应变协调性

对于二维无限小应变问题，其应变－位移关系为

${\displaystyle \varepsilon _{11}={\cfrac {\partial u_{1}}{\partial x_{1}}}~;~~\varepsilon _{12}={\cfrac {1}{2}}\left[{\cfrac {\partial u_{1}}{\partial x_{2}}}+{\cfrac {\partial u_{2}}{\partial x_{1}}}\right]~;~~\varepsilon _{22}={\cfrac {\partial u_{2}}{\partial x_{2}}}}$

其所对应的协调条件为

${\displaystyle {\cfrac {\partial ^{2}\varepsilon _{11}}{\partial x_{2}^{2}}}-2{\cfrac {\partial ^{2}\varepsilon _{12}}{\partial x_{1}\partial x_{2}}}+{\cfrac {\partial ^{2}\varepsilon _{22}}{\partial x_{1}^{2}}}=0}$

在三维问题中，共有六个条件需满足。除了二维问题中的一个协调条件扩展为三个条件之外，另外三个协调条件的形式为

${\displaystyle {\cfrac {\partial ^{2}\varepsilon _{33}}{\partial x_{1}\partial x_{2}}}={\cfrac {\partial }{\partial x_{3}}}\left[{\cfrac {\partial \varepsilon _{23}}{\partial x_{1}}}+{\cfrac {\partial \varepsilon _{31}}{\partial x_{2}}}-{\cfrac {\partial \varepsilon _{12}}{\partial x_{3}}}\right]}$

使用指标记号可以将所有六个条件合写为[2]

${\displaystyle e_{ikr}~e_{jls}~\varepsilon _{ij,kl}=0}$

其中${\displaystyle e_{ijk}}$为列维-奇维塔符号。使用张量符号则可以表示成

${\displaystyle {\boldsymbol {\nabla }}\times ({\boldsymbol {\nabla }}\times {\boldsymbol {\varepsilon }})={\boldsymbol {0}}}$

二阶张量

${\displaystyle {\boldsymbol {R}}:={\boldsymbol {\nabla }}\times ({\boldsymbol {\nabla }}\times {\boldsymbol {\varepsilon }})~;~~R_{rs}:=e_{ikr}~e_{jls}~\varepsilon _{ij,kl}}$

被称为不协调张量，即圣维南张量。

右柯西－格林变形张量的协调条件为

${\displaystyle R_{\alpha \beta \rho }^{\gamma }:={\frac {\partial }{\partial X^{\rho }}}[\Gamma _{\alpha \beta }^{\gamma }]-{\frac {\partial }{\partial X^{\beta }}}[\Gamma _{\alpha \rho }^{\gamma }]+\Gamma _{\mu \rho }^{\gamma }~\Gamma _{\alpha \beta }^{\mu }-\Gamma _{\mu \beta }^{\gamma }~\Gamma _{\alpha \rho }^{\mu }=0}$

其中${\displaystyle \Gamma _{ij}^{k}}$表示第二类克里斯托费尔符号，${\displaystyle R_{ijk}^{m}}$则表示黎曼－克里斯托费尔曲率张量。