Researchers at the University of Florida are turning to horses to produce lighter and stronger structures for aircraft and spacecraft. In particular, they're looking at a unique bone characteristic in these animals for design inspiration. The third metacarpus bone in a horse's leg absorbs much of the force resulting from its movement. And curiously, a pea-sized hole—which serves as the entry point for blood vessels—can be found on one side of the cucumber-sized bone.

As this unique bone proves, nature has found a way to outwit a design rule dictating that drilled holes make structures weaker. Usually, objects with drilled holes break more easily than solid structures do under pressure. With the goal of developing ways to imitate nature's answer to this design conundrum, the research group of graduate students and faculty led by Andrew Rapoff, assistant professor of mechanical and aerospace engineering at the University of Florida in Gainesville, has devoted the past three years to the NASA-funded project.

Holes accommodate wiring and fuel and hydraulic lines in airplanes, boats, cars and other structures. To make up for the loss in strength caused by holes, the areas around them are made thicker. But thickening the material increases the structure's weight. And one pound of weight removed from an airplane can reduce fuel needs by 10 pounds, according to an aerospace industry guideline for fuel efficiency described by Rapoff, formerly a designer and analyst of aircraft structures.

The engineers examined the microscopic makeup of the horse bone around the hole or foramen. Based on this information, they developed equations describing the bone's mechanical properties. For example, the engineers converted the bone's mineral density and porosity into an equation depicting the stiffness.

Collaborating with colleague, Raphael Haftka, who helped them with mathematical optimization, the engineers created a computer model that imitates the bone's behavior when stress is applied. According to Rapoff, the computer model's predictions fall in line with the measurements of an actual horse bone sample.

The research shows that the bone's configuration allows it to send the greatest stresses away from the foramen into an area of higher strength, says Rapoff. According to the professor, the hole in the bone is also more robust than an average drilled hole, better able to withstand cracks that could expand to destructive lengths.

Based on their analyses and computer model, the researchers developed a biologically inspired plate, with a hole encircled by polyurethane foam to imitate the composition of the horse bone surrounding the foramen.

Pacific Research Laboratories in Vashon, Washington manufactured the plate. The researchers then placed a sample in a materials-testing machine, which stretched the plate. They compared the results with those from a similar test of a plate with a drilled hole not encircled by the foam stabilizer. They found that the biologically inspired plate could take twice the tension before breaking. And when it did crack, the fracture did not pass through the hole like it did on the plate without the foam.

By adjusting a material's strength more accordingly to its applied stresses, nature has found a way to better protect against failure near the hole while utilizing materials more shrewdly, says Rapoff. In so doing, it can produce stronger structures with less extra weight, the professor explains.

The group is applying what it is discovering about horse bone structure to the creation of designs for fiber composites.

Source: Thoroughbred Structures
John DeGaspari
Mechanical Engineering, Design 2003
http://www.memagazine.org/medes03/thorobrd/thorobrd.html