thoroughbred structures
An unusual bone configuration in horses provides inspiration for lighter and stronger aerospace structures.
By John DeGaspari, Associate Editor
A curious bone characteristic in horses has given
researchers at the University of Florida some ideas of how aircraft and
spacecraft structures could be made lighter and stronger. The third metacarpus
bone in a horse's leg supports much of the force conveyed as the animal
moves. On one side of the cucumber-size bone is a pea-size hole where
blood vessels enter.
As a rule, drilled holes weaken structures, causing them to break more
easily than solid structures when pressure is applied, but nature has
found a way of circumventing this rule. The research group of graduate
students and faculty headed by Andrew Rapoff, an ASME member and assistant
professor of mechanical and aerospace engineering at the University of
Florida in Gainesville, has spent the last three years in the NASA-funded
project exploring ways to mimic nature's solution.
Airplanes, boats, automobiles, and other structures have holes for wiring,
and fuel and hydraulic lines. To compensate for weaknesses, regions around
holes are made thicker. The problem is that thickening the material adds
weight. A quick rule of efficiency in the aerospace industry is that one
pound of weight saved in a plane can save 10 pounds of fuel, said Rapoff,
who used to work as a designer and analyst of aircraft structures.
Researcher
Andrew Rapoff displays a horse's leg bone and a manmade structure inspired
by it.
The engineers analyzed the microscopic composition of the horse bone
around the hole, or foramen, and converted the information into equations
describing the bone's mechanical properties. For example, the bone's mineral
density and porosity were converted into an equation describing the stiffness.
Working with a colleague, Raphael Haftka, who provided mathematical optimization
expertise, the engineers developed a computer model that mimics the bone's
behavior under stress. Measurements of an actual horse bone sample compared
favorably to the predictions of the computer model, Rapoff said.
According to Rapoff, the research established that the bone was configured
in such a way that it pushed the highest stresses away from the foramen
into a region of higher strength. He said that the bone's hole is also
tougher than a typical drilled hole—more resistant to initial cracks
growing to catastrophic lengths.
The engineers used their analyses and computer model to design a biologically
inspired plate, with a hole surrounded by polyurethane foam to mimic the
compositional structure of the bone near the foramen.
The plate was fabricated by Pacific Research Laboratories of Vashon, Wash.
The researchers tested it by stretching the sample in a materials-testing
machine, and comparing the results with those from an identical test of
a plate with a drilled hole without the foam stabilizer. It took twice
the tension to break the biologically inspired plate. When it finally
did break, the fracture did not go through the hole as it did on the non-foam
plate with the drilled hole.
Rapoff explained that by bringing a material's strength more in line with
its applied stresses, nature provides a way of minimizing chances of failure
near the hole while using materials more judiciously. This makes it possible
to build stronger structures with less added weight, he said.
The group is taking what it is learning about horse bone structure and
generating designs for fiber composites.