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Residual stress and strain occur in CFRP pipes during the manufacturing process, which leads to defects in worst-case scenarios. Therefore, monitoring the state of composites and setting up an appropriate manufacturing process are essential. With the sheet winding method, prepregs are wound around a mandrel with each prepreg having an edge formed at the seam. Two forms of prepreg edges can exist: one where the prepreg edge has a gap, and the other, where the prepreg edge overlaps. This research examined the effects of the forms of the prepreg edges on the cure-induced internal states using a fiber-optic-based method. In the case of the edge involving a gap, strain level peaks at the prepreg edges were observed throughout the curing process. Meanwhile, the sample containing an overlap presented no strain level peaks during cooling. According to the results, it would be preferable to have overlapping present at the prepreg edges. [ABSTRACT FROM AUTHOR]

Vacuum-bag-only curing is an attractive out-of-autoclave method as an alternative to conventional autoclave curing. Previous extensive researches provided great insight into void formation during the vacuum-bag-only method and these findings are reflected in current vacuum-bag-only cure cycles to minimize void content. Cure process can be further improved by taking into consideration cure-induced residual stress/strain. The present paper proposed a residual stress/strain reduction method and evaluated its effectiveness using a commercially available vacuum-bag-only material by fiber-optic-based in-situ strain monitoring and tensile tests. First, cure process monitoring and tensile tests were conducted for the manufacturer’s recommended cure cycle. Cure process monitoring showed that the material vitrifies during post-cure temperature dwell. Furthermore, the tensile test revealed that the vacuum-bag-only material has lower strength than conventional autoclave materials, suggesting the importance of the effect of cure-induced residual stress/strain. Then, two cure cycles were proposed based on the findings from the manufacturer’s recommended cure cycle tests and a cure kinetics model. In the proposed cycles, resin vitrifies at a lower temperature than the manufacturer’s recommended cure cycle, leading to reduced residual stress/strain. Cure process monitoring and tensile test results for the new cycles showed that the residual strain was reduced by 12–18%, and the strength was increased by 26% in the best case. Moreover, void content was not significantly affected by changing the cure cycle. Although vacuum-bag-only material was used in this research, the proposed concept can be widely applied for autoclave cures and other types of vacuum-bag-only processes with slight modification. [ABSTRACT FROM AUTHOR]

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]