Single Side Stitching, an innovative textile ... - Mechanical Engineering

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For the insertion of standing ends into the braid, those yarns are led through input tubes positioned between the horngears or through the horngear axles (see Fig.1). In this way it is possible with this technique to produce braids with almost any fibre orientations and cross-section geometry in near net-shape with minimum waste. Due to the possibility to change the number of "active" bobbins the cross-section area and with that the geometry of the braid can be varied online. CAE Tools for Braid Development Experience has shown that producing a good 3D-braid is such a complex process that it must be supported by numerical tools. Consequently, four linked software tools (modules) have been developed to optimise the bobbins movements, visualise and check a chosen scheme, simulate the braiding process, and finally, analyse the mechanical performance of the composite part. Using these tools the design engineer is able to ‘virtually’ design the braid and generate an output file to control the 3D-Rotary braider. This approach to design allows the design engineer to optimise ‘virtually’ both the manufacturing process and final part without resort to difficult and time consuming testing methods. The CAE concept with the interaction and the function of the different software tools is explained in the diagram of Fig.2. ROUTE Automatic enumeration of possible bobbin movements under defined conditions CAB Visualisation of the bobbin movements (e.g. paths). General control by the user prints of bobbin setups etc. Unsatisfactory design - redesign PAM-SOLID 1. Dynamic Finite element simulation of the braiding process 2. Static and failure analysis of the textile braided composite Satisfactory design MANUFACTURE Generation of output file for the 3D-rotary braider Fig.2: Interactions of the different software tools and schematic representation of the process to design virtually 3D-braids The software tool ROUTE helps to select the best combination of bobbins movements and to activate as many bobbins as possible and in this way to maximise the machine efficiency. After the enumeration of all possible paths for each bobbins under the defined boundary conditions it produces an overall solution based on the compatibility of all paths. The solution can be visualised by the machine internal Computer Aided Braiding (CAB) simulation software, which acts manifold. On the one hand it allows the user to specify the boundary conditions (e.g. bobbin set-up, braid cross-section geometry, etc.) of the bobbin movement and to create and visualise the bobbin tracks interactively. On the other hand it creates the machine control files and it generates the informations about bobbin set-up and movement, which are necessary for the FEM simulation. The tool used therefore is the commercial FEM software PAM-SOLID, that has been adapted to simulate the braiding process. A dynamic simulation of the braiding process give a detailed prediction of the yarn architecture in the braid which then is used as a basis for a meso-mechanical stiffness analysis. 2
FEM-Simulation of the 3D-Braiding Process Simulation of the braiding process is a difficult application for finite element analysis. The calculation must permit very large displacements and a highly efficient contact algorithm is essential to treat the enormous amount of yarn interaction. Some special features are also needed such as an element to model the ‘feed-in’ and ‘feed-out’ of yarn from the bobbin as it transverses the base plate. The key features of a braiding FE model are shown in Figure 3. The yarns are represented as a fine mesh of bar (or membrane) elements with a typical length of 2 mm so that fine details of the braid may be captured. All nodes at the braid point are moved vertically with a constant ‘take-up’ velocity and each node at the lower end is assigned a velocity time history to describe the motion of the bobbin it represents. Stationary yarns are similarly modelled except that their lower nodes are held fixed. The yarns are given typical material properties and the special elements at the base, which represent the bobbins, are given a force-deflection behaviour that corresponds to the ‘feedout’ and ‘feed-in’ of yarn from the bobbin. Finally, potential contact, with friction, is defined for all yarns so that this may be identified and treated. take-up velocity bobbin path braiding point (110 mm, 220 mm, 700 mm) stationary thread braiding thread 220 mm bobbin element bar elements node j+1 node j+2, j+3 horn gear velocity node j bar n+1 bar n bar n-1 membrane elements node j+1, j membrane n+1 membrane n membrane n-1 Fig.3: Key features for the finite element modelling of 3D-braiding using bar and membrane elements Braiding needles Fig.4: Simulation of a rectangular braid using membrane elements The simulation result using membrane elements is shown in Figure 4 for a rectangular braid. A generally good interaction of the yarns is obtained and the contact between the yarns is seen to function well. These results allow the motion of yarns and their final positions to be studied and other valuable information, such as the total feed-out of yarn from each bobbin, to be predicted. Variations in bobbin tensile force, braid take-up velocity and alternative braiding schemes may be easily simulated and assessed. Mechanical analysis of braided composites From the braiding simulation a detailed model of yarn architecture is obtained which could provide a basis for a mechanical analysis of the textile reinforced composite. The approach being investigated is to extract a representative section of the braid from the virtual model which is then overlaid with a mesh of solid elements to approximate the matrix resin. These two independent finite element meshes are modified and coupled via constraint equations. This level of modelling is at the ‘meso-’ scale which can give a good local prediction of stress and strain distribution within each constituent material. The model will correctly capture the effects of the anisotropic yarn reinforcement and, furthermore, should provide a sound basis for failure pre- 3
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Single Side Stitching, an innovative textile ... - Mechanical Engineering

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