A new highly sensitive strain measurement method has been developed by applying the change of the electronic conductivity of CNTs. It is reported that most multi-walled carbon nanotubes (MWCNTs) show metallic conductivity and they are rather cheap comparing with single-walled carbon nanotubes (SWCNTs). The effect of the longitudinal axial strain on the band structures of electrons in CNTs was analyzed by applying the abinitio calculation based on the density functional theory. The change of the band structure of a MWCNT under uni-axial strain was analyzed. It was found that the electric conductivity of (MWCNTs) changes drastically because of the large change of their band gap. Therefore, the authors have focused on the possibility of the application of MWCNTs to a highly sensitive strain sensor. Multi-walled CNTs were dispersed in various kinds of resins such as epoxy, polycarbonate, and polyisoprene to form a thin film which can be easily attached to rounded surfaces. The length and diameter of the CNTs were about 5 μm and 50 nm, respectively. One of the base materials of resin employed was polycarbonate and the volumetric concentration of CNT dispersed was about 11.5%. The thickness of the film was about 500 μm. Uni-axial strain was applied to the CNT-dispersed resin by applying a 4 point bending method, and the change of the electric resistance was measured. The range of the applied strain was from −0.025% to 0.025%. The electric resistance changed almost linearly with the applied strain. The ratio of the resistance change under the tensile strain was about 400%/%strain and that under the compressive strain was about 150%/%strain. The CNTs were also dispersed in polyisoprene by about 5%. Uni-axial tesile strain was also applied to the CNT-dispersed rubber. The maximum strain was 240%. It was found that the resistance of the rubber increased monotonically with the increase of the amplitude of the applied strain. The increase rate also increased with the amplitude of the applied strain, and the maximum rate reached about 25%/%strain. Two-dimensional strain fields were evaluated by using finely area-arrayed CNT-dispersed resin made by MEMS technology with spatial resolution of 50 μm.

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