European FITNET Fitness-for-Service Procedure ... - Congrex

Workshop on Fracture Control of Spacecraft, Launchers and their Payloads and Experiments 

9-10 10 February 2009, ESA/ESTEC Noordwijk, Noordwijk, 

The Netherlands 

European FITNET Fitness-for 

Fitness for-Service Service Procedure: 

Application of its Fracture Module 

to the Stiffened 

Al-alloy Al alloy Panels of Airframe Structures 

M. Koçak Ko ak, , V. Uz 

GKSS Research Centre, Institute of Materials Research 

Materials Mechanics Section, Section, 

21502 Geesthacht, Geesthacht, 

Germany 

www.eurofitnet.org 

e-mail: mail: mustafa.kocak@gkss.de 

ESA/ESTEC Workshop on Fracture Control, Noordwijk, NL, 9-10 Feb. 2009 M. Kocak, V. Uz, GKSS 1

FITNET Fitness for Service Procedure 

• What is FITNET? 

• Laser beam welding in aircraft fuselage (A318, A340, A380) 

• FITNET residual strength prediction of 

• Flat un-stiffened panels with LBW and FSW butt joints 

• Flat stiffened panels with Laser Beam Weld T-joints 

Flat stiffened panel 

for laboratory testing 

Scope 


Metallic Airframe: Fabrication Technologies 

stringer 

frame 

clip 

From Riveting 

To 

Laser Beam Welding 


Laser Welding in Aircraft Structures 

Stringer 

Frame 

ESA/ESTEC Workshop on Fracture Control, Noordwijk, NL, 9-10 Feb. 2009 M. Kocak, V. Uz, GKSS 4 

Clip 

Skin 

Conventional Riveted 

Aircraft Structure

Source: AIRBUS 

Laser Beam Welding of Panels 


Differential Structure 

Frame 

Stringer 

Stringer Rivet 

Skin 

Seal 

Innovation and Challenges 

Clip 

Skin 

LBW 

skin-stringer 

joint 

Integral Structure 

Stringer 


Skin 

Laser Beam Weld (LBW)

Testing and Analysis Steps 

FULL SCALE TEST 

DEMONSTRATIVE PART 

CURVED PANEL TESTING 

STRUCTURAL COMPONENTS DETAILS 

TESTS ON STRUCTURAL DETAILS 

COUPON TESTS 

How to apply Fitness-for- 

Service Analysis Procedures ? 



FITNET was a European Thematic Network project funded to develop 

Unified European Fitness-for-Service (FFS) Procedure 

for assessing the structural integrity of metallic (welded and nonwelded) 

engineering structures transmitting loads. 

Project Duration: 

1 st Feb. 2002 - 31 st May 2006 

What is FITNET ? 

FITNET 

FFS 


• 39 Member organisations* 

from 17 EU Countries 

* FITNET TN has also covered 

expert organisations from 

Japan, Korea, USA, Switzerland 

• Over 60 experts worked 

4.5 years with 

2 Mio. Euro budget ! 

Network of FITNET 


Definition of FFS: 

Fitness-for-Service (FFS) 

FFS assessment is a quantitative engineering evaluation 

performed to demonstrate the structural integrity of 

in-service component containing a flaw & damage 

Output of a FFS assessment is a decision to run as is, 

alter, repair, monitor, or replace. Provide guidance on inspection 

interval 

It provides fabrication quality acceptance criteria, material selection 

for design and fabrication of new components 

FITNET FFS Procedure consists of analytical methods 

to assess structural significance of flaws & damage 


Fitness for Service 

Fracture 

Creep 

FITNET FFS Scope 

Fatigue 

Corrosion 


Structure of FITNET FFS 

1. Characterise the nature of the crack 

2. Establish cause of cracking 

3. Define service conditions 

• Stress 

• Service environment 

• Material characteristics 

4. Establish data for assessment 

5. Analysis 

Material 

Environment 

EAC 

Stress 


FFS 

Crack

FITNET FFS: Section 9 Corrosion 

• Simple equations have been developed for a range of 

pressure vessel and pipework geometries, including 

nozzles and bends. 


S>√D.t 

L 

w 

dmax or rmin


Limit state predictions of elbows in pipework 

Prediction of “Local Thin Area” (LTA): Surface Flaw ! 

Crown 

90° 

Extrados 

Intrados 

Crown 

30° 

30° 

30° 

30° 

30° 

30° 


Crown 

90° 

Extrados 

Intrados 

Crown

Comparison of finite element and FITNET limit state 

equation predictions 

Failure pressure (MPa) 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 


no corrosion d/t=0.25 d/t=0.5 d/t=0.75 

No LTA Pressure only 

Extrados - no moment 

Intrados - no moment 

Extados - opening moment 

Extrados - closing moment 

Intrados - opeining moment 

Intrados - closing moment 

Extrados - FITNET 

Intrados - FITNET 


FITNET FFS Available 

FITNET FFS Procedure FITNET FFS Annex FITNET Proceedings 


Crack 

After H-J. Schmidt, AeroStruc 

Damage Tolerance Analysis 

Differential Structure 

(Riveted) 

Integral Structure 

(Laser Welded) 


