The above mode of expressing [σ] in terms of [S] is valid for solids showing viscoelastic or plastic response as well, except that [S] is then to be regarded not only as a function of the present [E^M] and θ but also as dependent on the prior history of both. Assuming that such materials show elastic response to sudden stress changes or to small unloading from a plastically deforming state, [S] may still be expressed as a derivative of f, as above, but the derivative is understood as being taken with respect to an elastic variation of strain and is to be taken at fixed θ and with fixed prior inelastic deformation and temperature history. Such dependence on history is sometimes represented as a dependence of f on internal state variables whose laws of evolution are part of the inelastic constitutive description. There are also simpler models of inelastic response, and the most commonly employed forms for plasticity and creep in isotropic solids are presented next.

To a good approximation, plastic deformation of crystalline solids causes no change in volume; and hydrostatic changes in stress, amounting to equal change of all normal stresses, have no effect on plastic flow, at least for changes that are of the same order or magnitude as the strength of the solid in shear. Thus, plastic response is usually formulated in terms of deviatoric stress, which is defined by τ_ij = σ_ij − δ_ij(σ₁₁ + σ₂₂ + σ₃₃)/3. Following Richard von Mises, in a procedure that is found to agree moderately well with experiment, the plastic flow relation is formulated in terms of the second invariant of deviatoric stress, commonly rewritten as and called the equivalent tensile stress. The definition is made so that, for a state of uniaxial tension, σ equals the tensile stress, and the stress-strain relation for general stress states is formulated in terms of data from the tensile test. In particular, a plastic strain ε^p in a uniaxial tension test is defined from ε^p = ε − σ/E, where ε is interpreted as the strain in the tensile test according to the logarithmic definition ε = lnλ, the elastic modulus E is assumed to remain unchanged with deformation, and σ/E << 1.

Thus, in the rate-independent plasticity version of the theory, tensile data (or compressive, with appropriate sign reversals) from a monotonic load test is assumed to define a function ε ^p(σ ). In the viscoplastic or high-temperature creep versions of the theory, tensile data is interpreted to define dε ^P/dt as a function of σ in the simplest case, representing, for example, secondary creep, and as a function of σ and ε ^p in theories intended to represent transient creep effects or rate-sensitive response at lower temperatures. Consider first the rigid-plastic material model in which elastic deformability is ignored altogether, as is sometimes appropriate for problems of large plastic flow, as in metal forming or long-term creep in the Earth’s mantle or for analysis of plastic collapse loads on structures. The rate of deformation tensor D_ij is defined by 2D_ij = ∂v_i/∂x_j + ∂v_j/∂x_i, and in the rigid-plastic case [D] can be equated to what may be considered its plastic part [D^p], given as Dp/ij = 3(dε^p/dt)τ_ij/2σ. The numerical factors secure agreement between Dp/11 and dε^p/dt for uniaxial tension in the 1direction. Also, the equation implies that which must be integrated over previous history to get ε^p as required for viscoplastic models in which dε^p/dt is a function of σ and ε^p. In the rate-independent version, [D^p] is defined as zero whenever σ is less than the highest value that it has attained in the previous history or when the current value of σ is the highest value but dσ/dt < 0. (In the elastic-plastic context, this means that “unloading” involves only elastic response.) For the ideally plastic solid, which is idealized to be able to flow without increase of stress when σ equals the yield strength level, dε^p/dt is regarded as an undetermined but necessarily nonnegative parameter, which can be determined (sometimes not uniquely) only through the complete solution of a solid mechanics boundary-value problem.

The elastic-plastic material model is then formulated by writing D_ij = De/ij + Dp/ij, where Dp/ij is given in terms of stress and possibly stress rate as above and where the elastic deformation rates [D^e] are related to stresses by the usual linear elastic expression De/ij = (1 + ν)σ_ij^*/E − νδ_ij(σ₁₁^*+ σ₂₂^*+ σ₃₃^*)/E. Here the stress rates are expressed as the Jaumann co-rotational ratesis a derivative following the motion of a material point and where the spin Ω_ij is defined by 2Ω_ij = ∂v_i/∂x_j − ∂v_j/∂x_i. The co-rotational stress rates are those calculated by an observer who spins with the average angular velocity of a material element. The elastic part of the stress-strain relation should be consistent with the existence of a free energy f, as discussed above. This is not strictly satisfied by the form just given, but the differences between it and one which is consistent in that way involves additional terms that are on the order of σ/E² times the σ_kl^* and are negligible in typical cases in which the theory is used, since σ/E is usually an extremely small fraction of unity, say, 10⁻⁴ to 10⁻². A small-strain version of the theory is in common use for purposes of elastic-plastic stress analysis. In these cases, [D] is replaced with ∂[ε(X, t)]/∂t, where [ε] is the small-strain tensor, ∂/∂x with ∂/∂X in all equations, and [σ^*] with ∂[σ(X, t)]/∂t. The last two steps cannot always be justified, even in cases of very small strain when, for example, in a rate-independent material, dσ/dε^p is not large compared to σ or when rates of rotation of material fibres can become much larger than rates of stretching, which is a concern for buckling problems even in purely elastic solids.