1. CYCLIC LOADING and FAILURE
4/12/2018 4/12/2018
CYCLIC LOADING and FAILURE
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2. Loading and unloading a part is never completely reversible – energy is always lost – this fact is pronounced when loading is vibrational in nature
Damping coefficient η
Measure the degree to which a material dissipates vibrational energy
Loss coefficient η
Fraction of the stored elastic energy not returned on unloading

3. High-cycle fatigue loading is the most significant
Low amplitude acoustic vibration
High-cycle fatigue: cycling below the yield strength
Low-cycle fatigue: cycling above the yield strength but below the tensile strength
High-cycle fatigue loading is the most significant
in engineering terms

4. Fatigue failures occur due to cyclic loading at stresses below a material’s yield strength
Depends on the amplitude of the stress and the number of cycles
Loading cycles can be in the millions for an aircraft
Fatigue testing must employ millions of fatigue cycles to provide meaningful design data

5. Fatigue characteristics are measured and plotted on an S-N curve
Stress amplitude
Mean stress
R value of -1 indicates the mean stress is zero
Endurance limit σe
Stress amplitude below which fracture does not occur at all or only after a very large number of cycles (>107)

6. Coffin’s Law Basquin’s Law
For low-cycle fatigue
Basquin’s Law
For high-cycle fatigue
The laws describe the fatigue failure of uncracked components cycled at a constant amplitude about a mean stress of zero

7. Miner’s Rule of cumulative damage
Goodman’s Rule
The corrected stress range can be plugged into Basquin’s law
Miner’s Rule of cumulative damage
When the cyclic stress amplitude changes, the life is calculated using Miner’s rule

8. Fatigue crack growth is studied by cyclically loading specimens containing a sharp crack
Cyclic stress intensity range
The range ΔK increases with time under constant cyclic stress because the crack grows in length

9. Safe design requires calculating the number of loading cycles possible before the crack grows to a dangerous length

10. Endurance limit is the most important property characterizing fatigue strength
Metals/Polymers
Glasses/Ceramics

11. SUMMARY Static structures stand for a very long time Eiffel tower
Golden Gate Bridge
Moving structures don't last as long
Materials can support static loads much better than changing loads
Metals: endurance limit is ONE THIRD of tensile strength
Materials are very sensitive to cyclic loads
Cracks are almost invisible until final failure occurs without warning
Surface treatment to reduce surface crack formation are now standard practice.