A technique that enables on-demand control of composite behavior could enable a variety of new capabilities for future rotorcraft design, performance, and maintenance. The focus of the research was on controlling how molecules interact with each other; specifically, having them interact in such a way that changes at a small size, or nanoscale, could lead to observed changes at a larger size, or macroscale.

A motivation for this work was the desire to engineer new structures to enable advanced rotorcraft concepts that have been proposed in the past, but were infeasible due to limitations in current composites. One of the most important capabilities envisioned by these concepts is a significantly reduced maintenance burden due to compromises made to fly at high speeds. Enhanced mechanical properties with potentially low weight penalties enabled by the new technique could lead to nanocomposite-based structures that would enable rotorcraft concepts that cannot be built today.

A rotorcraft concept represents reactive reinforcements that when exposed to ultraviolet light will increase the mechanical behavior on demand. Control of mechanical behavior could potentially lead to increased aerodynamic stability in rotorcraft structures. (U.S. Army illustration)

These composite materials could become 93 percent stiffer and 35 percent stronger after a five-minute exposure to ultraviolet light. The technique consists of attaching ultraviolet light-reactive molecules to reinforcing agents like carbon nanotubes. These reactive reinforcing agents are then embedded in a polymer. Upon ultraviolet light exposure, a chemical reaction occurs such that the interaction between the reinforcing agents and the polymer increases, thus making the material stiffer and stronger. The chemistry used is generally applicable to a variety of reinforcement/polymer combinations, thereby expanding the utility of this control method to a wide range of material systems. It is possible to control the overall material property of the nanocomposites through molecular engineering at the interface between the composite components.

Some particularly attractive design options that correspond to lower mechanical loads and vibration are not currently achievable due to limitations in structural damping in hinge-less blade or wing structures. Future structures based on this work may help lead to new composites with controlled structural damping and low weight that could enable low-maintenance, highspeed rotorcraft concepts (e.g. soft inplane tiltrotors). In addition, controllable mechanical response will allow for the development of adaptive aerospace structures that could potentially accommodate mechanical loading conditions.

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