Colloquia

Summary

The over-braiding process is manufacturing process for the production of tubular preforms of the composite parts. Structural design of braid reinforced products requires prior knowledge of the fiber distribution in terms of the braiding angles and fiber density. Process simulations are useful to predict this fiber distribution for complex parts. A simulation model has been developed for biaxial over-braiding process. This model requires the coefficient of friction as input data. Therefore, the aim of this research is to study the frictional behavior of the dry carbon yarns and validate the single contact point model against the experimental data. There are three different parameters which might influence the frictional behavior of the yarns: the sliding velocity, the angle between yarns and the pre-tension on the carbon yarns. To achieve the influence of these parameters on the frictional behavior, an experimental setup is designed and validated, then the data was collected, and finally the data was analyzed and compared to the developed model. From the experiments, it can be concluded that the coefficient of friction is independent of the velocity, the same trend is found in the pre-tension. However, the coefficients of friction are dependent on the inner-angle. For the large inner-angle, the relation in these experiments is found to be close to the Coulombs law instead of the Hertz contact equations. This is due to the spreading of the bundle with an increasing normal force, which results in more contact points. This is not the case for small inner-angles. The small angles restricts the yarns from spreading, resulting in a more predictable behavior. The coefficient of friction did increase with a decrease of inner-angle. It is concluded that this is the result of the change of contact area. The comparison of the experimental results with the model showed a deviation of 10 % for the normal forces between the results and the model. The conducted data was also analyzed using force dependent model, the Howell friction. The results show that the exponent of the Howell model used for fitting on the force dependent data of small angles approximates much higher than 1/3 indicating a dependence on the number of fibers. So the friction behavior cannot be approximated with a cylinder on cylinder contact, but the number of fibers should be taken into account. Validations of the new model show a better agreement with experimental data. However, the predictions for small angles is not yet accurate as the model is not able to capture inner-angle dependence. Within this thesis, recommendations are given for the continuation of this study.