Fatigue Failure Through Bending David Burnette ME 498

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

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

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

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

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

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

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

Results - Chauvenet Nd/σ data point removed with Chauvenet’s: D=σ *(d/σ) StressAllowed Deviation (cycles)

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

σ e = 85 MPa

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

Results - Surface conditions Fatigue failure versus dynamic failure Crack Propagation

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

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