Ply-ply friction behavior between unidirectional thermoplastic prepreg plies during thermoforming: Characterization and modeling

The ply-ply friction behavior of unidirectional carbon fiber reinforced polyphenylene sulfide (UD CF/PPS) prepreg in thermoforming was investigated through an improved pull-through testing system. The effects of temperature, normal force and slipping velocity were considered in testing. The results indicated the existence of three key mechanisms underlying slipping: namely the resin shear, fiber–fiber contact and fiber-resin interaction. A three-stage division of the experimental curve was proposed, and several factors were defined to establish a quantitative definition of the ply-ply friction behavior. The results showed that the influence of slipping velocity is particularly evident. Its post-yield stress (12.8 kPa), steady-state CoF (0.118) and residual stress (0.705 kPa) were the highest, exhibiting the largest changes of 957.9 %, 736.9 % and 47.2 %, respectively. This indicated the strong contribution of resin in ply-ply friction. In addition, the Stribeck analysis also suggested the dominance of hydrodynamic lubrication condition in slipping. Based on the experimental results, a simple phenomenological model was proposed based on experimental curves to accurately describe the effects of processing parameters. This work presents a comprehensive characterization of the ply-ply friction behavior of UD CF/PPS prepreg in thermoforming, providing a substantial experimental foundation for the subsequent simulation research.