Isotropic materials are those that have the same value for a given property in all directions, normally this is the way the strain develops and structures form. A granite has the same response to applied stress in any direction.

Anisotropic materials are those that have different values for a given property in different directions. A granite gneiss respond in different manners if compressed parallel to the gneissosity planes and when compressed normal to gneissosity planes.

The synoptic diagram shows the kind of structures generated at different angles to the planes of anisotropy.

Also called Carey's rheid model, Maxwell and Kelvin body in parallel and involves both primary and secondary creep. If s is the stress (sigma)applied and G the terms then time strain e(t) or rheidity can be given by:

e(t)=(2s/9K) + (s/3G2) + (s/3G1) - (s/3G4) * (e^-(G1t/h1)) + (s t / 3h2)

where h (eta) is the Poisson's ratio.

Term 2: spring in Maxwell body

Term 3: spring in Kelvin body

Term 4: dashpot + spring in Kelvin body

Term 5: dashpot in Maxwell body

The rheidity is the time at which the permanent deformation equals 1000 times the recoverable deformation. At this point you can consider the last term to dominate and the material is acting like a viscous fluid for those terms. If deformation conditions persist for this amount of time then the finite deformation can be treated as viscous. Rheidity and viscosity are very sensitive to temperature.

Kelvin or Voigt model involves spring and dashpot in parallel and generally involves the primary or time elastic creep or time recoverable deformation. It can be used to model isostatic rebound and mantle viscosities. Works on time frames of thousands of years for earthly deformation conditions.

MATERIAL, KELVIN AND MAXWELL STRAIN TIME CURVES