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Durability Comparison and Optimization of Forged Steel and Ductile Cast Iron

Crankshafts

Jonathan Williams and Farzin MontazersadghGraduate Assistants

andAli Fatemi, Professor The University of Toledo

March 7, 2007

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• Overview and Overall Goal

• Forged Steel & Cast Iron Crankshafts and Comparisons– Experimental Work

• Specimen tests• Component tests

– Analytical Work• Life predictions• Dynamic load analysis and FEA• Optimization

– Conclusions

Outline

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Overview and Overall Goal

• Evaluate and compare fatigue performance of forged steel components with competing manufacturing process technologies.– Steering Knuckle

* Forged steel * Cast aluminum * Cast iron– Connecting Rod

* Forged steel * Powder metal– Crankshaft

* Forged steel * Ductile cast iron

• Evaluate life prediction techniques as compared with experimental results and perform optimization.

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Overview and Overall Goal

Steering KnuckleSteering Knuckle• Forged Steel: 2.5 kg, 11V37 Steel• Cast Aluminum: 2.4 kg, A356-T6• Cast Iron: 4.7 kg, 65-45-12

Connecting RodConnecting Rod• Forged steel: 0.93 lb• Powder metal: 1.2 lb

CrankshaftCrankshaft• Forged steel: 3.9 kg• Ductile cast iron: 3.7 kg

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Publications of Results(Steering Knuckle and Connecting Rod)

Steering Knuckle• “Fatigue Performance Evaluation of Forged vs. Competing Process Technologies: A Comparative Study”, A.

Fatemi and M. Zoroufi, 24th Forging Industry Technical Conference, Cleveland, OH, October 2002.

• “Fatigue Life Comparison of Competing Manufacturing Processes: A Study of Steering Knuckle”, M. Zoroufi and A. Fatemi, SAE Technical paper 2004-01-0628, SAE World Congress 2004, Detroit, MI, March 2004.

• “Durability Comparison and Life Predictions of Competing Manufacturing Processes: An Experimental Study of Steering Knuckle”, M. Zoroufi and A. Fatemi, 25th Forging Industry Technical Conference, Detroit, MI, April 2004.

• “Experimental Durability Assessment and Life Prediction of Vehicle Suspension Components: A Case Study of Steering Knuckles”, M. Zoroufi and A. Fatemi, Journal of Automobile Engineering, Vol. 220, pp. 1565-1579, 2006.

Connecting Rod• “A Comparative Study of Fatigue Behavior and Life Predictions of Forged Steel and PM Connecting Rods”, A.

Afzal and A. Fatemi, SAE paper 2004-01-1529, SAE World Congress 2004, Detroit, MI, March 2004.

• “Connecting Rod Optimization for Weight and Cost Reduction”, P. Shenoy and A. Fatemi, SAE Technical paper 2005-01-0987, SAE World Congress 2005, Detroit, MI, April 2005.

• "Dynamic Analysis of Loads and Stresses in Connecting Rods", P. Shenoy and A. Fatemi, Journal of Mechanical Engineering Science, Vol. 220, pp. 615-624, 2006.

• "Comparative Durability Study of Competing Manufacturing Process Technologies", 26th Forging Industry Technical Conference, A. Fatemi, M. Zoroufi, P. Shenoy and A. Afzal, Chicago, IL, November 2005.

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Crankshaft StudyOverall Objectives

• Evaluate and compare fatigue performance of forged steel and ductile cast iron crankshafts.

• Perform life predictions and compare with component test data.

• Perform dynamic load analysis and optimization.

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Outline

• Literature Survey on Crankshafts– Design and manufacturing considerations including cost analysis– Comparison of Competing manufacturing techniques– Durability assessment and optimization– Experimental techniques and bench testing

• Experimental Work– Specimen Testing (Forged steel and ductile cast iron)– Component Testing (Forged steel and ductile cast iron)

• Analytical Evaluations– Dynamic Load Analysis– Stress Analysis Using FEA– Durability Analysis and Life Predictions– Optimization

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Crankshaft Publication of Results

• A Literature Review on Durability Evaluation of Crankshafts Including Comparisons of Competing Manufacturing Processes and Cost Analysis, M. Zoroufi and A. Fatemi, 26th Forging Industry Technical Conference, Chicago, IL, November 2005.

