Anisotropic solids also are common in nature and technology. Examples are single crystals; polycrystals in which the grains are not completely random in their crystallographic orientation but have a “texture,” typically owing to some plastic or creep flow process that has left a preferred grain orientation; fibrous biological materials such as wood or bone; and composite materials that, on a microscale, either have the structure of reinforcing fibres in a matrix, with fibres oriented in a single direction or in multiple directions (e.g., to ensure strength along more than a single direction), or have the structure of a lamination of thin layers of separate materials. In the most general case, the application of any of the six components of stress induces all six components of strain, and there is no shortage of elastic constants. There would seem to be 6 × 6 = 36 in the most general case, but, as a consequence of the laws of thermodynamics, the maximum number of independent elastic constants is 21 (compared with 2 for isotropic solids). In many cases of practical interest, symmetry considerations reduce the number to far below 21. For example, crystals of cubic symmetry, such as rock salt (NaCl); face-centred cubic metals, such as aluminum, copper, or gold; body-centred cubic metals, such as iron at low temperatures or tungsten; and such nonmetals as diamond, germanium, or silicon have only three independent elastic constants. Solids with a special direction, and with identical properties along any direction perpendicular to that direction, are called transversely isotropic; they have five independent elastic constants. Examples are provided by fibre-reinforced composite materials, with fibres that are randomly emplaced but aligned in a single direction in an isotropic or transversely isotropic matrix, and by single crystals of hexagonal close packing such as zinc.

General linear elastic stress-strain relations have the formwhere the coefficients C_ijkl are known as the tensor elastic moduli. Because the ε_kl are symmetric, one may choose C_ijkl = C_ijlk, and, because the σ_ij are symmetric, C_ijkl = C_jikl. Hence the 3 × 3 × 3 × 3 = 81 components of C_ijkl reduce to the 6 × 6 = 36 mentioned. In cases of temperature change, the ε_ij above is replaced by ε_ij − ε_ij^thermal, where ε_ij^thermal = α_ij(θ − θ₀) and α_ij is the set of thermal strain coefficients, with α_ij = α_ji. An alternative matrix notation is sometimes employed, especially in the literature on single crystals. That approach introduces 6-element columns of stress and strain {σ} and {ε}, defined so that the columns, when transposed (superscript T) or laid out as rows, are {σ}^T = (σ₁₁, σ₂₂, σ₃₃, σ₁₂, σ₂₃, σ₃₁) and {ε}^T = (ε₁₁, ε₂₂, ε₃₃, 2ε₁₂, 2ε₂₃, 2ε₃₁). These forms assure that the scalar {σ}^T{dε} ≡ tr([σ][dε]) is an increment of stress working per unit volume. The stress-strain relations are then written {σ} = [c]{ε}, where [c] is the 6 × 6 matrix of elastic moduli. Thus, c₁₃ = C₁₁₃₃, c₁₅ = C₁₁₂₃, c₄₄ = C₁₂₁₂, and so on.

Thermodynamic considerations

In thermodynamic terminology, a state of purely elastic material response corresponds to an equilibrium state, and a process during which there is purely elastic response corresponds to a sequence of equilibrium states and hence to a reversible process. The second law of thermodynamics assures that the heat absorbed per unit mass can be written θds, where θ is the thermodynamic (absolute) temperature and s is the entropy per unit mass. Hence, writing the work per unit volume of reference configuration in a manner appropriate to cases when infinitesimal strain can be used, and letting ρ₀ be the density in that configuration, from the first law of thermodynamics it can be stated that ρ₀θds + tr([σ][dε]) = ρ₀de, where e is the internal energy per unit mass. This relation shows that if e is expressed as a function of entropy s and strains [ε], and if e is written so as to depend identically on ε_ij and ε_ji, then σ_ij = ρ₀∂e([ε], s)/∂ε_ij.

