Fatigue failure Flashcards
What can the fracture surface tell us about the stress condition that led to fatigue?
Big crack propagating area, and small catastrophic rupture area
-> Low stress (Later failure)
Small crack propagating area, and big catastrophic rupture area
-> High stress (Earlier failure)
What can the fracture surface tell us about the stress condition that led to fatigue?
Big crack propagating area, and small catastrophic rupture area
-> Low stress (Later failure)
Small crack propagating area, and big catastrophic rupture area
-> High stress (Earlier failure)
Four distinct phases can be identified in a fatigue damage process. Which?
- Nucleation
- Microstructurally governed crack growth (the field of “short cracks” or “small cracks”
regime, that will be discussed in a following section) - Crack growth governed by continuum mechanics through linear-elastic fracture
mechanics, elastoplastic-, or fully plastic fracture mechanics - Final crack instability.
Some of these can also be easily noticed on macroscopic views of fracture surfaces of parts broken under fatigue loads.
What happens if a stress lower than the fatigue limit is applied for more than 10^6 cycles?
No failure will be detected. (Infinite life)
What is da/dN and delta_K?
da/dN - The crack growth rate (mm/cycle)
delta_K - The rage of Stress Intensity Factor K (MPa*sqrt(m))
What is Low Cycle Fatigue and High Cycle Fatigue
Low Cycle Fatigue
Failure after few cycles(10^2 to 10^4 cycles)
Fatigue generally governed by mechanisms quite similar to those t
High Cycle Fatigue
Failure after many cycles.
Ruled by slow- progressing damage micromechanisms where the crack grows at rates even lower than 1 mirco-meter/cycle.
When is SN-curves used?
S-N and ε-N curves are properly used for materials that can be approximated as continuous media, free from defects. The approaches behind these criteria is to consider the service of perfect materials and to remove from service the structure as soon as defects are detected (by NdT techniques).
When is the stress intensity factor K used? How can we use it to determine when a fracture is occurring?
When considering defects on material behavior the reference parameter becomes the stress intensity factor K if linear elastic fracture mechanics is applicable.
Fracture should occur when the stress intensity factor K reaches the critical value KIC (or very close to it).
What is a microstructually-short crack?
Size of the crack is approximate the same as grain size / microstructural features.
Plastic zone is fully contained within a few grains.
What is a mechaniclly-short crack?
Size of the crack is smaller than the geometrical feature that induced the crack nucleation.
(Crack tip is fully surrounded by the plastic region). The growth is not affected much by microstructure variables.
What is a chemically-short crack?
Size of the crack is comparable to small imperfections in the material.
Starts growing when exposed to a chemical/corrosive environment
What is thermal fatigue? And how does it happen?
Fatigue from thermal conditions/stresses. Rapid thermal changes can induce cyclic strain to the component which over time can lead to fatigue
On high temp, the material is often subjected to oxidation, which can speed up the crack growth.