• Fatigue Performance Evaluation and Optimization of Crankshafts, J. Williams, F. Montazersadgh and A. Fatemi 27th Forging Industry Technical Conference, Ft. Worth, Texas, March 27, 2007.

• Fatigue Performance of Forged Steel and Ductile Cast Iron Crankshafts, J. Williams and A. Fatemi, SAE Technical Paper 2007-01-1001, SAE World Congress 2007, Innovations in Steel Bar Products and Processing, April 17, 2007, Detroit, Michigan.

• Dynamic Load and Stress Analysis of a Crankshaft, F. Montazersadgh and A. Fatemi, SAE Technical Paper 2007-01-0258, SAE World Congress 2007, New SI Engine and Component Design, April 17, 2007, Detroit, Michigan.

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Crankshaft Nomenclature

http://www.tpub.com/engine3/en3-53.htm

Loading: Bending and Torsion

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Crankshafts

Ductile Cast Iron Crankshaft– Similar engine type and size– 3.7 kg

Forged Steel Crankshaft– Outdoor power equipment engine– 460 cc, 12.5 HP– 3.9 kg

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Specimen Testing

• Specimen Tests– Strain-controlled tensile tests

– Strain-controlled fatigue tests

– Procedures and practices as outlined by ASTM

– Round specimens machined from unmachined crankshafts Closed-loop servo-hydraulic

axial load frame

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Specimen Geometry

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Material Stress-Strain Curves

0

100

200

300

400

500

600

700

800

900

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

True Strain (%)

Tru

e S

tres

s (

MP

a)

Forged Steel Monotonic

Forged Steel Cyclic

Cast Iron Cyclic

Cast Iron Monotonic

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Material Stress-Life Comparison

100

1000

1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8

Reversals to Failure, 2Nf

Tru

e S

tres

s A

mp

litu

de

(MP

a)

(2)

Forged Steel

Cast Iron

• Forged steel has better S-N fatigue performance than the ductile cast iron.

• At long lives, for a given stress, forged steel has a factor of 30 longer life.

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Material Comparison

• Fatigue life is often controlled by stress and strain ranges at root of the fillet.

• At long lives, the forged steel has a factor of 50 longer life.

100

1000

10000

1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8

Reversals to Failure, 2Nf

Neu

ber

Str

ess

Ran

ge

[(Δε )

(Δσ )

E]1/

2 , MP

a

(2)

(2)

Forged Steel

Cast Iron

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Charpy V-Notch Results

Results

Specimen Orientation

0

10

20

30

40

50

60

70

80

90

100

-100 -50 0 50 100 150 200 250

Test Temperature (˚C)

Ab

sorb

ed E

ner

gy

(J)

Forged steel L-T

Forged steel T-L

Cast Iron

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Mechanical Properties

Forged Steel Cast Iron Ratio*

YS (MPa) 625 412 0.66

UTS (MPa) 827 658 0.80

%RA 58 6 0.10

CVN (Room Temp) (J) 58 5 0.08

Sf (at Nf=106) (MPa) 359 263 0.73

* base of comparison is the forged steel

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Component Fatigue Tests

Schematic of test set-up Actual test set-up

• Load-controlled, constant amplitude fatigue tests• R-Ratio: -0.2• Test frequency: 1.4 – 3 Hz• Life range: 7 X 103 – 4 X 106

• Both crankshafts tested at the same load (moment) amplitudes

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Component Fatigue Test Results

• Lives based on crack initiation (small crack on the order of a few mm).

