Investigation on creation of fibrous rings and their influence on the braided preform quality

During braiding of carbon fiber yarns specific defects may occur, which influence the braiding process stability and the preform quality. One common defect is the fibrous ring, which is a ring-shaped accumulation of carbon filaments on the braiding spool that prohibits the yarn from unwinding. This defect therefore leads to enlarged yarn tension of a single yarn and thus to the creation of a yarn gap, which is a local irregularity in braided preforms. Within this study the origin of fibrous rings and its influence on yarn gap formation is investigated. Before braiding, the carbon fiber yarn has to be rewound from supplier coils onto braiding spools. The effect of rewinding parameters as well as yarn predamage on the creation of fibrous rings were investigated in a test bench. Therefore an artificial yarn predamage was recreated before rewinding the yarn. It was found that an increased yarn predamage has the most significant effect on the creation of fibrous rings and thus on increasing yarn tension. Based on the yarn tension investigations conducted at the test bench, a single spool in the braiding machine was impinged with controlled higher yarn tensions. This method created yarn gaps on purpose during the process. It was detected that increased yarn tension of a single yarn led to larger yarn gaps up to a width of 4 mm.