Strain and strain-displacement relations

The shape of a solid or structure changes with time during a deformation process. To characterize deformation, or strain, a certain reference configuration is adopted and called undeformed. Often, that reference configuration is chosen as an unstressed state, but such is neither necessary nor always convenient. If time is measured from zero at a moment when the body exists in that reference configuration, then the upper case X may be used to denote the position vectors of material points when t = 0. At some other time t, a material point that was at X will have moved to some spatial position x. The deformation is thus described as the mapping x = x(X, t), with x = x(X, 0) = X. The displacement vector u is then u = x(X, t) − X; also, v = ∂x(X, t)/∂t and a = ∂²x(X, t)/∂t².

It is simplest to write equations for strain in a form that, while approximate in general, is suitable for the case when any infinitesimal line element dX of the reference configuration undergoes extremely small rotations and fractional change in length, in deforming to the corresponding line element dx. These conditions are met when |∂u_i/∂X_j|<< 1. Many solids are often sufficiently rigid, at least under the loadings typically applied to them, that these conditions are realized in practice. Linearized expressions for strain in terms of [∂u/∂X], appropriate to this situation, are called small strain or infinitesimal strain. Expressions for strain will also be given that are valid for rotations and fractional length changes of arbitrary magnitude; such expressions are called finite strain.

Two simple types of strain are extensional strain and shear strain. Consider a rectangular parallelepiped, a bricklike block of material with mutually perpendicular planar faces, and let the edges of the block be parallel to the 1, 2, and 3 axes. If the block is deformed homogeneously, so that each planar face moves perpendicular to itself and so that the faces remain orthogonal (i.e., the parallelepiped is deformed into another rectangular parallelepiped), then the block is said to have undergone extensional strain relative to each of the 1, 2, and 3 axes but no shear strain relative to these axes. If the edge lengths of the undeformed parallelepiped are denoted as ΔX₁, ΔX₂, and ΔX₃, and those of the deformed parallelepiped as Δx₁, Δx₂, and Δx₃ (see Figure 5A, where the dashed-line figure represents the reference configuration and the solid-line figure the deformed configuration), then the quantities λ₁ = Δx₁/ΔX₁, λ₂ = Δx₂/ΔX₂, and λ = Δx₃/ΔX₃ are called stretch ratios. There are various ways that extensional strain can be defined in terms of them. Note that the change in displacement in, say, the x₁ direction between points at one end of the block and those at the other is Δu₁ = (λ₁ − 1)ΔX₁. For example, if E₁₁ denotes the extensional strain along the x₁ direction, then the most commonly understood definition of strain is E₁₁ = (change in length)/(initial length) = (Δx₁ − ΔX₁)/ΔX₁ = Δu₁/ΔX₁ = λ₁ − 1. A variety of other measures of extensional strain can be defined by E₁₁ = g(λ₁), where the function g(λ) satisfies g(1) = 0 and g′(1) = 1, so as to agree with the above definition when λ₁ is very near 1. Two such measures in common use are the strain E M = (λ 2/1 − 1)/2, based on the change of metric tensor, and the logarithmic strain E L = ln(λ₁).

To define a simple shear strain, consider the same rectangular parallelepiped, but now deform it so that every point on a plane of type X₂ = constant moves only in the x₁ direction by an amount that increases linearly with X₂. Thus, the deformation x₁ = γX₂ + X₁, x₂ = X₂, x₃ = X₃ defines a homogeneous simple shear strain of amount γ and is illustrated in Figure 5B. Note that this strain causes no change of volume. For small strain, the shear strain γ can be identified as the reduction in angle between two initially perpendicular lines.

Small-strain tensor

The small strains, or infinitesimal strains, ε_ij are appropriate for situations with |∂u_k/∂X_l|<< 1 for all k and l. Two infinitesimal material fibres, one initially in the 1 direction and the other in the 2 direction, are shown in Figure 6 as dashed lines in the reference configuration and as solid lines in the deformed configuration. To first-order accuracy in components of [∂u/∂X], the extensional strains of these fibres are ε₁₁ = ∂u₁/∂X₁ and ε₂₂ = ∂u₂/∂X₂, and the reduction of the angle between them is γ₁₂ = ∂u₂/∂X₁ + ∂u₁/∂X₂. For the shear strain denoted ε₁₂, however, half of γ₁₂ is used. Thus, considering all extensional and shear strains associated with infinitesimal fibres in the 1, 2, and 3 directions at a point of the material, the set of strains is given by

The ε_ij are symmetric—i.e., ε_ij = ε_ji—and form a second-rank tensor (that is, if Cartesian reference axes 1′, 2′, and 3′ were chosen instead and the ε_kl′ were determined, then the ε_kl′ are related to the ε_ij by the same equations that relate the stresses σ_kl′ to the σ_ij). These mathematical features require that there exist principal strain directions; at every point of the continuum it is possible to identify three mutually perpendicular directions along which there is purely extensional strain, with no shear strain between these special directions. The directions are the principal strain directions, and the corresponding strains include the least and greatest extensional strains experienced by fibres through the material point considered. Invariants of the strain tensor may be defined in a way paralleling those for the stress tensor.

An important fact to note is that the strains cannot vary in an arbitrary manner from point to point in the body. This is because the six strain components are all derivable from three displacement components. Restrictions on strain resulting from such considerations are called compatibility relations; the body would not fit together after deformation unless they were satisfied. Consider, for example, a state of plane strain in the 1, 2 plane (so that ε₃₃ = ε₂₃ = ε₃₁ = 0). The nonzero strains ε₁₁, ε₂₂, and ε₁₂ cannot vary arbitrarily from point to point but must satisfy

∂²ε₂₂/∂X 2/1 + ∂²ε₁₁/∂X 2/2 = 2∂²ε₁₂/∂X₁∂X₂,

as may be verified by directly inserting the relations for strains in terms of displacements.

When the smallness of stretch and rotation of line elements allows use of the infinitesimal strain tensor, a derivative ∂/∂X_i will be very nearly identical to ∂/∂x_i. Frequently, but not always, it will then be acceptable to ignore the distinction between the deformed and undeformed configurations in writing the governing equations of solid mechanics. For example, the differential equations of motion in terms of stress are rigorously correct only with derivatives relative to the deformed configuration, but, in the circumstances considered, the equations of motion can be written relative to the undeformed configuration. This is what is done in the most widely used variant of solid mechanics, in the form of the theory of linear elasticity. The procedure can be unsatisfactory and go badly wrong in some important cases, however, such as for columns that buckle under compressive loadings or for elastic-plastic materials when the slope of the stress versus strain relation is of the same order as existing stresses. Cases such as these are instead best approached through finite deformation theory.