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| '''Linear elasticity''' is the mathematical study of how solid objects deform and become internally stressed due to prescribed loading conditions. Linear elasticity models materials as [[Continuum mechanics|continua]]. Linear elasticity is a simplification of the more general [[Finite strain theory|nonlinear theory of elasticity]] and is a branch of [[continuum mechanics]]. The fundamental "linearizing" assumptions of linear elasticity are: [[Infinitesimal strain theory|infinitesimal strains]] or "small" [[Deformation (mechanics)|deformations]] (or strains) and linear relationships between the components of [[Stress (physics)|stress]] and strain. In addition linear elasticity is valid only for stress states that do not produce [[Yield (engineering)|yielding]]. These assumptions are reasonable for many engineering materials and engineering design scenarios. Linear elasticity is therefore used extensively in [[structural analysis]] and engineering design, often with the aid of [[finite element analysis]]. | | Hola. The author's name is Dalton but nonetheless , it's not the a lot of masucline name out that there. To drive is one of our own things he loves mainly. His wife and him chose to live a life in South Carolina and his family loves it. Auditing is even his primary income comes from. He can be running and [http://Www.Bbc.Co.uk/search/?q=maintaining maintaining] a functional blog here: http://prometeu.net<br><br> |
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| ==Mathematical formulation==
| | Feel free to surf to my blog post; hack clash of clans ([http://prometeu.net these details]) |
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| Equations governing a linear elastic [[boundary value problem]] are based on three [[tensor]] [[partial differential equation]]s for the [[conservation of momentum|balance of linear momentum]] and six [[infinitesimal strain]]-[[displacement field (mechanics)|displacement]] relations. The system of differential equations is completed by a set of [[linear equation|linear]] algebraic [[constitutive equations|constitutive relations]].
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| === Direct tensor form ===
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| In direct [[tensor]] form that is independent of the choice of coordinate system, these governing equations are:<ref name=Slau>Slaughter, W. S., (2002), ''The linearized theory of elasticity'', Birkhauser.</ref>
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| * [[Momentum#Linear_momentum_for_a_system|Equation of motion]], which is an expression of [[Newton's laws of motion#Newton's second law|Newton's second law]]:
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| :<math>\boldsymbol{\nabla}\cdot\boldsymbol{\sigma} + \mathbf{F} = \rho\ddot{\mathbf{u}} </math>
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| * [[Deformation (mechanics)#Infinitesimal strain|Strain-displacement]] equations:
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| :<math>\boldsymbol{\varepsilon} =\tfrac{1}{2} \left[\boldsymbol{\nabla}\mathbf{u}+(\boldsymbol{\nabla}\mathbf{u})^T\right]\,\!</math>
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| * [[Constitutive equations]]. For elastic materials, [[Hooke's law]] represents the material behavior and relates the unknown stresses and strains. The general equation for Hooke's law is
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| :<math> \boldsymbol{\sigma} = \mathsf{C}:\boldsymbol{\varepsilon},</math>
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| where <math>\boldsymbol{\sigma}</math> is the [[stress (mechanics)|Cauchy stress]] [[tensor]], <math>\boldsymbol{\varepsilon}</math> is the [[infinitesimal strain]] tensor, <math>\mathbf{u}</math> is the [[Displacement (vector)|displacement vector]], <math>\mathsf{C}</math> is the fourth-order [[stiffness tensor]], <math>\mathbf{F}</math> is the body force per unit volume, <math>\rho</math> is the mass density, <math>\boldsymbol{\nabla}</math> represents the [[nabla operator]] and <math>(\bullet)^T</math> represents a [[transpose]], <math>\ddot{(\bullet)}</math> represents the second derivative with respect to time, and <math>\mathbf{A}:\mathbf{B} = A_{ij}B_{ij}</math> is the inner product of two second-order tensors (summation over repeated indices is implied).
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| === Cartesian coordinate form ===
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| {{Einstein_summation_convention}}
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| Expressed in terms of components with respect to a rectangular [[Cartesian coordinate]] system, the governing equations of linear elasticity are:<ref name=Slau/>
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| * [[Cauchy momentum equation|Equation of motion]]:
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| :<math>\sigma_{ji,j}+ F_i = \rho \partial_{tt} u_i\,\!</math>
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| :{| class="collapsible collapsed" width="30%" style="text-align:left"
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| !Engineering notation
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| |<math>\frac{\partial \sigma_x}{\partial x} + \frac{\partial \tau_{yx}}{\partial y} + \frac{\partial \tau_{zx}}{\partial z} + F_x = \rho \frac{\partial^2 u_x}{\partial t^2}\,\!</math>
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| <math>\frac{\partial \tau_{xy}}{\partial x} + \frac{\partial \sigma_y}{\partial y} + \frac{\partial \tau_{zy}}{\partial z} + F_y = \rho \frac{\partial^2 u_y}{\partial t^2}\,\!</math>
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| <math>\frac{\partial \tau_{xz}}{\partial x} + \frac{\partial \tau_{yz}}{\partial y} + \frac{\partial \sigma_z}{\partial z} + F_z = \rho \frac{\partial^2 u_z}{\partial t^2}\,\!</math>
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| |}
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| :where the <math>{(\bullet)}_{,j}</math> subscript is a shorthand for <math>\partial{(\bullet)}/\partial x_j</math> and <math>\partial_{tt}</math> indicates <math>\partial^2/\partial t^2</math>, <math> \sigma_{ij}=\sigma_{ji}\,\!</math> is the Cauchy [[Stress (physics)|stress]] tensor, <math> F_i\,\!</math> are the body forces, <math> \rho\,\!</math> is the mass density, and <math> u_i\,\!</math> is the displacement.
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| :These are 3 [[System of linear equations#Independence|independent]] equations with 6 independent unknowns (stresses).
