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Factors Affecting Strain Life Fatigue Results
ForCarbon and Low Alloy Automotive Bar Steels
Thomas G. Oakwood
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AISI Bar Steel Fatigue ProjectParticipants
• Ground Vehicle Manufacturers.
• Ground Vehicle Suppliers.
• AISI Bar Steel Producers.
• University Testing Laboratories.– University of Toledo, Toledo, Ohio– University of Waterloo, Waterloo, Ontario
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Topics To Be Discussed
• Structure and Content of AISI Bar Steel Fatigue Database.
• Analysis of Fatigue Performance From Data.
• Work in Progress.
• Plans for Future Work.
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Topics To Be Discussed
• Structure and Content of AISI Bar Steel Fatigue Database.
• Analysis of Fatigue Performance From Data.
• Work in Progress.
• Plans for Future Work.
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• Steel Grades– Carbon– Low Alloy– Micro-alloy
• Processing– Normalized– Quenched and Tempered– Case Hardened– Hot Rolled, Controlled Cooled
Bar Steel Fatigue Database-Content
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Bar Steel Fatigue Database-Information
• Steel Characterization.– Composition– Processing History– Digital Photomicrographs of Microstructures
• Mechanical and Cyclic Properties.– Tabulation of Axial Fatigue Data
• Graphical Output of Strain-Life Curves and Neuber Curves.
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Bar Steel Fatigue Database-Format
• Direct-Access Internet Format.
• Multiple Screens.– Master Index– “Tabbed” Material Properties Screens– Report Format– Excel– SAE– Independent Access for Microstructures and Graphs
• Comparative Printing.
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Bar Steel Fatigue Database-Access
• “Private Access.”– AISI Bar Steel Fatigue Committee Subscribers– Private Access to Data for Two Years– 90 Iterations Complete
• 9 In Progress• 40 Planned
• “Public Access.”– General Public– Data Available After Two Year Restriction– 71 Iterations Available
• User Name and Password Required.
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Topics To Be Discussed
• Structure and Content of AISI Bar Steel Fatigue Database.
• Analysis of Fatigue Performance From Data.
• Work in Progress.
• Plans for Future Work.
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Strain Life Equation
Δε/2 = (σ′f /E)(2Nf)b + ε′f (2Nf)c
– Where:»Δε/2 = Strain Amplitude» E = Young’s Modulus» σ′f = Fatigue Strength Coefficient» ε′f = Fatigue Ductility Coefficient» b = Fatigue Strength Exponent» c = Fatigue Ductility Exponent» 2Nf = Reversals To Failure
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Strain Amplitude Curves Normalized Steels
Total Strain Amplitude vs. Reversals to Failure for SAE Normalized Grades
0.001
0.01
0.1
1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07
Reversals to Failure, 2Nf
True
Str
ain
Am
plitu
de
1050M
1090
1141Al
1141Nb
1141V
1141V17501541
1038
10V45
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Strain Life Curves Quenched and Tempered Steels
Total Strain Amplitude vs. Reversals to Failure for Quench & Tempered Grades
0.001
0.01
0.1
1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07
Reversals to Failure, 2Nf
Stra
in A
mpl
itude
1141Al
1141Nb
1141V (16)
1038 (20)
10B21 (24)
1045 (26)
4130Al (29)
4140 (30)
5140 (31)
51B60 (33)
9254V (34)
9254Al (35)
4140 (64)
4140 (65)
4140 (66)
4140 (67)
4140 (68)
4140 (69)
5160H (84)
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Fatigue Strength Determinations
• Experimental Data–Δσ/2 = σ′f (2Nf)b
»Δσ/2 Fatigue Strength»σ′f = Fatigue Strength Coefficient»b = Fatigue Strength Exponent»2Nf = Reversals To Failure
• Linear Regression–Δσ/2 = f(BHN, Composition)
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Fatigue Strength Normalized
Δσ/2 @ 106 Cycles (MPa) = 0.18BHN + 53.48 %C + 218.27 %Mn – 2052.98 %S-46.40 %Si + 1336.68 %Ni – 56.56 %Cr
-380.44 %Mo + 36.10 (+/-4.60)
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Fatigue Strength Normalized
Predicted Versus Experimental Fatigue Strength @ 10^6 CyclesNormalized Steels
y = 0.9372x - 3.6903R2 = 0.9624
200
210
220
230
240
250
260
270
280
290
200 220 240 260 280 300 320
Experimental Fatigue Strength, MPa
Pred
icte
d Fa
tigue
Str
engt
h, M
Pa
