Multidirectional Microstrain Sensors from MWCNT/Epoxy Nanocomposites for High-Sensitivity Structural Monitoring

The precise detection of multidirectional microstrains (<100 με) is critical for the early damage assessment of infrastructure. Nevertheless, it remains unresolved by conventional metal strain gauges (GF ≈ 2) or existing nanocomposite sensors, which are typically optimized for large-strain applications. This study introduces a class of multidirectional microstrain sensors fabricated through scalable screen-printing of an optimized multiwalled carbon nanotube/epoxy (MWCNT/EP) composite. With the purpose of overcoming the limitations of conventional gauges and previous nanocomposites, three architectures (separated-L, tridirectional, and circular) were designed and systematically evaluated to achieve angularly resolved strain detection with mechanical decoupling. All types of sensors demonstrated a marked 3.6–4.5× sensitivity increase within the target microstrain range (GF up to 32.23) compared to their performance at higher strain levels, which is attributed to a tunneling-dominated transport mechanism. While the separated-L architecture reached a gauge factor (GF) of 7.97 with exceptional linearity (R2 = 0.998), the tridirectional configuration exhibited clear directional discrimination (GF = 7.65, R2 = 0.987). The circular architecture further curtailed measurement anisotropy, contributing to the improved off-axis response through the optimized strain distribution. The nanocomposite itself features high electrical conductivity (75.65 S/m), a low percolation threshold (3.4 wt %), and robust cyclic stability over ten diverse loading cycles. Validation on a steel I-beam under four-point bending revealed a deviation of less than 10% from both theoretical predictions and conventional foil gauge readings, confirming the sensors’ high accuracy. By establishing a performance benchmark for multidirectional microstrain sensing, this study provides a validated design strategy for the deployment of advanced CNT/polymer nanocomposites in high-precision infrastructure health monitoring.