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| * [[Deformation (mechanics)#Infinitesimal strain|Strain-displacement]] equations:
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| :<math>\varepsilon_{ij} =\frac{1}{2} (u_{j,i}+u_{i,j})\,\!</math>
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| :{| class="collapsible collapsed" width="30%" style="text-align:left"
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| !Engineering notation
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| |<math>\epsilon_x=\frac{\partial u_x}{\partial x}\,\!</math>
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| |<math>\gamma_{xy}=\frac{\partial u_x}{\partial y}+\frac{\partial u_y}{\partial x}\,\!</math>
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| |<math>\epsilon_y=\frac{\partial u_y}{\partial y}\,\!</math>
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| |<math>\gamma_{yz}=\frac{\partial u_y}{\partial z}+\frac{\partial u_z}{\partial y}\,\!</math>
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| |<math>\epsilon_z=\frac{\partial u_z}{\partial z}\,\!</math>
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| |<math>\gamma_{zx}=\frac{\partial u_z}{\partial x}+\frac{\partial u_x}{\partial z}\,\!</math>
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| |}
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| :where <math> \varepsilon_{ij}=\varepsilon_{ji}\,\!</math> is the strain. These are 6 independent equations relating strains and displacements with 9 independent unknowns (strains and displacements).
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| * [[Constitutive equations]]. The equation for Hooke's law is:
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| :<math>
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| \sigma_{ij} = C_{ijkl} \, \varepsilon_{kl}
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| \,\!</math>
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| :where <math>C_{ijkl}</math> is the stiffness tensor. These are 6 independent equations relating stresses and strains. The coefficients of the stiffness tensor can always be specified so that <math> C_{ijkl}=C_{klij}=C_{jikl}=C_{ijlk}</math>.
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| An elastostatic boundary value problem for an isotropic-homogeneous media is a system of 15 independent equations and equal number of unknowns (3 equilibrium equations, 6 strain-displacement equations, and 6 constitutive equations). Specifying the boundary conditions, the boundary value problem is completely defined. To solve the system two approaches can be taken according to boundary conditions of the boundary value problem: a '''displacement formulation''', and a '''stress formulation'''.
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| ===Cylindrical coordinate form===
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| In cylindrical coordinates (<math>r,\theta,z</math>) the equations of motion are<ref name=Slau/>
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| :<math>
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| \begin{align}
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| & \frac{\partial \sigma_{rr}}{\partial r} + \cfrac{1}{r}\frac{\partial \sigma_{r\theta}}{\partial \theta} + \frac{\partial \sigma_{rz}}{\partial z} + \cfrac{1}{r}(\sigma_{rr}-\sigma_{\theta\theta}) + F_r = \rho~\frac{\partial^2 u_r}{\partial t^2} \\
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| & \frac{\partial \sigma_{r\theta}}{\partial r} + \cfrac{1}{r}\frac{\partial \sigma_{\theta\theta}}{\partial \theta} + \frac{\partial \sigma_{\theta z}}{\partial z} + \cfrac{2}{r}\sigma_{r\theta} + F_\theta = \rho~\frac{\partial^2 u_\theta}{\partial t^2} \\
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| & \frac{\partial \sigma_{rz}}{\partial r} + \cfrac{1}{r}\frac{\partial \sigma_{\theta z}}{\partial \theta} + \frac{\partial \sigma_{zz}}{\partial z} + \cfrac{1}{r}\sigma_{rz} + F_z = \rho~\frac{\partial^2 u_z}{\partial t^2}
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| \end{align}
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| </math>
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| The strain-displacement relations are
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| :<math>
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| \begin{align}
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| \varepsilon_{rr} & = \cfrac{\partial u_r}{\partial r} ~;~~
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| \varepsilon_{\theta\theta} = \cfrac{1}{r}\left(\cfrac{\partial u_\theta}{\partial \theta} + u_r\right) ~;~~
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| \varepsilon_{zz} = \cfrac{\partial u_z}{\partial z} \\
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| \varepsilon_{r\theta} & = \cfrac{1}{2}\left(\cfrac{1}{r}\cfrac{\partial u_r}{\partial \theta} + \cfrac{\partial u_\theta}{\partial r}- \cfrac{u_\theta}{r}\right) ~;~~
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| \varepsilon_{\theta z} = \cfrac{1}{2}\left(\cfrac{\partial u_\theta}{\partial z} + \cfrac{1}{r}\cfrac{\partial u_z}{\partial \theta}\right) ~;~~
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| \varepsilon_{zr} = \cfrac{1}{2}\left(\cfrac{\partial u_r}{\partial z} + \cfrac{\partial u_z}{\partial r}\right)
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| \end{align}
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| </math>
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| and the constitutive relations are the same as in Cartesian coordinates, except that the indices <math>1</math>,<math>2</math>,<math>3</math> now stand for <math>r</math>,<math>\theta</math>,<math>z</math>, respectively.
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| ===Spherical coordinate form ===
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| In spherical coordinates (<math>r,\theta,\phi</math>) the equations of motion are<ref name=Slau/>
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| :<math>
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| \begin{align}
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| & \frac{\partial \sigma_{rr}}{\partial r} + \cfrac{1}{r}\frac{\partial \sigma_{r\theta}}{\partial \theta} + \cfrac{1}{r\sin\theta}\frac{\partial \sigma_{r\phi}}{\partial \phi} + \cfrac{1}{r}(2\sigma_{rr}-\sigma_{\theta\theta}-\sigma_{\phi\phi}+\sigma_{r\theta}\cot\theta) + F_r = \rho~\frac{\partial^2 u_r}{\partial t^2} \\
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| & \frac{\partial \sigma_{r\theta}}{\partial r} + \cfrac{1}{r}\frac{\partial \sigma_{\theta\theta}}{\partial \theta} + \cfrac{1}{r\sin\theta}\frac{\partial \sigma_{\theta \phi}}{\partial \phi} + \cfrac{1}{r}[(\sigma_{\theta\theta}-\sigma_{\phi\phi})\cot\theta + 3\sigma_{r\theta}] + F_\theta = \rho~\frac{\partial^2 u_\theta}{\partial t^2} \\
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| & \frac{\partial \sigma_{r\phi}}{\partial r} + \cfrac{1}{r}\frac{\partial \sigma_{\theta \phi}}{\partial \theta} + \cfrac{1}{r\sin\theta}\frac{\partial \sigma_{\phi\phi}}{\partial \phi} + \cfrac{1}{r}(2\sigma_{\theta\phi}\cot\theta+3\sigma_{r\phi}) + F_\phi = \rho~\frac{\partial^2 u_\phi}{\partial t^2}
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| \end{align}
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| </math>
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| [[File:3D Spherical.svg|thumb|240px|right|Spherical coordinates (''r'', ''θ'', ''φ'') as commonly used in ''physics'': radial distance ''r'', polar angle ''θ'' ([[theta]]), and azimuthal angle ''φ'' ([[phi]]). The symbol ''ρ'' ([[rho]]) is often used instead of ''r''.]]
