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In this article, a strain sensor made from an epoxy-based nanocomposite using a kind of multi-walled carbon nanotubes (MWNTs), i.e., LMWNT-10, was investigated. It was found that the piezoresistivity of this strain sensor is linear and anti-symmetric in tensile and compressive states within a large strain range, which is different from that of the strain sensor using another type of MWNTs, i.e., MWNT-7, in our previous reports. This linear and anti-symmetric piezoresistivity is very suitable for practical sensor applications. In our experiments, both static and dynamic responses of the two sensors were measured. The experimental results indicate that the completely different piezoresistivity characteristics of the two sensors are probably due to the complete different working mechanisms. For the sensor made from LMWNT-10, the present experimental investigations reveal that its working mechanism should be the piezoresistivity of MWNTs due to deformation of MWNTs. On the other hand, for the strain sensor made from MWNT-7, the dominant working mechanism of this sensor is the tunneling effect caused by the distance changes among carbon nanotubes.

To improve the reliability of delamination detection techniques based on Lamb waves for carbon fibre reinforced plastics (CFRP) composite laminates, the influence of excitation frequency of Lamb waves on the intensity of reflected waves from a delamination in cross-ply CFRP delaminated beams was investigated. The A0 Lamb wave mode was chosen although the conclusion in this work is also applicable to S0 mode. A Chebyshev pseudospectral Mindlin plate element proposed by authors was employed to simulate the A 0 mode propagation and the interaction of the A0 mode with the delamination. Numerical results show that the sensitivity of A0 mode to the delamination at different frequencies is remarkably different. Each delamination with different lengths or different locations along the through-thickness direction of the laminates generally has at least one or more optimal excitation frequencies which can generate the strongest reflected wave signals from the delamination. The reason for this phenomenon was also investigated by analyzing the natural vibration of the local upper- and lower-delaminated regions. The optimal excitation frequencies were identified to be close to the resonant frequencies of the local delaminated regions with pure flexural vibration modes similar to the deformation pattern of A0 mode.