Introduction to special issue

The Right Combination

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Science  17 Nov 2006:
Vol. 314, Issue 5802, pp. 1099
DOI: 10.1126/science.314.5802.1099
CREDIT: BALAZS ET AL

In the simplest sense, a composite is an object made up of two or more distinct parts. A common example is a composite image that may come from a series of photographs that together tell a bigger story than could be achieved with only one image. Within materials science, composite materials are put together from two or more components that remain distinct or separate within the final product. They can be as simple as a matrix material that envelops a reinforcing material, as when concrete surrounds steel bars that help prevent the concrete from failing under tension. Beyond this simple construct, composite forms now include layered structures and reinforcing agents that act in all three dimensions. The challenge is that the options for making a composite are almost limitless, but only a few combinations of materials will combine synergistically, and the design criteria may not be obvious. To design a material that will absorb more energy before breaking, a weaker reinforcing material may be added. When this composite fails, it may form a much larger number of cracks, and it is the additional crack length that makes the material tougher.

This special section examines contemporary composite materials from three different perspectives. Hogg (p. 1100) describes their very practical and important use in crafting energy-absorbing armor to protect vehicles and people. This application demands a remarkably broad range of properties, in that stopping a knife attack is very different from stopping a bullet; and in the case of projectiles, stopping the second bullet or mortar is as important as stopping the first.

Imagine a single combination of matrix and reinforcing fiber. Aside from the weight fraction of fibers used, variables include the fibers' distribution pattern in the matrix, their orientation, and their length profile, among other characteristics. Every permutation may respond differently to the range of repeated stresses that, for example, an airplane must withstand. The challenge of certification in this context is a major impediment to the widespread use of composites in critical applications. However, the tide may be turning. Cox and Yang (p. 1102) review the improving accuracy of models and simulations to make it possible to accurately extrapolate the failure properties of a range of composites from a limited data set.

Balazs et al. (p. 1107) focus on the challenges of designing new composites. They offer a detailed look at what happens when inorganic nanoparticles are embedded in a polymer matrix. Because of the comparable dimensions of the components, both enthalpic and entropic effects come into play.

As the design of new materials becomes more costly and complex, the synergistic fusion of existing materials into a better composite becomes an increasingly attractive fabrication strategy. It is hoped that the perspectives presented here may encourage new thinking toward bringing diverse materials and scientists together to make better composites.

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