adi days - giovanni meneghetti

Austempering, A Technology for Substitution

ADI DAYS 2016 6th – 7th October Minerbe

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FATIGUE BEHAVIOUR OF AS-‐CAST AND AUSTEMPERED DUCTILE IRONS

G. Meneghetti

Department of Industrial Engineering, University of Padova



2

Fatigue assessment approaches for components and joints made of metal materials

• Stress-‐based approach (either high-‐cycle or infinite life) -‐ Woehler curve with fatigue limit concept (k, σA,10%, σA,50%, σA,90%) -‐ Nominal stresses (elastically calculated)

• Strain-‐based approach (finite life assumed) -‐ Manson-‐Coffin curve (n’, K, σ’f, ε’f, b, c; Ed) -‐ Local elasto-‐plastic strains

• Linear Elastic Fracture Mechanics (either finite or infinite life) -‐ Paris curve with threshold concept (ΔKth, C, n) -‐ Stress intensity factor

• Component testing -‐ Fatigue curve (k, FA,90%)

• Linear elastic notch mechanics -‐ Non-‐conventional extension of LEFM to notches -‐ Fatigue design curves based on N-‐SIFs (Notch-‐Stress intensity factors) -‐ Short/long cracks, sharp/blunt notches (whatever notch root radius and opening angle)



3

Fatigue limit of cracks and notches

ρ

2⋅a

D

a

D

ρ

d

α1, Ktg

α2, Ktg

a ( scala log)

Fatigue limit Δσth (scala log)

tg

0K

σΔ

0Δσ

aα2⋅π

K thΔ

aα1⋅π

K thΔ

α2a from linear elastic FE analysesα2a (scala log)

aα2⋅π

K thΔ

⋅

)aa(

K

0

th

+⋅2α⋅π

Δ

Atzori B., Lazzarin P., Meneghetti G., Fracture mechanics and notch sensitivity, Fatigue Fract. Engng Mater. Struct., 2003.

a0 a*

2

0

th0

K1a !!"

#$$%

&

σΔ

Δ⋅

π=



4

Analysed ADIs and specimens preparation

Specimens preparation Raw Lynchburg specimens φ=25 mm, L=300 mm GS 600 separately cast (85% pearlite, 15% ferrite). Austempering treatments to obtain ADI 900, ADI 1050 e ADI 1200. Machining.



5

Experimental test program

• Rotating bending (f=200 Hz); • Short stair-‐case procedure (5÷7 specimens) to

determine the fatigue limit. • SEM analysis of fatigue damage



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Results (selection)

50

1E7 N° di cicli

σa,g [MPa]

40

30

20

10 1E6 1E5 1E4

R σg,th 50% k

-1 23 4,49 60 70

ADI900-12-3-0,1

1E8

80 90

N° di cicli

σa,g [MPa]

1E4 1E5 1E6 1E7 1E8

100

200

ADI1200-8-1-0,1

-1 77 5.93 R σg,th 50% k

ADI 1200 – 8 – 1 - 0,1

MATERIALE

Da

ρ

45°

a

D

ρ

PS 10%

PS 90%

PS 10%

PS 90%



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The diagram for fatigue limit assessments

10

100

1000

0.01 0.1 1 10 100α2a [mm]

Δσg,th

[MPa]

( )[ ] 5,02thth,g aK

−= απΔσΔ

a0 =0,091mm

Δσ0 =848 MPa

tg

0th,g K

σΔσΔ =

Theoretical model Exp. results

Atzori B., Lazzarin P., Meneghetti G., Fracture mechanics and notch sensitivity, Fatigue Fract. Eng Mater. Struct., 2003



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Comparison ADI 900 -‐ ADI 1050

10

100

1000

0,01 0,1 1 10 100α2 a [mm]

Δσ

g,th

[MPa

]

ADI900

ADI1050

Δσ0,ADI1050=848MPa

a0=147 µm (ADI 900)a0=91 µm (ADI 1050)

Δσ0,ADI900=687MPa



9

Damage analysis

SEM (300x)

SEM (950x)ottico (500x)

l

Run-out specimens showed non-propagating cracks.

Point of view



10

Non-‐propagating crack length

• NPC length : -‐ Increases as material parameter a0 increases



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A synthesis of the low-‐ and medium-‐cycle fatigue behaviour of as-‐cast and austempered ductile irons

based on the plastic strain energy

G. Meneghetti, M. Ricotta, B. Atzori

Department of Industrial Engineering, University of Padova (Italy)



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• Ductile irons (DIs) are structural materials suitable to manufacture mechanical components of complex geometry.

