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DURABILITY AND OPTIMIZATION OF CONNECTING RODS

Adila Afzal and Pravardhan ShenoyGraduate Research Assistants

and Ali Fatemi, ProfessorThe University of Toledo

Funded by:

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OVERALL OBJECTIVES

• Evaluate and compare fatigue performance of forged steel and powder metal connecting rods.

• Perform life predictions of connecting rods and validate by experiments.

• Perform dynamic load analysis and optimization

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PROJECT OUTLINE

• Literature Survey on Connecting Rods– Load and Stress Analyses– Durability and Optimization Aspects– Manufacturing Processes and Economic Costs– Comparative Studies of Forged Steel & PM Connecting Rods

• Experimental Work– Specimen Testing (Material Evaluation)

• Forged steel• Powder metal• C-70 steel

– Testing of Forged Steel and Powder Metal Connecting Rods • Manufacturing fixtures• Load-control fatigue testing

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PROJECT OUTLINE

• Analytical Evaluations– Digitizing Connecting Rod Geometry– Stress (FEA) Analysis– Durability (Fatigue) Analysis and Life Predictions– Dynamic Load and Stress (FEA) Analysis – Optimization Analysis

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PUBLICATIONS AND PRESENTATIONS

• Durability and Optimization of Connecting Rods, Great Designs in Steel Seminar 2004, Livonia, MI (February 18th

2:30 PM).

• A Comparative Study of Fatigue Behavior and Life Predictions of Forged Steel and PM Connecting Rods, SAE Technical paper 2004-01-1529, SAE World Congress 2004, Innovations in Steel Bar Products and Processing Session (M2), Detroit, MI (March 11th,11:00 AM).

• Dynamic Load and Optimization Aspects, to be published.

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CONNECTING ROD NOMENCLATURE

From “Tension and Compression in Connecting Rods, Schreier 1999”

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LOADING OF CONNECTING ROD

Connecting Rods Are Subjected to:• Inertia forces due to mass• Forces generated from the combustion process• Forces due to wearing of forging flashes

These Forces Produce:• Cyclic axial force and stress• Cyclic bending moment and stress

(perpendicular to the crankshaft axis)• Cyclic bending moment and stress

(parallel to the crankshaft axis)

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LOADING OF CONNECTING ROD (SONSINO, 1996)

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Conventional forged steel

• Sedan, 150 HP @ 5700 RPM

• Weight: 0.93 lb

Powder metal

• Minivan, 150 HP @ 5200 RPM

• Weight: 1.2 lb

CONNECTING RODS

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Specimen TestingStrain-controlled tensile tests

Strain-controlled fatigue tests

Based on ASTM standards and recommended practices

Flat specimens from forged steel and powder metal connecting rods

Instron closed-loop servo-controlled hydraulic axial load

frame and digital servo controller are utilized for the tests.

SPECIMEN TESTING

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2.54

76

6.985

9.53

2.5

24.5

R12.

7

Flat plate specimen configuration and dimensions

according to ASTM Standard E606

(All dimensions in mm)

SPECIMEN GEOMETRY

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COMPARISON OF MATERIAL MECHANICAL PROPERTIES

Forged Steel Powder Metal

YS (MPa) 700 588YS Ratio 1 0.84UTS (MPa) 938 866UTS Ratio 1 0.92YS’ (MPa) 620 609YS’ Ratio 1 0.98Sf (at Nf = 106) 423 334Sf Ratio 1 0.79

* Base of comparison is Forged Steel.

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MATERIAL CYCLIC & MONOTONIC STRESS-STRAIN BEHAVIORS

0

100

200

300

400

500

600

700

800

900

1000

0.0% 0.5% 1.0% 1.5% 2.0%

True Strain (%)

Tru

e St

ress

(M

Pa)

FS mo no to nic curveFS cyclic curveP M mo no to nic curveP M cyclic curve

Monotonic

Monotonic

Cyclic

Cyclic

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MATERIALSTRESS-LIFE COMPARISON

Better overall S-N fatigue resistance of the forged steel

Long-life fatigue life of forged steel more than an order of magnitude longer than powder metal

O Forged steelX Powder metal

1E+4 1E+5 1E+6 1E+7

200

1000

1E+3

Tru

e St

ress

Am

plitu

de, Δ

σ/2

(MPa

)

Reversals to Failure, 2Nf

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MATERIALTOTAL STRAIN-LIFE COMPARISON

For a connecting rod, subjected to many millions of cycles, high cycle regime is of primary interest

In the high cycle regime, forged steel results in about a factor of seven longer life than the powder metal material. 0.10%

1.00%

1E+3 1E+4 1E+5 1E+6 1E+7 1E+8Reversals to Failure, 2Nf

Tru

e St

rain

Am

plitu

de, Δ

ε/2,

%

O Forged steelX Powder metal

1E+4 1E+5 1E+6 E+7

0.10%

1.00%

1E+3

Reversals to Failure, 2Nf

1E+8

Tru

e St

rain

Am

plitu

de, Δε/

2, %

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DIGITIZED MODEL OF FS CONNECTING ROD

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BOUNDARY CONDITIONSFOR STRESS (FEA) ANALYSIS

Tension and compression loads applied as pressure on bearing surfaces.

In tension cosine function over 180 deg, in compression uniform distribution over 120 deg (Webster 1983).

Four cases considered:

Crank end Pin endtension restrainedcompression restrainedrestrained tensionrestrained compression

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MESH DETAILS

7.77 mm2.54 mm

1.52 mm 1.27 mm (used)80,000 elements130,000 nodes

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CONTOURS OF VM STRESS FOR FS CONNECTING ROD (Pmax = 26.7 kN or 6 kip)

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CONNECTING ROD TEST FIXTURES

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CONNECTING ROD TEST CONDITIONS

Load-controlled axial tests

Load ratio Pmin/ Pmax = - 1.25

Test frequency 2-5 Hz

Life range 4 X 104 – 4 X 106 Cycles

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COMPONENT TEST RESULTS& STRESS-LIFE COMPARISONS

Forged steel connecting rod fatigue strength is 387 MPa, while for PM connecting rod it is 282 MPa.

About two orders of magnitude longer life for the forged steel connecting rod.

Sa = 852 (Nf)-0.0572

Sa = 971 (Nf)-0.0895

O Forged steelX Powder metal

R = - 1.25

(2)

1E+4

(2)

1E+5 1E+6 1E+7

200

600

Cycles, Nf

Stre

ss A

mpl

itude

(Sa),

MPa

(MPa

)

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FRACTURE SURFACES

FORGEDSTEEL

PM

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FS FRACTURE SURFACES

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Sponsored by: American Iron and Steel Institute