Characterization of the tool/fabric and fabric/fabric friction for woven fabrics: Static and dynamic

This research characterizes the static and dynamic coefficients of friction for TwintexRTM commingled glass-polypropylene balanced plain-weave and unbalanced twill-weave fabrics at the tool/fabric and fabric/fabric interfaces during the composite thermostamping process. The effects of fabric orientation, fabric shearing, fabric and tool temperatures, fabric velocity, and normal pressure on the coefficients of friction between the metal tool and the fabric and between adjacent layers of fabrics are studied individually. The parameters investigated correspond to a range of thermostamping process parameters. Based on the individual parameter investigations, velocity and normal pressure were found to have the greatest effect on the coefficients of friction. A phenomenological friction model was developed based on the experimental results and implemented into the commercial finite element codes ABAQUS/Explicit and LS-DYNA via user-defined subroutines. The friction subroutines were validated with a finite element model of the experimental friction test, where the numerically obtained coefficients of friction correlated well with the experimentally obtained coefficients of friction at various combinations of velocity and normal pressure. The fabric-friction model was then used in conjunction with a user-defined material model in a finite element model of the thermostamping process. The static coefficient of friction was determined to be insignificant to the outcome of the finite element model results relative to the dynamic coefficient of friction. Agreement was found between ABAQUS/Explicit and LS-DYNA when comparing the punch forces as a function of punch displacement for a hemispherical dome. The effect of varying friction at the tool/fabric and fabric/fabric interfaces on the punch forces, fabric stresses, and the shearing of the fabric were investigated and justified the need for a varying friction model to predict part quality accurately and to design the stamping process to minimize the occurrence of fabric defects such as tearing and/or wrinkling.