The multiaxial architecture permits double bias fabrics ±45° (X-designation); triaxial with 0°/ ±45°and 90°/±45°; and quadraxial fabrics with 0°/90°/45°. It is possible to have ±60°and ±45°,but these constructions necessitate major machine changes. Manufacturers can join layers together with an adhesive, or more probably, stitch with a yarn such as polyster or glass. The polyester stitching may give a poor bond with some resin systems. Stitched multiaxial fabrics have better mechanical properties than woven fabrics, since the fibers are always straight and not crimped.
The most common fabric is (+45°, 0°, -45°), in which the 0°lay-up contains 50% of the fiber, with the other 50% spread equally between the ±45° layers. Angles of ±60°and ±30° do necessitate major changes to the machine setup, which becomes time consuming and expensive. The width of a given machine is fixed and produces good quality fabrics above 1000g/m2.
The fabric is penetrated by a stitch bar containing up to 700 needles and the method of stitching used is important:
- The stitch gage is the frequency of knitting across the fabric width and smaller gages provide greater stability, improving the drape, and is suitable for fabrics needling more handing.
- A smaller stitch length will improve drape, but raises production costs.
- The stitch style affects the handing of the fabric. A chain stitch gives good drapeability, whilst a tricot stitch gives a more stable fabric. A modified tricot stitch can provide optimum drape and stability.
After stitching, the fabric can be slit to specified widths for use in, say, pultrusion.
Multilayered fabrics can be constructed, where up to eight layers can be assembled, with the warp yarns travelling to and fro from the bottom surface of the combined lay-up to the top.
- Producing a stitched fabric by the simultaneous stitch process: Multiaxial architecture of stitched reinforcements permit up to 5 plies of virtually any angle in the range 0-90°, such as 45, 90, 30, 60, and 22°, where the 0°ply runs in the direction of the warp and is fed from a creel, whilst the cross plies are fed by carriages, which move to and fro across the warp direction. The stitching is carried out on Karl Mayer machines and the NVG a stitch bonded variable layer process undertaken on Malimo machines are examples. The NVG process displays lower mechanical properties due to fiber damage which occurs during the insertion of the 0° filaments via a small tube that is strongly bent during insertion into the knitting elements.
The Liba process is non-impaled and has the stitches inserted around the insert yarns. The malimo process is termed impaled with stitches piercing the insert yarns. The non-impaled knitted structure is useful for high modulus fibers, with good fiber positioning and little build up of stress in the load bearing system. The impaled fabric can give 3D toughened structures through interlocking by stitch bonding.
The liba initially used modified lace making machinery, which produced a thick glass fiber fabric and when adapted for carbon fiber, used a low weight thermoplastic thread to hold the tows together with a minimum of crimp.
- Producing a stitched fabric by the weave and stitch process: There is another method of producing ±45°stitched fabrics by weaving a weft unidirectional fabric and then pulling the fiber on a special machine to introduce a 45° The process does introduce a measure of crimp when the fiber is skewed, but heavy tows can be used, keeping cost down. A quadraxial lay-up could consist of 45°, 0°, 90°, -45°, where the 0° and 90° can be a conventional woven fabric used with the skewed +45° and -45° layers and after assembly, the three layers are then stitched together. The disadvantage is that the woven layer will introduce a measure of crimp.
- Double bias fabrics: Double bias fabrics with ±45° reinforcements have good conformability, torsional strength and enhanced surface cosmetics and can be used to provide off-axis strength for pultrusion products.
- Triaxial weave: Triaial weave originated in the 1970s and feature a 90± 60° hexagonal planar arrangement. A more popular construction is a unidirectional + and – layer, with eigher a weft or a warp unidirectional layer. All three layers are normally combined by stitchbonding into a single fabric, which gives better drapeability than an adhesive bonded fabric.
- Quadraxial: Quadraxial fabrics are quasi-isotropic, providing strength in all four fiber axes.