This paper concentrates on conductive composites with conductive fillers and their use for strain sensing based on piezoresistivity. Percolation curves of resistivity were summarized and validated as the basic for the design of such conductive composites-made piezoresistivity-based strain sensors, with percolation zoon and percolation slope as the key. A detailed comparison between cement-based and resin-based conductive composites with fibrous and particulate fillers were made and discussed in this work for further understanding the design of such sensors based on percolation curves. Magnetic field was introduced as an extrinsic factor for the design of the percolation curves and further for the design of such sensors’ piezoresistivity curves, which was proved to be quite an effective way. Concluded speaking, when making more full use of the designability of percolation curves, we can make better strain sensors used for structural health monitoring.
This paper investigates properties about electrical resistivity and piezoresistivity of multi-wall carbon nanotubes (MWCNTs)-filled epoxy-based composite and its further use for strain sensing. The MWCNTs dispersed epoxy resin, using MWCNTs in the amount of 1.5~3.0 vol.%, was first prepared by combined high-speed stirring and sonication methods. Then, the MWCNTs dispersed epoxy resin was cast into an aluminum mold to form specimens measuring 10×10×36 mm. After curing, DC electrical resistance measurements were performed along the longitudinal axis using the four-probe method, in which copper nets served as electrical contacts. The percolation threshold zone of resistivity was got as MWCNTs in the amount of 2.00–2.50 vol.%. Further compressive testing of these specimens was conducted with four-probe method for resistance measurements at the same time. Testing results show that the electrical resistivity of the composites changes with the strain’s development, namely piezoresistivity. While for practical strain sensing use, signals of electric resistance and current in the acquisition circuits were both studied. Results show that the signal of current, compared with that of resistance, had better linear relationship with the compressive strain, better stability and longer effective section to reflect the whole deformation process of the specimens under pressure. Further works about the effects of low magnetic field on the electrical resistivity and piezoresistivity of Ni-CNTs filled epoxy-based composites were presented briefly at the end of the paper.