Process improvement for out-of-autoclave prepreg curing supported by in-situ strain monitoring

Abstract

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.

Effects of Processing Parameters for Vacuum-Bagging-Only Method on Shape Conformation of Laminated Composites

“…Hence, excessive and unnecessary corner suppression is avoided. A similar finding is reported by Yijia Ma et al [26]; the CoV results reported in this study are in good agreement to theirs.…”

Effects of processing parameters for vacuum‐bag‐only method on void content and mechanical properties of laminated composites

“…[ 20,21 ] Studies have shown that the MRCC for commercially available prepregs could further improve mechanical properties by considering residual stress/strain induced during curing. [ 21–23 ] Takagaki et al [ 23 ] proposed two new improved cure cycles by examining in situ cure‐induced residual stress/strain of a commercially available prepreg. They demonstrated that after C‐stage, the resin is vitrified and 100% degree of cure (DOC) was achievable.…”

Effects of thermal cycling on phenylethynyl-terminated PMDA-type asymmetric polyimide composites

“…Because T650-35 8HS laminates were cured above T g , during cure, polymer chains were in a rubbery state with high mobility and short relaxation time, allowing relaxation of stress induced by cure shrinkage. [5][6][7] Thus, the internal stress at 371 C was assumed to be zero. The thermal stress at a temperature (T) in a cross-ply region was calculated by integrating equation 2 Numerical calculation of thermal stress in T650-35 8HS/ PMR-15 composites revealed that the thermal stress in the crimped region was 12% greater than in the cross-ply region.…”

“…Thus, it is generally assumed that composites are in a stress-free state during cure above T g . 6,7 After cure, the polymer matrix is vitrified and bonded to the fibers. Coefficients of thermal expansion (CTEs) of polymers generally are much greater than those of carbon fibers.…”