Unit 7 - Creep

Question 1

What is creep?

Answer 1

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.

Question 2

What are the key aspects of low temperature or logarithmic creep?

Answer 2

• 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

Question 3

What are the key aspects of high temperature creep?

Answer 3

• 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)

Question 4

Where does creep occur in a gas turbine?

Answer 4

• High pressure compressor
• Combustor

Question 5

Where does creep occur in a power station?

Answer 5

Steam piping - very slowly. Can take up tp 10 years for an issue to occur

Question 6

What is creep testing?

Answer 6

Subjecting a specimen to constant stress or constant
load while maintaining the temperature constant.

Question 7

How does a vacancy form in a solid material?

Answer 7

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.

Question 8

How does vacancy diffusion work?

Answer 8

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.

Question 9

Which regions of a microstructure bond energy/strength vary?

Answer 9

• 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.

Question 10

What is the Nabarro-Herring (lattic vacancy diffusion) mechanism?

Answer 10

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.

Question 11

What is the Coble (grain boundary vacancy diffusion) mechanism?

Answer 11

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).

Question 12

What are the relationships between Nabarro-Herring and Coble creep and lattice self-diffusion?

Answer 12

• Nabarro-Herring creep will be dominant to self-diffusion at high temperatures
• Coble creep will be dominant to sefl-diffusion at low stress levels

Question 13

What is the basis for strain hardening?

Answer 13

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.

Question 14

According to the Hall-Petch relation, how can a polycrystalline material be strengthened?

Answer 14

By increasing the grain size through control of the manufacturing and processing routes. E.g. increasing colling rates during casting or deformation and heat-treatment options.

Question 15

What is the basis for dislocation hardening?

Answer 15

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.

Question 16

What is the basis for dislocation softening?

Answer 16

In high temperature creep, the pile ups of dislocations can be relieved by recovery processes:
* cross slip
* climb

Recovery processes allow dislocations to become more mobile and driven by diffusion.

Question 17

What are hard regions and soft regions?

Answer 17

Hard regions have high dislocation density and hece high stress. Soft regions have low dislocation density and hence low stress.

Question 18

What are the key aspects of creep fracture?

Answer 18

• Failure mode is generally ductile but can appear brittle
• Local instabilities can cause necking to occur which increases deformation rate
• As deformation proceeds, creep damage can occur
• Along with dislocation softening processes, this damage causes the creep rate to increase -> tertiary creep

Question 19

How can creep rates be lowered?

Answer 19

Via strengthening techniques such as precipitation hardening