In recent years multiaxial fabrics have begun to find favour in the construction of composite components. These fabrics consist of one or more layers of long fibres held in place by a secondary non-structural stitching tread. The main fibres can be any of the structural fibres available in any combination. The stitching thread is usually polyester due to its combination of appropriate fibre properties (for binding the fabric together) and cost. The stitching process allows a variety of fibre orientations, beyond the simple 0/90° of woven fabrics, to be combined into one fabric. Multiaxial fabrics have the following main characteristics:
The two key improvements with stitched multiaxial fabrics over woven types are:
(i) Better mechanical properties, primarily from the fact that the fibres are always straight and non-crimped, and that more orientations of fibre are available from the increased number of layers of fabric.
(ii) Improved component build speed based on the fact that fabrics can be made thicker and with multiple fibre orientations so that fewer layers need to be included in the laminate sequence.
Polyester fibre does not bond very well to some resin systems and so the stitching can be a starting point for wicking or other failure initiation. The fabric production process can also be slow and the cost of the machinery high. This, together with the fact that the more expensive, low tex fibres are required to get good surface coverage for the low weight fabrics, means the cost of good quality, stitched fabrics can be relatively high compared to wovens. Extremely heavy weight fabrics, while enabling large quantities of fibre to be incorporated rapidly into the component, can also be difficult to impregnate with resin without some automated process. Finally, the stitching process, unless carefully controlled, can bunch together the fibres, particularly in the 0° direction, creating resin-rich areas in the laminate.
The most common forms of this type of fabric are shown in the following diagrams:
There are two basic ways of manufacturing multiaxial fabrics:
Weave & Stitch
With the 'Weave & Stitch' method the +45° and -45° layers can be made by weaving weft Unidirectionals and then skewing the fabric, on a special machine, to 45°. A warp unidirectional or a weft unidirectional can also be used unskewed to make a 0° and 90° layer If both 0° and 90° layers are present in a multi-layer stitched fabric then this can be provided by a conventional 0/90° woven fabric. Due to the fact that heavy rovings can be used to make each layer the weaving process is relatively fast, as is the subsequent stitching together of the layers via a simple stitching frame.
To make a quadraxial (four-layer: +45°, 0°, 90°, -45°) fabric by this method, a weft unidirectional would be woven and skewed in one direction to make the +45° layer, and in the other to make the -45° layer. The 0° and 90° layers would appear as a single woven fabric. These three elements would then be stitched together on a stitching frame to produce the final four-axis fabric.
Simultaneous stitch manufacture is carried out on special machines based on the knitting process, such as those made by Liba, Malimo, Mayer, etc. Each machine varies in the precision with which the fibres are laid down, particularly with reference to keeping the fibres parallel. These types of machine have a frame which simultaneously draws in fibres for each axis/layer, until the required layers have been assembled, and then stitches them together, as shown in the diagram below.
Published courtesy of David Cripps, Gurit