• Austempering heat treatment improve their mechanical properties and can be adjusted to obtain, as an example, high fatigue performances or high wear resistance.

• Push-pull, strain-controlled fatigue tests were carried out on ferritic, pearlitic, isothermed and austempered ductile irons covering a range of tensile strengths σR from about 400 MPa to 1300 MPa.

Introduction



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Materials investigatedMaterial Microstructure nod/

mm2 Es

[GPa] σR

[MPa] σp02

[MPa] A [%]

HB

DI 400 100% Ferrite 220 160 440 305 19 150

100% Ferrite 290 160 449 315 >30 168

DI 600 35% Ferrite-65% Pearlite 310 165 722 426 10 220

25% Ferrite-75% Pearlite 240 165 727 435 12 239

DI 700 5% Ferrite-95% Pearlite 244 161 805 487 8.0 244

5% Ferrite-95% Pearlite 310 161 862 500 8.8 275

IDI Pearl.-Ferr. Interconnected 220 170 758 455 10 240

Pearl.-Ferr. Interconnected 290 170 957 645 15 292

ADI 800 Ausferrite 244 170 858 551 15 270

Ausferrite 310 170 1084 757 17 321

ADI 1050 Ausferrite 244 163 1110 794 13 330

Ausferrite 244 163 1160 831 12 350

Ausferrite 310 163 1118 805 10 345

ADI 1200 Ausferrite 310 148 1355 1150 10 410

Ausferrite 310 148 1363 1046 11 410



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Strain-‐controlled fatigue testing (3)

Strain, ε

Stress, σ

Δεplastic

Δε

Δεelastic= Δε - Δεplastic

Applied Strain amplitude, Δε/2

elastic

plastic

total

One testFitting curve: cyclic σ−ε curve

!'n1

'plelKE !!"

#$$%

& σ+

σ=ε+ε=ε

Fitting curve: Manson-Coffin curve

!( ) ( )cf'

fb

f

'f

pl,ael,aa N2N2E

ε+σ

=ε+ε=ε

Nf

# 5 material parameters

# 2 material paramenters



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Compatibility conditionsOnly 3 among the 7 material parameters are independent because two compatibility conditions must be satisfied:

!( ) ( )cf'

fb

f

'f

pl,ael,aa N2N2E

ε+σ

=ε+ε=ε

!'n1

'aa

pl,ael,aa KE!"

#$%

& σ+σ

=ε+ε=ε

Compatibility equations (must be valid for any stress amplitude and Nf):

!'n1

'aa

pl,ael,aa KE!"

#$%

& σ+σ

=ε+ε=ε!

( ) ( )cf'f

bf

'f

pl,ael,aa N2N2E

ε+σ

=ε+ε=ε=

!'n1

'aa

pl,ael,aa KE!"

#$%

& σ+σ

=ε+ε=ε =!

( ) ( )cf'f

bf

'f

pl,ael,aa N2N2E

ε+σ

=ε+ε=ε

!cbn' =

!

( ) c/b'f'f'K

ε

σ=

Cyclic curve

Manson-Coffin curve

If the Cyclic stress-strain curve and the Manson-Coffin curve are fitted separately, then compatibility is satisfied only approximately



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A graphical representation of the analysis procedure

A. Niesłony, C. Dsoki, H. Kaufmann, P. Krug: Int J Fatigue, 2008. Meneghetti G, et al., Fatigue Fract. Eng. Mater. Struct. 2014

stra

in

stress life

Stress-strain

Stress-life

Strain-life



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Procedure to analyse the experimental data

b, σf’ c, ε’f

n’, K’ according to compatibility conditions

E, dynamic elastic modulus

Stress-life curve (σa-Nf) Plastic strain-‐life curve (εa,pl-N)

2D statistical analysis (ASTM E 739)

Compatibility conditions are satisfied !

! ( )bf'fa N2⋅σ=σ ( )cf

'fpl,a N2ε=ε



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Procedure to analyse the experimental dataStatic and strain-controlled fatigue tests were performed on 7 different ductile irons (about 120 fatigue experiments).



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Specimens’ preparation

• Cast Lynchburg specimens (L=200 mm, φ=25 mm) • Austempering Heat treatment (for ADI and IDI grades)

✓ Austenitization between 800 and 900 °C; ✓ Quenching in salt bath (NaNO3 and NaNO2) between 300-400 °C;

✓ Cooling at room temperature. • Machining of specimens to final geometry

Adopted specimens’ geometry for static and constant amplitude, strain-controlled fatigue tests (5<Ф<7 mm).



