Equations of motion of linear elastic bodies.

The final equations of the purely mechanical theory of linear elasticity (i.e., when coupling with the temperature field is neglected, or when either isothermal or isentropic response is assumed) are obtained as follows. The stress-strain relations are used, and the strains are written in terms of displacement gradients. The final expressions for stress are inserted into the equations of motion, replacing ∂/∂x with ∂/∂X in those equations. In the case of an isotropic and homogenous solid, these reduce toknown as the Navier equations (here, ∇ = e₁∂/∂X₁ + e₂∂/∂X₂ + e₃∂/∂X₃, and ∇² is the Laplacian operator defined by ∇·∇, or ∂²/∂x₁² + ∂²/∂x₂² + ∂²/∂x₃², and, as described earlier, λ and μ are the Lamé constants, u the displacement, f the body force, and ρ the density of the material). Such equations hold in the region V occupied by the solid; on the surface S one prescribes each component of u, or each component of the stress vector T (expressed in terms of [∂u/∂X]), or sometimes mixtures of components or relations between them. For example, along a freely slipping planar interface with a rigid solid, the normal component of u and the two tangential components of T would be prescribed, all as zero.

Body wave solutions

By looking for body wave solutions in the form u(X, t) = pf (n · X − ct), where unit vector n is the propagation direction, p is the polarization, or direction of particle motion, and c is the wave speed, one may show for the isotropic material that solutions exist for arbitrary functions f if either The first case, with particle displacements in the propagation direction, describes longitudinal, or dilatational, waves; and the latter case, which corresponds to two linearly independent displacement directions, both transverse to the propagation direction, describes transverse, or shear, waves.

Linear elastic beam

The case of a beam treated as a linear elastic line may also be considered. Let the line along the 1-axis (see Figure 7), have properties that are uniform along its length and have sufficient symmetry that bending it by applying a torque about the 3-direction causes the line to deform into an arc lying in the 1,2-plane. Make an imaginary cut through the line, and let the forces and torque acting at that section on the part lying in the direction of decreasing X₁ be denoted as a shear force V in the positive 2-direction, an axial force P in the positive 1-direction, and torque M, commonly called a bending moment, about the positive 3-direction. The linear and angular momentum principles then require that the actions at that section on the part of the line lying along the direction of increasing X₁ be of equal magnitude but opposite sign.

Now let the line be loaded by transverse force F per unit length, directed in the 2-direction, and make assumptions on the smallness of deformation consistent with those of linear elasticity. Let ρA be the mass per unit length (so that A can be interpreted as the cross-sectional area of a homogeneous beam of density ρ) and let u be the transverse displacement in the 2-direction. Then, writing X for X₁, the linear and angular momentum principles require that ∂V/∂X + F = ρA ∂²u/∂t² and ∂M/∂X + V = 0, where rotary inertia has been neglected in the second equation, as is appropriate for disturbances which are of a wavelength that is long compared to cross-sectional dimensions. The curvature κ of the elastic line can be approximated by κ = ∂²u/∂X² for the small deformation situation considered, and the equivalent of the stress-strain relation is to assume that κ is a function of M at each point along the line. The function can be derived by the analysis of stress and strain in pure bending and is M = EIκ, with the moment of inertia I = ∫_A(X₂)²dA for uniform elastic properties over all the cross section and with the 1-axis passing through the section centroid. Hence, the equation relating transverse load and displacement of a linear elastic beam is −∂²(EI∂²u/∂X²)/∂X² + F = ρA∂²u/∂t², and this is to be solved subject to two boundary conditions at each end of the elastic line. Examples are u = ∂u/∂X = 0 at a completely restrained (“built in”) end, u = M = 0 at an end that is restrained against displacement but not rotation, and V = M = 0 at a completely unrestrained (free) end. The beam will be reconsidered later in an analysis of response with initial stress present.

The preceding derivation was presented in the spirit of the model of a beam as the elastic line of Euler. The same equations of motion may be obtained by the following five steps: (1) integrate the three-dimensional equations of motion over a section, writing V = ∫_Aσ₁₂dA; (2) integrate the product of X₂ and those equations over a section, writing M = −∫_AX₂σ₁₁dA; (3) assume that planes initially perpendicular to fibres lying along the 1-axis remain perpendicular during deformation, so that ε₁₁ = ε₀(X, t) − X₂κ(X, t), where X ≡ X₁, ε₀(X, t) is the strain of the fibre along the 1-axis, and κ(X, t) = ∂²u/∂X², where u(X, t) is u₂ for the fibre initially along the 1-axis; (4) assume that the stress σ₁₁ relates to strain as if each point were under uniaxial tension, so that σ₁₁ = Eε₁₁; and (5) neglect terms of order h²/L² compared to unity, where h is a typical cross-section dimension and L is a scale length for variations along the direction of the 1-axis. In step (1) the average of u₂ over area A enters but may be interpreted as the displacement u of step (3) to the order retained in (5). The kinematic assumption (3) together with (5), if implemented under conditions such that there are no loadings to generate a net axial force P, requires that ε₀(X, t) = 0 and that κ(X, t) = M(X, t)/EI when the 1-axis has been chosen to pass through the centroid of the cross section. Hence, according to these approximations, σ₁₁ = −X₂M(X, t)/I = −X₂E∂²u(X, t)/∂X². The expression for σ₁₁ is exact for static equilibrium under pure bending, since assumptions (3) and (4) are exact and (5) is then irrelevant. This motivates the use of assumptions (3) and (4) in a situation that does not correspond to pure bending.

Sometimes it is necessary to deal with solids that are already under stress in the reference configuration that is chosen for measuring strain. As a simple example, suppose that the beam just discussed is under an initial uniform tensile stress σ₁₁ = σ⁰—that is, the axial force P = σ⁰A. If σ⁰ is negative and of significant magnitude, one generally refers to the beam as a column; if it is large and positive, the beam might respond more like a taut string. The initial stress σ⁰ contributes a term to the equations of small transverse motion, which now becomes −∂²(EI∂²u/∂X²)/∂X² + σ⁰A∂²u/∂X² + F = ρA∂²u/∂t².

Free vibrations

Suppose that the beam is of length L, is of uniform properties, and is hinge-supported at its ends at X = 0 and X = L so that u = M = 0 there. Then free transverse motions of the beam, solving the above equation with F = 0, are described by any linear combination of the real part of solutions that have the form u = C_n exp (iω_nt)sin(nπX/L), where n is any positive integer, C_n is an arbitrary complex constant, and wheredefines the angular vibration frequency ω_n associated with the nth mode, in units of radians per unit time. The number of vibration cycles per unit time is ω_n/2π. Equation (117) is arranged so that the term in the brackets shows the correction, from unity, of what would be the expression giving the frequencies of free vibration for a beam when there is no σ⁰. The correction from unity can be quite significant, even though σ⁰/E is always much smaller than unity (for interesting cases, 10⁻⁶ to, say, 10⁻³ would be a representative range; few materials in bulk form would remain elastic or resist fracture at higher σ⁰/E, although good piano wire could reach about 10⁻²). The correction term’s significance results because σ⁰/E is multiplied by a term that can become enormous for a beam that is long compared to its thickness; for a square section of side length h, that term (at its largest, when n = 1) is AL²/π²I ≈ 1.2L²/h², which can combine with a small σ⁰/E to produce a correction term within the brackets that is quite non-negligible compared to unity. When σ⁰ > 0 and L is large enough to make the bracketed expression much larger than unity, the EI term cancels out and the beam simply responds like a stretched string (here, string denotes an object that is unable to support a bending moment). When the vibration mode number n is large enough, however, the stringlike effects become negligible and beamlike response takes over; at sufficiently high n that L/n is reduced to the same order as h, the simple beam theory becomes inaccurate and should be replaced by three-dimensional elasticity or, at least, an improved beam theory that takes into account rotary inertia and shear deformability. (While the option of using three-dimensional elasticity for such a problem posed an insurmountable obstacle over most of the history of the subject, by 1990 the availability of computing power and easily used software reduced it to a routine problem that could be studied by an undergraduate engineer or physicist using the finite-element method or some other computational mechanics technique.)