Rate-Dependent, Large-Displacement Deformation of Vertically Aligned Carbon Nanotube Arrays

Rate dependent mechanical response of the vertically aligned carbon nanotube arrays (VA-CNTs) has been examined with large-displacement indentation tests. The VA-CNTs are observed to exhibit elastic deformation at small displacement and then plastic deformation at large displacement. Under the cylindrical, flat-ended punch, the nanotube arrays collapse plastically at positions of immediately beneath the indenter face. The plastic zone remains stable at large displacement, because the stress/strain field under a flat cylindrical punch is relatively constant. From the normalized indentation stress-displacement curve, the critical indentation pressure (Pm), a measure of collapsing stress of the CNT arrays, is obtained. The speeds of the indenter have been varied, from 0.5 to 4 μm/s. The large displacement deformation is influenced by the effective strain rate of the material. The critical indentation pressure increases with the increase with the strain rates.