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Material response to cyclic strains: DI and IDI

• Strain hardening occurs, the higher the applied strain amplitude. • Stabilization is not attained (half-life stress amplitude has been considered). • This behaviour was observed for DI 400, DI 600, DI 700 and IDIs.

DI-400 0

100

200

300

400

0.0E+00 2.0E-03 4.0E-03 6.0E-03 8.0E-03ε [m/m]

σ [ M

Pa]

Serie3

Serie1

Best fitting method: Es= 160000 MPa; n'=0.063; K'=601 MPaPresent method: E=159300 MPa; n'=0.069; K'=624 MPaMonotonic curve: Es=160000 MPa, n=0.08K=500 MPaσ

a-εa data

monotonic

cyclic

0

100

200

300

400

0.0E+00 2.0E-03 4.0E-03 6.0E-03 8.0E-03ε [m/m]

σ [

MPa

]

Serie3

Serie1

Best fitting method: Es= 160000 MPa; n'=0.063; K'=601 MPaPresent method: E=159300 MPa; n'=0.069; K'=624 MPaMonotonic curve: Es=160000 MPa, n=0.08K=500 MPaσ

a-εa data

Cyclic parameters:



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Material response to cyclic strains: ADIs

600

700

800

900

1000

1100

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

2N, number of reversals

σ a [M

Pa]

0.80%0.70%0.60%0.50%0.40%

εa=0.80%ε

a=0.70%ε

a=0.60%ε

a=0.50%ε

a=0.40%

ADI 1200

• Initial strain hardening and then softening (for εa>0.5%), overall hardening behaviour

• This behaviour was observed for ADI 800, ADI 1050 and ADI 1200.

0

200

400

600

800

1000

1200

0.0E+0 2.0E-3 4.0E-3 6.0E-3 8.0E-3 1.0E-2ε [m/m]

σ [ M

Pa]

Classical method

Present method

Serie3

Serie1

Best fitting method: Es= 148000 MPa; n'=0.039; K'=1412 MPaPresent method: E=167200 MPa; n'=0.175; K'=3361 MPaMonotonic curve: Es=148000 MPa; n=0.123; K=2100 MPaσ

a-εa data

monotonic

cyclic

Cyclic parameters:

0

200

400

600

800

1000

1200

0.0E+0 2.0E-3 4.0E-3 6.0E-3 8.0E-3 1.0E-2ε [m/m]

σ [ M

Pa]

Classical method

Present method

Serie3

Serie1

Best fitting method: Es= 148000 MPa; n'=0.039; K'=1412 MPaPresent method: E=167200 MPa; n'=0.175; K'=3361 MPaMonotonic curve: Es=148000 MPa; n=0.123; K=2100 MPaσ

a-εa data



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Example of Manson-‐Coffin curves: DI 400

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+072Nf, number of reversals to failure

ε a [m

/m]

ea

ea,pl

ea,el

ea<e0

εa totalε

a plasticε

a elasticε

a<ε0

ASTM procedureb = -0.052σ'

f = 568 MPaEs=160000 MPac = -0.667ε'

f = 0.194

Present methodb = -0.046σ'

f = 557 MPaE=164000 MPac = -0.667ε'

f = 0.194

Williams, C.R., Lee, Y.-L., Rilly, J.T., “ International Journal of Fatigue, 25, pp. 427-436 (2003).

Evaluated material parameters

1.E-03

1.E-02

1.E+02 1.E+04 1.E+06

ε a[m

/m]

2Nf, number of reversals to failure

GS-400

GS-700

IDI PD 2006

ADI 1000 2nd series

ADI 1200

42CrMo4



23

Comparison of fatigue performancesIDI microstructure

provides high fatigue ductility (ε’f)

ADI 1050 optimised for high cycle fatigue



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Fatigue-‐related material parameters of tested materials

Material E [MPa] n’ K’ [MPa]

σ’p02 [MPa]

σ’f [MPa]

b ε’f c

DI-400 164000 0.069 624 406 557 -0.046 0.194 -0.667

DI-600 165300 0.092 906 511 770 -0.056 0.171 -0.610

DI-700 164700 0.082 903 542 807 -0.057 0.254 -0.690

IDI 159300 0.134 1265 550 935 -0.070 0.104 -0.521

ADI 800 157000 0.125 1420 653 995 -0.060 0.057 -0.476

ADI 1050 156000 0.173 2598 887 1789 -0.106 0.116 -0.613

ADI 1200 167200 0.175 3361 1133 2029 -0.097 0.083 -0.629

2029/557=3.64

!

