Surface instabilities during straining of anisotropic materials

The development of instabilities in traction-free surfaces is investigated numerically using a unit cell model. Full finite strain analyses are conducted using isotropic as well as anisotropic yield criteria and both plane strain tension and compression are considered. In the load range of tension investigated, it is found that isotropic plasticity can only predict surface instabilities if non-associated plastic flow is accounted for. However, for anisotropic plasticity a surface instability is observed for associated plastic flow if the principal axes of anisotropy coincide with the directions of principal overall strain. For other orientations, surface instabilities are seen when non-associated plastic flow is taken into account. Compared to tension, smaller compressive deformations are needed in order to initiate a surface instability.