Viscoelastic and cyclic compaction response of prepregs tested under isothermal temperatures and various compaction speeds

Abstract Viscoelastic compaction/relaxation and cyclic compaction studies of uncured prepregs are of importance to understand the process parameters, such as mold closing forces and speed and the clamping force required to maintain a given thickness during molding, and so forth, for many composites manufacturing processes, including compression molding. In this study, viscoelastic compaction/relaxation, and multi‐cycle‐compaction tests were performed on uncured glass/epoxy prepreg specimens at elevated temperatures and variable compaction speeds up to a target fiber volume fraction of 0.65. At low temperatures, the relaxation process was found to be less sensitive to compaction speed. At lower compaction speeds (e.g., 0.1 mm/min) and elevated temperatures, such as 90°C, higher maximum stresses, and resin bleed out were observed as compared to a relatively faster compaction speed of 1 mm/min. Specimens tested under multi‐cyclic‐compaction exhibited a lower final thickness, owing to cyclic hysteresis, which caused increased fiber nesting compared to that observed in stress relaxation tests. As a result, the percentage of permanent deformation is higher in the former case than in the viscoelastic compaction/relaxation tests. An investigation of the microstructure through X‐ray computed tomography aided voxel models of the cured specimens confirmed nesting of the fiber tows. Moreover, at a compaction speed of 0.1 mm/min, a decrease in thickness and an elongation of void sizes were also observed as the temperature increased.