変形勾配
e = 1 2 ( I − B − 1 ) , e i j = 1 2 ( ∂ u i ∂ x j + ∂ u j ∂ x i − ∂ u k ∂ x i ∂ u k ∂ x j ) {\displaystyle {\begin{aligned}e&={\frac {1}{2}}(I-B^{-1}),\\e_{ij}&={\frac {1}{2}}\left({\frac {\partial u_{i}}{\partial x_{j}}}+{\frac {\partial u_{j}}{\partial x_{i}}}-{\frac {\partial u_{k}}{\partial x_{i}}}{\frac {\partial u_{k}}{\partial x_{j}}}\right)\end{aligned}}}