Fracture II Flashcards
What does fatigue occur in?
Fatigue occurs in structures subject to dynamic and varying stress – metals,
composites, ceramics, plastics…
When does fatigue occur?
- Occurs after a long time of cyclic stress
What stress levels can fatigue occur at?
Failure at stress levels below yield or tensile strength of the material due to imperfections
What are the three stages of fatigue?
- Initiation
- Propagation
- Final fracture (K=Kc)
What can be defined from a S-N curve?
Fatigue strength and fatigue life can be defined from an S-N curve
What is the fatigue limit?
the stress amplitude at which fatigue
failure does not occur; it is exhibited at low stress amplitudes in some
materials (eg. steel) but not others
What do cyclic stresses cause that leads to fatigue?
crack nucleation and growth and crack propagation, leading to fatigue fracture.
What are methods that improve fatigue life/reduce fatigue failure?:
- Shot-Peening
- Case hardening
- Mechanical Design (reducing sharp concerns)
What can stress concentrations arise from( i.e. sources of stress concentration) ?
- mechanical surface
damage - residual stress from
manufacturing - corrosion pitting
- sharp corners (design)
What happens during shot-peening?
- Residual compressive
stresses are generated
on the surface of a
metal by shooting hard
particles at it - Residual compressive
stresses delay crack
initiation (may retard
crack growth)
What happens during case hardening?
- Hardens the surface of a metal object
while allowing the metal underneath
to remain tough (high Kc) - A carbon- or nitrogen-rich surface
layer is created through atomic
diffusion by exposing the metal to a
carbon or nitrogen rich atmosphere at
a high temperature
What types of cyclic loading are imposed on
components of a jet-engine?
- Temperature cycles (engine start-up)
- Loading cycles (take off, cruise, landing)
- Blade flutter and vibrations (rotation)
When do cracks usually nucleate?
*Cracks usually nucleate in the first 10-20% of
service life
Why is there a range in the probability of fatigue failure?
There is variability in number and distribution of defects
within a component due to processing
What is the relationship between mean stress and fatigue life?
increasing the mean stress
level leads to a decrease in fatigue life.
Name the factors that affect fatigue life:
Mean Stress, Stress Amplitude, Surface Treatment, and Mechanical Design
How can fatigue life be improved with mechanical design? Provide an example covered in class.
By incorporating
rounded fillet into a rotating shaft where there is a change in diameter.
How do sharp corners impact fatigue failure?
Sharp corners are stress raisers, and can render the component
more susceptible to fatigue failure
Where are beach marks found?
Found where crack propagation occurred
What is beachmark spacing indicative of?
The rate of crack propagation
What is the single largest cause of failure?
Fatigue failure
Where do cracks start for fatigue failure?
Cracks start at stress concentration points, usually
on surface – dents, scratches, sharp fillets, keyways, machined threads etc
What is the length at which fatigue failure occurs?
- Failure occurs when crack grows to critical length –
ie. when Kc is exceeded
What type of stress is needed for crack nucleation and growth and why?
Tensile stress required – cracks will not form if there are
only compressive stresses present
Where can cracks nucleate for fatigue failure?
Cracks can nucleate internally on microstructure discontinuities (eg. porosity, inclusions etc.) or on the surface –dents, scratches, sharp fillets, key-ways, machined threads etc.
Why is significant time important for initiation and crack growth?
- This way, cracks can be found by inspection of structures:
This is why we design structures with critical crack
lengths that are large and easily detectable.
What is the difficulty with internal fatigue cracks?
Internal fatigue cracks can be a lot more difficult to
detect
What are beachmarks?
Macro-scale
features usually associated
with the periodic start-up of
machinery; they do not
reflect load cycles, but
indicate rate of crack
propagation
What are striations?
Micro-scale
features indicating crack
origin and growth
characteristics; indicative
of load cycles
What are the physical features of fatigue failure?
Beachmarks and striations.
What is the relationship between mean stress and fatigue life?
Increasing the mean stress
level leads to a decrease in fatigue life.
How can striations be observed?
Using electron microscopy
What do beachmarks result from?
Interruptions in the stress cycles.
What does each fatigue striation correspond to?
The advance of a fatigue crack during a single load cycle.
