Lecture 18 Fatigue Mechanical Behavior of Materials Sec. 9.6-9.7 Jiangyu Li

1. Mechanics of Materials Lab Lecture 18 Fatigue Mechanical Behavior of Materials Sec. 9.6-9.7 Jiangyu Li University of Washington Jiangyu Li, University of Washington

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

3. S-N Diagram Endurance limit Jiangyu Li, University of Washington

4. Endurance Limit For steel Jiangyu Li, University of Washington

5. Fatigue Failure Criteria Jiangyu Li, University of Washington

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

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

8. Fatigue Failure Criteria Multiply the stress By safety factor n Jiangyu Li, University of Washington

9. 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 Jiangyu Li, University of Washington

10. Example: ASME Elliptic Table 7-11 2 3 1 Jiangyu Li, University of Washington

11. 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 Jiangyu Li, University of Washington

12. Safety Factor with Mean Stress Jiangyu Li, University of Washington

13. 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? Jiangyu Li, University of Washington

14. Torsional Fatigue Strength Jiangyu Li, University of Washington

15. 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 Jiangyu Li, University of Washington

16. 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 Jiangyu Li, University of Washington

17. Example Tm=(20+160)/2=90Nm; Ta=(160-20)/2=70Nm Jiangyu Li, University of Washington

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