Figure 2. Schematic of fabrics: (a) plain weave; (b) five-harness satin weave (reprinted with permission from ref. 7).

laminate may be oriented so that the fibers in one layer are directed at 45° to those in the next layer. Thus, a 2-D CC structure can have fibers oriented in more than two directions.

A 2-D CC is suitable for most applications in which interlaminar shear strength and out-of-plane tensile strength are not critical; 2-D CC also is typically less costly than multidirectionally reinforced (i.e., n > 2) CC, discussed subsequently. The cost differential originates in weaving-related expenses, since fabrics typically used to reinforce 2-D CC are mass produced to give economies not normally achievable with the weaving of individual fiber reinforcements used for 3-D preforms. The higher cost of 3-D CC has inspired alternative fabrication schemes to permit the use of 2-D material in situations in which 3-D would otherwise be necessary.

Through-the-Thickness Reinforced CC

Through-the-thickness reinforcement (typically termed 3-D) overcomes some of the limitations of 2-D CC. Braiding and weaving are the two methodologies employed to fabricate preform structures having through-the-thickness reinforcement.

Braiding is appropriate for making curved thin-walled structures such as the small rocket motor exit cone, shown in figure 3.

Braided components can be fabricated with either through-the-thickness reinforcement or triaxial reinforcement which consists of helical and axial fibers. A triaxial braided fiber pattern is shown in figure 4. The braid pattern imparts good longitudinal strength to a thin-walled tube, as would a uniaxial structure resulting from axial fibers. The off-axis fibers in a braided construction tie the structure together and impart bending and torsional stiffness to the structure. These attributes represent significant improvements that make braided tubes preferable to uniaxially reinforced tubes.

Thin-walled parts can be constructed of 3-D tape, as illustrated in figure 5. Tape thickness for a single-layer construction depends on the cross section of the yarn used. Typical 3-D tapes are multilayered and can be varied in thickness. Threedirectional tape constructions are appropriate for flat panels applications in which improved interlaminar shear strength or resistance to thermal shock is required.

Thick-Walled Constructions

Thick-wall multidirectional CC structures include nose tips for atmospheric reentry vehicles and components of solid propellant rocket engines. Weave

-Helical yarns

-Axial yarns

Tube -

Braid angle

Figure 4. Schematic of braid pattern.

architectures for the former are, for the most part, orthogonal 3-D constructions with fine yarn spacings. A typical 3-D construction is illustrated in figure 6 (ref. 7). In addition to the high-temperature mechanical property retention afforded by CC composites, reentry vehicle nose tips are required to exhibit unparalleled resistance to thermal shock, predictable thermochemical ablation performance, and the capacity to survive erosive conditions presented to high-velocity vehicles by ice, snow, rain, and dust. Atmospheric reentry environments and their impacts on CC design have been studied extensively over the years (refs. 8 to 11). For optimum mechanical properties, high fiber volume fraction (42 to 48 percent) is desired. Small center-to-center fiber spacings (<0.040 in.) are preferred (especially in the axial direction) for smooth erosion/ablation performance. Low matrix porosity (<6 percent) is also desired for resistance to both thermochemical ablation and particle erosion. A woven orthogonal 3-D preform is shown in figure 7.

Carbon-carbon reentry applications other than nose tips may have other performance requirements. Control surface components requiring higher in-plane shear moduli may require a 4-D architecture, such as shown in figure 8, or a 5-D architecture, as shown in figure 9. In both cases the additional in-plane fibers impart higher X-Y shear moduli and overall greater isotropy than can be realized from an orthogonal 3-D architecture. The result of these fiber architectures is a lower fiber volume fraction in each in-plane direction of reinforcement with a resultant loss of tensile and compressive properties in those directions.

Figure 6. Typical 3-D block construction (reprinted with permission from ref. 7).

Where off-axis properties are of primary importance and on-axis property requirements can be sacrificed, a 4-D architecture of the type shown in figure 10 may be appropriate. A seven-directional CC has been manufactured, and 11-D CC constructions are feasible. In general, an increase in the number of directions of reinforcement serves to enhance off-axis properties at the expense of on-axis properties, rendering the CC composite more isotropic in its properties and lowering the maximum possible fiber volume fraction, hence, the preceding conditions influence the directional properties in any one direction.

For thick-walled constructions that are cylindrical or conical in shape, a polar 3-D architecture, shown in figure 11, is appropriate. A fully densified cylindrical ring having this fiber architecture is shown in figure 12. The Z-direction fibers of a polar preform provide axial tensile and axial compressive strength; the circumferential fibers provide hoop tensile strength; the radial fibers impart radial compressive strength and torsional shear strength to the construction.

While weaving of dry carbon yarns is one method of constructing 3-D cylindrical preforms, it is not the only method that has been used. An alternative approach uses prefabricated (i.e., pultruded) rods that form the radial array and automatic winding of the axial and hoop directions using dry carbon yarn (ref. 5). The cylindrical preform is then processed to yield a CC structure.