1. Mechanics of Materials Lab
Lecture 24
Fatigue
Mechanical Behavior of Materials Sec
Jiangyu Li
University of Washington 1
Jiangyu Li, University of Washington

2. Jiangyu Li, University of Washington
S-N Diagram 2
Jiangyu Li, University of Washington

3. Fatigue Failure Criteria3
Jiangyu Li, University of Washington

4. Jiangyu Li, University of Washington
Effect of Mean Stress 4
Jiangyu Li, University of Washington

5. Jiangyu Li, University of Washington
Effect of Mean Stress 5
Jiangyu Li, University of Washington

6. Jiangyu Li, University of Washington
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 6
Jiangyu Li, University of Washington

7. Jiangyu Li, University of Washington
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 7
Jiangyu Li, University of Washington

8. Safety Factor with Mean Stress8
Jiangyu Li, University of Washington

9. Jiangyu Li, University of Washington
Example
Man-Ten steel subject to stress amplitude of 180 Mpa and mean stress of 100PMa for cycles
What is the safety factor in life and stress? 9
Jiangyu Li, University of Washington

10. Safety Factor with Mean Stress10
Jiangyu Li, University of Washington

11. Jiangyu Li, University of Washington
Example
Man-Ten steel (sf’=1089MPa, b=-0.115) subject to stress amplitude of 180 Mpa and mean stress of 100PMa for cycles
What is the safety factor in life and stress? 11
Jiangyu Li, University of Washington

12. Jiangyu Li, University of Washington
Assignment
Mechanical Behavior of Materials
9.30, 9.31,9.33 , 9.35 12
Jiangyu Li, University of Washington