Unit 7 - Creep Flashcards
What is creep?
Creep is the process of strain accumulation under static loading. It is a time dependant process which is more common at high temperatures but can occur at any temperature above absolute zero.
What are the key aspects of low temperature or logarithmic creep?
- It occurs at low temperature where diffusion processes do not occur or are very slow
- Total creep strains are typically below 1% so rarely leads to failure
- Loading to yield or beyond, plastic deformation follows elastic
What are the key aspects of high temperature creep?
- Levels of strain accumulated can be very large
- Can lead to damage processes resulting in an eventual increase in creep rate to ultimate failure
- Curve can be separated into three distinct stages according to creep rate:
- Primary (decreasing strain rate)
- Secondary (minimum strain rate)
- Tertiary (increasing strain rate)
Where does creep occur in a gas turbine?
- High pressure compressor
- Combustor
Where does creep occur in a power station?
Steam piping - very slowly. Can take up tp 10 years for an issue to occur
What is creep testing?
Subjecting a specimen to constant stress or constant
load while maintaining the temperature constant.
How does a vacancy form in a solid material?
As atoms vibrate they collide, transferring energy
and resulting in some atoms having more
energy than others. If an atom has sufficient energy, it may become displaced from it’s lattice site
resulting in the formation of a vacancy. This occurs more at higher temperatures.
How does vacancy diffusion work?
The presence of vacancies allow atoms to change site within the lattice. An atom with sufficient thermal energy can jump from its lattice site provided
there is a vacancy adjacent to it. This means the atoms will move in one direction and the vacancy in the opposite direction.
Which regions of a microstructure bond energy/strength vary?
- Bonds are weaker on the surface of grains
- Bonds are weaker on grain boundaries
This makes grain boundaries and surface regions easy paths for diffusion.
What is the Nabarro-Herring (lattic vacancy diffusion) mechanism?
Vacancy flow from boundaries under tension to
those under compression, giving a counterflow
of atoms, so that diffusional creep processes
can cause a grain in a polycrystalline material to
extend with time.
What is the Coble (grain boundary vacancy diffusion) mechanism?
Creep occurs stress-directed flow of vacancies,
the vacancies may flow both through the grains
(Nabarro-Herring creep) or along the grain
boundaries (Coble creep).
What are the relationships between Nabarro-Herring and Coble creep and lattice self-diffusion?
- Nabarro-Herring creep will be dominant to self-diffusion at high temperatures
- Coble creep will be dominant to sefl-diffusion at low stress levels
What is the basis for strain hardening?
Dislocations move through a material causing slip. As slip continues, the dislocations multiply and the average distance between dislocations decreases. Dislocation-disclocation strain interractions are repulsive so the motion of any one dislocation gets increaisngly more difficult. Therefore, to continue movement, higher stresses must be applied.
According to the Hall-Petch relation, how can a polycrystalline material be strengthened?
What is the basis for dislocation hardening?
The material deforms due to initial creation and motion of dislocations under static loading. Dislocations then form pile ups at precipitates, grain boundaries and interlocking tangles. Deformation stops.