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| The strain tensor in spherical coordinates is
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| :<math>
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| \begin{align}
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| \varepsilon_{rr} & = \frac{\partial u_r}{\partial r}\\
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| \varepsilon_{\theta\theta}& = \frac{1}{r}\left(\frac{\partial u_\theta}{\partial \theta} + u_r\right)\\
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| \varepsilon_{\phi\phi} & = \frac{1}{r\sin\theta}\left(\frac{\partial u_\phi}{\partial \phi} + u_r\sin\theta + u_\theta\cos\theta\right)\\
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| \varepsilon_{r\theta} & = \frac{1}{2}\left(\frac{1}{r}\frac{\partial u_r}{\partial \theta} + \frac{\partial u_\theta}{\partial r}- \frac{u_\theta}{r}\right) \\
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| \varepsilon_{\theta \phi} & = \frac{1}{2r}\left[\frac{1}{\sin\theta}\frac{\partial u_\theta}{\partial \phi} +\left(\frac{\partial u_\phi}{\partial \theta}-u_\phi \cot\theta\right)\right]\\
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| \varepsilon_{r \phi} & = \frac{1}{2} \left(\frac{1}{r \sin \theta} \frac{\partial u_r}{\partial \phi} + \frac{\partial u_\phi}{\partial r} - \frac{u_\phi}{r}\right).
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| \end{align}
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| </math>
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| == Isotropic homogeneous media ==
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| In [[Hooke's Law#Isotropic materials|isotropic]] media, the stiffness tensor gives the relationship between the stresses (resulting internal stresses) and the strains (resulting deformations). For an isotropic medium, the stiffness tensor has no preferred direction: an applied force will give the same displacements (relative to the direction of the force) no matter the direction in which the force is applied. In the isotropic case, the stiffness tensor may be written:
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| :<math> C_{ijkl}
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| = K \, \delta_{ij}\, \delta_{kl}
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| +\mu\, (\delta_{ik}\delta_{jl}+\delta_{il}\delta_{jk}-\textstyle{\frac{2}{3}}\, \delta_{ij}\,\delta_{kl})
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| \,\!</math>{{citation needed|date=June 2012}}
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| where <math>\delta_{ij}\,\!</math> is the [[Kronecker delta]], ''K'' is the [[bulk modulus]] (or incompressibility), and <math>\mu\,\!</math> is the [[shear modulus]] (or rigidity), two [[elastic moduli]]. If the medium is inhomogeneous, the isotropic model is sensible if either the medium is piecewise-constant or weakly inhomogeneous; in the strongly inhomogeneous smooth model, anisotropy has to be accounted for. If the medium is [[Homogeneous (chemistry)|homogeneous]], then the elastic moduli will be independent of the position in the medium. The constitutive equation may now be written as:
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| :<math> \sigma_{ij}
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| =K\delta_{ij}\varepsilon_{kk}+2\mu(\varepsilon_{ij}-\textstyle{\frac{1}{3}}\delta_{ij}\varepsilon_{kk}).
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| \,\!</math>
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| This expression separates the stress into a scalar part on the left which may be associated with a scalar pressure, and a traceless part on the right which may be associated with shear forces. A simpler expression is:
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| :<math> \sigma_{ij}
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| =\lambda \delta_{ij} \varepsilon_{kk}+2\mu\varepsilon_{ij}
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| \,\!</math>{{citation needed|date=June 2012}}
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| where λ is [[Lamé parameters|Lamé's first parameter]]. Since the constitutive equation is simply a set of linear equations, the strain may be expressed as a function of the stresses as:<ref name=sommerfeld>{{cite book |title= Mechanics of Deformable Bodies |last=Sommerfeld|first=Arnold |authorlink=Arnold Sommerfeld|year=1964 |publisher=Academic Press |location=New York}}</ref>
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| :<math>\varepsilon_{ij}
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| = \frac{1}{9K}\delta_{ij}\sigma_{kk} + \frac{1}{2\mu}\left(\sigma_{ij}-\textstyle{\frac{1}{3}}\delta_{ij}\sigma_{kk}\right)
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| \,\!</math>
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| which is again, a scalar part on the left and a traceless shear part on the right. More simply:
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| :<math>\varepsilon_{ij}
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| =\frac{1}{2\mu}\sigma_{ij}-\frac{\nu}{E}\delta_{ij}\sigma_{kk}=\frac{1}{E}[(1+\nu)\sigma_{ij}-\nu\delta_{ij}\sigma_{kk}]
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| \,\!</math>
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| where ν is [[Poisson's ratio]] and ''E'' is [[Young's modulus]].
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| ===Elastostatics===
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| Elastostatics is the study of linear elasticity under the conditions of equilibrium, in which all forces on the elastic body sum to zero, and the displacements are not a function of time. The [[Momentum#Linear_momentum_for_a_system|equilibrium equations]] are then
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| :<math>
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| \sigma_{ji,j}+ F_i = 0.
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| \,\!</math>
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| :{| class="collapsible collapsed" width="30%" style="text-align:left"
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| !Engineering notation
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| |-
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| |<math>\frac{\partial \sigma_x}{\partial x} + \frac{\partial \tau_{yx}}{\partial y} + \frac{\partial \tau_{zx}}{\partial z} + F_x = 0\,\!</math>
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| <math>\frac{\partial \tau_{xy}}{\partial x} + \frac{\partial \sigma_y}{\partial y} + \frac{\partial \tau_{zy}}{\partial z} + F_y = 0\,\!</math>
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| <math>\frac{\partial \tau_{xz}}{\partial x} + \frac{\partial \tau_{yz}}{\partial y} + \frac{\partial \sigma_z}{\partial z} + F_z = 0\,\!</math>
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| |}
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| This section will discuss only the isotropic homogeneous case.
