1. Mechanics of Materials Lab
Lecture 17
Fatigue
Mechanical Behavior of Materials Sec
Jiangyu Li
University of Washington 1
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2. Jiangyu Li, University of Washington
S-N Diagram 2
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S-N Diagram
Endurance limit 3
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Endurance Limit
For steel 4
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5. Fatigue Failure Criteria5
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Effect of Mean Stress 6
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Effect of Mean Stress 7
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8. Fatigue Failure Criteria
Multiply the stress
By safety factor n 8
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Example: Gerber Line
AISI 1050 cold-drawn bar, withstand a fluctuating axial load varying from 0 to16 kip. Kf=1.85; Find Sa and Sm and the safety factor using Gerber relation
Sut=100kpsi; Sy=84kpsi; Se’=0.504Sut kpsi
Table 7-10 2
Change
over 1 3 9
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10. Example: ASME Elliptic
Table 7-11 2 3 1 10
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11. Torsional Fatigue Strength11
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Combination of Loads
Use Se from bending;
Apply appropriate Kf for each mode;
Multiply axial stress component by 1/kc
Find the principle stresses
Find von Mises effective alternative stress
Use the fatigue failure criteria to determine safety factor 12
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Example
A rotating AISI 1018 cold-drawn steel tube (42x4 mm) has a 6mm diameter
hole drilled transversely through it. The shaft is subjected to a torque
Fluctuating from 20Nm to 160Nm, and a stead bending moment 150Nm
Estimate safety factor
Sut=440 Mpa, Syt=370MPa, Se’=0.504x440 =222 Mpa, Se=166MPa
Stress concentration factor Kt=2.366, from Table A-16, for bending,
Kts=1.75 for torsion; from the Fig. 7-20, 7-21, notch sensitivity q is
0.78 for bending, and 0.96 for torsion.
Thus Kf=2.07 for bending, and Kfs=1.72 for torsion 13
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Example
Tm=(20+160)/2=90Nm; Ta=(160-20)/2=70Nm 14
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Assignment
Mechanical Behavior of Materials
9.30, 9.33 15
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