The mechanical behaviour of graphene flakes under both tension and compression is examined using a cantilever-beam arrangement. Two different sets of samples were employed involving flakes just supported on a plastic bar but also embedded within the plastic substrate. By monitoring the shift of the 2D Raman line with strain, information on the stress transfer efficiency as a function of stress sign and monolayer support were obtained. In tension, the embedded flake seems to sustain strains up to 1.3%, whereas in compression there is an indication of flake buckling at about 0.7% strain. The retainment of such a high critical buckling strain confirms the relative high flexural rigidity of the embedded monolayer. 1The mechanical strength and stiffness of crystalline materials are normally governed by the strength and stiffness of their interatomic bonds. In brittle materials, defects present at the microscale are responsible for the severe reduction of tensile strengths from those predicted theoretically. However, as the loaded volume of a given brittle material is reduced and the number of microscopic defects diminishes, the material strength approaches the intrinsicmolecular-strength. This effect was first described by Griffith in 1921 [1] and the best manifestation of its validity is the manufacture and use of thin glass and carbon fibres that nowadays reinforce a whole variety of commercial plastic products such as sports goods, boats, aircrafts, etc.With reference to material stiffness, the presence of defects plays a minor role and is rather the degree of order and molecular orientation that provide the amount of stiffness along a given axis. In other words, in order to exploit the high stiffness in crystals, the stress direction should coincide with the eigenvector of a given bond. [2] Pure stretching of covalent or ionic bonds is normally responsible for high material stiffness whereas bending or twisting provides high compliance. This is why commercial-amorphous-polymers are compliant materials since an external stress is mainly consumed in the unfolding of entropic macromolecular chains rather than stretching of individual bonds. [2] Graphene is a two-dimensional crystal, consisting of hexagonally-arranged covalently bonded carbon atoms and is the template for one dimensional CNTs, three dimensional graphite, and also of important commercial products, such as polycrystalline carbon fibres (CF). As a single, virtually defect-free crystal, graphene is predicted to have an intrinsic tensile strength higher than any other known materials [3] and tensile stiffness similar to graphite. [4] Recent experiments [4] , have confirmed the extreme tensile strength of graphene of 130 GPa and the similar in-plane Young's modulus of graphene and graphite, of about 1TPa. [4] One way to assess how effective a material is in the uptake of applied stress or strain 2 along a given axis is to probe the variation of phonon frequencies upon loading. Raman spectroscopy has proven very successful in monitoring phonons of a whole ...

Central to most applications involving monolayer graphenes is its mechanical response under various stress states. To date most of the work reported is of theoretical nature and refers to tension and compression loading of model graphenes. Most of the experimental work is indeed limited to the bending of single flakes in air and the stretching of flakes up to typically approximately 1% using plastic substrates. Recently we have shown that by employing a cantilever beam we can subject single graphenes to various degrees of axial compression. Here we extend this work much further by measuring in detail both stress uptake and compression buckling strain in single flakes of different geometries. In all cases the mechanical response is monitored by simultaneous Raman measurements through the shift of either the G or 2D phonons of graphene. Despite the infinitely small thickness of the monolayers, the results show that graphenes embedded in plastic beams exhibit remarkable compression buckling strains. For large length (l)-to-width (w) ratios (> or =0.2) the buckling strain is of the order of -0.5% to -0.6%. However, for l/w < 0.2 no failure is observed for strains even higher than -1%. Calculations based on classical Euler analysis show that the buckling strain enhancement provided by the polymer lateral support is more than 6 orders of magnitude compared to that of suspended graphene in air.

We present a systematic experimental and theoretical study of the two-phonon (2D) Raman scattering in graphene under uniaxial tension. The external perturbation unveils that the 2D mode excited with 785 nm has a complex line-shape mainly due to the contribution of two distinct double resonance scattering processes (inner and outer) in the Raman signal. The splitting depends on the direction of the applied strain and the polarization of the incident light. The results give new insight into the nature of the 2D band and have significant implications for the use of graphene as reinforcement in composites since the 2D mode is crucial to assess how effectively graphene uptakes an applied stress or strain.