When Sung Hoon Kang looks out his office window and sees a pigeon or a blue jay fly by, he sees a feat of engineering. An assistant professor of mechanical engineering in the Whiting School of Engineering, Kang focuses his research on how nature—plants, animals, the human body—can provide inspiration for engineering breakthroughs. Through a four-year, roughly $600,000 Air Force grant, he is studying how the lightweight, adaptable, irregular structure of bird bones could provide a blueprint for more efficient and resilient aerospace and automotive materials.
An Engineer's Quandary
When drawing up a new aerospace or automotive design, engineers face an inevitable push and pull, according to Kang. They want to build the vehicle to stand up to the worst-case scenario—hurricane-force winds, for example—leading them to use materials that are way sturdier than what's necessary for the typical day-to-day. But such durability often comes with a hefty drawback. "Every addition of weight requires more money, less fuel efficiency, and a greater environmental effect," Kang says.
That's where our feathered friends come in. Birds' bones are hollow, and yet they have an amazing capability to efficiently withstand force, Kang says. Moreover, their skeleton's irregular internal structure minimizes the spread of damage when they are injured. Kang, along with a team of JHU students and Air Force researchers, is currently working on a proof of concept for a new, avian bone–inspired material.
"We thought, if we can adapt the capability of the natural system to get engineering materials to behave like our body or bird bones, we can prevent overdesign," he says. "It's a more sustainable and efficient approach that allows the system to adapt to those kinds of changes."
A bird's bones—and even the bones inside the human body—can adapt in response to mechanical stress. Imagine a tennis player, Kang says. Over time, as the player exercises her forehand, the material density of the bone in her arm increases to allow it to withstand the mechanical load of the racket hitting the ball and apply more force to her hits.
An avian bone–inspired material used in a drone or airplane would similarly be able to adapt to changing conditions such as wind or rain. The hollow interior would act almost like a fuel tank, holding a metal solution. Electricity generated via applied force—a phenomenon called piezoelectricity—would convert the metal solution into a coating that could bolster an area on the wing that is being hit by particularly high winds, for example. The stronger the wind, the more coating will be formed.
"They can autonomously add more reinforcement where it's needed and remove the material where not much force is applied. It's mechanically efficient," Kang says. "If there's a material that can adapt to different loading conditions, we don't have to put a huge amount of materials, or overly expensive ones. Essentially, this material can train itself such that it can still work for those changes in loading conditions."
Strength in Imperfections
There's even more magic to a swan's skeletal structure. If you look at a cross section of a bird's bone, you'll find a porous, random design. Surprisingly, Kang says, that's a good thing. "If you look at something like honeycomb, it's a very regular structure," he says. "But we actually learned that while a honeycomb's structure is very efficient, if there's damage, it can be catastrophic."
With a gridlike structure, Kang explains, damage has an obvious path to follow. With a more random design, not so much. "Essentially, it slows down the propagation of damage so that it can potentially be localized," he says. "It could mitigate critical failure to the system."