Reduction of self-weight in all structures is a necessity in modern day developments. It is a well-known fact that weight reductions of up to 75% can be achieved with fiber-reinforced polymer composites in comparison to steel parts. Dry fiber placement is an emerging technology that enables outstandingly high mechanical performance of composite parts, as preform structures with load-related fiber orientations can be created with minimum of fiber crimp and undulation. However, the poor impregnation characteristics of dry fiber placement preforms during liquid composite molding processing methods offer a significant drawback. Alternative techniques have been sought, such as the out-of-plane impregnation in vacuum assisted or compression resin transfer molding processes, to reduce the flow length within the preform. So far, this is the first step towards reducing cycle times. Regardless, further improvements are still needed in order to enable the application of dry fiber placement preforming methods in serial production.
Recently, Oliver Rimmel, David May and Peter Mitschang from the Institut für Verbundwerkstoffe GmbH (Institute of Composite Materials) in Kaiserslautern investigated the influence of a stitching step on the out-of-plane permeability of dry fiber placement preforms as well as the mechanical properties of the finished laminate depending on a variety of stitching parameters such as thread tension during stitching, denier of the used thread, seam distance, seam displacement, stitch distance and treated dry fiber placement layup. They hoped that their work will help fill the gap regarding the impregnation behavior of dry fiber placement preforms and the possibilities to influence it which presently has only been superficially studied. Their work is currently published in the research journal, Polymer Composites.
In brief, the research method used commenced with the preform manufacture using standard tools and procedures. Next, the researchers undertook the out-of-plane permeability measurements where, first, they determined the effects of stitching at different fiber volume contents. Secondly, they determined the effect of different input parameters. Lastly, they assessed the influence of the stitch on mechanical properties.
The authors observed that out-of-plane permeability of dry fiber placement preforms were largely increased by inserting flow channels through stitching. In addition, they noted that the examined parameter space, K3 increased linearly with increasing stitching density. It was also seen that the used layup pattern has a large influence on K3 as well. Moreover, the bending tests undertaken showed that the modulus decreased with increasing stitching density.
In summary, Oliver Rimmel and colleagues presented an in-depth examination of the influence of stitching seams on the impregnation behavior of dry fiber placement preforms and the mechanical properties of the laminate. Generally, they showed that the biggest impact on out-of-plane permeability resulted from the achieved stitching density and denier while thread tension and seam displacement showed negligible effects. Altogether, stitching is a promising measure to enhance K3 and thus impregnability of dry fiber placement preforms to a level comparable to conventional textile semi-finished materials. “Stitching of preforms for composite parts is the key to combine maximum lightweight potential with high process efficiency” Oliver Rimmel said in a statement to Advances in Engineering.
Oliver Rimmel, David May, Peter Mitschang. Impact of Stitching on Permeability and Mechanical Properties of Preforms Manufactured by Dry Fiber Placement. Polymer Composites-2018.Go To Polymer Composites