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Fatigue Strength Quenched and Tempered
Δσ/2 @ 106 Cycles (MPa) = 2.52 BHN + 674.41 %C + 388.56 %Mn – 4062.07 %S-1225.32 %Si + 210.07 %Cr – 2260.21 %Mo
+355.04 %Ni + 570.18 %Al - 1590.00 %V – 476.48 (+/- 27.94)
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Fatigue Strength Quenched and Tempered
Predicted Versus Experimental Fatigue Strength @ 10^ 6 Cycles Quenched and Tempered Steels
y = 0.9672x - 7.3316R2 = 0.9858
200
250
300
350
400
450
500
550
600
650
700
200 250 300 350 400 450 500 550 600 650 700
Experimental Fatigue Strength, MPa
Pred
icte
d Fa
tigue
Str
engt
h, M
Pa
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Modified Strain Life Equation(Reference: M. Roessle and A. Fatemi, Int. Journ. Fatigue, 22, 2000)
Δε/2 = [(4.25BHN+225)/E](2Nf)-0.09
+ [(0.32BHN2 - 487BHN + 191000)/E](2Nf)-0.56
– Where: » BHN = Brinell Hardness Number»Δε/2 = Strain Amplitude» E = Young’s Modulus» 2Nf = Reversals To Failure
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Fatigue Prediction 1038 Normalized
Fatigue Approximation Iteration No. 18 1038 Normalized to 163 BHN
0.01
0.1
1
10
100 1000 10000 100000 1000000 10000000 100000000
Reversals to Failure
Stra
in A
mpl
itude
, %
Experimental DataPredicted From BHN
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Fatigue Prediction 1141 Normalized
Fatigue Approximation Iteration No. 11 (1141 AlFG Normalized to 223 BHN)
0.01
0.1
1
10
100 1000 10000 100000 1000000 10000000 100000000
Reversals to Failure
Stra
in A
mpl
itude
, %
Experimental DataPredicted From BHN
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Fatigue Prediction 9254 Quenched and Tempered
Fatigue Approximation Iteration No. 35 (9254 Quenched and Tempered to 584 BHN)
0.01
0.1
1
10
100 1000 10000 100000 1000000 10000000 100000000
Reversals to Failure
Stra
in A
mpl
itude
, %
Experimental DataPredicted From BHN
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Fatigue Prediction C70 Controlled Cooled
Fatigue Approximation Iteration No. 43 (C70 As-Rolled, Controlled Cooled BHN 241)
0.01
0.1
1
10
100 1000 10000 100000 1000000 10000000 100000000
Reversals to Failure
Stra
in A
mpl
itude
, %
Predicted From BHNExperimental Data
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Fatigue Prediction 4140 Quenched and Tempered
Fatigue Approximation Iteration Nos. 64-69 (4140 Quenched and Tempered to BHN 375 avg.)
0.01
0.1
1
10
100 1000 10000 100000 1000000 10000000 100000000
Reversals to Failure
Stra
in A
mpl
itude
, %
4140 Q&T It. 644140 Q&T It. 654140 Q&T It. 66Predicted From BHN4140 Q&T It. 674140 Q&T It. 684140 Q&T It. 69
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Summary
• Fatigue Strength Can Be Predicted From Composition And Hardness.– Limits:
• Simple Linear Regression• Composition Range of the Data• Constant Processing and Microstructure
• Strain Life Curve Can Be Predicted From Hardness.– Limits:
• Microstructure• Surface Condition• Residual Stress• Prior Austenite Grain Size• Inclusions
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Topics To Be Discussed
• Structure and Content of AISI Bar Steel Fatigue Database.
• Analysis of Fatigue Performance From Data.
• Work in Progress.
• Plans for Future Work.
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Work in Progress
• Comparison of Fatigue Behavior of Atmosphere and Vacuum Carburized Steels.– Simulated Carburized Case and Core– Surface Effects– Prior Austenite Grain Size
• Effects of Sulfur Level on Fatigue Properties.– Transverse Testing
• Effects of Varying Amounts of Bainite on the Fatigue Properties of Carburized Steels.– Simulated Carburized Case
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Topics To Be Discussed
• Structure and Content of AISI Bar Steel Fatigue Database.
• Analysis of Fatigue Performance From Data.
• Work in Progress.
• Plans for Future Work.
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Future Work
• Effect of Variation in Composition Within SAE Limits on Fatigue Properties.
• Residual Stress Measurements.• Effect of Varying Hardness Due to Change in Microstructure
on Fatigue Properties.– Simulated Carburized Core
• Comparison of Fatigue Properties of As-Rolled and Forged. Micro-alloyed Steels.
• The Effects of Random Overloads on Fatigue Behavior.• Continued Comparisons of the Fatigue Properties of
Atmosphere Carburized and Vacuum Carburized Steels.