Strength Under-Match in LBW and FSW 

LBW 

Undermatch 

FSW 

Undermatch 


LBW 

clip-to-skin 

T-joint 

LB Welded Stiffened Panels 

Al 6156 T6 Al 6013 T6 

1.9 

clip 

skin 


2.0 

stringer 

LBW 

stringer-to-skin 

T-joint

Material related parameters 

(experiment): 

• Tensile properties 

• Fracture toughness (R-curve) 

• Mis-match ratio (M) 

a 

W 

M=YSWM / YSBM Mis-match ratio 

FITNET FFS: Section 6 Fracture 

Assessment for Postulated or Real Flaw 

FITNET FFS 

Fracture Analysis 

Prediction of critical structural condition: 

• stress level & max. load 

• critical crack size 

• min. fracture toughness 

Component related parameters 

(handbook, FEM): 

• Stress analysis, Flaw size 

• K-factor and plastic limit load soln 


Principle of residual strength prediction 

CDF = crack driving force 

K J 

δ 5 

F 1 

F 2 

F 3 = F max 

a 0 

1 

3 

2 

Δa cr 

R-curve 

4 

CDF 

Crack length, a 

Crack extension, Δa 

K J = K I ⋅ [ f (Lr ) ] −1 

δ5 =δ5e ⋅ [ f (Lr) ] −2 

FITNET 

FFS 

Crack extension, Δa 


F max 

Load, F 

L r = 

1 

2 

3 

Δa cr 

or CTOD δ 5 

F 

? 

4 

F Y (R p 0.2,a)

Un-stiffened base metal panel 

Base material Al 6013 T6 

Base material Al 6013 T6 

FITNET Option 3 

• Very good agreement with experiment 

• Used as validation of the FITNET FFS Procedure 

Test 



Yield Load 

E. Seib, PhD Thesis, 2005 


Un-stiffened LBW and FSW panels 

LBW FSW 

a 0 

• CTOD δ 5 R-curve obtained from C(T)50 

5 R-curve obtained from C(T)50 

• m = 2.0 yields best prediction 


δ 5 = 

2 

KI mER p 0.2W 

a 0 

⋅ [ f (Lr) ] −2 

E. Seib, PhD Thesis, 2005

Experimental procedure for stiffened panels 

Two-Stringer Panels Testing with Anti-Buckling Guides 


Large Panel Testing Programme at the GKSS 


Crack branching 

Stringer 

Skin 

a str = a sk-beyond 

a str = a sk-beyond 


K-factor in stiffened panels 

Stringer-load-factor 

Ls = Fstr * 

Fstr welded 

β sk = K Isk 

σ πa 

riveted 

crack branching 

Normalized 

K-factor for skin crack 

FEM results 

FEM results 

Normalized 

K-factor for stringer crack 

β str = K Istr 

σ πa 



Residual Strength Prediction 

2-stringer 3-stringer 

• method enables the separation of skin and stringer failure analysis 

• 2- and 3-str cases the panel failure is isgoverned governed by the skin failure 

ESA/ESTEC Workshop on Fracture Control, Noordwijk, NL, 9-10 Feb. 2009 M. Kocak, V. Uz, GKSS 


28


„simplified“ approach 

using equivalent thickness 

2-stringer 3-stringer 

• treatment of stiffened panels as simple M(T) panel with 

respective K-factor and limit load solution. 

• approach yields predictions with reasonable accuracy 



29


Panels with clips 

• Crack extends along the clip weld in in panel center: 

Aim: To investigate the sensitivity of residual strength to Clip-LBW 

• R-curve obtained from „structure-like“ C(T) specimen with T-joint 


EU-Project WEL-AIR, 2007 

30


• FITNET FFS Procedure Fracture Module provides good 

predictions of residual strengths of the stiffened panels with 

and without welded clips 

Panels without clips 



31


RS Test on Panel of 2139 T8 / 3.2 mm and FITNET Prediction 

After FCP test (Two Central Stringer are Broken: LARGE CRACKED Panel 

With only two intact stringers) 

The panel with large damage successfully predicted by FITNET FFS 



32 

3.2 

30 

20 

R8 

3.2


Residual Strength Test on Panel of AlLi Alloy 2198-T8 2198 T8 and FITNET Prediction 



33

Conclusions 

• Investigation has demonstrated the ability of the FITNET FFS 

fracture module to predict the residual strength of complex thinwalled 

structural components. 

• Methodology presented for stiffened panels incorporating both 

skin and stiffener failure criteria enables a prediction whether 

skin or stringer will fail. However, stringer crack K-factor 

solution is required. 

• The „simplified approach“ by treating the stiffened panels as 

simple M(T) panels with equivalent thickness has shown a 

potential to predict the residual strength of such complex 

structures with a reasonable accuracy. 

• FITNET FFS analysis of 4-stringer panels yielded good 

estimates of the max load justifying the approximation of 

stiffened panels to simple base metal plate. 



FITNET FFS Procedure FITNET FFS Annex FITNET Proceedings 


Thank you… 

you 


Transferability of R-curves 

Base material 

Base material 

a 0 

δ 5 + 5mm 

Δa 

LBW & FSW 

LBW & FSW 


δ 5 

R-curve 

Δa 

BM

LBW of Large Panels 

at the AIRBUS 

LBW system for stringer-skin panel joints 

• 2 CO2-Laser with 3.5 kW beam power 

• Speed: max. 10m/min 

• Working area: 11m x 3m x 0.75m

European FITNET Fitness-for-Service Procedure ... - Congrex

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