Fatigue Failure Through Bending David Burnette ME 498

1. Fatigue Failure Through Bending David Burnette ME 498

2. Overview • Objectives of experiment • Importance and theory • Experimental details • Result • Conclusions and recommendations

3. Objectives • To become familiar with fatigue testing procedures • Develop fatigue data for AA 6061-T6 specimens • Extrapolate the endurance limit from the S-N curve (at 5x10^8 cycles) • Compare estimated endurance limit and cycles to failure to known • Evaluate the surface characteristics of fatigue failure

4. What is Fatigue? Crack Propagation • Examples of Fatigue Factors • Size, loading types • Stress concentration factors • temperature, corrosion

5. Testing Procedures - Application of Stresses

6. Test Specimens – Cycles to Failure Comparison Aluminum Alloys: Nearly pure (>95%), precipitation hardening, tempering, lack of carbon

7. 3 1 D C B A 4 2 Experimental Setup Bending Stress LOAD Cantilever Arm Motor 6061-T6 specimen

8. Results • Endurance limit for 6061-T6 alloy at 5x108 • Predicted Cycles to Failure v. Observed • Fracture Surface

9. Results - Chauvenet N d/σ 5 1.65 6 1.73 7 1.81 8 1.86 9 1.91 10 1.96 12 2.04 14 2.10 16 2.15 18 2.20 20 2.24 1 data point removed with Chauvenet’s: D=σ *(d/σ)

10. Results - Predicted Cycles Causes of Error eccentricity (set screw), yield strength, diameter, number of points (12)

11. σe = 85 MPa

12. Results - Diameter Uncertainty I =

13. Results - Surface conditions • Fatigue failure versus dynamic failure • Crack Propagation

14. Conclusions • Fatigue failure is very different than static or dynamic failures • A small change in diameter can significantly increase the stress • Wide range of deviations (Factor of Safety) • Difference of only 10.5% with eccentricity (human error), diameter uncertainty, alloy uncertainty, etc

15. Recommendations • Replace set screws with chuck or threaded specimens • Increase size of aluminum specimens (fewer points)