In the field of Structural Health Monitoring, there is growing interest in continuous volume measurements of material properties using embedded sensors organized in wireless sensor networks (1,2,3). In this work we propose a novel, highly reproducible piezoresistive sensor based on Carbon Nanotube (CNT) networks deposited on polymer to be used for embedded strain monitoring and crack detection in concrete. We highlight the originality of the fabrication process and describe the modalities for signal conditioning and for integration into a RFID-based wireless sensor network. We have achieved the first reported strain gauge fabricated by direct inkjet printing of CNTs on polymeric substrates, namely Ethylene tetrafluoroethylene (ETFE). The strain gauge we propose is of interest for its flexibility and sensitivity (Gauge Factor) which is higher than that of metallic sensors. For these reasons we believe that the proposed strain gauge is a promising alternative to the rigid metallic strain gauges currently on the market. The method is based on the dispersion of CNTs in a solution of 1,2-dichlorobenzene, with sodium dodecyl benzene sulphonate added as surfactant to enhance the solution’s wettability on the polymer. The solution is used as ink to print resistive layers of randomly oriented CNTs on polymer. We optimized the printing and the rinsing process for maximum control of the resistivity and uniformity of the CNT layers. The sensors show a Gauge Factor (GF) of 2.54 which is superior to commercial metallic strain gauges by 25%. Our technology is characterized by its high reproducibility, with only a 6.8% variation of GF values for different sensors fabricated on ETFE. Tests on multiple loading cycles prove the repeatability of the measurements over several loading cycles. Our results suggest that such devices could be used in real life applications. With this goal in mind, we compare several electronic circuits for signal conditioning (from Wheatstone bridge with amplification chain to Sigma-Delta conditioner) in order to select the more appropriate for embedded sensing in concrete. The application dictates two constraints on the electronics: low power consumption and small dimension. We suggest that a configuration based on RFID (Radio Frequency Identification) is the optimum solution given the imposed constraints (4,5). We propose an architecture that can be exploited in conditioning elements sensitive to other parameters of interested for SHM as humidity and pH sensors (6). Although the sensor has its own intrinsic merits, our project presents a complete system (sensor with conditioning electronics) which is innovative, original and intended for a practical application of great importance to modern societies.
Wireless flexible strain sensor based on Carbon Nanotube piezoresistive networks for embedded measurement of strain in concrete