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Normal deviatoric solid stress components, \( {\bf S}_s\), are set to zero in tangential directions:

\begin{align} {\bf t_1} \cdot {\bf S_{s}} \cdot {\bf n} = 0 \\ {\bf t_2} \cdot {\bf S_{s}} \cdot {\bf n} = 0\end{align} \(\bf n\) is the boundary normal, \(\bf t_1\) and \(\bf t_2\) are the associated tangentials, i.e.

\begin{align} {\bf t_1} \cdot {\bf n} = 0 \\ {\bf t_2} \cdot {\bf n} = 0\end{align}

with

\begin{align} \left\Vert{\bf t_1}\right\Vert = \left\Vert{\bf t_2}\right\Vert = \left\Vert{\bf n}\right\Vert = 1\end{align}