  This work is devoted to establishing a regularity result for the stress
tensor in quasi-static planar isotropic linearly elastic - perfectly plastic
materials obeying a Drucker-Prager or Mohr-Coulomb yield criterion. Under
suitable assumptions on the data, it is proved that the stress tensor has a
spatial gradient that is locally squared integrable. As a corollary, the usual
measure theoretical flow rule is expressed in a strong form using the
quasi-continuous representative of the stress.
