Crack Nucleation and Propagation
Fatigue failure is due to crack nucleation and propagation
A fatigue crack will start in areas of localized stresses
This happens often at discontinuities
Cracks start at grains failing with tensile loading where the irregularities increase as normal stresses cycle where irregularities that go in are called intrusions and the ones that pop out are called extrusions
Stage 1 Crack Growth (Shear Mode)
- Bonds are breaking
- Growth rate is slow
- Cracks propagate throughout grains
Stage 2 Crack Growth (Tensile Mode)
- A large stress concentration forms when the crack propagates across enough grains
- The size of the tensile plastic zone increases
- The crack is vulnerable to being opened by tensile normal stress
Fatigue Life Methods
Crack nucleation and growth as a portion of total fatigue life is shown
At higher stress levels, cracks initiate quickly
At lower stress levels, nucleation happens across the crack life and then it quickly cracks
Focus
During this course, we are focusing on the stress-life method and finding (N) which is the number of cycles to failure.
High Cycle Fatigue
Long lives, low cyclic loads
Low Cycle Fatigue
Short lives, high cyclic loads
Fatigue Design Criteria
Infinite Life Design
- Designed such that stresses never exceed the endurance limit
- Plastic strain is avoided Safe-life Design
- Finite life (< 1 million cycles)
- Applications may have limited stress cycles or imprecisions in estimating fatigue life
- Uses significant safety margins Fail Safe Design
- Approach to design where if one part fails, the entire system does not fail
- Not trying to prevent cracking Damage-tolerant Design
- Assumed a crack is present always
- Aimed at dictating an inspection and replacement schedule
S-N DIagram
Models stress vs Number of cycles to see different things like:
- The crack growth region
- The slip band region
- Final Fracture
- Microcrack nucleation