Abstract

Thermoforming of composite materials includes deforming the flat sheets of prepreg laminates into complex shaped composite parts. This deformation of flat laminates into complex shapes can cause wrinkles in the final part. External processing conditions of forming process must be chosen in a way to minimize wrinkles. This dissertation describes the experimental technique to measure friction between two plies of thermoset prepreg. A test rig was designed and constructed which can provide different temperatures and different normal pressures to the prepreg sample. This test rig was installed on a tensile testing machine which can provide different pulling rates and measure the frictional resistance between prepreg layers. It was observed that a yield shear stress level associated with each set of external processing conditions exists which has to be overcome to begin the layer slippage. Further continuing the slippage requires less force than the initial yield shear force. The results show that the temperature is the most influencing external processing parameter as the coefficient of friction decreases significantly with increase in temperature. This is due to the resin viscosity reduction. Forming rate which was simulated by pulling velocity in experimental set up has less effect on the interply friction. Larger friction coefficient was found at higher pulling velocities and vice versa. With the increase in normal pressure, the coefficient of friction decreases. By comparing the 8HS, 5HS and UD prepregs, frictional resistance was found to be the largest in unidirectional composites, followed by 8-harness and 5-harness composites respectively. It was observed that change in the fabric (fiber) orientation of the prepreg plies has effect on interply friction. Interestingly, experimental results show that friction mechanism between prepreg plies is mixed friction i.e. it is dominated by coulomb friction as well as hydrodynamic friction. To draw Stribeck curves, Hersey number was calculated and plotted against the coefficient of friction. A friction model was developed based on the linear relationship between friction coefficient and Hersey number. By deriving the relationship between angle θ and the two variables from linear equations, a general equation was developed which could be used to predict coefficient of friction for any set of processing conditions at any fabric (fiber) orientation. A design of experiments technique, Taguchi method, was adopted to find the optimum processing parameters in thermoforming of composites. Analysis of Variance was used to find the contribution of each parameter.