The structural geometry of the MWK fabric systems consists of warp (0°), weft (90°), and bias (10) yarns held together by a chain or tricot stitch through the thickness of the fabric, as illustrated in figure 19. The major distinctions of these fabrics are the linearity of the bias yarns, the number of axes, and the precision of the stitching process. Depending on the number of guidebars available and the yarninsertion mechanism, the warp-knit fabric can consist of predominantly uniaxial, biaxial, triaxial, or quadraxial yarns. The latest commercial nonimpaled MWK fabric is produced by the Mayer Textile Corporation using a multiaxial magazine, weft-insertion mechanism. The attractive feature of this system is the precision of yarn placement with four layers of linear or nonlinear bias yarns plus a short fiber mat arranged in a wide range of orientations. Furthermore, stitches are formed without piercing through the reinforcement yarns (hence the term, "nonimpaled") at a production rate of 100 m/hr. The latest development in the impaled MWK is the LIBA or Hexcel system, shown in figure 20. Six layers of linear yarns can be assembled in various stacking sequences and stitched together by knitting needles piercing through the yarn layers. While this piercing action unavoidably damages the reinforcing fiber, the powerful knitting needles incorporate a fiber mat as a surface layer for the composite. The 3-D braiding technology is an extension of the well-established 2-D braiding technology wherein the intertwining or orthogonal interlacing of two or more yarn systems to form an integral structure constructs the fabric. The 3-D braiding is one of the textile processes in which a wide variety of solid complex structural shapes can be produced integrally resulting in a highly damageresistant structural preform. Figure 21 shows two basic loom setups in circular and rectangular configurations (ref. 21). The 3-D braids are produced by a number of processes including the Track and Column (3-D circular loom) method (ref. 22) (fig. 22), the two-step braiding method (refs. 23 and 24) (fig. 23), and a variety of displacement braiding techniques. The basic braiding motion includes the alternate X and Y displacement of yarn carriers followed by a compacting motion. The proper positioning of the carriers and the joining of various rectangular groups through selected carrier movements accomplish shape formation. Examples of the structural shapes successfully demonstrated in the Fibrous Materials Research Laboratory at Drexel University are shown in figure 24. Structure and Properties of Textile-Reinforced CCC The properties of carbon-carbon are not well publicized. For structural applications, the goal is to produce CCC having a density level of 1.8-2.0 g/cc. Increased density calls for a careful selection of yarn bundle size and fiber architecture. A fine weave is more desirable in obtaining high density, although an interconnected, three-dimensional fiber network is preferred for a chemical vapor deposition (CVD) system. As the density of the composite increases, strength and modulus are expected to increase as well. Because of the low strength of the carbon Figure 21. Basic loom setups for circular and rectangular configurations. Figure 22. Atlantic Research Corporation's Track and Column 3-D circular loom. matrix (less than 40 MPa) and the weak interface between the fiber and matrix, one of the key issues in the area of CCC is the improvement of composite throughthe-thickness strength. For structural applications, most reinforcement preforms are 3-D fabrics. The article by McAllister and Lachman (ref. 25) and the book by Tarnpol'ski et al. (ref. 26) provide an excellent summary of the structure and properties of 3-D fabric-reinforced CCC. In a review article by Ko (ref. 16), the properties of 3-D fabric-reinforced carbon matrix composites were reviewed and summarized as shown in table IV. In Table IV, p is density, Vƒ is fiber volume fraction, σt is tensile strength, Et is tensile modulus, σ is compressive strength, Ec is compressive modulus of the composite, of is strength of the fiber, Ef is elastic modulus of the fiber, 7 is shear strength, and a is coefficient of thermal expansion. The dependence of mechanical properties on fiber architecture can be illustrated in table V. |