21 4 Rock dynamics and time-dependent aspectswhilst the theory of elasticity is of assistance in understanding and
analysing the mechanics of rock masses, a theory is also required for time-
dependent effects.
Words used to describe time-dependent behaviour are clarified in the
glossary below.
Glossury of Terms
Elastic
Stresses are related to strains in a time-independent manner (i.e. o = SE,
where S is the elastic compliance matrix). All strain energy is recoverable.
It is assumed, in this context, that elastic materials remain elastic and so
have infinite strength.
Plastic
Stresses are related to strains in a time-independent manner, but the
material undergoes plasticflow when stressed (i.e. do = I'd&, where P is a
6 x 6 plasticity matrix whose coefficients are stress- or strain-dependent).
Deformation continues indefinitely without any further increase in
stress. Strain energy is lost through permanent plastic straining.
Generally, plastic behaviour is a function of distortional strains and
deviatoric stresses.
Viscous
Stresses are related to strain rate (i.e. o = vi, where 17 is a 6 x 6 viscosity
matrix). Generally, viscous behaviour is also a function of distortional
strains and deviatoric stresses.
Elastoplasticity
Time-independent theory combining elasticity and plasticity: materials
behave elastically up to certain stress states and plastically thereafter.
Viscoelasticity
A generic term for a time-dependent theory in which strains are related to
stresses and time. Instantaneously, viscoelastic materials have effectively
infinite strength.
Viscoplasticity
Time-dependent behaviour in which the deviatoric stresses (or distortional
strains) give rise to viscous behaviour, or plastic behaviour if the
instantaneous strength of the material is temporarily exceeded.
Elastoviscoplasticity
This is the same as viscoplasticity, except that the instantaneous response
of the material is purely elastic.
Creep
Under the action of a constant stress state, straining continues (see Fig. 6.16).
Relaxation
Under the action of a constant strain state, the stress within a material
reduces (also see Fig. 6.16).
Fatigue
A generic term generally used to describe the increase in strain (or decrease
in strength) due to cyclical loading.
Rheology
The study of flow.