Often it is convenient to decompose the stress tensor into volumetric and
deviatoric (distortional) parts. Applications of such decompositions can be
found in metal plasticity, soil mechanics, and biomechanics.

Decomposition of the Cauchy stress Edit
The Cauchy stress can be additively decomposed as

${\boldsymbol {\sigma }}=\mathbf {s} -p~{\boldsymbol {\mathit {1}}}$ where $\mathbf {s}$ is the deviatoric stress and $p$ is the pressure and

${\begin{aligned}p&=-{\frac {1}{3}}~{\text{tr}}({\boldsymbol {\sigma }})=-{\frac {1}{3}}~{\boldsymbol {\sigma }}:{\boldsymbol {\mathit {1}}}\\\mathbf {s} &={\boldsymbol {\sigma }}+p~{\boldsymbol {\mathit {1}}}~;~~\mathbf {s} :{\boldsymbol {\mathit {1}}}={\text{tr}}(\mathbf {s} )=0\end{aligned}}$ In index notation,

${\begin{aligned}p&=-{\frac {1}{3}}~\sigma _{ii}\\s_{ij}&=\sigma _{ij}-{\frac {1}{3}}~\sigma _{kk}~\delta _{ij}\end{aligned}}$ Decomposition of the 2nd P-K stress Edit
The second Piola-Kirchhoff stress can be decomposed into volumetric and
distortional parts as

${\boldsymbol {S}}={\boldsymbol {S}}'-p~J~{\boldsymbol {C}}^{-1}$ where

${\begin{aligned}p&=-{\frac {1}{3}}~J^{-1}~{\boldsymbol {S}}:{\boldsymbol {C}}\\{\boldsymbol {S}}'&=J~{\boldsymbol {F}}^{-1}\cdot \mathbf {s} \cdot {\boldsymbol {F}}^{-T}~;~~{\boldsymbol {S}}':{\boldsymbol {C}}&=0\end{aligned}}$