13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 0 0 0 , L L L L L ε − Δ = = (1) where L0 and L are the equilibrium and the current length of the entire systems in the direction of loading. Under uniaxial tension, the transverse repositioning of nanostructures perpendicular to the load axis gives a measure of the strain from which the Poisson’s ratio can be calculated T L d d ε ν ε =− , (2) where Tε and Lε are the transverse and longitudinal strains, respectively. 3. Results and Discussions A uniaxial tension test has been carried out to study the mechanical properties of fullerene 3D foam-like nanostructures formed by CNTs and fullerene. The basic 3D bcc-lattice nano-truss unit is displayed in Figure 1d. Figure 2 shows the stress-strain evolutions of 3D nanotruss with CNT length N varying from 0 to 10. Except for small N where linear elasticity was observed, the 3D nanostructures exhibits nonlinear elasticity and the CNT length plays a key role on the mechanical properties. The dense 3D network (small N) has more constraints that limit the realignment and bending of CNTs, and results in a linear behavior. A nonlinear up-swing behavior appears in the sparse network due to the increase of CNTs length, as in Figure 2. Such nonlinear stiffening behavior results from the different deformation mechanisms at different strain level, such as realignment and bending dominated deformation at smaller strain, and the stretching dominated deformation at larger strain. Subsequent sharp stress load drop reveals a permanent failure due to bond-breaking at the junction for all tests. An in-depth investigation on the Poisson’s ratio is then performed. Figure 3 presents the calculated strain-dependent Poisson’s ratio of the 3D networks. It can be seen that the CNTs length dominates the Poisson’s ratio of the 3D networks. With N = 0, the 3D covalent pack of fullerenes exhibits a negative Poisson’s ratio. The negative Poisson’s ratio of 3D network is attributed to a reduction of absolute curvature in fullerene hub due to high constraint from direct covalent connection between fullerenes without participation of CNTs. The linkage of CNTs would actively involve the bending and stretching dominated deformation, and causes a different stress concentration on the fullerene Figure 2 Stress–strain curves of 3D nano-truss networks with N varying from 0 to 10 under uniaxial tension. Figure 3 Deformation dependence of the ratio of the transverse and longitudinal strains for the 3D networks under uniaxial tension.
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