Impact of Stitching Processes on the Compaction Behavior of Glass Fiber Reinforcements

Abstract

The objective of this project is to investigate the application influences of stitching techniques on the compaction and relaxation behavior of preforms made of three different semi-finished glass fabric types. Particular attention is directed to the impact of stitching parameters (i.e., thread tension and seam spacing) and the compaction procedure (single-stage vs multi-stage) on the experimental results. The experiments prove that there is no difference between single-stage and multi-stage compaction. Furthermore, it is shown that the stitching of the preforms leads to an increase in compaction force due to the additional volume of thread to be compacted. Comparing high and low thread tension reveals that an increasing thread tension causes pre-compaction of the preform, thus reducing the required compaction force, though the degree of pre-compaction seems to be confined to a limit above which no reduction in compaction force can be observed any more. Regarding the spacing of the seams, it turns out that only the narrow spacing (5 mm) has a significant influence on compaction, whereas the wide spacing (20 mm) does not lead to a notable change in compaction behavior compared to the unstitched preforms. Finally, visual inspection of the preforms during compaction and relaxation shows that the morphologic change of the preform surfaces is a very complex phenomenon which cannot easily be modeled. No visual change of the preform surfaces are observed during relaxation. Therefore, it is assumed that relaxation is caused by invisible microstructural reorientation of glass filaments within single rovings.