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| ====Displacement formulation====
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| In this case, the displacements are prescribed everywhere in the boundary. In this approach, the strains and stresses are eliminated from the formulation, leaving the displacements as the unknowns to be solved for in the governing equations.
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| First, the strain-displacement equations are substituted into the constitutive equations (Hooke's Law), eliminating the strains as unknowns:
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| :<math>\begin{align}
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| \sigma_{ij} &= \lambda \delta_{ij} \varepsilon_{kk}+2\mu\varepsilon_{ij} \\
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| &= \lambda\delta_{ij}u_{k,k}+\mu\left(u_{i,j}+u_{j,i}\right). \\
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| \end{align}
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| \,\!</math>
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| Differentiating yields:
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| :<math>
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| \sigma_{ij,j} = \lambda u_{k,ki}+\mu\left(u_{i,jj}+u_{j,ij}\right).\,\!</math>
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| Substituting into the equilibrium equation yields:
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| :<math>\lambda u_{k,ki}+\mu\left(u_{i,jj}+u_{j,ij}\right) +F_i=0\,\!</math>
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| or
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| :<math>\mu u_{i,jj}+(\mu+\lambda)u_{j,ij}+F_i=0\,\!</math>
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| where <math>\lambda\,\!</math> and <math>\mu\,\!</math> are [[Lamé parameters]].
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| In this way, the only unknowns left are the displacements, hence the name for this formulation. The governing equations obtained in this manner are called '''Navier-Cauchy equations''' or, alternatively, the ''elastostatic equations''.
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| :{| class="toccolours collapsible collapsed" width="60%" style="text-align:left"
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| !Derivation of Navier-Cauchy equations in Engineering notation
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| |First, the <math>x\,\!</math>-direction will be considered. Substituting the strain-displacement equations into the equilibrium equation in the <math>x\,\!</math>-direction we have
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| :<math>\sigma_x = 2\mu\varepsilon_x + \lambda(\varepsilon_x + \varepsilon_y +\varepsilon_z)=2\mu\frac{\partial u_x}{\partial x}+\lambda\left(\frac{\partial u_x}{\partial x}+\frac{\partial u_y}{\partial y}+\frac{\partial u_z}{\partial z}\right)\,\!</math>
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| :<math>\tau_{xy} = \mu\gamma_{xy}=\mu\left(\frac{\partial u_x}{\partial y}+\frac{\partial u_y}{\partial x}\right)\,\!</math>
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| :<math>\tau_{xz} = \mu\gamma_{zx}=\mu\left(\frac{\partial u_z}{\partial x}+\frac{\partial u_x}{\partial z}\right)\,\!</math>
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| Then substituting these equations into the equilibrium equation in the <math>x\,\!</math>-direction we have
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| :<math>\frac{\partial \sigma_x}{\partial x} + \frac{\partial \tau_{yx}}{\partial y} + \frac{\partial \tau_{zx}}{\partial z} + F_x = 0\,\!</math>
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| :<math>\frac{\partial}{\partial x}\left( 2\mu\frac{\partial u_x}{\partial x}+\lambda\left(\frac{\partial u_x}{\partial x}+\frac{\partial u_y}{\partial y}+\frac{\partial u_z}{\partial z}\right)\right) + \mu\frac{\partial}{\partial y}\left(\frac{\partial u_x}{\partial y}+\frac{\partial u_y}{\partial x}\right)+ \mu\frac{\partial}{\partial z}\left(\frac{\partial u_z}{\partial x}+\frac{\partial u_x}{\partial z}\right)+F_x=0\,\!</math>
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| Using the assumption that <math>\mu</math> and <math>\lambda</math> are constant we can rearrange and get:
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| :<math>\left(\lambda+\mu\right)\frac{\partial}{\partial x}\left(\frac{\partial u_x}{\partial x}+\frac{\partial u_y}{\partial y}+\frac{\partial u_z}{\partial z}\right)+\mu\left(\frac{\partial^2 u_x}{\partial x^2}+\frac{\partial^2 u_x}{\partial y^2}+\frac{\partial^2 u_x}{\partial z^2}\right)+F_x=0\,\!</math>
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| Following the same procedure for the <math>y\,\!</math>-direction and <math>z\,\!</math>-direction we have
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| :<math>\left(\lambda+\mu\right)\frac{\partial}{\partial y}\left(\frac{\partial u_x}{\partial x}+\frac{\partial u_y}{\partial y}+\frac{\partial u_z}{\partial z}\right)+\mu\left(\frac{\partial^2 u_y}{\partial x^2}+\frac{\partial^2 u_y}{\partial y^2}+\frac{\partial^2 u_y}{\partial z^2}\right)+F_y=0\,\!</math>
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| :<math>\left(\lambda+\mu\right)\frac{\partial}{\partial z}\left(\frac{\partial u_x}{\partial x}+\frac{\partial u_y}{\partial y}+\frac{\partial u_z}{\partial z}\right)+\mu\left(\frac{\partial^2 u_z}{\partial x^2}+\frac{\partial^2 u_z}{\partial y^2}+\frac{\partial^2 u_z}{\partial z^2}\right)+F_z=0\,\!</math>
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| These last 3 equations are the Navier-Cauchy equations, which can be also expressed in vector notation as
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| :<math>(\lambda+\mu)\nabla(\nabla\cdot\mathbf{u})+\mu\nabla^2\mathbf{u}+\mathbf{F}=0\,\!</math>
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| |}
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| Once the displacement field has been calculated, the displacements can be replaced into the strain-displacement equations to solve for strains, which later are used in the constitutive equations to solve for stresses.
