Fatigue Life Assessment of 2024 Aluminum Alloy Specimens ...meso2006/presentations/Meso_36.pdf ·...

Laboratory of Technology and Strength of Materials 8th MesoMechanics Conference, 19-22 July 2006, Porto, Portugal

Fatigue Life Assessment of 2024 Aluminum Alloy Specimens by means of Hardness

Measurements at the Meso Scale

By

Sp.G. PantelakisLaboratory of Technology and Strength of Materials

Dept. of Mechanical Engineering & AeronauticsUniversity of Patras, Greece

P.V. PetroyiannisLaboratory of Technology and Strength of Materials

Dept. of Mechanical Engineering & AeronauticsUniversity of Patras, Greece

K.-D. BouzakisLaboratory for Machine Tools and Manufacturing Engineering

Dept. of Mechanical EngineeringAristoteles University of Thessaloniki, Greece

I. MirisidisLaboratory for Machine Tools and Manufacturing Engineering

Dept. of Mechanical EngineeringAristoteles University of Thessaloniki, Greece

Laboratory of Technology and Strength of MaterialsUniversity of Patras, Greece


Introduction Introduction

Failure by fatigue remains the most serious concern for structurFailure by fatigue remains the most serious concern for structural failure of al failure of aircraft components despite the exhaustive amount of past researaircraft components despite the exhaustive amount of past research.ch.

An accurate fatigue failure prediction is difficult:An accurate fatigue failure prediction is difficult:

-- different physical processes which are complex and interrelateddifferent physical processes which are complex and interrelated, develop , develop with increasing number of fatigue cycles from atomic, over nanowith increasing number of fatigue cycles from atomic, over nano--, meso, meso--and microand micro--, to macro, to macro--scale damage mechanisms,scale damage mechanisms,

-- entail a host of material, geometric and loading parameters.entail a host of material, geometric and loading parameters.



Introduction Introduction (continued)(continued)


Damage tolerance which is the present day design concept for modDamage tolerance which is the present day design concept for modern ern metallic aircraft structures, relies on fracture mechanics, i.e.metallic aircraft structures, relies on fracture mechanics, i.e. accounts for accounts for fatigue damage accumulation after the structure under consideratfatigue damage accumulation after the structure under consideration ion involves fatigue macroinvolves fatigue macro--crackscracks

Growth of a fatigue macroGrowth of a fatigue macro--crack represents the last stage of the entire crack represents the last stage of the entire fatigue damage accumulation process which refers to a small percfatigue damage accumulation process which refers to a small percentage of entage of the materialthe material’’s fatigue life s fatigue life

The increasing trend in aeronautical industry to replace the exiThe increasing trend in aeronautical industry to replace the existing sting differential by integral structures makes the need to early detedifferential by integral structures makes the need to early detect possible ct possible fatigue damage urgentfatigue damage urgent



The objective of the present workThe objective of the present work, is to exploit the ability of measuring , is to exploit the ability of measuring material hardness changes at the nanomaterial hardness changes at the nano-- and mesoand meso--scale in order to develop a scale in order to develop a concept for the early detection of fatigue damage accumulation fconcept for the early detection of fatigue damage accumulation for the aircraft or the aircraft aluminum alloy 2024 T3.aluminum alloy 2024 T3.

To accomplish the above objective interrupted fatigue tests at RTo accomplish the above objective interrupted fatigue tests at R=0.1 followed =0.1 followed

by hardness measurements at the mesoscale were conductedby hardness measurements at the mesoscale were conducted

The determination of the specimen hardness was carried out by meThe determination of the specimen hardness was carried out by means of ans of nanoidentationsnanoidentations

The evolution of the surface hardness with increasing number of The evolution of the surface hardness with increasing number of fatigue fatigue cycles cycles has been monitoredhas been monitored

The effect of the applied fatigue stress value on the surface haThe effect of the applied fatigue stress value on the surface hardness change rdness change has been accounted forhas been accounted for

Present WorkPresent Work


Basic ConsiderationsBasic Considerations


Cyclic plasticity at the near surface material layers at the atoCyclic plasticity at the near surface material layers at the atomicmic--, nano, nano--and mesoand meso--scale is prevailing damage mechanism at the early stages of scale is prevailing damage mechanism at the early stages of high cycle fatigue damage accumulation.high cycle fatigue damage accumulation.

