| Author: | Wong, Ting Yui |
| Title: | Nano-engineered polymer composite for structural health monitoring in fiber-reinforced polymer structures |
| Advisors: | Yu, Tao (CEE) Zou, Fangxin (AAE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xviii, 196 pages : color illustrations |
| Language: | English |
| Abstract: | Fiber reinforced polymer (FRP) composites have found their structural applications in many sectors like aerospace, automotive, marine, and construction because of their high strength-to-weight ratio, good resistance to corrosion and fatigue, and flexibility in design. Durable and cost-effective monitoring techniques are required to help gain long-term observations of structural deterioration and provide solutions for structural health monitoring (SHM). Self-sensing polymer nanocomposites are alternative SHM methodology with attractive advantages including inherently integrated nature with trivial weight penalty, great potential of multifunctionality, and capability to detect matrix-dominated damages. Despite the excellence and great potential shown in using self-sensing nanocomposites for SHM in FRP composites, more work is required to promote the application of this methodology in real-life industrial structures. The aim of this PhD study is to develop SHM methodology for FRP structures using nano-engineered polymer composites through a mechanistic study on the piezoresistivity of carbon nanotube (CNT)/epoxy nanocomposites and investigations into the application of CNT/epoxy nanocomposites for self-sensing strain and damage in FRP structures. This study started with investigation on the effect of curing conditions on the piezoresistivity of CNT/epoxy nanocomposites under tensile loading. CNT/epoxy nanocomposite samples were fabricated using different curing cycles, including room temperature curing followed by post-curing and direct heat curing at temperatures ranging from 40 to 100 °C. Quasi-static tensile tests with simultaneous electrical resistance measurements revealed that the piezoresistivity of the nanocomposites is significantly affected by the curing conditions. Samples cured at room temperature exhibited non-monotonic piezoresistive behavior, with an inversion in piezoresistivity occurring at critical strains. In contrast, samples cured at elevated temperature demonstrated monotonic piezoresistive behavior until failure. The monotony of the piezoresistive behavior and the critical strain were found to be independent of the nanofiller content. The microstructural origin of the nonmonotonic piezoresistive behavior in the nanocomposites was investigated using coarse-grained molecular dynamics (CGMD) simulations. The experimental results of the piezoresistive behavior of CNT/epoxy nanocomposites transitioning from nonmonotonic to monotonic with increasing curing temperature were reproduced by the CGMD simulations. Analysis revealed that the inter-nanofiller junction geometry governs the monotony of the piezoresistive behavior. Mechanistically, monotonically increasing resistance responses under tension can be achieved by promoting active diffusion that causes van der Waals force-driven barrier crossing of nanofillers (resulting in direct contact between nanofillers, e.g., at elevated curing temperatures) during curing; thus, during deformation, nanofillers primarily move away from one another. Conversely, suppressing diffusion during curing causes barrier crossing of nanofillers, which results in resistance reduction, under deformation owing to stress-driven local rearrangement of polymer molecules in heterogeneous shear transformation zones. The molecular structure of the polymer matrix also influenced nanofiller diffusion, modulating the inter-nanofiller junction geometry and piezoresistive behavior. The mechanistic insights provided by this study can guide the design of next-generation, advanced strain-sensing materials in the future. Then, the piezoresistive behaviors of glass fiber reinforced polymer (GFRP) composite laminates and tubes using CNT-dispersed epoxy as matrix were investigated, focusing on the effect of fiber orientation. CNT/GFRP laminates and tubes with different fiber orientations were prepared using a wet lay-up process and a filament winding process, respectively. The results revealed that the piezoresistive behavior of CNT/GFRP composites is significantly affected by fiber orientation. Laminates with fibers oriented nearly perpendicular to the tensile direction exhibited a monotonic increase in resistance with strain, while those with off-axis fibers, i.e. oriented around 45°, showed a nonmonotonic response with a relatively low critical strain. Laminates with fibers aligned nearly parallel to the tensile axis demonstrated a nonmonotonic response with higher critical strains. Similarly, tubes with fibers oriented parallel or perpendicular to the tensile direction displayed monotonic piezoresistivity, whereas those with off-axis fibers exhibited a nonmonotonic response. The microstructural origin of the fiber orientation dependent piezoresistive behavior in CNT/GFRP composites was investigated using a multiscale finite element analysis (FEA) based electromechanical framework. The model included microscale for unit cells of yarn and matrix, and mesoscale for geometric pattern of yarn and matrix, involving both mechanical and electrical domains. Digital image correlation (DIC) and FEA revealed that the strain distribution heterogeneity varies with fiber orientation, and localized transverse compression may lead to the inversion in piezoresistivity. The means of tunneling distance provided microscopic insights into the changes in the CNT network during the transition from positive to negative piezoresistivity, which was accompanied by increase and then decrease in tunneling distance between CNTs, for composites with off-axis fibers. The monotonic or nearly monotonic positive piezoresistive behaviors of composites with parallel or perpendicular fibers were attributed to the increase in tunneling distance under CNT separation. The study highlighted the intricate relationship between fiber orientation, deformation mechanisms, and CNT network reconfiguration in CNT/GFRP composites under tensile loading, contributing to the design of tailored composites with optimized piezoresistive performance for SHM applications. The nonmonotonic piezoresistive behavior of [±55] GFRP composites, which are widely used in real-world structures, poses challenges for self-sensing. Design of CNT/GFRP composites with desired monotonic piezoresistive behavior using the multiscale FE modeling was attempted with support of experimental validations. The effects of yarn width, matrix Poisson's ratio, and stacking sequence on the piezoresistive response of the composites were explored. Numerical results suggested that narrower yarns and lower matrix Poisson's ratios improve the monotony of the piezoresistive behavior, but these parameters alone may be insufficient for achieving a monotonic response. The inclusion of [0] layer was found to be crucial for obtaining monotonic piezoresistive behavior. Experimental validations using multi-layered composite tubes with different stacking sequences confirmed the effectiveness of the [0] layer in improving the monotony of the piezoresistive response. The [±55/90/0] stacking sequence exhibited the best self-sensing performance with a monotonic piezoresistive behavior, demonstrating its potential for practical sensing applications. The resistance change-stress relationships of the CNT/GFRP composite tubes revealed two distinct stages: an initial smooth increase in resistance associated with elastic deformation, followed by fluctuations and step increases indicating damage initiation and propagation. This study highlighted the importance of fiber orientation and stacking sequence in tailoring the piezoresistive behavior of CNT/GFRP composites for SHM applications. In summary, this PhD study has performed in-depth investigations on the piezoresistive properties of CNT/epoxy nanocomposites and the development of self-sensing CNT/GFRP composite structures. The study contributes to the development of advanced composites with optimized piezoresistive performance for SHM applications. |
| Rights: | All rights reserved |
| Access: | open access |
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