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| ===== The biharmonic equation =====
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| The elastostatic equation may be written:
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| :<math>(\alpha^2-\beta^2)u_{j,ij}+
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| \beta^2u_{i,mm}=-F_i.\,\!</math>
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| Taking the [[divergence]] of both sides of the elastostatic equation and assuming the body forces has zero divergence (homogeneous in domain) (<math>F_{i,i}=0\,\!</math>) we have
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| :<math>(\alpha^2-\beta^2)u_{j,iij}+\beta^2u_{i,imm} = 0.\,\!</math>
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| Noting that summed indices need not match, and that the partial derivatives commute, the two differential terms are seen to be the same and we have:
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| :<math>\alpha^2u_{j,iij} = 0\,\!</math>
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| from which we conclude that:
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| :<math>u_{j,iij} = 0.\,\!</math>
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| Taking the [[Laplacian]] of both sides of the elastostatic equation, and assuming in addition <math>F_{i,kk}=0\,\!</math>, we have
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| :<math>(\alpha^2-\beta^2)u_{j,kkij}+\beta^2u_{i,kkmm}=0.\,\!</math>
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| From the divergence equation, the first term on the left is zero (Note: again, the summed indices need not match) and we have:
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| :<math>\beta^2u_{i,kkmm}=0\,\!</math>
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| from which we conclude that:
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| :<math>u_{i,kkmm}=0\,\!</math>
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| or, in coordinate free notation <math>\nabla^4 \mathbf{u}=0\,\!</math> which is just the [[biharmonic equation]] in <math>\mathbf{u}\,\!</math>.
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| ====Stress formulation====
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| In this case, the surface tractions are prescribed everywhere on the surface boundary. In this approach, the strains and displacements are eliminated leaving the stresses as the unknowns to be solved for in the governing equations. Once the stress field is found, the strains are then found using the constitutive equations.
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| There are six independent components of the stress tensor which need to be determined, yet in the displacement formulation, there are only three components of the displacement vector which need to be determined. This means that there are some constraints which must be placed upon the stress tensor, to reduce the number of degrees of freedom to three. Using the constitutive equations, these constraints are derived directly from corresponding constraints which must hold for the strain tensor, which also has six independent components. The constraints on the strain tensor are derivable directly from the definition of the strain tensor as a function of the displacement vector field, which means that these constraints introduce no new concepts or information. It is the constraints on the strain tensor that are most easily understood. If the elastic medium is visualized as a set of infinitesimal cubes in the unstrained state, then after the medium is strained, an arbitrary strain tensor must yield a situation in which the distorted cubes still fit together without overlapping. In other words, for a given strain, there must exist a continuous vector field (the displacement) from which that strain tensor can be derived. The constraints on the strain tensor that are required to assure that this is the case were discovered by Saint Venant, and are called the "[[Saint-Venant's compatibility condition|Saint Venant compatibility equations]]". These are 81 equations, 6 of which are independent non-trivial equations, which relate the different strain components. These are expressed in index notation as:
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| :<math>\varepsilon_{ij,km}+\varepsilon_{km,ij}-\varepsilon_{ik,jm}-\varepsilon_{jm,ik}=0.\,\!</math>
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| :{| class="collapsible collapsed" width="30%" style="text-align:left"
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| !Engineering notation
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| |-
| |
| |<math>\frac{\partial^2 \epsilon_x}{\partial y^2} + \frac{\partial^2 \epsilon_y}{\partial x^2} = 2 \frac{\partial^2 \epsilon_{xy}}{\partial x \partial y}\,\!</math>
| |
| | |
| <math>\frac{\partial^2 \epsilon_y}{\partial z^2} + \frac{\partial^2 \epsilon_z}{\partial y^2} = 2 \frac{\partial^2 \epsilon_{yz}}{\partial y \partial z}\,\!</math>
| |
| | |
| <math>\frac{\partial^2 \epsilon_x}{\partial z^2} + \frac{\partial^2 \epsilon_z}{\partial x^2} = 2 \frac{\partial^2 \epsilon_{zx}}{\partial z \partial x}\,\!</math>
| |
| | |
| <math>\frac{\partial^2 \epsilon_x}{\partial y \partial z} = \frac{\partial}{\partial x} \left ( -\frac{\partial \epsilon_{yz}}{\partial x} + \frac{\partial \epsilon_{zx}}{\partial y} + \frac{\partial \epsilon_{xy}}{\partial z}\right)\,\!</math>
| |
| | |
| <math>\frac{\partial^2 \epsilon_y}{\partial z \partial x} = \frac{\partial}{\partial y} \left ( \frac{\partial \epsilon_{yz}}{\partial x} - \frac{\partial \epsilon_{zx}}{\partial y} + \frac{\partial \epsilon_{xy}}{\partial z}\right)\,\!</math>
| |
| | |
| <math>\frac{\partial^2 \epsilon_z}{\partial x \partial y} = \frac{\partial}{\partial z} \left ( \frac{\partial \epsilon_{yz}}{\partial x} + \frac{\partial \epsilon_{zx}}{\partial y} - \frac{\partial \epsilon_{xy}}{\partial z}\right)\,\!</math>
| |
| |}
| |
| | |
| The strains in this equation are then expressed in terms of the stresses using the constitutive equations, which yields the corresponding constraints on the stress tensor. These constraints on the stress tensor are known as the ''Beltrami-Michell'' equations of compatibility:
| |
| | |
| :<math>\sigma_{ij,kk}+\frac{1}{1+\nu}\sigma_{kk,ij}+F_{i,j}+F_{j,i}+\frac{\nu}{1-\nu}\delta_{i,j}F_{k,k}=0.\,\!</math> | |
| In the special situation where the body force is homogeneous, the above equations reduce to
| |
| :<math> (1+\nu)\sigma_{ij,kk}+\sigma_{kk,ij}=0.\,\!</math>
| |
| A necessary, but insufficient, condition for compatibility under this situation is <math>\boldsymbol{\nabla}^4\boldsymbol{\sigma} = \boldsymbol{0}</math> or <math>\sigma_{ij,kk\ell\ell} = 0</math>.<ref name=Slau/>
| |
| | |
| These constraints, along with the equilibrium equation (or equation of motion for elastodynamics) allow the calculation of the stress tensor field. Once the stress field has been calculated from these equations, the strains can be obtained from the constitutive equations, and the displacement field from the strain-displacement equations.
| |
| | |
| An alternative solution technique is to express the stress tensor in terms of [[stress functions]] which automatically yield a solution to the equilibrium equation. The stress functions then obey a single differential equation which corresponds to the compatibility equations.
