Non-Eulerian behavior of graphitic materials under compression

Abstract

The mechanical behavior of graphitic materials is greatly affected by the\nweak interlayer bonding with van der Waals forces for a range of thickness from\nnano to macroscale. Herein, we present a comprehensive study of the effect of\nlayer thickness on the compression behavior of graphitic materials such as\ngraphene which are fully embedded in polymer matrices. Raman Spectroscopy was\nemployed to identify experimentally the critical strain to failure of the\ngraphitic specimens. The most striking finding is that, contrary to what would\nbe expected from Eulerian mechanics, the critical compressive strain to failure\ndecreases with increase of flake thickness. This is due to the layered\nstructure of the material and in particular the weak cohesive forces that hold\nthe layers together. The plate phenomenology breaks down for the case of\nmulti-layer graphene, which can be approached as discrete single layers weakly\nbonded to each other. This behavior is modelled here by considering the\ninterlayer bonding (van der Waals forces) as springs in series, and very good\nagreement was found between theory and experiment. Finally, it will be shown\nthat in the post failure regime multi-layer graphenes exhibit negative\nstiffness and thus behave as mechanical metamaterials.\n