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. Desired StressStress (MPa) Required Load (g).9247.52337.7.82202190.7192.51916.61651641 Testing Procedures - Application of Stresses

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

7. Experimental Setup Cantilever Arm 6061-T6 specimen Motor LOAD Bending Stress

8. Results Endurance limit for 6061-T6 alloy at 5x10 8 Predicted Cycles to Failure v. Observed Fracture Surface

9. Results - Chauvenet Nd/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) StressAllowed Deviation (cycles).946659.8142573.7331482.61367672

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

11. σ e = 85 MPa

12. Results - Diameter Uncertainty I = Radius (mm) Endurance limit (at 5x10^8 cycles) Difference from published values 2.5085 MPa10.5% 2.4589 MPa6.3%

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

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