Lezione 9: Cyclic loading, damage and failure Flashcards
Talk about vibration and resonance
Loading and unloading a part is never completely reversible, because energy is always lost. This is even more pronounced when loading is in vibration. This is expressed with 2 parameters:
- Damping coefficient: measures the degree to which a material dissipates vibrational energy
- Loss coefficient: fraction of the stored energy not returned on unloading
How many types cycling loading exist?
-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
Second type is the most relevant in engineering terms
Talk about fatigue failure
Fatigue failures occur due to cyclic loading at stresses below a material’s yield strength. It depends on the amplitude of the stress and the number of cycles.
What are S-N curves?
S-N curve (stress amplitude and cycles to failure) is a graph that measures and plots fatigue characteristics. Thanks to the graph we can also find out the endurance limit of a material, which is the stress amplitude below which fracture does not occur at all or only after a very large number of cycles.
How is it possible to predict fatigue life?
It can be done through two laws:
-Coffin’s law (for low-cycle fatigue)
-Basquin’s law (for high-cycle fatigue)
These laws descrive the fatigue failure of uncracked components cycled at a constant amplitude about a mean stress of zero.
The stress range for failure under a mean stress σm is related to zero mean failure stress σ0. The corrected stress range can be plugged into Basquin’s law.
What happens when amplitude is variable?
When the cyclic stress amplitude changes, the life is calculated using Miner’s rule.
Miner’s rule of cumulative damage: if N1 cycles are spent at stress amplitude ∆σ1 a fraction N1/Nf,1 of the available life is used up, where Nf,1 is the number of cycles to failure at that stress amplitude.
How does fatigue loading work on cracked components?
Fatigue crack growth is studied by cyclically loading specimens containing a sharp crack. We use the cyclic stress intensity range.
Below a stress intensity ∆K threshold, the rate is zero.
If ∆K is higher, it stays in a steady state regime (controlled crack growth rate with Paris law).
For higher ∆K, fast fracture starts up to Kmax=K1c
Safe design requires calculating the number of leading cycles possible before the crack grows to a dangerous length.
Talk about fatigue damage and cracking
During high-cycle fatigue the tension cycle produces a tiny plastic zone which is folded furward during compression.
During low-cycle fatigue the plastic zone is large enough for voids to nucleate and coalesce which advances the crack
What is fatigue ratio?
It’s the correlation between endurance limit and yield strength not as strong as with tensile strength
How is it possible to prevent cracks?
Crack only propagate during the tensile part of a stress cycle, compressive stress forces the rack faces together, clamping it shut. Hacing compressive forces on the surgace of a material make ti resist crack growth. One method is shot peening.
