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Lecture 24Fatigue
Mechanical Behavior of Materials Sec. 9.6-9.8
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Mechanics of Materials Lab
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S-N Diagram
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Fatigue Failure Criteria
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Effect of Mean Stress
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Effect of Mean Stress
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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
1
1)( 2
r
SS
SS
ut
m
e
a
kpsiK
kpsid
F
aofma
moa
ao
38.8
,53.443
Changeover
Table 7-10
1
2
3
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Example
• AISI 4340 steel is subjected to cyclic load with a tensile mean stress of 200 MPA– What is the fatigue life for stress amplitude
of 450 MPA– Estimate the relationship between stress
amplitude and fatigue life for this mean stress
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Safety Factor with Mean Stress
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Example
• Man-Ten steel subject to stress amplitude of 180 Mpa and mean stress of 100PMa for 20000 cycles– What is the safety factor in life and
stress?
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Safety Factor with Mean Stress
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Example
• Man-Ten steel (f’=1089MPa, b=-0.115) subject to stress amplitude of 180 Mpa and mean stress of 100PMa for 20000 cycles– What is the safety factor in life and stress?
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Assignment
Mechanical Behavior of Materials
9.30, 9.31,9.33 , 9.35