Paulo M S T de Castro, Paulo F P de Matos, Pedro M G P ...meso2006/presentations/Meso_42.pdf ·...

Mesomechanics’2006 Conference, Porto, FEUP, July 19-22, 2006 1

FATIGUE ANALYSES OF AERONAUTICAL

STRUCTURAL DETAILSOF DIFFERENT COMPLEXITY

Paulo M S T de Castro, Paulo F P de Matos, Pedro M G P Moreira, Lucas F M da Silva

IDMEC and Faculdade de Engenharia,Universidade do Porto, Portugal


de Havilland : Spitfire, Mosquito, ..., Comet

de Havilland Comet I : the first commercial jet airliner

context - Comet

After several catastrophic accidents Comet was taken from commercial use

Testing revealed that these planes had a major fatigue weakness ... the designers insisted on square windows ...


Wreckage of the fuselage of the Comet:The primary cracks are shown propagating well beyond the fastener lines of several fuselage frames, without arrest

context - Comet


……..in order to save costs, the same fuselage that had been tested until the ultimate design load, was later used for the fatigue test. Residual compressive stresses, arising from the ultimate loading, retarded the development of the fatigue cracks

As a result, there were no indications from the fatigue test that the fuselage design was inadequate ……

context - Comet

Abe Brot, ‘Innovation often leads to new problems’, priv. com. Feb 23, 2004


Aloha flight 243, Boeing 737, 1988

http://www.facstaff.bucknell.edu/rich/MECH353/TN353A2.htm

context - Aloha 737


http://www.aloha.net/~icarus/

context - Aloha 737


Presentation outline

1. Cold-worked hole specimens

2. Single rivet lap-joint

3. Lap-joint panel crack growth

modelling

4. Conclusions


Cold worked specimens - contents1.1 What are the questions?

1.2 Geometry and cold working process

1.3 Fatigue tests

1.4 Residual stresses: cold working technique

1.5 Fractography analysis

1.6 Conclusions


1.1 What are the questions?

What is the effect of the residual stress on fatiguecrack initiation?

1) SEM measurements2) Reconstitution of the fatigue life

How do experimental measurements and numericalmodelling of the residual stresses agree?

1) X-Ray measurements2) FEM modelling

Why do the specimens with residual stresses last longer?1) Later crack initiation? 2) Braking effect on crack propagation?

What is the effect of residual stress on the fatigue life?1) S-N


1.2 Geometry and cold working process

Specimen geometry

Cold working process:

Entrance face

Exit face

Material: 2024-T3 Alclad


1.3 Fatigue tests


1.4 X-ray measurements

• Al clad is used to decreasecorrosion problems

• Al clad has to be removed because of its low yield strength

X-ray measurements for:θ=0º and θ=90º

Clad thickness


1.4.1 X-ray measurements - θ=0º

Entrance face Exit face


1.4.2 Residual stress: 3D FEA / model42200 3D elements (8 nodes)

Mesh details

mandrelsleeve


1.4.2 Residual stress: 3D FEA / model


1.4.3 FEA 3D vs exp. meas. - θ=0º


1.5 Fractography analysis □ Inspect the specimen fracture surfaces

→ Crack growth rate assessment□ Measure the fatigue striation spacing


1.5.1 Fractography: SEM measurements σmax=140MPa

Nor

mal

hol

e sp

ecim

encr

ack

leng

th=2

.71m

m (

20

000)

Col

d w

orke

d ho

le s

peci

men

crac

k le

ngth

=2.5

0mm

( 4

0 00

0)

×

×


1.5.1 Fractographic analysis

σmax=120MPa σmax=160MPa

σmax=180MPa

σmax=200MPa


1.5.2 Fractographic fatigue crack history

Nedbal et al.


1.5.2 D-factor: fat. crack history recons.D reflects the interactions of different micromechanisms on the crack front linking micro and macro features of the fatigue process.

D-factor depends essentially on three simultaneously interaction factors:

□ the existence of idle cycles;

□ spatial dispersion of localdirections of crack growth;

□ influence of fatigue crack growthmechanisms other than the striating one.


1.5.3 Surface crack measurements


1.5.4 Fractographic reconstitution and macroscopic surface measurements

Normal hole Expanded hole


1.5.4 - Fractographic reconstitution and macroscopic surface measurements


1. X-ray technique measurements1.6 Conclusions

□ Inevitably the stresses are averaged over theirradiated volume of material

□ It is not possible to resolve the steep stress gradients in the vicinity of the hole

□ The peak values predicted by FEM are averagedout in the X-ray measurements

2. FEM simulations

□ FEM simulations are in general good agreement with experimental measurements


1.6 Conclusions

□ Striation spacing increase with increasing in cracklength and with the stress level

□ Lower striation spacing measurements show thecrack growth rate decreases due to the cold working process.

