Geometrical modelling of braided textiles

Increasingly great pressures on design engineers to develop and manufacture lightweight and stiff structures can be seen across multiple industries. This has been achieved through the increase in the use of fibre reinforced composite materials. This is expected to continue with the added pressure of new and innovative solutions to such problems. Current trends have been towards using pre-impregnated woven or unidirectional fabrics with an increase in out-of-autoclave solutions being adopted. Alternative fabric geometries have been used in some small areas of the industry with examples of 3D woven and braided fabrics being used in some niche applications. Typically, the lack of use is due to the difficulty in the prediction and uncertainty surrounding the use of non-standard fabric architectures. The main objective of this study is to address this problem with the development of novel tools for design engineers for the prediction of biaxial braided fabric geometries and resultant composite mechanical properties. To address this objective a series of studies are presented, aiming to initially understand the fibre architecture and the effects of pre-preparation of the yarns on the resultant fabric. This study has shown the significant effect of winding twist on the geometry of the yarns post-braiding. The inclusion of 5 twists per metre of yarn can be seen to reduce the yarn width by 20% with a corresponding 27% increase in the thickness of the braided fabric. This can lead to significant effects during the infusion process with an increase in the number and severity of resin-rich regions within the composite panel. Additional imaging of the surface of the braid has shown increased damage to the surface of the yarns when twist is not applied, caused by an increase in friction between yarns during the braiding process. In addition to yarn preparation techniques, mandrel complexity has been investigated with the inclusion of converging and diverging conical sections to the mandrel. Observations have shown a substantial impact on the braid geometry from the inclusion of a conical section. Fabric thickness and braid angle are both affected, showing instability in the braid during these regions. Diverging sections are shown to lead to the most significant impact on the braid geometry. Large levels of yarn slippage on the mandrel are observed in these regions. This leads to difficulties in the prediction of properties and the repeatability of results. The detrimental effects on the braid architecture are seen to reduce for a reduction in the slope angle of the conical section, indicating gradual changes in mandrel geometry can lead to minor or negligible effects on the braid geometry. Further to these studies, the scale of the braiding machine has been investigated with the use of a 48-carrier and 192-carrier braider. Investigations into yarn width and braid angle for the range of braiders on both circular and conical mandrels have shown minor differences between the braiders. This result is significant as it allows for results from braid geometry on reduced scale, and pre-production models to be interpolated for full-sized components, reducing the requirement for expensive manufacturing trails. The research presented has been developed into a braiding tool for design engineers using TexGen modelling software. Through novel functions users are able to input process variables into TexGen, such as braider speeds and configurations, to simulate a 2D biaxial braided textile. Key parameters of the braid such as braid angle, coverage and yarn paths are predicted using established models. Custom refinement functions have been implemented to accurately model changes in the cross-section on the yarn along the undulations within the fabric. These fabrics have been compared to braided samples using a variety of inspection techniques such as optical microscopy as well as novel non-contact methods such as structured white light scanning and laser scanning. Studies into the mechanical performance of sample fabrics have been investigated, with studies into mandrel geometry and yarn twist levels presented. A novel approach to modelling fabrics with a small radius of curvature has been implemented. CTextileBraidCurved has been developed to model unit cells which follow the curvature of the mandrel using a polar coordinate system, which is vital when modelling fabrics on small radius mandrels. Evaluation of elastic properties compared to equivalent flat unit cells show a difference of up to 10.5% in the axial modulus for radii of 10mm, reducing significantly for larger radii. Further studies present the effects of yarn twist level on the mechanical performance of the composites with a good level of agreement between experimental and simulated results. Results show minor reductions in the mechanical performance in tensile modulus and strength with the inclusion of twist