Braiding Simulation for Rtm Preforms

Braiding is a manufacturing process that is increasingly being used to manufacture pre-forms for Resin Transfer Moulding. A fast simulation method is presented for the prediction of the fibre distribution on complex braided parts and complex kinetic situations (e.g. changes in velocity, orientation). The implementation is suited for triangular surface representations as generated by many CAD software packages in use. Experimental results are presented to validate the model predictions, showing an acceptable correlation with the data predicted by the simulation method. The guide ring dimensions and spacing appear to have a significant effect on the accuracy of the predicted fibre orientations INTRODUCTION Automated braiding is a suitable process for manufacturing reproducible preforms for resin transfer moulding (RTM). It provides a fast fibre lay-down due to the simultaneous fibre deposition. The highly interlaced structure of braids makes it possible to cover components with sharp curvatures and non-circular cross-sections, varying along the length of the component. Furthermore, the interlaced nature of braids provides high levels of impact strength. Typical examples of these RTM components are propeller blades, trailing arms for a helicopter landing gear and automotive space frame components. So far, it was by no means trivial to predict the mechanical properties of an arbitrary braid reinforced product, firstly because the fibre directions could not be predicted in advance. Most braid simulation models (1,2,3) are not suitable for the preforms indicated, with a non-axisymmetric cross-section, varying along the length of the component. Here, we will refer to these as ‘complex shapes’. Kessels and Akkerman (4) presented a model for the prediction of the yarn trajectories on these complex shaped mandrels, based on geometrical primitives (such as planar, cylindrical and spherical surfaces). A triangular surface mesh is a more versatile description, however, which is suitable for coupling to most CAD systems. This paper presents the algorithms, results, and experimental validation of such an implementation. PROCESS DESCRIPTION An illustration of a horn-gear braiding machine is given in fig.1. The mandrel, supported by a holder (not shown in the illustration), is located between the spools. The mandrel moves with an axial velocity, V. The yarns are driven by spools in the spool plane. One group of yarns, denoted as the warp yarns, moves clockwise while the weft yarns move counter-clockwise, both with an angular velocity of ±ω. The two yarn groups interlock, forming a closed biaxial fabric on the mandrel. Optionally a third group of yarns can be inserted through the horn gears (see fig.2). These stem yarns will be deposited in parallel to the mandrel axis, providing extra stiffness and strength in the axial direction of the now triaxially braided preform. A pair of guide rings leads the yarns towards the mandrel. The yarns converge to the mandrel and touch the mandrel at a distance H from the guide ring. The point where a yarn touches the mandrel is denoted as the fell point. In operation, the mandrel with its holder can be driven to the right and left alternately for both forward and reverse braiding, and the layer of braid formed previously is covered (or ‘overbraided’) by the newly formed one. In this manner, multilayered products can be braided in one run. Figure 1. Braiding machine (schematically). Figure 2. Yarn supply in the spool plane. MATHEMATICAL MODEL Fig.3 shows a model of a braiding machine with a complex mandrel. The frame of reference is chosen to be fixed to the mandrel. The yarn supply point, q , is located on the guide ring in this case. The figure further shows the fell point of the yarn, p , on the surface of the mandrel, Q, with for every point x on the surface: ( ) 0. Q x = r (1) The angle between the path of the yarn and the tangent line of the surface in z-direction is the braid angle, α. The path of the yarn can be defined as a trajectory ( ) f λ r where the scalar λ increases monotonically along the fibre path. The fell point, ) (t p , moves along this trajectory in time. Figure 3. Model of a braiding machine with a complex mandrel. Taking into account the conditions present in the manufacture of most of the braided RTMpreforms, the following assumptions were made to simplify the model without severely compromising the accuracy of the results: Yarns Spool plane weft spool Convergence Zone V Mandrel Fell point