13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- hub. The Poisson’s ratio is strikingly strain-dependent, sign change from negative to positive when an extensional strain reach around 0.09, which suggests that the transverse strain begins to act in an opposite sense to the longitudinal strain. Many cubic metals when stretched along the specific 110 off-axis direction become auxetic [24]. Accordingly, we conduct tension tests on the bcc-lattice 3D nano-truss network along the <110> orientation and the resulting stress–strain relations with N varying from 0 to 10 are shown in Figure 4. We again see clearly that the CNT length dominates the mechanical properties. The tensile stress, tensile stain and stiffness all decreases with the increase of the segment length N for the case under tension along the <110> direction. Compared with the Figure 3 we observe that both the tensile strain and tensile stress are smaller than those under <100> direction, resulting from a pure stretching of octagon rings in the fullerene hub. For large CNT segment length of nanotruss, the fracture strain, however, is significantly larger than those under <100> direction. Figure 4 Stress–strain curves of 3D nano-truss networks with N varying from 0 to 10 under uniaxial tension. The corresponding Poisson’s ratio is determined and plotted in Figure 5. It can be seen that this bcc-latticed 3D network also exhibits a negative Poisson’s ratio when tension is applied along the <110> direction. Figure 5c and 5d compare the value of 110 110 / ε ε <− > < > − and 001 110 / ε ε < > < > − as a function of CNTs length at a specific small strain 110 ε< > = 0.01. It must be noted that this 3D network yields a very wide range of Poisson’s ratios that are both controlled and tunable by virtue of the microstructural parameter — the length of CNTs. The magnitude of 110 110 / ε ε <− > < > − and 001 110 / ε ε < > < > − roughly covers a wide range from 0.4 to 0.3, and from 0.1 to 1.2, respectively. The spectacular change of 001 110 / ε ε < > < > − appears in this 3D network exceeds 0.5, which is the threshold value for isotropic materials in the theory of elasticity. Interestingly, a dramatic “flip” behaviour of the Poisson’s ratio from negative → positive → negative with increase of CNTs length appears (Figure 5c). The findings can be utilized on tailor-making auxetic CNT-fullerene networked nanostructures with the wanted properties. Two different mechanisms, involving curvature flattening and rigid mechanical model, are identified to be responsible for the “flipping” of Poisson’s ratio.
RkJQdWJsaXNoZXIy MjM0NDE=