The Emergence of Machine-Augmented Composites
Embedding simple machines in a polymer matrix yields complex materials suitable for applications ranging from launch vehicle fairings to golf clubs.
In response to a request by the California Department of Transportation (Caltrans) in 2000, a team of materials scientists from The Aerospace Corporation considered burying shock absorbers in the rubber dampers located on top of bridge columns. This simple concept—burying a mechanism in a material—was later refined to develop materials useful for applications as diverse as launch vehicles and sports equipment.
An Idea is Born
Caltrans had asked Aerospace to help wrap composite materials around the columns to ensure they would remain standing after an earthquake. The tops of the bridge columns already had huge alternating layers of rubber and lead to help dampen earthquake vibrations. The scientists proposed burying automotive shock absorbers in the rubber to control the damping. The remedy could have worked, but more important, it prompted the scientists to consider what would happen if mechanisms were buried in a material on a much smaller scale. What if many small (millimeter-sized) mechanisms were buried in a flexible matrix? Would this material yield properties that could not be obtained any other way?
The team began developing this new concept. Traditional composites are made with a matrix material that holds together many fibers. Such composite materials are stiff and strong because the fibers they contain are stiff and strong. But, the team postulated, if many small, simple machines were embedded in a matrix, the resulting composite would have properties like the machines. These machines could augment the properties of the composite; thus, the scientists named their new concept the machine-augmented composite (MAC).
Rather than start with shock absorbers, which are quite complicated, the team decided to start with a very simple mechanism: a four-bar linkage. This simple machine converts compressive (perpendicular) forces into shear (tangential or parallel) forces and vice versa. Most normal materials simply compress when subjected to a compressive load, but a MAC made with embedded four-bar linkages would, in theory, generate substantial shear motion when compressed.
Using Aerospace’s rapid prototyping machine, the team made the first proof-of-concept samples with four-bar compliant mechanisms buried in a polyurethane matrix. They performed simple tests to prove that the material would respond as expected, based on mechanics. They used the data in a proposal to obtain their first Aerospace research and development funds, which allowed them to create mathematical models to describe the material, manufacture more realistic samples, test the accuracy of those models, and explore potential applications to space systems.
In essence, this Z-MAC—named because of the shape of the machines—diverts forces from one direction to another. The team first focused on applications where a preexisting shear force can be used to clamp down on a part. They found the clamping could be useful for locking down components during launch.
The models and experience gained in building and testing the Z-MAC inspired development of a MAC with a more complicated machine—a fluid-filled shock absorber shaped like an hourglass that would collapse during compression. The volume inside of the hourglass would be filled with a variety of fluids, each with a different viscosity, ranging from water to high-viscosity silicone. An outside vendor was contracted to manufacture these machines. The samples were tested to determine the damping capability of the different versions. The results were promising when compared with naturally good dampers (e.g., rubber), but more work was needed to mature this concept to a point where a particular application would specify this material over existing technologies.
DARPA (Defense Advanced Research Agency), which was looking for technology similar to what the Aerospace scientists had already accomplished, granted the team funding for further research. Joining a group from Texas A&M University, the team expanded research into more complicated designs.
At the same time, researchers from the University of Wisconsin, Madison, published a paper showing how they had achieved a “negative” stiffness (or negative modulus), taking advantage of an unusual phase change in a magnetic material. Unlike a typical material that deforms in the direction of the applied force, a material with negative stiffness deforms in the direction opposite that of the applied force. The Aerospace team took it as a challenge to make a more useful negative-modulus material by designing a set of machines that reverse the direction of the applied force. Pushing on the front surface of this MAC causes the reverse side to deflect toward the observer (rather than away, as a normal material would).
The negative-modulus MAC turned out to have a useful application. Sound waves passing through it would be shifted in phase by 180 degrees. If a plate combined normal material with these MACs, some of the sound waves would be phase shifted and some would not. When designed correctly, the two waves would cancel each other out through destructive interference. An entire wall of material could be made that worked like sound-canceling headphones without any electronics. A launch vehicle fairing made of the material could help reduce acoustic loads on spacecraft during launch.
