Abstract
CFRP pipes are used in spacecrafts to support heavy optical instruments. Low coefficients of thermal expansion and high stiffness of CFRPs improve the performance of the instruments. However, it is known that significant through-thickness strain arises in thick CFRP pipes during the cure process and the operation in low temperature, resulting in delamination failure. This study developed a fiber-optic-based life cycle monitoring system to measure the through-thickness strain development. First, we addressed the mechanism of the strain development by a theoretical approach and finite element analysis (FEA). It was confirmed that geometric constraint arising from cylindrical shapes of pipes causes significant out-of-plane stress in the through-thickness center, and the stress value increases with increase in the thickness and stiffness. A fiber Bragg grating (FBG) sensor was then embedded at the through-thickness center of the pipe during the lay-up process. “Non-axisymmetric strain” change in the FBG sensor was continuously measured using birefringence effect throughout the life cycle including cure process and simulated operation in low temperature environment. It was clearly demonstrated that through-thickness strain and delamination failure can be monitored using the non-axisymmetric strain. FEA and comparison with plate specimen were also conducted, confirming the validity of the experimental results.
