Lecture 2 - Rheology Flashcards
To what extent can rocks sustain elastic strain?
Different for different rocks, 1-2% is the maximum for all.
What happens to a rock when the level of strain that it can withstand is surpassed (passes the yield point)?
The rock either breaks (brittle) or they deform internally (ductile)
A ductile material is still cohesive (holds together) after the yield point. If it behaves purely plastically it deforms without a change in stress. What happens when the strain hardens and softens?
Strain hardening means that an increase in stress is needed for further deformation.
Strain softening means that less stress is needed for further deformation.
Name and describe 5 deformation mechanisms, From brittle (elastic) to ductile (plastic)
- Fracturing & Frictional sliding - breaking lattices, interconnecting microcracks
- Mechanical twinning - bending lattices
- Dissolution Creep - dissolution and reprecipitation of material at grain edges
- Dislocation Creep - intercrystalline slip of the lattice structure
- Diffusion Creep - shape change by migration of vacancies, either at the Grain boundaries or within the Volume of the crystal.
What is a defect?
A defect can be defined as a break in the periodicity of the lattice of a crystal. All natural crystals and most synthetic ones contain defects.
Name and describe three classifications of defect
Point defects -> lattice periodicity is broken at a point. The most important ones are vacancies.
Line defects or dislocations -> lattice imperfections occuring along a line in a crystal. Most important agents of ductile deformation.
Planar defects -> two-dimensional break in lattice periodicity. An important example is a subgrain boundary
Under what conditions does vacancy induced diffusion occur?
Since diffusion is involved it is a very slow process, it only occurs on the local scale or at very high temperatures.
What conditions are brittle deformations associated with?
High shear stress, temperature independent.