| |
| | |
| ====Solutions for elastostatic cases====
| |
| | |
| :{| class="toccolours collapsible collapsed" width="60%" style="text-align:left"
| |
| !Thomson's solution - point force in an infinite isotropic medium
| |
| |-
| |
| |The most important solution of the Navier-Cauchy or elastostatic equation is for that of a force acting at a point in an infinite isotropic medium. This solution was found by [[William Thomson, 1st Baron Kelvin|William Thomson]] (later Lord Kelvin) in 1848 (Thomson 1848). This solution is the analog of [[Coulomb's law]] in [[electrostatics]]. A derivation is given in Landau & Lifshitz.<ref name=LL>{{cite book |title=Theory of Elasticity |edition=3rd|last=Landau |first=L.D. |authorlink=Lev Landau |coauthors=[[Evgeny Lifshitz|Lifshitz, E. M.]] |year=1986 |publisher=Butterworth Heinemann |location=Oxford, England |isbn=0-7506-2633-X }}</ref>{{rp|§8}} Defining
| |
| | |
| :<math>a=1-2\nu\,\!</math>
| |
| :<math>b=2(1-\nu)=a+1\,\!</math>
| |
| | |
| where <math>\nu\,\!</math> is Poisson's ratio, the solution may be expressed as
| |
| | |
| :<math>u_i=G_{ik}F_k\,\!</math>
| |
| | |
| where <math>F_k\,\!</math> is the force vector being applied at the point, and <math>G_{ik}\,\!</math> is a tensor [[Green's function]] which may be written in [[Cartesian coordinates]] as:
| |
| | |
| :<math>G_{ik}=
| |
| \frac{1}{4\pi\mu r}\left[
| |
| \left(1-\frac{1}{2b}\right)\delta_{ik}+\frac{1}{2b}\frac{x_i x_k}{r^2}
| |
| \right]
| |
| \,\!</math>
| |
| | |
| It may be also compactly written as:
| |
| | |
| :<math>G_{ik}=
| |
| \frac{1}{4\pi\mu}\left[\frac{\delta_{ik}}{r}-\frac{1}{2b}\frac{\partial^2 r}{\partial x_i\partial x_k}\right]
| |
| \,\!</math>
| |
| | |
| and it may be explicitly written as:
| |
| | |
| :<math>G_{ik}=\frac{1}{4\pi\mu r}\begin{bmatrix}
| |
| | |
| 1-\frac{1}{2b}+\frac{1}{2b}\frac{x^2}{r^2} &
| |
| \frac{1}{2b}\frac{xy} {r^2} &
| |
| \frac{1}{2b}\frac{xz} {r^2} \\
| |
| | |
| \frac{1}{2b}\frac{yx} {r^2} &
| |
| 1-\frac{1}{2b}+\frac{1}{2b}\frac{y^2}{r^2} &
| |
| \frac{1}{2b}\frac{yz} {r^2} \\
| |
| | |
| \frac{1}{2b}\frac{zx} {r^2} &
| |
| \frac{1}{2b}\frac{zy} {r^2} &
| |
| 1-\frac{1}{2b}+\frac{1}{2b}\frac{z^2}{r^2}
| |
| \end{bmatrix}
| |
| \,\!</math>
| |
| | |
| In cylindrical coordinates (<math>\rho,\phi,z\,\!</math>) it may be written as:
| |
| | |
| :<math>G_{ik}=\frac{1}{4\pi \mu r}\begin{bmatrix}
| |
| 1-\frac{1}{2b}\frac{z^2}{r^2}&0&\frac{1}{2b}\frac{\rho z}{r^2}\\
| |
| 0&1-\frac{1}{2b}&0\\
| |
| \frac{1}{2b}\frac{z \rho}{r^2}&0&1-\frac{1}{2b}\frac{\rho^2}{r^2}
| |
| \end{bmatrix}
| |
| \,\!</math>
| |
| | |
| where r is total distance to point.
| |
| | |
| It is particularly helpful to write the displacement in cylindrical coordinates for a point force <math>F_z\,\!</math> directed along the z-axis. Defining <math>\hat{\mathbf{\rho}}\,\!</math> and <math>\hat{\mathbf{z}}\,\!</math> as unit vectors in the <math>\rho\,\!</math> and <math>z\,\!</math> directions respectively yields:
| |
| | |
| :<math>
| |
| \mathbf{u}=\frac{F_z}{4\pi\mu r}\left[\frac{1}{4(1-\nu)}\,\frac{\rho z}{r^2}\hat{\mathbf{\rho}} + \left(1-\frac{1}{4(1-\nu)}\,\frac{\rho^2}{r^2}\right)\hat{\mathbf{z}}\right]
| |
| \,\!</math>
| |
| | |
| It can be seen that there is a component of the displacement in the direction of the force, which diminishes, as is the case for the potential in electrostatics, as 1/r for large r. There is also an additional ρ-directed component.
