Multipoint cure monitoring of temperature and strain of carbon fibre-reinforced plastic shafts using fibre Bragg grating sensors

ABSTRACT Carbon fibre-reinforced plastic (CFRP) shafts feature lightweight, high intensity, good dimensional stability and high drive efficiency. However, built-in stress formed during curing process may cause defects in the product before actual use. In this paper, fibre Bragg grating (FBG) sensors were adopted for real-time temperature and strain monitoring of CFRP shafts while curing. A 22-layer CFRP shaft served as the test specimen and embedding positions of FBG sensors were decided based on its design. In order to bury all four FBG sensors (FBGa, FBGb, FBG1 and FBG2) into the CFRP shaft, a separating mandrel was prepared. FBGa and FBGb were sensitive to both temperature and strain, while FBG1 and FBG2 were encapsulated and only sensitive to temperature. FBGa and FBG1 were buried between the first and second layers, while FBGb and FBG2 were between the ninth and tenth layers. Sensitivity coefficient of each FBG sensor was calibrated: stress sensitivity coefficients of FBGa and FBGb were 1.33 × 10−5 and 1.37 × 10−5 pm/Pa; temperature sensitivity coefficients of FBG1 and FBG2 were 8.58 and 8.52 pm/℃. Finally, cure monitoring of CFRP shaft using FBG sensors was carried out and results showed: a 57.8 με residual strain due to axial contraction was detected between the first and second layers of CFRP shaft (when cooled to 85℃), but that between the ninth and tenth layers was nearly zero (when cooled to 55℃); residual strains of the inner surface and middle layers were different during curing process when specimen’s diameter–thickness ratio was 28.4; it was reliable to have FBG sensors embedded into CFRP shaft using a separating mandrel; FBG sensors can precisely detect the changes of temperature and strain inside the CFRP shaft during curing process.