a kilogram of fibers formed from this rigid polyaramide molecule to be five times stronger than a kilogram of steel and still be five times as stiff. Since the density of aramide fiber is only one-fifth that of steel, this new class of synthetic high-performance fibers already is an obvious replacement for metal in many applications.

Brittle materials, like carbon, also have a higher strength and stiffness when formed into fibers. High-performance carbon fibers formed from pitch are now available commercially with a tensile strength of 3.9 GPa. This is approximately 1000 times greater than the strength of unoriented carbon in bulk form. In the case of brittle materials, the higher strength of fibers is caused by two factors. First, like polymeric fibers, the molecular structure and orientation are improved by the fiber-formation process. Second, since the failure of brittle objects is dominated by flaws, the small size of fibers limits the size of the flaws that can exist. Thus, in addition to forming a more perfect structure, brittle materials in fiber form contain smaller flaws, further enhancing the tensile strength.²

Unfortunately, the increased tensile strength of fibers does not come without a penalty. Fibers, like rope, display this increased strength only when the load is applied parallel to the fiber axis. Even though the tensile strength parallel to the fiber axis increases as the orientation and structure become more perfect in the fiber direction, this same increase causes a decrease in strength perpendicular to the fiber axis. For example, the strength of a carbon fiber perpendicular to the fiber axis is 10 times less than the strength parallel to the axis. Also, as the orientation of a fiber increases, it often becomes brittle, making it more susceptible to damage by abrasion. Thus, to take advantage of the high strength of fibrous materials in a structure, the fibers must be oriented in the direction of the applied load and separated to prevent damage by abrasion.

Mechanical reinforcement of matrices can also be accomplished by short, randomly oriented fibers, by crystal ''whiskers,'' or by particulates. These types of reinforcement offer directionally independent (isotropic) reinforcement, but the degree of reinforcement is not as great as that obtainable from longer continuous filamentary fibers. This report is concerned principally with high-performance continuous fibers.

Advances in the performance of fibers have come about because of continuity of effort on fiber materials and fiber-processing research and development. This is illustrated by Figure 1.2 for the case of organic polymer fibers; and similar illustrations apply to other types of fibers. In the early 1920s the first synthetic fibers were produced from cellulose. Because this natural polymer degrades before it ever melts, this early synthetic fiber was precipitated from a concentrated polymer solution. After precipitation the cellulose fibers had to be drawn in order to orient the polymer molecules and improve the mechanical properties. Nylon, one of the

Front Matter (R1-R16)

Executive Summary, Conclusions, and Recommendations (1-8)

1 High-Performance Synthetic Fibers for Composites (9-20)

2 High-Performance Fiber Materials: Applications, Needs, and Opportunities (21-48)

3 Fiber-Forming Processes: Current and Potential Methods (49-102)

4 Important Issues in Fiber Science/Technology (103-108)

5 Important Policy Issues (109-118)

Glossary of Terms (119-126)

Appendix (127-129)