The convention adopted in the present investigation for the scalThe convention adopted in the present investigation for the scales is: 1*10es is: 1*10--44

to 1*10to 1*10--33 µµm atomicm atomic--scale, 1*10scale, 1*10--33 to 1*10to 1*10--11 µµmm nanonano--scale, 1*10scale, 1*10--11 to 10 to 10 µµmm

mesomeso--scale, 10 to 200 scale, 10 to 200 µµmm micromicro--scale and >200 scale and >200 µµmm macromacro--scale.scale.

Cyclic loading of metals leads to the formation of bands of concCyclic loading of metals leads to the formation of bands of concentrated entrated slip.slip.

Cyclic slip occurs almost immediately after a cyclic stress abovCyclic slip occurs almost immediately after a cyclic stress above the fatigue e the fatigue limit is applied.limit is applied.

The occurrence of surface slip markings lying along traces of thThe occurrence of surface slip markings lying along traces of the active slip e active slip planes is a general feature of cyclic plastic deformation.planes is a general feature of cyclic plastic deformation.



Not all metals subjected to cyclic loading harden, as cyclic sofNot all metals subjected to cyclic loading harden, as cyclic softening has tening has been observed as well.been observed as well.

Cyclic plasticity of engineering polycrystalline alloys is complCyclic plasticity of engineering polycrystalline alloys is complex, thus ex, thus making the prediction of the mechanisms of cyclic damage and themaking the prediction of the mechanisms of cyclic damage and theassociated cyclic hardening or softening difficult.associated cyclic hardening or softening difficult.




With regard to the engineering alloy under consideration and theWith regard to the engineering alloy under consideration and the aims of the present aims of the present study the following observations are of importance:study the following observations are of importance:

-- Experiments on FCC single crystals subjected to fully reversed Experiments on FCC single crystals subjected to fully reversed fatigue loading, fatigue loading, have shown a rapid hardening already in the initial few cycles.have shown a rapid hardening already in the initial few cycles.

-- The high value of stacking fault energy favors the activation oThe high value of stacking fault energy favors the activation of multiple glide f multiple glide systems and the formation of threesystems and the formation of three--dimensional dislocation structures, which result in dimensional dislocation structures, which result in cyclic hardening during plastic deformation.cyclic hardening during plastic deformation.

In precipitation hardened alloys, cyclic hardening occurs due toIn precipitation hardened alloys, cyclic hardening occurs due to an increase in an increase in

dislocation density and dislocation dislocation density and dislocation –– precipitate interactions.precipitate interactions.

Cyclic hardening is highly favored if the precipitates in the agCyclic hardening is highly favored if the precipitates in the agee--hardened alloy are not hardened alloy are not

easily shearable by the dislocations. This is the case for the Aeasily shearable by the dislocations. This is the case for the All--Cu alloys, like the Cu alloys, like the

investigated 2024, containing noninvestigated 2024, containing non--shearable shearable θθ (Al2Cu) precipitates.(Al2Cu) precipitates.

Available experimental data for the 2024 T4, 7075 T6 aluminum alAvailable experimental data for the 2024 T4, 7075 T6 aluminum alloys showed cyclic loys showed cyclic

hardening due to cyclic loading.hardening due to cyclic loading.




