Modeling of CNT alignment state on the strain sensing characteristics of MWCNT-based nanocomposites

The strain sensing characteristics of MWCNT-based nanocomposite sensors are highly dependent on the orientation and distribution of CNTs. Most CNT sensors are neither perfectly aligned nor randomly oriented; they are just the two limiting state of partially aligned sensors. To cover this wider distribution, an anisotropic homogenization model is established to study the axial and transverse multi-field coupled performances of MWCNT-based nanocomposite strain sensors from perfectly aligned to randomly oriented state. The transversely isotropic conductivity and mechanical moduli are utilized as the dual homogenization parameters in the present anisotropic analysis. The effective mechanical moduli are evaluated via the Mori-Tanaka method, while the electrical conductivities are calculated by the effective-medium approximation. The loadingdependent tunneling distance among the partially aligned MWCNTs is established under the axial or transverse loading to assess the contribution of electron tunneling. The predicted axial and transverse strain sensing capacities, including the resistance change ratios and strain sensitivity factors, of partially aligned MWCNT-based nanocomposite sensors are demonstrated and shown to agree with experiments. The axial sensing characteristics of a partially aligned strain sensor is higher than that of a randomly oriented one due to the alignment of MWCNTs. The optimal MWCNT volume fraction of high strain sensing characteristics is determined to be near the percolation threshold. The revealed anisotropic electromechanically coupled behaviors can provide guidance to tailor the axial and transverse sensing characteristics for the general class of partially aligned nanocomposite strain sensors.