presenetd in IIW recommendations-Fatigue life calculation and comparison

5. Results

provided a new value of FAT could be considered as the design value or lower bound.The new case with fillet radius not consider in IIW and in BS. (FAT=71 Mpa.)

Radius R t

1

10

100

1000

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

Log N (Cycle)

Str

ess

Ran

g (M

pa.)

Experimental

FAT63.No toe radius.IIW

FRANC2D.C=5E-13.m=3.ai=0.1mm.with toe radius

C=1,85E-13.m=3.mean values

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1) Accurate SIF is calculated through FEM FRANC2D

2) Different weld geometries which are not yet listed in previous recommendations could be easily modeled to evaluate the fatigue life strength. Which means that,

2) The new approach saves the time and costs need for experimental testes

6. Conclusions

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Thank you for your attention

Vielen Dank für Ihre Aufmerksamkeit