□ The effect of the hole cold expansion: 1) the main source of the life prolongation is not alonger crack initiation time, 2) but anintensive braking of early crack growth stage

3. Fractographic measurements


2. Single rivet lap-joint – contents

2.1 Geometry

2.2 SEM analysis

2.3 Stress intensity factor calibration

2.4 Load transfer

2.5 Conclusions


specimen type: lap splice with 3 rivet rows and 1 rivet columnmaterial: 2024-T3 Alclad sheetssheet thickness: 1.2 mmspecimen length: 260 mm, 60 mm overlapspecimen width: 20 mmrivets: 2117-T4, ∅ 3.2 x 6.4 mmriveting force: according to typical production standards

2.1 Specimens geometry


specimens: 93, 95, 224 and 229

specimen 224

2.2 SEM analysis


longitudinal

transversal

Fatigue striation - specimen 93

16 striations

2.2 SEM analysis


The transversal crack growth rates are almostconstant

The longitudinal crack growth rates are similar to thetransversal crack growth rates for small cracks

The longitudinal crack growth rate increasesexponentially with increasing crack length

2.2 SEM analysis - remarks


Assembled fatigue striations spacingThe fatigue striation spacing was converted in crack growth rate da/dN

2.2 SEM analysis - comparison


3D Mesh detail

Boundary constraints

Rivets8-node brick elements (C3D8)

7344 elements

Plates20-node brick isoparametric elements (C3D20)7296 elements

2.3 Stress intensity factor calibration using FEM


2.3 Stress intensity factor calibration using FEM - notation


Non dimensional Keff / Symmetric crack and Asymmetric

2.3 Stress intensity factor calibration using FEM


2.4 Load transfer


2.5 Conclusions

□ Striation spacing increase with increasing in cracklength and approximately constant along thethickness

□ the stress intensity factor calibration for a symmetric and asymmetric crack was presented

□ the load transfer behaviour for different crack lengths, for the case of a symmetric and asymmetric crack, was presented


3. Lap-joint crack growth simulationcontents

3.1 Geometry

3.2 Franc2D/L model

3.3 Crack growth

3.4 Crack growth results

3.5 Conclusions


3.1 Lap-joint - geometry

Cavallini, G. and R. Lazzeri, Report on experimental fatigue crack growth in medium and complexcomponents. ADMIRE project report: ADMIRE-TR-3.0-67-3.1/UP, 2003, Dipartimento di Ingegneria, Università diPisa


3.2 Franc2D/L modelMeshFranc2D/L model


Mesh – rivets are modeled by spring elements (triangles)

Crack scenario at 137950cycles - mesh

3.2 Franc2D/L mesh details


3.3 Crack growth inspected sites

crack shape – experimentallyobserved


3.3 Crack growth – crack shape

cracks were modeled as through cracks - approximation


scenario at different stages of the fatigue test

3.3 Crack growth overall scenario


3.3 Crack growth – model


3.4 Crack growth – results


L1

L

L3

Lc

L2

p

sd

eb

ei

L 555

L1 299,5

L2 299,5

L3 440

Lc 44

ei 3,2

eb 1,6

Number of rows 2

Number of columns 20

p 22

s 22

d 4,8

other projects

Specimens tested, SMAAC projectL F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000, pp.319-338


Ductile fracture

Fatigue

1

20

Rivet hole

1 mm interval to compensate the cut

Rivet hole


Rivet hole


Rivet hole


Rivet hole


Rivet hole


Rivet hole


Crack pathSamples for sem

1 20Direction

visualisation ofFigure 40

Fracture surface (hole is only visible for columns 1-5 and 14-20 – see below crack path); specimen #1

L F M Silva, F M F Oliveira, P M S T de Castro, “Task3.2: Individual report on the test results of lap jointspecimens’, SMAAC report SMAAC-TR-3.2-11-1.3/IDMEC,

Sept 1998


L F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000,

pp.319-338

Fracture surface; specimen #3


L F M Silva, J P M Gonçalves, F M F Oliveira, P M S T de Castro, 'Multiple-site damage in riveted lap-joints: experimental simulation and finite element prediction', International Journal of Fatigue, vol.22, (4), 2000, pp.319-338



Swift criterion for link-up


leftblank...others...

modelling problems remain …….

J S Domingues, L F M Silva,P M S T de Castro, ‘Results of Task5 problems ‘type II and ‘type V’ of the round robin programme – a comparison’, SMAAC report SMAAC-WP-5-ZZ-1.3/IDMEC, Sept 1998


3.5 Lap - joint conclusions

□ Crack growth modelling was done in a lap-joint panel from 132150 to 138450 cycles;

□ A good agreement was found between theexperimentation and the prediction of FRANC2D/Lbased on the Paris Law;

Limitations:□ Cracks grow with an elliptical shape and not as

through cracks;□ Rivets were treated as spring elements. It is only a

simplification that has some limitations


4. Conclusions – open hole□ The improvement of the fatigue live due to the cold

working process was quantified

□ The assessment of the residual stresses using the X-ray diffraction and FEM modelling are in general good agreement

□ Striation spacing increase with increasing incrack length and with the stress level

□ Lower striation spacing meas. show the crack growth rate decreases due to cold expansion


□ Striation spacing increase with increasing in cracklength and approximately constant along thethickness

□ the stress intensity factor calibration for a symmetric and asymmetric crack was presented

□ the load transfer behaviour for different crack lengths for the case of a symmetric and asymmetric crack was presented

4. Conclusions – single rivet lap-joint


□ Crack growth in a lap-joint panel was modeled

□ A good agreement was found between theexperimentation and the prediction of FRANC2D/Lbased on the Paris Law;

Limitations:

□ Cracks grow with an elliptical shape and not as through cracks;

□ Rivets were treated as spring elements. It is only a simplification that has some limitations

4. Conclusions – Lap-joint crack growth modelling


Acknowledgments:

The authors would like to acknowledge inparticular the European Union projects:

• SMAAC project BRITE BE95-1053, leader: Dr Peter Horst (then at DASA - Hamburg)

• ADMIRE project contract no. G4RD-CT-2000-00396, leader: Dr Alfonso Apicella, ALENIA

Paulo M S T de Castro, Paulo F P de Matos, Pedro M G P ...meso2006/presentations/Meso_42.pdf ·...

Documents

Transcript of Paulo M S T de Castro, Paulo F P de Matos, Pedro M G P ...meso2006/presentations/Meso_42.pdf ·...