Alternatively, one may introduce the Helmholtz free energy f per unit mass, where f = e − θs = f([ε], θ), and show that σ_ij = ρ₀∂f([ε], θ)/∂ε_ij. The latter form corresponds to the variables with which the stress-strain relations were written above. Sometimes ρ₀f is called the strain energy for states of isothermal (constant θ) elastic deformation; ρ₀e has the same interpretation for adiabatic (s = constant) elastic deformation, achieved when the time scale is too short to allow heat transfer to or from a deforming element. Since the mixed partial derivatives must be independent of order, a consequence of the last equation is that ∂σ_ij([ε], θ)/∂ε_kl = ∂σ_kl([ε], θ)/∂ε_ij, which requires that C_ijkl = C_klij, or equivalently that the matrix [c] be symmetric, [c] = [c]^T, reducing the maximum possible number of independent elastic constraints from 36 to 21. The strain energy W([ε]) at constant temperature θ₀ is W([ε]) ≡ ρ₀f([ε], θ₀) = (¹/₂){ε}^T[c]{ε}.

The elastic moduli for adiabatic response are slightly different from those for isothermal response. In the case of the isotropic material, it is convenient to give results in terms of G and K, the isothermal shear and bulk moduli. The adiabatic moduli G and K̄ are then G = G and K̄ = K(1 + 9θ₀K α²/ρ₀c_ε), where c_ε = θ₀∂s([ε],θ)/∂θ, evaluated at θ = θ₀ and [ε] = [0], is the specific heat at constant strain. The fractional change in the bulk modulus, given by the second term in the parentheses, is very small, typically on the order of 1 percent or less, even for metals and ceramics of relatively high α, on the order of 10⁻⁵/kelvin.

The fractional change in absolute temperature during an adiabatic deformation is found to involve the same small parameter: [(θ − θ₀)/θ₀]_{s = const} = −(9θ₀Kα²/ρ₀c_ε) [(ε₁₁ + ε₂₂ + ε₃₃)/3αθ₀]. Values of α for most solid elements and inorganic compounds are in the range of 10⁻⁶ to 4 × 10⁻⁵/kelvin; room temperature is about 300 kelvins, so 3αθ₀ is typically in the range 10⁻³ to 4 × 10⁻². Thus, if the fractional change in volume is on the order of 1 percent, which is quite large for a metal or ceramic deforming in its elastic range, the fractional change in absolute temperature is also on the order of 1 percent. For those reasons, it is usually appropriate to neglect the alteration of the temperature field due to elastic deformation and hence to use purely mechanical formulations of elasticity in which distinctions between adiabatic and isothermal response are neglected.

Finite elastic deformations

When elastic response under arbitrary deformation gradients is considered—because rotations, if not strains, are large or, in a material such as rubber, because the strains are large too—it is necessary to dispense with the infinitesimal strain theory. In such cases, the combined first and second laws of thermodynamics have the form ρ₀θds + det[F]tr([F]⁻¹[σ][dF]) = ρ₀de, where [F]⁻¹ is the matrix inverse of the deformation gradient [F]. If a parcel of material is deformed by [F] and then given some additional rigid rotation, the free energy f must be unchanged in that rotation. In terms of the polar decomposition [F] = [R][U], this is equivalent to saying that f is independent of the rotation part [R] of [F], which is then equivalent to saying that f is a function of the finite strain measure [E^M] = (¹/₂)([F]^T[F] − [I]) based on change of metric or, for that matter, on any member of the family of material strain tensors. Thus,is sometimes called the second Piola-Kirchhoff stress and is given by S_kl = ρ₀∂f([E^M],θ)/∂E^M/_kl, it being assumed that f has been written so as to have identical dependence on E^M/_kl and E^M/_lk.

Inelastic response

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 asand 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 D^p/_ij = 3(dε^p/dt)τ_ij/2σ. The numerical factors secure agreement between D^p/₁₁ and dε^p/dt for uniaxial tension in the 1direction. Also, the equation implies thatwhich 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 = D^e/_ij + D^p/_ij, where D^p/_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 D^e/_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.