| |
| |}
| |
| | |
| :{| class="toccolours collapsible collapsed" width="60%" style="text-align:left"
| |
| !Boussinesq-Cerruti solution - point force at the origin of an infinite isotropic half-space
| |
| |-
| |
| |Another useful solution is that of a point force acting on the surface of an infinite half-space. It was derived by Boussinesq<ref>{{cite book |title= Application des potentiels à l'étude de l'équilibre et du mouvement des solides élastiques |last=Boussinesq|first=Joseph |authorlink=Joseph Boussinesq |year=1885 |publisher=Gauthier-Villars |location=Paris, France |url=http://name.umdl.umich.edu/ABV5032.0001.001 }}</ref> and a derivation is given in Landau & Lifshitz.<ref name=LL/>{{rp|§8}} In this case, the solution is again written as a Green's tensor which goes to zero at infinity, and the component of the stress tensor normal to the surface vanishes. This solution may be written in Cartesian coordinates as [note: a=(1-2ν) and b=2(1-ν), ν== Poissons ratio]:
| |
| | |
| :<math>G_{ik}=\frac{1}{4\pi\mu}\begin{bmatrix}
| |
| | |
| \frac{b}{r}+\frac{x^2}{r^3}-\frac{ax^2}{r(r+z)^2}-\frac{az}{r(r+z)} &
| |
| \frac{xy}{r^3}-\frac{axy}{r(r+z)^2}&
| |
| \frac{xz}{r^3}-\frac{ax}{r(r+z)}\\
| |
| | |
| \frac{yx}{r^3} -\frac{ayx}{r(r+z)^2}&
| |
| \frac{b}{r}+\frac{y^2}{r^3}-\frac{ay^2}{r(r+z)^2}-\frac{az}{r(r+z)} &
| |
| \frac{yz}{r^3} -\frac{ay}{r(r+z)}\\
| |
| | |
| \frac{zx}{r^3}+\frac{ax}{r(r+z)}&
| |
| \frac{zy}{r^3}+\frac{ay}{r(r+z)}&
| |
| \frac{b}{r}+\frac{z^2}{r^3}
| |
| \end{bmatrix}
| |
| \,\!</math>
| |
| |}
| |
| | |
| Other solutions:
| |
| | |
| * Point force inside an infinite isotropic half-space<ref>{{cite journal |last=Mindlin |first= R. D.|authorlink=Raymond D. Mindlin |year=1936|title=Force at a point in the interior of a semi-infinite solid |journal=Physics |volume=7|issue= 5|pages=195–202 |id= |url= |quote=|doi=10.1063/1.1745385 |bibcode = 1936Physi...7..195M }}</ref>
| |
| * Contact of two elastic bodies: the Hertz solution.<ref>{{cite journal |last=Hertz |first= Heinrich|authorlink=Heinrich Hertz |year=1882|title=Contact between solid elastic bodies |journal=Journ. Für reine und angewandte Math.|volume=92}}</ref> See also the page on [[Contact mechanics]].
| |
| | |
| === Elastodynamics – the wave equation ===
| |
| {{Expand section|more principles, a brief explanation to each type of wave|discuss=Talk:Linear elasticity#New section needed|date=September 2010}}
| |
| Elastodynamics is the study of '''elastic waves''' and involves linear elasticity with variation in time. An '''elastic wave''' is a type of [[mechanical wave]] that propagates in elastic or [[viscoelasticity|viscoelastic]] materials. The elasticity of the material provides the restoring [[force]] of the wave. When they occur in the [[Earth]] as the result of an [[earthquake]] or other disturbance, elastic waves are usually called [[seismic wave]]s.
| |
| | |
| The '''[[wave equation]]''' of elastodynamics is simply the equilibrium equation of elastostatics with an additional inertial term:
| |
| :<math>
| |
| \sigma_{ji,j}+ F_i = \rho\,\ddot{u}_i = \rho\,\partial_{tt}u_i.
| |
| \,\!</math>
| |
| | |
| If the material is isotropic and homogeneous (i.e. the stiffness tensor is constant throughout the material), the '''elastodynamic wave equation''' has the form:
| |
| | |
| :<math>
| |
| \mu u_{i,jj}+(\mu+\lambda)u_{j,ij}+F_i=\rho\partial_{tt}u_i
| |
| \quad \mathrm{or}\quad
| |
| \mu\nabla^2\mathbf{u}+(\mu+\lambda)\nabla(\nabla\cdot\mathbf{u})+\mathbf{F}=\rho\frac{\partial^2\mathbf{u}}{\partial t^2}.
| |
| \,\!</math>
| |
| | |
| The elastodynamic wave equation can also be expressed as
| |
| :<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]].
| |
| | |
| In [[Hooke's Law#Isotropic materials|isotropic]] media, the stiffness tensor has the form
| |
| :<math> C_{ijkl}
| |
| = K \, \delta_{ij}\, \delta_{kl}
| |
| +\mu\, (\delta_{ik}\delta_{jl}+\delta_{il}\delta_{jk}-\frac{2}{3}\, \delta_{ij}\,\delta_{kl})\,\!</math>
| |
| where
| |
| <math>K\,\!</math> is the [[bulk modulus]] (or incompressibility), and
| |
| <math>\mu\,\!</math> is the [[shear modulus]] (or rigidity), two [[elastic moduli]]. If the material is homogeneous (i.e. the stiffness tensor is constant throughout the material), the acoustic operator becomes:
| |
| :<math>A_{ij}[\nabla]=\alpha^2 \partial_i\partial_j+\beta^2(\partial_m\partial_m\delta_{ij}-\partial_i\partial_j)\,\!</math>
| |
| | |
| For [[plane waves]], the above differential operator becomes the ''acoustic algebraic operator'':
| |
| :<math>A_{ij}[\mathbf{k}]=\alpha^2 k_ik_j+\beta^2(k_mk_m\delta_{ij}-k_ik_j)\,\!</math>
| |
| where
| |
| :<math> \alpha^2=\left(K+\frac{4}{3}\mu\right)/\rho \qquad \beta^2=\mu/\rho\,\!</math>
| |
| are the [[eigenvalue]]s of <math>A[\hat{\mathbf{k}}]\,\!</math> with [[eigenvector]]s <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]]).
| |
| | |
| ==Anisotropic homogeneous media ==
| |
| | |
| {{Main|Hooke's law}}
| |
| | |
| For anisotropic media, the stiffness tensor <math> C_{ijkl}\,\!</math> is more complicated. The symmetry of the stress tensor <math>\sigma_{ij}\,\!</math> means that there are at most 6 different elements of stress. Similarly, there are at most 6 different elements of the strain tensor <math>\varepsilon_{ij}\,\!</math> . Hence the fourth-order stiffness tensor <math> C_{ijkl}\,\!</math> may be written as a matrix <math>C_{\alpha \beta}\,\!</math> (a tensor of second order). [[Voigt notation]] is the standard mapping for tensor indices,
| |
| :<math>
| |
| \begin{matrix}
| |
| ij & =\\
| |
| \Downarrow & \\
| |
| \alpha & =
| |
| \end{matrix}
| |
| | |
| \begin{matrix}
| |
| 11 & 22 & 33 & 23,32 & 13,31 & 12,21 \\
| |
| \Downarrow & \Downarrow & \Downarrow & \Downarrow & \Downarrow & \Downarrow & \\
| |
| 1 &2 & 3 & 4 & 5 & 6
| |
| \end{matrix}\,\!</math>
| |
| | |
| With this notation, one can write the elasticity matrix for any linearly elastic medium as:
| |
| | |
| :<math> C_{ijkl} \Rightarrow C_{\alpha \beta} =\begin{bmatrix}
| |
| C_{11} & C_{12} & C_{13} & C_{14} & C_{15} & C_{16} \\
| |
| C_{12} & C_{22} & C_{23} & C_{24} & C_{25} & C_{26} \\
| |
| C_{13} & C_{23} & C_{33} & C_{34} & C_{35} & C_{36} \\
| |
| C_{14} & C_{24} & C_{34} & C_{44} & C_{45} & C_{46} \\
| |
| C_{15} & C_{25} & C_{35} & C_{45} & C_{55} & C_{56} \\
| |
| C_{16} & C_{26} & C_{36} & C_{46} & C_{56} & C_{66}
| |
| \end{bmatrix}.
