be controlled in the synthesis of optical fibers, which in turn support nonlinear soliton communications technology.

The most promising concepts for high-performance materials involve composites. Current technology typically uses two-pot synthesis of constituents with complementary properties (reinforcing filler and matrix) followed by mechanical mixing, often involving hand lay-up. In several demonstrated cases, however, composite properties have been achieved through in situ filling via chemical reactions. These new synthetic strategies enable new manufacturing concepts including net shape processing and low-temperature synthesis of ceramics.

Self-assembling materials seem to apply local rules of assembly to generate rather complicated long-range structures. These materials, as found for example in natural living organisms, would offer outstanding properties in synthetic materials if the rules of assembly could be deciphered and manipulated. Since biological systems are adaptable (local conditions modify growth rules, leading to variability), the exploitation of self-assembly could open pathways to novel agile materials whose properties are easily tuned. This is an area in which recent discoveries in fundamental nonlinear science—studies of cellular automata, neural nets, and other adaptive complex systems—should provide valuable insights.

Fracture

Despite the importance of fracture to almost every technology, from ancient to modern, the fundamentals of crack growth have eluded materials scientists. Although linear regions of crack growth exist, failure usually involves nonlinear processes. Classical theories introduce a velocity-dependent fracture energy to parameterize experimental data. This approach, however, has proven inadequate to account for even the qualitative aspects of crack propagation.

In brittle materials, thresholds separate regions of nonpropagation, steady crack growth, and unstable growth. Unstable growth occurs at about 40 percent of the speed of sound, where the acceleration of cracks slows sharply and they emit high-frequency acoustic waves. At this point, the fracture surface shows periodic structure correlated with velocity oscillations. The time scale of the velocity oscillations is greater than the time scale on which bonds break and remains unexplained. These basic features are found in materials from network glasses to linear polymers.

Recent work employing nonlinear mathematics and computer simulation accounts for the qualitative features of crack growth but still fails to account for the observed rates of crack propagation. Experiments designed to elucidate the nonlinear phenomena underlying crack growth are nearly nonexistent.

Rubbery materials fail by complex processes that are also poorly understood. Crack propagation in such materials is related closely to adhesion. As opposed to brittle materials, bond breaking is a minor contributor to fracture energy. Rather, nonlinear phenomena associated with the pullout of polymer chains dominate crack propagation. Because of the complexity of polymer dynamics, the viscoelastic processes occurring over 10 decades in time are active in crack propagation. Once again, experiments designed to probe the fundamental processes underlying crack propagation are limited.

Numerous attempts have been made to use fractal analysis to describe the fracture interface. This exercise has yet to lead either to conclusive evidence for fractal roughness or to insights into the fracture process. Interfaces arising from diffusion processes, on the other hand, do display fractal characteristics, but the universal nature of the diffusion process guarantees similar geometric structures despite radical differences in interface strength.

Smart Materials

Smart materials have duties beyond simple structural support and containment. Such materials, although nearly nonexistent in man-made structures, are abundant in living structures. Biomaterials not only heal but also perform a variety of sensing and control functions. It is not surprising that biomimetic materials are recognized as a major emerging route to new-generation materials. Although



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement