Linear elasticity
The linear theory of elasticity models the macroscopic mechanical properties of solids assuming "small" deformations.
Basic equations
Linear elastodynamics is based on three tensor equations:
<math>
\partial_j T_{ij} + f_i =\rho \, \partial_{tt} u_i
<math>
<math>
T_{ij} = C_{ijkl} \, E_{kl}
<math>
<math>
E_{ij} =\frac{1}{2} (\partial_i u_j+\partial_j u_i)
<math>
where:
- <math> T_{ij}=T_{ji} <math> is stress
- <math> f_i <math> is body force
- <math> \rho <math> is density
- <math> u_i <math> is displacement
- <math> C_{ijkl}=C_{klij}=C_{jikl}=C_{ijlk} <math> is the stiffness tensor
- <math> E_{ij}=E_{ji} <math> is strain
Wave equation
From the basic equations one gets the wave equation
- <math> (\delta_{kl} \partial_{tt}-A_{kl}[\nabla]) \, u_l
= \frac{1}{\rho} f_k <math>
where
- <math> A_{kl}[\nabla]=\frac{1}{\rho} \, \partial_i \, C_{iklj} \, \partial_j <math>
is the acoustic differential operator, and <math> \delta_{kl}<math> is Kronecker delta.
Plane waves
A plane wave has the form
- <math> \mathbf{u}[\mathbf{x}, \, t] = U[\mathbf{k} \cdot \mathbf{x} - \omega \, t] \, \hat{\mathbf{u}} <math>
with <math>\hat{\mathbf{u}}<math> of unit length.
It is a solution of the wave equation with zero forcing, if and only if
<math> \omega^2 <math> and <math>\hat{\mathbf{u}}<math> constitute an eigenvalue/eigenvector pair of the
acoustic algebraic operator
- <math> A_{kl}[\mathbf{k}]=\frac{1}{\rho} \, k_i \, C_{iklj} \, k_j <math>
This propagation condition may be written as
- <math>A[\hat{\mathbf{k}}] \, \hat{\mathbf{u}}=c^2 \, \hat{\mathbf{u}}<math>
where
<math>\hat{\mathbf{k}} = \mathbf{k} / \sqrt{\mathbf{k}\cdot\mathbf{k}}<math>
denotes propagation direction
and <math>c=\omega/\sqrt{\mathbf{k}\cdot\mathbf{k}}<math> is phase velocity.
Isotropic media
In isotropic media, the elasticity tensor has the form
- <math> C_{ijkl}
= \kappa \, \delta_{ij}\, \delta_{kl}
+\mu\, (\delta_{ik}\delta_{jl}+\delta_{il}\delta_{jk}-\frac{2}{3}\, \delta_{ij}\,\delta_{kl})<math>
where
<math>\kappa<math> is incompressibility, and
<math>\mu<math> is rigidity.
Hence the acoustic algebraic operator becomes
- <math>A[\hat{\mathbf{k}}]=
\alpha^2 \,\hat{\mathbf{k}}\otimes\hat{\mathbf{k}}
+\beta^2 \, (\mathbf{I}-\hat{\mathbf{k}}\otimes\hat{\mathbf{k}} )
<math>
where
<math> \otimes <math> denotes the tensor product,
<math> \mathbf{I} <math> is the identity matrix, and
- <math> \alpha^2=(\kappa+\frac{4}{3}\mu)/\rho
\qquad \beta^2=\mu/\rho <math>
are the eigenvalues of
<math>A[\hat{\mathbf{k}}]<math>
with eigenvectors <math>\hat{\mathbf{u}}<math> parallel and orthogonal to the propagation direction
<math>\hat{\mathbf{k}}<math>, respectively.
In the seismological literature, the corresponding plane waves are called P-waves and S-waves (see Seismic wave).
References
- Gurtin M. E., Introduction to Continuum Mechanics, Academic Press 1981
- L. D. Landau & E. M. Lifschitz, Theory of Elasticity, Butterworth 1986
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