Impact of conductive-filler content on the static sensing performance of spray-coated flexible strain sensors for steel-bridge strain monitoring

Abstract Flexible strain sensors fabricated using spray coating enable efficient and reliable strain monitoring of steel bridges. This study aimed to develop a flexible strain sensor with an extremely low detection limit, high linearity, and high sensitivity. The sensing layer materials were prepared by simply mixing silver-coated copper powder particles and water-based acrylic emulsion. Commercial moisture-proof insulating paint for printed circuit boards was used as the insulating layer, and modified polyurethane resin was used as the protective layer. Sensors were fabricated using the spray coating process, and the influence of the conductive-filler content on their static sensing performance was investigated. The results showed that the sensor exhibited the highest sensitivity near the percolation threshold, with a gauge factor of 272.54 and minimum strain detection limit of 0.000 98% but suffered from high resistance relaxation and hysteresis. At higher conductive-filler contents, the resistance relaxation and hysteresis decreased; however, the sensitivity decreased, and the minimum detection limit of strain increased. The linear correlation coefficient of the sensor first increased and then decreased with increasing filler content; however, all values were above 0.940. A comparison of the conductivity and sensing performance revealed that the performance changes were significant near the percolation threshold and stabilized away from it, whereas the linearity coincided with the conductivity changes. This indicated that the sensor performance was closely related to the initial conductive network state of the sensing layer.