Materials science

Viscoelasticity

Molding (decorative)

Volume fraction

Thermodynamics

Physics

Composite material

Compaction

Isothermal process

Polymer Composites

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.

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