Conventional fiber Bragg grating (FBG) sensors used for tensile monitoring have a limited measurement range and therefore cannot cover the entire service stage of prestressed carbon-fiber-reinforced polymer (CFRP)–strengthened members. In this study, a smart CFRP plate is developed by embedding a wide-range FBG sensor in a prestressed CFRP plate. Based on strain-transfer theory for the grating region, an analytical expression for the average strain-transfer rate is derived and then used to inversely design the groove geometry and bonding parameters; the resulting groove size is 0.5 mm × 0.5 mm. During bonding, a tensile force of 0.3Pc is applied and maintained, where Pc denotes the theoretical ultimate tensile capacity of the plate, so that the embedded FBG remains in tension and potential slack at the bonded interfaces is eliminated. Three CFRP specimens (100 mm × 2 mm, width × thickness) are fabricated, and three rounds of cyclic calibration followed by monotonic tensile tests are conducted. In accordance with national standards for static performance, the sensitivity, hysteresis, linearity, repeatability, and root-sum-square (RSS) combined accuracy are quantified. The measured FBG strain sensitivity agrees well with the theoretical value of 1.21 pm/με. The measurable strain range reaches approximately 12,088.71 με, covering about 90% of the working interval up to the theoretical ultimate load of the member. The static performance indices are as follows: hysteresis ≤ 0.94%, linearity ≤ 0.57%, repeatability ≤ 2.82%, and overall accuracy ≤ 2.95%. The proposed approach markedly extends the monitoring range while maintaining accuracy, enabling continuous strain monitoring of CFRP-strengthened members from normal service to the pre-ultimate stage and meeting engineering requirements for both a wide range and high precision.
Li et al. (Thu,) studied this question.