Fatigue Failure Through Bending

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 5x108 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 (gt95),
precipitation hardening, tempering, lack of carbon
7
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/s 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 Chauvenets Ds (d/s)
10
Results - Predicted Cycles
Causes of Error eccentricity (set screw), yield
strength, diameter, number of points (12)
11
se 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)