Abstract: In this thesis we investigate the loading of elastic sheets, which are patterned with a square array of circular holes. An elastic buckling instability due to this loading induces a pattern transformation in the material, which breaks symmetries of the material. The pattern transformation is accompanied by a drastic change in the mechanical response, which we investigate through extensive FEM simulations, applying uni-axial as well as bi-axial loading. We find that the pre- and post-buckling response shows similarities to (thick) beam-behaviour, and therefore, we propose to model the material as single effective beams under a load. In the post-buckling phase a negative stiffness is found for certain hole sizes, which results in snap-through buckling behaviour.