Thermomechanical performance of pultruded GFRP rods for SHM using in situ fibre optics

Purpose: Assess the thermomechanical performance of pultruded glass-fibre-reinforced polymer (GFRP) rods containing an embedded distributed fibre-optic sensor (DFOS) for structural health monitoring (SHM). Quantify the impact of integrating the sensor on the curing behaviour, thermal stability and mechanical properties, and compare this with nominally identical rods without a sensor. Design/methodology/approach: Two rod variants were pultruded: one containing a sensor (OS-GFRP) and one without (No-OS-GFRP). The epoxy matrix was tuned using rheology, and the DFOS was embedded on axis without a protective coating. Differential scanning calorimetry (DSC) was employed to analyse the glass transition, residual curing, and degradation onset. It was followed by quasi-static tests (tensile, three-point bending and short-beam shear), while Charpy impact tests were used to quantify energy absorption. Microscopic studies (optical and SEM-EDS) examined the quality of the interphase and the presence of defects. Findings: The integration of embedded Distributed Fibre Optic Sensing (DFOS) was achieved without materially compromising the composite’s structural integrity or load-bearing capacity. Thermally, DFOS did not systematically reduce the glass transition temperature or thermal stability. Mechanically, while the axial modulus decreased by approximately 6% and shear properties were modestly lower, the mean tensile strength was maintained, flexural strength increased by ~8%, and impact energy absorption rose by 9% with less scatter. Microscopy revealed that minor local interphase perturbations, such as micro-voids, were consistent with the small penalties to shear properties. Overall, the presence of DFOS enables sensing functionality, with any compromises being minor and predictable, and process tuning can effectively mitigate manufacturing inconsistencies. Research limitations/implications: The observed differences, particularly with regard to shear and scatter, suggest that the interphase is sensitive to both curing kinetics and local defects. Therefore, future work should focus on optimising the curing process around the optical fibre, refining the interphase chemistry, and investigating fatigue, creep, and durability in alkaline and concrete environments. Practical implications: Pultruded GFRP rods with on-axis distributed fibre-optic sensors (DFOS) are a robust, easy-to-install solution for structural health monitoring (SHM) of concrete and civil infrastructure. They provide distributed strain and temperature measurements with minimal impact on structural performance, enabling data-driven maintenance and safety. Originality/value: Demonstrates a production-feasible route to integrate DFOS during pultrusion, links DSC-diagnosed local curing phenomena to small but interpretable mechanical deltas, and evidences that embedded sensing can coexist with near-baseline thermomechanical properties in GFRP rods.