From The Bone Of A Horse, A New Idea For Aircraft Structures

So says a group of University of Florida engineers who have recreated part of a unique bone in the horse's leg with an eye toward lighter, stronger materials for planes and spacecraft.

The third metacarpus bone in the horse's leg supports much of the force conveyed as the animal moves. One side of the cucumber-sized bone has a pea-sized hole where blood vessels enter the bone. Holes naturally weaken structures, causing them to break more easily than solid structures when pressure is applied. Yet while the third metacarpus does fracture, particularly in racehorses, it doesn't break near the hole - not even when the bone is subjected to laboratory stress tests.

UF engineering researchers think they've figured out why - and they've built and are testing a plate that mimics the bone's uncanny strength in a form potentially useful for airplanes and spacecraft.

"Holes are a classic source of failure in engineered structures, but nature has found a way around that in this bone," said Andrew Rapoff, an assistant professor of aerospace and mechanical engineering and the lead researcher on the project. "We're mimicking nature's solution."

The researchers have published at least five papers on their work, which they've been conducting for three years with the assistance of a $675,000 NASA grant. Most recently, they were invited to submit a paper to a special issue of the Journal of Biomechanics to appear next year.

Airplanes have holes for wiring, fuel and hydraulic lines. Similar holes are common in boats, buildings, automobiles, homes and virtually any other structure that has functions beyond simply sheltering or containing something. Engineers typically compensate for the weaknesses caused by these holes by increasing the thickness of the material around them. In a classic example, ship builders add extra material around portholes in hulls to guard against structural weakness or failure, said Stephen Cowin, a distinguished professor of mechanical engineering and director of the New York Center for Biomedical Engineering at The City College of New York.

The shortcoming of that approach is that it adds weight, a problem for airplanes and spacecraft that need to be as light as possible, Rapoff said. The rule of thumb in the aerospace industry is that reducing the weight of a plane by one pound saves 10 pounds of fuel, so techniques to maintain aircraft strength without adding weight are sorely needed. This is true particularly for spacecraft that have extremely high launch costs, he said.

The engineers analyzed the structure of the horse bone around its hole - or foramen - with microscopy and microradiography, techniques that render the details of its microscopic composition. They converted the resulting information into equations describing the bone's mechanical properties - for example, converting the bone's mineral density and porosity into an equation describing its stiffness. The engineers then developed a computer model that mimics the bone's behavior under stress, proving the model's accuracy by testing it against laboratory tests of the bone.

The upshot of their analyses: The bone was configured in such a way that it pushed the highest stresses away from the foramen into a region of higher strength. For example, the position of its osteons, or structural units created when the bone first developed, routed stress around the foramen.

The engineers used their analyses and computer model to create a "biomimetic plate," with a hole surrounded by several different grades of polyurethane foam to mimic the compositional structure of the bone near the foramen. Biomimetics describes the increasingly common engineering trend of mimicking natural solutions in manmade materials.

The researchers tested the plate by placing it across two upright pillars and weighing it down, comparing the results with those from an identical test of a plate with a drilled hole without the foam stabilizer. It took twice the weight to break the biomimetic plate. Moreover, when it did finally break, the fracture did not go through the hole as occurred with the plate with the drilled hole.

UF master's student Barbara Garita has recently taken the work a step further. Garita, one of several master's and doctoral students working on the project, has demonstrated the foramen in the natural bone is stronger than a drilled hole when thin sections of the bone are subjected to repeated stress over time. The engineering researchers plan to subject the biomimetic plate to similar cyclical stress conditions that are common in real life, occurring, for example, when a boat pounds waves or an airplane experiences repeated turbulence. "We'll solve many problems using this bone," Garita said.

Rapoff and Cowin said applications for the research will grow as manufacturing techniques to create products composed of different grades of material improve.

"We'll be able to manufacture materials with modern machinery in a very elegant way that allows us to vary the properties the way that nature does," Cowin said, adding that the University of Florida researchers are the only researchers he's aware of who are examining the problem using a biomimetic approach.

Basic versions of so-called "functionally graded materials" already are used in airplanes and spacecraft, for example, in wing flaps made from material arranged in a honeycomb pattern overlaid with metal, Rapoff said. Future versions will replace the honeycomb pattern with continuous gradations of differently composed materials. If the flap or other elements of the structure built with such materials need holes, it would make sense to draw on the UF research on the horse bone foramen, he said.

"We've told the world 'this is how you design near holes for minimum weight and maximum strength,'" he said. "Now it's up to the designers and manufacturers to make these sorts of things."