| |
| \,\!</math>
| |
| | |
| As shown, the matrix <math> C_{\alpha \beta}\,\!</math> is symmetric, this is a result of the existence of a strain energy density function which satisfies <math>\sigma_{ij}=\frac{\partial W}{\partial\varepsilon_{ij}}</math>. Hence, there are at most 21 different elements of <math> C_{\alpha \beta}\,\!</math>.
| |
| | |
| The isotropic special case has 2 independent elements:
| |
| :<math> C_{\alpha \beta} =\begin{bmatrix}
| |
| K+4 \mu\ /3 & K-2 \mu\ /3 & K-2 \mu\ /3 & 0 & 0 & 0 \\
| |
| K-2 \mu\ /3 & K+4 \mu\ /3 & K-2 \mu\ /3 & 0 & 0 & 0 \\
| |
| K-2 \mu\ /3 & K-2 \mu\ /3 & K+4 \mu\ /3 & 0 & 0 & 0 \\
| |
| 0 & 0 & 0 & \mu\ & 0 & 0 \\
| |
| 0 & 0 & 0 & 0 & \mu\ & 0 \\
| |
| 0 & 0 & 0 & 0 & 0 & \mu\
| |
| \end{bmatrix}.
| |
| \,\!</math>
| |
| | |
| The simplest anisotropic case, that of cubic symmetry has 3 independent elements:
| |
| :<math> C_{\alpha \beta} =\begin{bmatrix}
| |
| C_{11} & C_{12} & C_{12} & 0 & 0 & 0 \\
| |
| C_{12} & C_{11} & C_{12} & 0 & 0 & 0 \\
| |
| C_{12} & C_{12} & C_{11} & 0 & 0 & 0 \\
| |
| 0 & 0 & 0 & C_{44} & 0 & 0 \\
| |
| 0 & 0 & 0 & 0 & C_{44} & 0 \\
| |
| 0 & 0 & 0 & 0 & 0 & C_{44}
| |
| \end{bmatrix}.
| |
| \,\!</math>
| |
| | |
| The case of [[transverse isotropy]], also called polar anisotropy, (with a single axis (the 3-axis) of symmetry) has 5 independent elements:
| |
| :<math> C_{\alpha \beta} =\begin{bmatrix}
| |
| C_{11} & C_{11}-2C_{66} & C_{13} & 0 & 0 & 0 \\
| |
| C_{11}-2C_{66} & C_{11} & C_{13} & 0 & 0 & 0 \\
| |
| C_{13} & C_{13} & C_{33} & 0 & 0 & 0 \\
| |
| 0 & 0 & 0 & C_{44} & 0 & 0 \\
| |
| 0 & 0 & 0 & 0 & C_{44} & 0 \\
| |
| 0 & 0 & 0 & 0 & 0 & C_{66}
| |
| \end{bmatrix}.
| |
| \,\!</math>
| |
| | |
| When the transverse isotropy is weak (i.e. close to isotropy), an alternative parametrization utilizing [[Thomsen parameters]], is convenient for the formulas for wave speeds.
| |
| | |
| The case of orthotropy (the symmetry of a brick) has 9 independent elements:
| |
| :<math> C_{\alpha \beta} =\begin{bmatrix}
| |
| C_{11} & C_{12} & C_{13} & 0 & 0 & 0 \\
| |
| C_{12} & C_{22} & C_{23} & 0 & 0 & 0 \\
| |
| C_{13} & C_{23} & C_{33} & 0 & 0 & 0 \\
| |
| 0 & 0 & 0 & C_{44} & 0 & 0 \\
| |
| 0 & 0 & 0 & 0 & C_{55} & 0 \\
| |
| 0 & 0 & 0 & 0 & 0 & C_{66}
| |
| \end{bmatrix}.
| |
| \,\!</math>
| |
| | |
| === Elastodynamics ===
| |
| The elastodynamic wave equation for anisotropic media can be expressed as
| |
| :<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 and Christoffel equation ====
| |
| 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'' (also known as the '''Christoffel equation''') 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.
| |
| | |
| ==See also==
| |
| {{Continuum mechanics|cTopic=[[Solid mechanics]]}}
| |
| <div style="-moz-column-count:4; column-count:4;">
| |
| *[[Castigliano's method]]
| |
| *[[Clapeyron's theorem (elasticity)]]
| |
| *[[Contact mechanics]]
| |
| *[[Deformation (mechanics)|Deformation]]
| |
| *[[Elasticity (physics)]]
| |
| *[[Hooke's law]]
| |
| *[[Infinitesimal strain theory]]
| |
| *[[Michell solution]]
| |
| *[[Plasticity (physics)]]
| |
| *[[Signorini problem]]
| |
| *[[Spring system]]
| |
| *[[Stress (mechanics)]]
| |
| *[[Stress functions]]
| |
| </div>
| |
| | |
| == References ==
| |
| {{Reflist}}
| |
| {{More footnotes|date=September 2010}}
| |
| | |
| {{DEFAULTSORT:Linear Elasticity}}
| |
| [[Category:Elasticity (physics)]]
| |
| [[Category:Solid mechanics]]
| |