ductile iron at high strain rate

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Constitutive response of austempered ductile iron at high strain rate

A. Ruggiero

¹

, G. Iannitti

¹

, N. Bonora

¹

, E. Veneri

²

, F. Vettore

²

, S.

Masaggia

²

1University of Cassino and Southern Lazio, Cassino I-03043 (FR)

2Zanardi Fonderie, I-37046 Minerbe (VR)

ECF21 - Catania, Italy 2016

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OUTLINE

• Background

• Scope of the work

• Experimental characterization

• Material modelling

• Model verification

• Conclusions

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BACKGROUND

• Austempered ductile iron has a unique

microstructure called ausferrite (acidular ferrite and retained austenite) [1].

• This ausferrite microstructure sets ADI apart from as-cast ductile iron providing excellent

property combinations of strength, ductility, and toughness

• Austempered Ductile Iron was first commercially applied in 1972. Today, it is estimated that over 50,000 tons per year of austempered ductile iron components are installed in cars and trucks world-wide. That production appears to be

growing at a rate of exceeding 10% per year [2].

ECF21 - Catania, Italy 2016 References

[1] Böhme* and Reissig, Adv. Eng. Mat. 2015, 17, No. 8 [2] Keough and Hayrynen, SAE, 2000, paper no 248871

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BACKGROUND

• Austempered ductile irons (ADI) market represents nearly all segments of

manufacturing.

• The use of ADIs in Mobility, Mining and Defense industry is still limited also because of the

general perception and concern about their intrinsic brittleness eventually promoted by dynamic loading conditions.

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SCOPE OF THE WORK

• To investigate the constitutive response of

selected ADIs under combinations of high strain rate and temperature.

• To compare material response with that of a reference steel

• Develop a constitutive model for simulation based design of ADIs manufactured

components operating under dynamic loading conditions

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MATERIAL

• Materials under investigation:

• ADI 1050-6

• ADI 1200

• ADI HSIADI (high Si)

• 42CrMo4 (ref material)

DESIGNATION UTS YIELD STRESS ELONGATION HARDNESS

MPa MPa % BR

ADI 1050 1050 700 6 320-380

ADI 1200 1200 850 3 340-420

HSiADI 1450 - - 400

References

[3] J. R. Keough, K. L. Hayrynen and G. L. Pioszak, AFS Proc. 2010, Schaumburg, IL USA.

[3]

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EXPERIMENTAL CHARACTERIZATION

TEST STRAIN RATE TEMPERATURE

QUASI-STATIC TENSILE 0.001/s -60°C, 25°C and +70°C

DYNAMIC TENSILE 600/s -60°C, 25°C and +70°C

DYNAMIC TENSILE 1200/s -60°C, 25°C and +70°C

QUASI STATIC TESTING

• Low temperature:

nitrogen

• High temperature:

induction coil

• Strain measurement:

clip and DIC

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EXPERIMENTAL CHARACTERIZATION

DYNAMIC TENSILE 600/s -60°C and 25°C

DYNAMIC TESTING

nitrogen

induction coil

clip and DIC

8000mm

3000mm 3000mm

1000mm A

A

A-A

1200mm

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EXPERIMENTAL CHARACTERIZATION

DYNAMIC TESTING

nitrogen

induction coil

clip and DIC

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EXPERIMENTAL CHARACTERIZATION: RESULTS

QUASI STATIC TESTING

0 0.05 0.1 0.15 0.2

0 200 400 600 800 1000 1200 1400 1600

Deformazione

Sforzo (MPa)

T= 298 K T= 213 K T= 343 K

LOG. STRAIN

TRUE STRESS [MPa]

343K 298K

213K

OBSERVATION:

• Lowering the temperature, the work hardening rate increases while the

«apparent» yield stress (s_02%) decreases

0.2% off-set

References

[4] Bonora and Ruggiero , Int. J. Solid and Structure, 2005, 43(5)

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EXPERIMENTAL CHARACTERIZATION: RESULTS

QUASI STATIC TESTING NOTE:

• Strain is uniform up to rupture which occurs without necking.

