Fiber-optic-based life-cycle monitoring of through-thickness strain in thick CFRP pipes.

CFRP pipes are used in spacecraft to support heavy optical instruments. CFRPs' low coefficients of thermal expansion and high stiffness improve the instrument performance. However, significant through-thickness strain arises in thick CFRP pipes in curing and operation at low temperature, resulting in delamination failure. In this study, we developed a fiber-optic-based life-cycle monitoring system to measure through-thickness strain development. First, we addressed the mechanism of strain development using a theoretical approach and finite element analysis (FEA). We confirmed that geometrical constraint arising from the cylindrical shapes causes significant out-of-plane stress in the through-thickness center, and the stress increases with increased thickness and stiffness. A fiber Bragg grating (FBG) sensor was then embedded at the through-thickness center of a pipe during lay-up. Non-axisymmetric strain change in the FBG sensor was continuously measured using the birefringence effect throughout the life cycle, including curing and simulated operation in a low temperature environment. Through comparison between pipe and plate specimens, it was clearly demonstrated that the system could sensitively capture through-thickness strain development and detect delamination failure in the pipe. In addition, FEA was conducted and the result agreed well with the experiment data, confirming the validity of the experiment results. [ABSTRACT FROM AUTHOR]