Please cite the article as: W Hu, LK Grunenfelder, T Centea, and S Nutt, “In-situ monitoring of void evolution in unidirectional prepreg”, J Composite Materials (2018) DOI: 10.1177/0021998318759183 In situ monitoring and analysis of void evolution in unidirectional prepreg Wei Hu, Lessa K Grunenfelder, Timotei Centea and Steven R. Nutt* M.C. Gill Composites Center, Department of Chemical Engineering and Materials Science, University of Southern California, 3651 Watt Way VHE-406 Los Angeles, CA 90089-0241, USA * E-mail: nutt@usc.edu Abstract: In the layup of prepreg laminates, air is inevitably entrapped between adjacent prepreg plies, yet the removal of this inter- ply air under vacuum bag cure conditions is not well understood. In this study, an in situ visualization technique was developed to dynamically observe inter-ply air removal during the cure of an out-of-autoclave prepreg. The technique was used to investigate mechanisms of air removal and void evolution in unidirectional prepreg. Prepreg impregnation was also tracked by inspection of laminate cross-sections prepared at different times during the cure cycle. From these data, a three-stage air removal mechanism was documented based on the relationships between void content, resin properties, and tow impregnation as functions of time. Furthermore, a positive correlation was observed between the rate of evacuation and bubble elongation. Though discussed here in the specific context of voids in unidirectional laminates, the in situ observation technique developed for this work has broad potential to enhance understanding of processing phenomena associated with out-of-autoclave prepregs. Key words: Prepreg, porosity/voids, out-of-autoclave processing