Lecture Nine - Stress and strain Flashcards
Define strain.
Strain is a measure of change in shape.
Strain is dimensionless.
Strain words - shortening vs extension and contraction vs expansion.
Define stress.
Stress is a measure of force over area, also called pressure.
Stress has dimensions - Nm^-2, Pa or bar.
Stress words - compression vs tension.
Note: 100MPa = 1Kbar.
What types of stresses exisit in the Earth’s crust?
Lithostatic - Pressure of the loverlying rock. Density of rock (kg.m^-3) x depth (m) x gravity (m.s^-2).
Hydrostatic - Pressure of the overlying water.
Tectonic - A result of tectonic forces.
Normal - Stress at right angles to surface - perpendicular stress.
Shear - Stress parallel to a surface.
What are the different types of strains?
Elastic - Recoverable, like a sponge. Linear relationship between stress and strain, until failure = earthquake.
Plastic - Non-recoverable repture (brittle behaviour).
Viscous - Non-recoverable rock flow.
Elasto-plastic - Elastic strain followed by brittle failure. (seismic events).
Visco-plastic - Folding and faulting.
Rocks can bee all three of viscous, elastic and plastic at some point or another.
Strain rate, temperature, rock composition and pressure among other factor will dictate whether a rock acts with elastic, plastic (brittle) or viscous (ductile) behaviour.
High strain rate promotes brittle behaviour, low strain rate promotes ductility.
High temperaure promotes ductility, low temperature promotes britallity.
High pressure promotes ductile behaviour and low pressure promotes brittle behaviour.
What is the effect of depth on britallity or ductility of the rocks in the Earth’s crust?
In the Earth’s crust, temperature and pressure increase with depth.
Shallow - low temperature and pressure = brittle deformation.
Deep - high temperaure and pressure = ductile deformation.
The depth at which brittle behaviour changes to ductile behaviour is known as the brittle/ductile transition zone, this is where folding and faulting can occur in the same area.
Note - there is a fair bit of gold in this zone.
The depth of the brittle/ductile transition depends on:
Geothermal gradient, density of rock, tectonic stresses and strain rate.
Generally between 10-15km deep.
Note - depth of brittle ductile transition is usually the deepest depth at which earthquakes can occur.
What is a joint?
A joint is a fracture WITH NO movement.
Compared to a fault, which is a fracture WITH movement.
A joint can be filled with material, such as gold.
Hydrothermal fluids flow into joints.
What is a boudinage?
Occurs due to the stretching and necking of layers.
When some rocks stretch at different rates in the same area.
Pinches at either end = necking.
When folding and extension occurs quickly enough to cause the rock to become brittle and neck, and potentially separate.
Show the process of boudinage.
Sigma 1 = maximum compressional stress.
Sigma 3 = minimum compressional stress.
Note that the axis of stress are not always parallel to the strain axis. Can rotate in relatino to one another.
Draw the diagram of the faults and strain axes orientations.
Draw the strain ellipse.
Strain is three dimensional so we can meaure change in shape by change in length of X, Y and Z or ellipse x, y and z.
