Modelling the seam architectures for textile performing
Sewing of textile preforms offers a lot of opportunities for the future production of FRP parts. Although out of plane properties and damage tolerance abilities of the preform improve by stitching, stitching degrades the in plane properties of the preform. The choice of material and process parameters is crucial to realise the benefits of stitching. This demands modelling and simulation of stitched textile structures. Currently simple textile structures like multi layer woven fabrics and multi layer non-crimp fabrics can be modelled. This paper shows a concept for a mathematical modelling of stitching architectures including the generation of the thread surfaces. The model can help in realising two functions. On the one hand different material and process parameters can be taken into account and on the other hand the computed surface description including geometrical dependencies can be transferred into a structural mechanic model (e.g. WiseTex).
On Surface Fibre-Free Zones and Irregularity of Piercing Pattern in Structurally Stitched NCF Preforms
The paper addresses the surface imperfections (openings in fibrous plies) and irregularity of the piercing pattern in structurally stitched multilayer carbon-fibre preforms. Each layer is a multiaxial multiply non-crimp fabric (NCF) with a non-structural stitching. The term “structural” presumes here that the stitching yarn does not only consolidate the plies (as the non-structural one does) but forms also a through-the-thickness (3D) reinforcement. Four stitching methods are studied: ordinary CNC-sewing, tufting, dual-needle, and curved needle techniques with multifilament yarns: aramide (60 or 120 tex) or glass (204 tex). In total, 14 stitching cases are investigated experimentally. It is observed that a non-negligible variability exists in the meso-level geometry of the reinforcement. The geometrical characterization provides important input data for analysis of the permeability and mechanical properties of the preforms.
A mechanistic approach to the properties of stitched laminates
Abstract New insights are presented into the mechanisms responsible for changes to the in-plane mechanical properties of polymer matrix laminates as a result of through-the-thickness stitching. A critical appraisal of a large amount of published mechanical property data reveals that stitching usually reduces the stiffness, strength and fatigue resistance of a laminate by not more than 10–20%, although in a few cases the properties remain unchanged or increase slightly. This range of changes is observed for loading in compression, tension, bending or shear. Softening and strengthening mechanisms are proposed to account for the changes in properties due to stitching. Areas of research that are needed to further the understanding of the relationships between mechanisms and properties are identified. These include more detailed reporting of the physical properties of the stitched and unstitched laminates (e.g. fiber content, fiber distortions), the stitching conditions (e.g. yarn tension) and more thorough examination of observed mechanisms.