Friction Characterization of UHMWPE Cross-Ply Composite Sheets for Thermoforming Processes

Thermoforming is an attractive process for the low-cost high-volume manufacture of textile-reinforced composite structures with complicated geometries. Tool/ply and ply/ply frictions play critical roles during forming. The friction between the binder ring and the blank induce an in-plane tensile stress that mitigates wrinkling. Unwanted wrinkling can develop across the part if the in-plane stresses are too low but tearing of the material can occur if the applied stresses are too high. Understanding the role that friction plays during thermoforming can give insight on how to mitigate these manufacturing-induced defects in the part. In the current work, the coefficients of friction for two unidirectional cross-ply ultra-high molecular weight polyethylene (UHMWPE) materials are characterized as a function of pressure, fiber orientation, side of material, and pulling rate for [0/90/0/90] cross-ply sheets. The materials are tested at multiple fiber orientations to understand the influence that fiber direction has with respect to the coefficients of friction and on each respective side of the material to understand how surface topology influences the coefficients of friction. The results of the testing are found to correlate with modified Hersey numbers.