I'm researching CNTs and am currently describing Graphene as I lead from carbon atoms to nano-tubes. I've come accross this: "The force-displacement behavior is interpreted within a framework of nonlinear elastic stress-strain response, and yields second- and third-order elastic stiffnesses of 340 newtons per meter (N m–1) and –690 Nm–1, respectively." http://www.sciencemag.org/content/321/5887/385.full The text says that the the "Elastic Stiffness" = Young's Modulus, but what about second- and third-order elastic stiffnesses? I know elasticity is a materials ability to return to its previous shape, and stiffness is resistance to deformation or strain. Thank you for your help.