ADVANCED MATERIALS & PROCESSES | MARCH 2025 27 braided vary widely and include glass, carbon, ceramic, metal, thermoplastic, thermoset, aramid, cotton, flax, polyester, and many other fiber types. Further, braided structures may be formed with combinations of these materials. For example, one or more tows of a braided structure may be comprised of glass, while all of the other tows may be comprised of carbon for increased stiffness. consider the method of production of a braided product versus a traditional woven product. A woven product is produced such that the fibers are oriented at 0° and 90° to the longitudinal machine axis, while a braided product is produced such that the fibers are oriented at oblique angles to the longitudinal machine axis. Further, a woven product will never be produced with fibers oriented at any orientation other than 0° and 90°, while biaxial braided structures may comprise bias angles commonly ranging from ±15° to ±75°. The nominal angle that a biaxial or triaxial braided structure may be produced with depends on the final application of the structure, diameters to which the structure may be applied or required to achieve, desired performance proper- ties, and desired coverage. Current test methods are devised to accurately capture woven product properties, and considerations must be made for braided products. In the case of a 0°/90° woven product, testing will always occur in the fiber direction; however, for a ±45° braided biaxial product, the same testing may occur off-axis and may result in a perceived knockdown in properties. The weaving process naturally results in the creation of products with varying amounts of crimp and yarn tension in the warp (0°) and weft (90°) directions. The warp yarns in a woven product tend to have less induced crimp, while the weft yarns that intertwine over and under the warp yarns tend to have comparatively more crimp. Crimp is defined as the undulation induced into a tow as a result of the intertwining of tows. The differences in yarn tension and crimp between the warp and weft yarns may result in a 10% knockdown of properties in the weft direction, which must be accounted for in part design and layup. Unlike a woven product, a biaxial braided product will demonstrate the same properties in either of the bias directions. In the case of a ±45° biaxial structure, the braid in testing may be simply rotated to mirror woven product test methods; however, due to the range of bias angles that a braided structure Similar hybrids can be created to tune stiffness and strength, while dissimilar materials such as thermoplastics can be added for increased toughness. In addition, bundles of two or more tows may be braided together. Figure 6 illustrates several examples of braided structures with unique tow compositions. BRAIDED STRUCTURES: PROPERTIES AND TEST METHODS The use of braid in advanced composite structures has become increasingly common since the early 1990s. In particular, braided quasi- isotropic fabric has enabled easy lay-up by reducing the number of plies and eliminating the need for ply rotation and has provided increased performance properties for a variety of composite applications[4]. When evaluating braid mechanical pro- perties, one must Fig. 5 — Gallagher 3D radial braiding machine. Courtesy of Gallagher Custom Machines & Automation. Fig. 6 — Braids with varying tow and bundle sizes: (a) Braided wide tows; (b) braided tow bundles; (c) braided tows of variable bundle sizes; and (d) braided tows of varying sizes. Courtesy of A&P Technology. (a) (b) (c) (d)
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