Probing Intrinsic Elastic Properties of Multilayer Graphene -- a New Mechanical Constant
We present measurements on in-plane Young's modulus and the Grüneisen parameter of multilayer graphene with varying number of layers, obtained through {\it in situ} bulge tests. Accurate determination of their elastic parameters poses a significant experimental challenge due to the substantial differences in mechanical behavior between intra- and inter-layers. To address this, we develop a novel theoretical model with first-principles calculations to investigate thickness-dependent incomplete strain transfer between the layers. Our findings show that the experimentally measured elastic constants, which deviate from computed intrinsic values, fail to fully capture ideal mechanical couplings between layers. As a solution, we propose a new mechanical modulus that integrates the Grüneisen parameter and in-plane Young's modulus, providing a more reliable representation of their mechanical properties, independent of unavoidable interlayer effects.