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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