Aspects of Prepreg Layer Friction on Defect Formation on Thick-Walled Composite Manufacturing

Abstract

As composite manufacturing processes evolve and composite structures become ever more complex the role of geometry and built-in geometric features increase the role of ply movement during mold closure and compaction. With a greater amount of constraints and ply movement, the friction that develops between the prepreg plies and the tooling surfaces and/or between one ply to another become a significant force which is a factor in formation of defects such as ply wrinkles. As such a greater understanding of ply friction and its implementation into process modeling is justified. In this work Hexcel IM7/8552 8HS, is characterized for ply friction using a custom characterization rig. The rig compresses a layer of prepreg onto a tooling surface with a prescribed pressure and heats the prepreg layer in accordance to an autoclave heating cycle while pulling the tooling surface out in a testing frame capable of measuring the required force to do so. Current simulation tools for modeling the autoclave process in order to predict outcomes such as a structures spring-in due to manufacturing utilize only simple models for this frictional behavior. This work uses an approach with currently available tools to implement experimentally determined ply friction data for modeling the manufacturing process using user subroutines. Results of simulation predicts that compression along the fiber direction during the compaction stage of the autoclave cycle is concentrated in the radius region which is experimentally shown to cause ply wrinkling. In conclusion this work will present various aspects of ply friction characterization, coupon level experimentation with curved beam samples, and process modeling.