carbon fiber reinforcement and architecture-(3)

Inlaid fabrics: The tows in inlaid warp fabrics are not crimped and are held in place by a looped fine thread. Tows can run in the warp direction or in the weft direction. The insertion of a weft yarn requires a weft insertion device. Tows can be balanced biaxially using two sets of tows to give a stable fabric with good mechanical properties. A fabric with extremely good stability in all directions is provided by a multiaxial structure with four sets of tows held by looped threads. To insert the diagonal yarns, the machine is required to be fitted with a rotating platform.

Braiding: In many ways, braiding can be compared to filament winding, but cannot achieve such high volume fractions in carbon fiber composite. A braid is a system of three or more yarns intertwined such that no two yarns are twisted around one another. Braiding machines can be made with 24-800 carriers. The machine circumference increases in proportion to the number of carriers, whilst the braid area is proportional to the square of the number of carriers. When a braid is opened out, in practical terms, it is equivalent to a fabric woven on the bias. Braids tend to be made to order and are supplied on reels or piddled into boxes. Hybrid formats are available, incorporating aramid and glass.

Using computer controlled 3D braiding, near-net shape structures such as I–beams, T-sectons, hat-shaped and rocket motor exit cones can be produced, where the braid can conform to almost the exact shape of the sections. The final preforms should require no trimming or finishing before molding. Composites made from braiding have good shear resistance and torsional rigidity with very good resistance to fatigue and impact damage.

Forms of braiding: There are four main forms of braiding:

  1. Flat braids: These use one set of yarns, where each yarn is interwoven in a zig-zag pattern with every other yarn from the set, to insert the diagonal yarns.
  2. Sleevings: These use two sets of continuous yarn, one applied clockwise and the other counter-clockwise, where each fiber from one set is interwoven with every fiber from the second set in a continuous spiral pattern.
  3. Wide braided fabric: This is produced on machines permitting a biaxial lay-up or triaxial such as and in the one layer. Interestingly, a lay-up ensures equal mechanical properties in all directions.
  4. Overbraids: Braids can be manufactured onto a mandrel made out of a foam, a low melting point metal, or a blow molded thermoplastic. Braiding permits computer controlled lay-up and is used for making consistent complex shapes with accurate control of fiber angles, between 20 and 85C. A true 90C angle can be achieved by overwrapping using additional equipment. Fibers fed in the 0C direction help to control bending loads. The preform can then be impregnated with resin transfer moclding.

Braid architecture: There are two main forms of braid architecture:

  1. Biaxial 2D braid: A biaxial braid represents the most common form of braid. The construction of a flat braid is the processing parameters are defined as 0, the braiding angle, which is half the angle of the interlacing between yarn systems, also called bias angle and fiber angle; pick spacing, which is the distance between interlacing points; d, the diameter of the braid. Tightness of braid is controlled by the frequency of the interlacings and the fiber tex. The process permits the introduction of an axial yarn between the bias yarns without crimping occurring.
  2. Triaxial 3D braid: Triaxial braid is made with three sets of yarn whose intersections with a 0, +60, -60 setup form equilateral triangles, ensuring equal strength in every direction. The third yarn in a sleeving locks the diameter and prevents the braiding from expanding or contracting, imparting bending and torsional stiffness to a composite. Due to the significant increase in fiber volume of yarns laid in the axial direction, there will be added crimp in the bias yarns.

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