Inter-Ply Friction Measurement of Out-of-Autoclave Thermosetting Prepreg Composites

In thermoforming of composite materials, different layers of prepreg slide on each other to attain prescribed shape of the tool. To avoid the formation of wrinkles in the final product, inter-ply friction should be as low as... [ view full abstract ]

In thermoforming of composite materials, different layers of prepreg slide on each other to attain prescribed shape of the tool. To avoid the formation of wrinkles in the final product, inter-ply friction should be as low as possible and it can be made possible by varying processing conditions like temperature, normal pressure and forming rate. This paper 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 MTS tensile testing machine which can provide different pulling rates and measure the frictional resistance between prepreg layers. The whole experimental set up can simulate actual processing conditions of thermoforming, especially hot drape forming. It was found in the study that a yield shear stress level is associated with each set of external processing conditions which has to be overcome to begin slipping mechanism. Further continuing in the slipping mechanism requires a little less force than the initial yield shear force. The results show that temperature is the most influencing external processing parameter as the coefficient of friction decreases significantly with little increase in temperature; which is in agreement with the fact that viscosity decreases exponentially with increase in temperature. Forming rate which is simulated by pulling velocity in experimental set up has lesser effect on the inter-ply friction. Larger friction coefficient was found at higher pulling velocities and vice versa. With the increase in normal pressure, the coefficient of friction decreases. 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. Dependence of frictional resistance on pulling velocity shows that it is hydrodynamic in nature and it is coulomb friction as the frictional force reaches a maximum value i.e. yield stress value and then gradually decreases and attains a constant value.