Assessing Reliability of cm-Scale Optical Fiber Strain Sensing in High Gradient Configurations through Benchmarking and Mechanical Modelling
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Distributed strain monitoring via Optical Fiber Sensors (OFS) is widely used in Structural Health Monitoring (SHM) across sectors such as oil and gas, civil engineering, railways, aeronautics, and energy. OFS are immune to electromagnetic interference, provide long-range detection, and their small size allows embedding in materials for internal strain measurement. Structural features like bolted joints produce local stress concentration (LSC), which can initiate localised failures. Cracks and delaminations also weaken structures, with LSC contributing to crack propagation and degradation. Accurately assessing internal defects is essential for SHM of high-value metal, reinforced concrete, and composite structures, making distributed optical sensing a valuable tool. However, LSC leads to high strain gradients (HSG), which cause underestimation, measurement artifacts, and faulty data—issues rarely addressed in current research. This study provides experimental evidence of these effects and benchmarks the performance of three OFS readout techniques—Fiber Bragg Grating (FBG), Optical Frequency-Domain Reflectometry (OFDR), and Frequency-Modulated Continuous Wave (FMCW)—against Finite Element Method (FEM) predictions. A tensile test was conducted on a 2-mm-thick aluminium plate featuring three holes of varying diameters to purposely induce HSG. OFDR data align well with FEM, but correlation fails in HSG zones due to Rayleigh pattern deformation. FMCW demonstrates stability but underestimates strain. FBG data closely match FEM predictions, though HSG chirps the Bragg peak, affecting accuracy. This research provides key insights into accurate strain measurement under HSG and improves understanding of OFS capabilities and limitations in complex structures.