Cauchy stress tensor
In continuum mechanics, the Cauchy stress tensor (symbol σ {\displaystyle {\boldsymbol {\sigma }}} , named after Augustin-Louis Cauchy), also called true stress tensor or simply stress tensor, completely defines the state of stress at a point inside a material in the deformed state, placement, or configuration. The second order tensor consists of nine components σ i j {\displaystyle \sigma _{ij}} and relates a unit-length direction vector e to the traction vector T(e) across a surface perpendicular to e: T ( e ) = e ⋅ σ or T j ( e ) = ∑ i σ i j e i .