The elastoviscoplastic behavior of saltrocks may be interpreted in terms of a number of different fundamental deformation mechanisms that act at different rates at different shear and normal stress levels. The paper presents a new approach to a constitutive law for these materials, recognising the complex behavior, and making distinctions between different types of yield criteria and viscous flow laws. An attempt is made to develop this behavior as an analogue to the behavior of rock at high temperatures and stresses, where viscous mechanisms became possible. It is empasised that an appreciation of the dominant deformation mechanisms acting at particular conditions will guide the form of macroscopic constitutive laws for non-saltrocks.
Understanding rock behavior at great depth and elevated temperature requires quantification of deformation processes which do not occur in ordinary conditions. Study of these time-dependent (creep) processes requires high temperature (T) and stress (σm¹) tests at slow strain rates (dE/dt). Shear stresses (r=(σ¹-σ3)/2) and pore pressures must also be controlled. It may be of practical interest to explore analogues which display similar mechanisms at lower T, σm¹, and r conditions. Salt and potash may be useful analogues in some conditions. Halite (NaCl) and sylvite (KC1) display different deformation processes depending on σm¹, T, and r, but in conditions accessible in the laboratory and the field (σm¹ <70MPa; r<40MPa; T<150°C). Provided that it can be shown that mechanisms are similar, analogies may be developed to other rock types. The critical issues are that behavioral fields, activation energies, and mechanism transition points be quantified to permit comparisons and extrapolations; deformation maps (Frost and Ashby, 1982) are essential for this purpose. A behavioral model for saltrocks will be described and treated as an analogue to the behavior of rocks at great depth. Emphasis is on the effects of stress and strain rate on mechanisms in saltrocks. Saltrocks display most aspects of brittle viscous- ductile behavior at strain rates of d /dt=10-2-10-¹2 S-¹: brittle yield and dilation at low σm, ideally elastoplastic and non-dilative behavior at high σm' transition from viscoelastic to viscoplastic creep as r rises, of steady-state cracking (with an-nealing), rate dependence of the yield criterion, and primary creep with recovery.
Elastoplastic (EP) yield is the conventional yield criterion (Y) of rock mechanics. Fig. 1 shows Brazilian and triaxial test data giving a curved criterion with these features: at low σ3, brittle behavior with dilatancy and debonding, small to failure, dependency of Y on σ3, UCS about 12 times To; at high σ3, cataclastic flow without σ3 dependence, dilatancy supression, crystal rupture; and, at intermediate confining stresses, transitional behavior. Typically, for halite, the transition to fully cataclastic plastic yield occurs at σ3= 10–20 MPa, σ1-σ3]EP≈ 35- 40 MPa, with rate effects. Consider a test at low σ3: if r<Y, steady state creep will occur, but r>Y can be applied for short times before strain-weakening. Thus, Y is a lower bound, defined as the lowest τ for strain-weakening yield in any time.
