歪tensor

$ \pmb{\varepsilon}:=\frac{1}{2}\left(\pmb{\nabla u}+\pmb{\nabla u}^\top\right)

$ \gdef\pdef#1#2{\frac{\partial #1}{\partial #2}} [\pmb{\varepsilon}]^\mathcal{SS}=\begin{pmatrix}\pdef{u_x}{x}&\frac{1}{2}\left(\pdef{u_x}{y}+\pdef{u_y}{x}\right)&\frac{1}{2}\left(\pdef{u_x}{z}+\pdef{u_z}{x}\right)\\\frac{1}{2}\left(\pdef{u_x}{y}+\pdef{u_y}{x}\right)&\pdef{u_y}{y}&\frac{1}{2}\left(\pdef{u_y}{z}+\pdef{u_z}{y}\right)\\\frac{1}{2}\left(\pdef{u_x}{z}+\pdef{u_z}{x}\right)&\frac{1}{2}\left(\pdef{u_y}{z}+\pdef{u_z}{y}\right)&\pdef{u_z}{z}\end{pmatrix}