While the sound material was under development, the scientists continued developing samples of Z-MACs in the laboratory. To watch how one such material would respond to impact loading, they dropped a golf ball on it. Instead of bouncing off at an angle as expected, the ball bounced off with a surprisingly high rate of spin. The top of the Z-MAC was shifting sideways from the impact and putting a torque on the ball. The torque caused the spin. By changing the stiffness of the machines, the team could control the magnitude and direction of the spin. Besides applications in golf clubs, this material should be able to put a torque on the nose of a bullet as it entered an armor plate. Tumbling bullets are easier to stop in armor.
The team wrote a series of invention disclosures, and as the patent applications wound their way through the U.S. Patent and Trademark Office, Aerospace’s Intellectual Property Programs office started marketing the MAC material—or “MACterial.” This generated interest from several golf club manufacturers as well as armor manufacturers, car companies (for bumpers), high-end bicycle manufacturers, and automotive tire companies.
From Concept to Application
DARPA issued another request for a material that could change shape on command. The material would act similarly to heliotropic plants, such as sunflowers, that follow the sun as it passes overhead (the biology term is “nastic motion”). The team looked at making little machines that would change shape inside a material. They explored using battery technology to generate hydrogen gas in a cell to change its internal pressure (plants move by changing pressure in their cells), but calculations showed the limitations of this concept. Then, some colleagues suggested that hydrogen and oxygen could be generated by electrolysis of water; then, to institute a pressure increase, the gases could be ignited to explode. When hydrogen and oxygen burn, they create water—so, the researchers could make a closed system that repeats the cycle again and again.
The team proposed this idea for DARPA’s nastic program and offered to make a generic material that contained many small cells that generated an explosive gas mixture that could be ignited on command. DARPA awarded them the contract, this time surprising the team with full funding. The team’s surprise turned to panic, however, when they realized that to fulfill the contract, they would have to change the shape (or morph) of a helicopter blade. What started as a general open-ended development of a new material turned into a point design. The design of a typical helicopter blade involves a compromise between its performance in hover, where a high blade twist is desired for improved lift, and in forward flight, where a low blade twist is needed for high speed. A morphing blade could effectively overcome this trade-off by adapting its shape to each of the two flight regimes.
After locating a group that knew about helicopter blades, the team was able to demonstrate changing the shape of a one-quarter scale V22 helicopter blade at the Bell Helicopter factory in Dallas. The work for DARPA on helicopter blades was successful. On the way to this milestone, the team manufactured the industry’s most efficient actuator, which produced 160 horsepower per pound. This success led to a follow-on contract to build a material that changed shape and caused large acoustic pressure waves in water. The Aerospace team constructed and instrumented a large cistern outside its laboratory to test this acoustic source.
The researchers are now working to develop more mature versions of the MACterials into useful applications as they design more complicated machines with exotic properties. A recent proposal involves combining negative and positive springs. In theory, when these two springs are attached in series, a material with potentially infinite modulus would result. The researchers are confident the material modulus won’t be infinite, but they are working to see how large a modulus can be obtained.
- G. Hawkins, “Augmenting the Mechanical Properties of Materials by Embedding Simple Machines,” Journal of Advanced Materials, Vol. 34, No. 3, pp. 16–20 (July 2002).
- G. Hawkins, “Embedding Micromachines in Materials,” 14th International Conference on Composite Materials (San Diego, CA, July 2003).
- G. Hawkins and M. O’Brien, “Embedding Micromachines in Materials,” Composites in Manufacturing, Society of Manufacturing Engineers, Vol. 20, No. 3 (2004).
- G. Hawkins, M. O’Brien, R. Zaldivar, J. Schurr, and H. von Bremen, “Composites Containing Embedded Simple Machines,” 47th International SAMPE Symposium, Vol. 1, pp. 124–130 (May 12–16, 2002).
- G. Hawkins, H. von Bremen, and M. O’Brien, “Using Embedded Micromachines to Enhance the Properties of Materials,” Advances in Plastic Components, Society of Automotive Engineers International, SP-1763, pp. 109–113 (2003).
- R. Lakes et al., “Extreme Damping in Composite Materials with Negative Stiffness Inclusions,” Nature, Vol. 410, pp. 565–567 (2001).
- D. McCutcheon, J. Reddy, M. O’Brien, T. Creasy, and G. Hawkins, “Damping Composite Materials by Machine Augmentation,” Journal of Sound and Vibration, Vol. 294, p. 828 (2006).
- C.-Y. Tang, M. O’Brien, and G. Hawkins, “Embedding Simple Machines to Add Novel Dynamic Functions to Composites,” Journal of Materials, p. 25 (March 2005).