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Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
1
FATIGUE BEHAVIOUR OF AS-‐CAST AND AUSTEMPERED DUCTILE IRONS
G. Meneghetti
Department of Industrial Engineering, University of Padova
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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)
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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=
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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.
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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%
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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Damage analysis
SEM (300x)
SEM (950x)ottico (500x)
l
Run-out specimens showed non-propagating cracks.
Point of view
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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Non-‐propagating crack length
• NPC length : -‐ Increases as material parameter a0 increases
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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)
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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).
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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).
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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:
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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Comparison of fatigue performancesIDI microstructure
provides high fatigue ductility (ε’f)
ADI 1050 optimised for high cycle fatigue
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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]
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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°)
Austempering, A Technology for Substitution
ADI DAYS 2016 6th – 7th October Minerbe
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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)
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