Week 2 - Fatigue & polymers Flashcards
When does fatigue occur?
When a material or component is subjected to cyclic loading or cyclic stress.
What is the key part of fatigue failure?
Crack initiation
What are the 3 main differences in stress?
Reversed stress, repeated/fluctuating stress or irregular stress.
What kinds can stresses be?
axial, flexural or torsional
What method is fatigue usually tested by?
By using a S-N curve which is stress vs cycles to failure and looking at the fatigue limit/ endurance limit
What are common symptoms of cracks in materials?
Beach marks, striations can appear from crack initiation to have a final crack after.
Why is the fatigue limit/ endurance limit useful for designing products life?
if we keep the stress below the fatigue limit, the component will not fail regardless of the number of cycles.
What happens if there is no well defined fatigue limit?
for example aluminium, titanium and copper, will fail at any stress and number of cycles
Therfore consider fatigue strength or fatigue life
What is fatigue strength?
The stress that produces failure in a given number of cycles usually 10^7
What is fatigue life?
The number of cycles required for the material to fail at a given stress
What are the 3 main in service crack initiators?
Surface defects, fabrication defects and operating temperature
What are surface defects for crack initiation? and how can it be mitigated?
- Any surface defects that act as stress raisers and can reduce the
fatigue properties. This could include scratches, machining marks or
other surface imperfections for example inclusions - Some surface finishing operations for example electroplating may
have an impact on fatigue properties. - Design of the component which includes sharp corners or holes for
example can reduce the fatigue limit.
What are fabrication defects in crack intiation?
- Residual stress which may be produced during the fabrication of the
component can reduce the fatigue properties - heat treatments and surface hardening treatments can also affect the fatigue properties
What are operating temperature effects on crack initiation?
lecture on high temperature oxidation, oxide formation on the component surface may impact fatigue properties
What is shot peening? and why is it used?
It is used to modify the surface to further protect against fatigue.
Little balls are thrown at the surface with hard particles to produce a compressive residual stress on that surface area to close up any fatigue/cracks in there
How can fatigue be mitigated?
- Avoid areas of stress concentration and stress raises through
careful design of the component (sharp corners, unnecessary
holes, unnecessary markings) - Control the surface finish to avoid any surface damage from
operations for example machining etc. - Control corrosion, oxidation and erosion in service
- Prevent any surface changes for example decarburization during
heat treatment of the metal. - Consider surface treatment of the metal to give a compressive
stress at the surface
What is creep?
The deformation of materials under a static load over a long period of time.
Where and why does creep appear? what does it depend on?
Elevated Temperature (T>0.4Tm) and a static mechanical stress.
Creep is time dependant.
Prevalent in materials that are subjected to a constant load or stress (examples include turbine blades in gas turbine engines)
How does creep vary with low and high temperatures?
At low temperature we know that ε = f (σ)where ε - stain and σ - stress
But at high temperature, temperature (T) and time (t) become important so:
ε = f (σ, T, t)
Which three materials undergo creep at room temperature?
Tin, Lead and Zinc
What are the 3 stages of creep?
Stage I- this is where initial strain takes place
Stage II - can determine the strain rate here which is the minimum creep rate
Stage III- where the strain sharply increases and fractures
What factors can advance creep?
Temperature and stress. The activation energy can be found from these.
What are the 3 main creep mechanisms and what do they do?
- Coble creep
Diffusion of atoms along grain boundaries or along dislocation cores - Nabarro-Herring creep
Diffusion of atoms through the lattice
3.Dislocation climb/creep
Movement of dislocations and defects
These are all material dependent
What are the main 3 considerations to avoid creep?
1.Reduce the amount of grain boundaries:
-Both Coble and Nabarro-Herring creep decrease as grain
size increases
- Use single crystals or very large (usually columnar) grains
- Addition of solid solutions to eliminate vacancies
2. Use materials of high melting temperatures
3. Consider Creep Data during materials Selection
- In-service application/conditions
- Inspection.