• For a given bending moment, forged steel crankshaft has a factor of 6 longer life.

y = 2555.8x-0.1331

R2 = 0.8128

y = 2144.6x-0.1389

R2 = 0.9536

100

1000

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Cycles to Failure (Nf)

Mo

men

t A

mp

litu

de

(N-m

)

Forged Steel

Cast Iron

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Component Fatigue Test Results Based on 5% Change

• Lives based on 5% change in displacement amplitude (large crack).

• For a given bending moment, the forged steel crankshaft has an order of magnitude longer life.

y = 2401.8x-0.1218

R2 = 0.8656

y = 3110.8x-0.1651

R2 = 0.9579

100

1000

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Cycles to Failure (Nf)

Mo

men

t A

mp

litu

de

(N-m

)

Forged Steel

Cast Iron

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Fractured Components

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S-N Life Prediction Results

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Experimental Cycles to Failure

Pre

dic

ted

Cyc

les

to F

ailu

re

Forged Steel

Cast Iron

( )bffNf

u

m

Nf

a

NS

SS

SS

2

1

'σ=

=+

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Dynamic Load Analysis

0

5

10

15

20

25

2000 2800 3600

Engine Speed (RPM)

Forc

e M

agni

tude

(kN

)

Max Bending Max Torsion Range of Bending Range of Torsion

Pressure vs. Crankshaft Angle

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600 700

Crankshaft Angle (Deg)

Pres

sure

(bar

)

Slider-Crank Mechanism (ADAMS Simulation)

-10000

-5000

0

5000

10000

15000

20000

0 90 180 270 360 450 540 630 720

Crankshaft Angle (Deg)

Forc

e (N

) Resultant

TorsionalBending

Critical Speed

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Digitized Models

Forged Steel Cast Iron

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Boundary Conditions for FEA

Service Life

(1 of 4 steps)

Fixed surface in all degrees of freedom over 180°

Fixed edge in directions 1 & 2 over 180°

Applied load; constant pressure over 120°

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Mesh Generation

0.762

3.81

3.81

0.7623.812.54 3.81

1.27

3.81

3.81

0.762

3.81

3.81

3.812.54

3.81

Forged Steel

Cast Iron

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FEA and Critical Location

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-50

0

50

100

150

200

250

1 2 3 4 5 6

Location Number

Stre

ss M

agni

tude

(MPa

)

Range Mean

FEA and Critical Location

-50

0

50

100

150

200

0 180 360 540 720

Crankshaft Angle (Deg)

Stre

ss M

agni

tude

(MPa

)

1 2 3 4 5 6

Critical Location

Critical Location

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Comparison Between FEA and Experimental

Fixed surface in all degrees of freedom

Fixed surface in all degrees of freedom

Applied load; concentrated force

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Optimization Flowchart

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Geometry Optimization Process

Final Optimized Geometry

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Material and Manufacturing Optimization

Fillet Rolling (900 kgf)

Standard Bare Sample

• Manufacturing– Adding fillet

rolling• Material

– Using Micro-Alloyed Steel

Specimen Rig Test (Park et al. 2001)

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Conclusions

• Yield strength of the forged steel is 50% higher than that of the cast iron, while the ultimate strength is 26% higher. Ductility and impact toughness of the forged steel is also significantly higher.

• Material fatigue strength at 106 cycles for the forged steel is 37% higher than that of the cast iron, resulting in 30 times longer life.

• Component fatigue tests show fatigue strength based on crack initiation for the forged steel crankshaft to be 27% higher thanthat of the cast iron. This results in a factor of 6 longer life.

• Fatigue crack growth was a significant portion of the life for both crankshafts. The crack growth rate for the forged steel was slower than that of the cast iron.

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Conclusions

• Life predictions using the S-N approach provided very reasonable estimations for the forged steel crankshafts. Predictions for the cast iron crankshafts were less accurate but were conservative.

• Dynamic load analysis results in more realistic stresses, whereas static analysis overestimates the results.

• Considering the torsional load in the overall dynamics analysis has no effect on von Mises stress at the critically stressed location.

• Geometry optimization resulted in 18% weight reduction of the forged steel crankshaft. Fillet rolling results in significant increase of the crankshaft fatigue strength.