Aspect Ratio–Dependent Percolation and Strain-Sensing Behavior of Polyurethane and MWCNT Nanocomposites for Real-Time Health Monitoring of Seismic Isolation Bearings

Laminated polyurethane elastomeric bearings (LPEBs) in bridges are susceptible to interlayer delamination and other hidden damage given large deformations or fatigue loading, compromising long-term safety. To address this, this study developed a strain-sensing system based on polyurethane/multiwalled carbon nanotube (PU/MWCNT) nanocomposites for real-time LPEB health monitoring. By controlling the MWCNT aspect ratio, this research systematically investigated its effects on percolation behavior, network stability, and piezoresistive response. The results show that short-aspect-ratio MWCNTs enhance dispersion homogeneity and strain sensitivity, forming superior conductive networks in PU/8 wt% S-MWCNT composites. Given tensile deformation, resistance changes caused by MWCNT network disruption/reconstruction accurately map strain, enabling real-time LPEB deformation monitoring. During biaxial shear loading, sensors exhibit characteristic dual-peak resistance responses (P2≥P1). The P1 peak corresponds to instantaneous shear deformation, whereas P2 peak amplification stems from residual strain accumulation. The sensor maintains stable resistance in complex conditions, including variable frequency, strain, compressive pressure, temperature, thermal oxidative aging, and fatigue, demonstrating reliable LPEB monitoring capability. This work not only provides a novel approach for LPEB health monitoring but also offers theoretical guidance for optimizing flexible nanocomposite sensors through elucidated MWCNT aspect ratio-resistance response mechanisms.