Microhardness measurements by means of Vickers microMicrohardness measurements by means of Vickers micro--hardness tests hardness tests conducted on 2024 T4 alloy specimens fatigued at reversed bendinconducted on 2024 T4 alloy specimens fatigued at reversed bending have g have confirmed the increase of hardness at the microscale following fconfirmed the increase of hardness at the microscale following fatigue atigue loading.loading.

As changes are limited within a narrow surface material layer clAs changes are limited within a narrow surface material layer classical assical hardness measurements lack the necessary sensitivity to reliablyhardness measurements lack the necessary sensitivity to reliably quantify quantify this phenomenon.this phenomenon.

the use of the use of mesohardnessmesohardness measurements overcomes thismeasurements overcomes this

disadvantage.disadvantage.


Experimental investigation Experimental investigation Experimental conditionsExperimental conditions

MaterialMaterial

Sheet 2024 T3 aluminum alloy of 3mm nominal thicknessSheet 2024 T3 aluminum alloy of 3mm nominal thickness

Fatigue tests (ASTM E466)Fatigue tests (ASTM E466)


Locations of hardness measurementsLocations of hardness measurements


Experimental investigation Experimental investigation (continued)(continued)

Experimental conditionsExperimental conditions


8, 16.5, 24.5, 38, 57.5, 68.518236515000, 30000, 45000, 70000, 105000, 1250002503c

8, 16, 24, 38, 57, 6818458215000, 30000, 45000, 70000, 105000, 1250002502c

7, 14, 21.5, 33.5, 50, 60208692

15, 30, 45, 70, 105, 125100000

15000, 30000, 45000, 70000, 105000, 1250002501c

4.5, 9, 13, 20.7, 31, 37134929060000, 120000, 180000,

280000, 420000, 500000

2003b

6.5, 13, 19.5, 30, 45.5, 5491986560000, 120000, 180000,

280000, 420000, 500000

2002b

14, 28, 42, 65, 98428600

15, 30, 45, 70, 105, 125400000

60000, 120000, 180000, 280000, 4200002001b

3, 6, 9, 14, 21, 25, 1005000000 (interrupted)

150000, 300000, 450000, 700000,

1050000, 1250000, 5000000

1803a

9.5, 19, 28.5, 44, 66.5, 791577460150000, 300000, 450000, 700000,

1050000, 12500001802a

3, 6, 9, 14, 21, 25, 1005000000 (interrupted)

15, 30, 45, 70, 105,125, 5001000000

150000, 300000, 450000, 700000,

1050000, 1250000, 5000000

1801a

Consumed percentages of fatigue life corresponding to the

experimental fatigue lives[%]

Experimental fatigue lifeNf [cycles]

Estimated percentages of expected fatigue life

[%]

Expected fatigue life according to

[17]

Selected numbers of cycles for interrupting

the fatigue tests

Maximum applied stressσmax [MPa]

Specimen

Nr.


Experimental investigation Experimental investigation (continued)(continued)

Experimental conditionsExperimental conditions

Mesohardness measurementsMesohardness measurements


material surface layer

bulk material

bulk material




Experimental results Experimental results Fatigue testsFatigue tests

105 106 107

200

300

Max

imum

app

lied

stres

s - σ

max

[MPa

]

Number of cycles to failure - Nf [cycles]

7.29C4

5.09C3

425C2

182.60C1

Value of the coefficientFitting Coefficient

4c3 )(logN/c

121max

ecccσ −

+=



Experimental results Experimental results Hardness measurementsHardness measurements

Nanoindentation results conducted in the unloaded area 1 Measurement areas and corresponding nanoindentationresults for one specimen before fatigue loading



Experimental results Experimental results Stress field in the specimens at various loads

Fmax= 6.75kN

Fmax= 5.40kN

Fmax= 4.85kN



Experimental results Experimental results Nanoindentation depth results in all areas examined at various

operational cycles – Maximum stress at area 4: σmax = 180 MPa


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20

Specimen 1(a) Specimen 2(a) Specimen 3(a)

Inde

ntat

ion

dept

h - h

p [µ

m]

Percentage of consumed fatigue life - n/Nf

σlocal = 56 MPa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20


Inde

ntat

ion

dept

h - h

p [µ

m]


σlocal = 116 MPa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20


Inde

ntat

ion

dept

h - h

p [µ

m]


σlocal = 180 MPa

Area 4

Area 2 Area 3





0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20

Specimen 1(b) Specimen 2(b) Specimen 3(b)

Inde

ntat

ion

dept

h - h

p [µ

m]


σlocal = 62 MPa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18


Inde

ntat

ion

dept

h - h

p [ µ

m]


σlocal = 128 MPa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20


Inde

ntat

ion

dept

h - h

p [µ

m]


σlocal = 200 MPa

Area 4

Area 2 Area 3





0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20

Specimen 1(c) Specimen 2(c) Specimen 3(c)

Inde

ntat

ion

dept

h - h

p [ µ

m]


σlocal = 78 MPa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.05.00

5.02

5.04

5.06

5.08

5.10

5.12

5.14

5.16

5.18

5.20


Inde

ntat

ion

dept

h - h

p [µ

m]


σlocal = 160 MPa

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.04.950

4.975

5.000

5.025

5.050

5.075

5.100

5.125

5.150

5.175

5.200


Inde

ntat

ion

dept

h - h

p [µ

m]


σlocal = 250 MPa

Area 4

Area 2 Area 3


Experimental results Experimental results Indentation depth alteration versus the operational cycles at Indentation depth alteration versus the operational cycles at

various specimen areas and loadsvarious specimen areas and loads


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

2

4

6

8

10

12

14

16

18

20

22

24

σlocal

=76MPa σlocal=62MPa σlocal=56MPa

Inde

ntat

ion

dept

h al

tera

tion

(IDA

)


area 2

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

2

4

6

8

10

12

14

16

18

20

22

24

σlocal=160MPa σlocal=128MPa σlocal=116MPa

Inde

ntat

ion

dept

h al

tera

tion

(IDA

)


area 3

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.002468

10121416182022242628

σlocal=250MPa σlocal=200MPa σlocal=180MPa

Inde

ntat

ion

dept

h al

tera

tion

(IDA

)


area 4


Experimental results Experimental results

−

⋅+=3

21

P

Nn

p

f

ePPh

The observed hardness behaviour may be expressed by empirical The observed hardness behaviour may be expressed by empirical

equations of the formequations of the form

hhpp : : remaining penetration depth after the load is fully removed, remaining penetration depth after the load is fully removed,

n/Nn/Nff: consumed percentage fatigue life : consumed percentage fatigue life

PP11, , PP22 and and PP33 : material and fatigue stress dependent constants: material and fatigue stress dependent constants


ConclusionsConclusions

The surface hardness of the alloy 2024 increases at different raThe surface hardness of the alloy 2024 increases at different rates tes

depending on the applied fatigue stress and number of appliedepending on the applied fatigue stress and number of applied fatigue d fatigue

cycles.cycles.

The dependency of indentation hardness increase on the number ofThe dependency of indentation hardness increase on the number of fatigue fatigue

cycles is noncycles is non--linear and seems to tend to a hardness saturation value.linear and seems to tend to a hardness saturation value.

The surface hardness increase process is accelerated at higher fThe surface hardness increase process is accelerated at higher fatigue atigue

stress amplitudes.stress amplitudes.

Further experimental investigation is needed to derive reliableFurther experimental investigation is needed to derive reliable equations of equations of

the evolution of hardness increase with the number of appliethe evolution of hardness increase with the number of applied fatigue cycles d fatigue cycles

as well as the dependency of hardness increase on the applieas well as the dependency of hardness increase on the applied fatigue d fatigue

stress valuestress value



AcknowledgementsAcknowledgements

The authors gratefully acknowledge the support of Deutsche Airbus for performing

this work.


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