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EXPERIMENTAL CHARACTERIZATION: RESULTS

QUASI STATIC VS DYNAMIC TESTING

OBSERVATIONS:

• Strain rate effect between QS and dynamic: increase of the apparent yield stress (+18%)

• In dynamic traction, development of necking prior rupture

• Same effect also in temperature effect

• Small difference between two investigated high strain rates (600/s and 1200/s)

0 0.05 0.1 0.15 0.2

0 200 400 600 800 1000 1200 1400 1600

Deformazione

Sforzo (MPa)

T= 298 K T= 213 K T= 343 K 343K 298K

213K

0 0.05 0.1 0.15 0.2 0.25

0 200 400 600 800 1000 1200 1400 1600

Deformazione

Sforzo (MPa)

HpkGS_2 HpkGS_7 HpkGS_11 HpkGS_15

298K 600/s 213K

600/s 1200/s 1200/s 600/s

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EXPERIMENTAL RESULTS

ECF21 - Catania, Italy 2016 10^-4 10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

600 700 800 900 1000 1100 1200 1300 1400

ADI 1050 (present work)

ADI1200 (Bohmer and Reissig, 2015) Linear Fit of Sheet1 B"R02"

Yield stress, R 0.2% [MPa]

STRAIN RATE [1/s]

10^-4 10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴ 0.00

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

ADI1050 T=298K

ADI1200 (Bohmer and Reissig, 2015) ADI1050 T=213K

FAILURE STRAIN

STRAIN RATE [1/s]

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EXPERIMENTAL RESULTS

200x

HpkGS5 RT 1200s^-1 6.5x 25x

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MATERIAL MODELLING

• Modified Johnson-Cook law:

   

  ^ ^

s

 A T B T



ⁿ  1 Cln *



  

    

 

 

  

 

S

a

0

b

A A 1 exp T

m B B exp T

m

0 100 200 300 400

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Temperatura (K)

A e B (MPa)

A Fit A B Fit B

A_S(MPa) m_a B₀ (MPa) m_b n C

1207 160 10805 160 0.6 0.014 1.0



₀(s )^1

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MODEL VERIFICATION: FEM RESULTS

0 0.5 1 1.5 2 2.5

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

QsGS_2 FEM

0 0.5 1 1.5 2 2.5

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

QsGS_7 FEM

0 0.2 0.4 0.6 0.8 1

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

HpkGS_12 FEM

QS, T=-60°C QS, T=+25°C QS, T=+70°C

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MODEL VERIFICATION: FEM RESULTS

0 0.5 1 1.5

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

HpkGS_2 FEM

 740s ;T 25 C^¹  

0 0.5 1 1.5

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

HpkGS_7 FEM

 1200s ;T 25 C^¹  

0 0.5 1 1.5

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

HpkGS_15 FEM

 613s ;T^¹   60 C

0 0.5 1 1.5

0 2000 4000 6000 8000 10000 12000 14000

Allungamento (mm)

Forza (N)

HpkGS_11 FEM

 1100s ;T^¹   60 C

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CONCLUSIONS

• In spite of the concerns about intrinsic brittleness eventually promoted by

dynamic loading conditions, an increase of ductility (strain to failure) is observed in ADI 1050 in the strain rate range up to 10³/s

• These results are consistent with other source data.

• No difference was observed in the failure mechanisms at low and high strain rate which occurs by coalescence of cavities nucleated at debonded spheroids.

• The reduction of apparent yield with temperature is probably an effect of the recovery of the stresses at the matrix-spheroid which causes an anticipated spheroid debonding.

• The proposed model, which is phenomenological in nature, seems to accuraterly reproduce the temperature and strain rate effect at least over the respective

range investigated