100

1000

Experimental data Fitting line Eq. (4a)

b =-0.046 σ'f= 557 MPa

σ a [M

Pa]

102 103 104 105 106 107 2Nf, number of reversals to failure

DI-400

!

100

1000

Experimental data Fitting line Eq. (4a)

b=-0.097 σ'f= 2029 MPa

σ a [M

Pa]

102 103 104 105 106 107 2Nf, number of reversals to failure

ADI 1200



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Synthesis based on the plastic strain hysteresis energy

1k

k=1/0.730=1.37

constNWconstNW

f

fk

≠⋅Δ

=⋅Δ

Intrinsic scatter T10/90=2.2

DW

T10/90



26

G. Meneghetti*, S. Masaggia°

*Department of Industrial Engineering – University of Padova (Italy)°Zanardi Fonderie Spa, Minerbe, Verona (Italy)

Estimation of the fatigue limit of components made of

Austempered Ductile Iron

weakened by V-‐shaped notches



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Fatigue crack growth rate tests (da/dN vs DK)

•C(T) specimens according to ASTM E 647-00;

• Constant load amplitude tests (Paris’ curve);

• Load decreasing tests (ΔKth);

• Crack length “a” measured by means of a Leica® stereoscope operating at maximum 50 X;

• calibration function to determine ΔK from “a” and applied “ΔP” according to ASTM E 647-00.

•ADI 800 material

150

120 96 24

33

33 6

thickness 12 mm

11.2 42.8

R 15

30° 144

ΔP

a



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Experimental evaluation of DKth

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1 10 100 ΔK [MPa√m]

da/dN [m/cycle]

ΔΚth=8.1 MPa√m

da/dN=8.0⋅10-13⋅ΔK3.38

[ΔK]=MPa√m R=0.1

Δσg,th = 440 MPa, ΔKth = 8.1 MPa√m ⇒ a0 =108 µm

Threshold for crack propagation according to ASTM E-647



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Crack propagation rates: ADI 800 vs GS/Steels/Al alloyTab. 1: Costanti della curva di Paris per alcuni materiali da costruzione per rapporto di sollecitazione R=0.1 [35].

Fig. 14: Curve di Paris delle ghise sferoidali confrontato con quello di altri materiali (rapporto di ciclo R=0.1) [35].

1E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1 10 100

da/dN [m/ciclo]

ASTM A553 B1 steel

HSLA steel

AISI 304 steel

7075 T6

GS convenzionale

GSA EN-GJS-800-8

ΔK [MPa⋅m0.5]



30

Example 1: structural durability of a gearName Symbol Value

Normal modulus mn [mm] 4.5 N° of teeth Z [-] 24

N° of teeth between clamp. n [-] 4 Bending arm hFe [mm] 5.992

Normal pressure angle αn [°] 20° Load angle αFen [°] 22.5° Helix angle β [°] 0 Face width b [mm] 14

Base diameter Db [mm] 101.59 Tip diameter Dt [mm] 118.38

Std. pitch diameter Dp [mm] 108 Radius at 30° tangent ρF [mm] 2.12 Chord at 30° tangent sFn [mm] 9.34

Roughness Rz [µm] 4.7 Vickers hardness HV [-] 337 Residual stresses [MPa] +40

Material: ADI 1050 (static tests according to UNI EN ISO 6892)

σR [MPa] σp0.2 [MPa] A% 1166 823 12.4



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Results of material characterisation and fatigue assessmentAfter experimental evaluation of material fatigue properties, the Kitagawa diagram for estimating the fatigue limit of ADI components weakened by 120° V-notches can be determined.

120°

Assessment procedure: Evaluation of aeff by means of finite element analysis Comparison between theory and experiments

exp-‐. result

U-‐notch (0°)

V-‐notch (120°)



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Conclusions (I)Materials characterisation should fit structural design purposes to take into account: - Materials’ intrinsic microstructural features

- pores, nodules, inclusions… and matrix microstructure

- Technological processes and geometry of a component / joint - Surface state - Surface treatments

- Notches/cracks

- Loading conditions and stress state - Local elasto-plastic

- Static and/or fatigue

- Multiaxiality

- Spectrum loading (variable amplitude)