Versatile materials add functions and expand possibilities.

Industrial designers and engineers value composite materials primarily for structural properties such as light weight, stiffness and high strength. With rapid advances in the field of multifunctional composites, however, they may soon be looking at composites to provide many other properties as well.

Multifunctional materials and systems have more than one primary function occurring simultaneously or sequentially in time, according to James Thomas, section head of the Naval Research Laboratory’s (NRL) Multifunctional Materials Branch. They enhance system level performance by eliminating the redundancy between subsystem materials and functions. One function is usually structural – maintaining a shape or carrying a load – while the others could be anything from self-healing, thermal conduction or energy storage to damping, sensing or electromagnetic conductivity.

The distinction between multifunctional materials and systems has become blurred, especially as systems become smaller and smaller. Thomas says one difference is that multifunctional materials preserve the various functionalities even if the material is divided into parts. A multifunctional system, on the other hand, might lose some functionality if certain sections of the material are removed.

“The distinction is, in some sense, the size scale,” says Scott White, a professor in the Department of Aerospace Engineering at the University of Illinois at Urbana-Champaign (UIUC) and at the Beckman Institute for Advanced Science and Technology. He compared multifunctional systems to a bridge built with a network of sensors and electromagnetic dampers that could be signaled to clench when undesirable vibrations threaten the bridge structure. With a multifunctional material, “rather than a bridge that has a sensor and actuator attached to it, the material you build the bridge from itself is monitoring vibrations and damping them automatically.”

Thomas’ group at the NRL began working with multifunctional composite systems in 1999 under contracts with the Defense Advanced Research Projects Agency (DARPA). Researchers embedded custom-shaped lithium ion cell batteries into the composite wing structure of unmanned air vehicles (UAVs), removing some wing structural weight. They also embedded battery cells into composite structural beams for an unmanned underwater vehicle application to free up volume in the hull and provide space for additional energy storage or payload.

Thomas and his colleagues are currently studying poro-vascular composites. These consist of a thin laminate skin material with internal, fluid-filled vascular channels that connect to pores on the surface. The researchers control the liquid, moving it in and out of the pores to produce changes in the surface from flat to rough with spherical bumps or dimples like a golf ball. Changing surface roughness could provide a new method of aerodynamic control of UAVs.