With Navy grant for novel hull coating, McDaniel takes inspiration from a sweet fruit with a tough skin
By Patrick L. Kennedy
If a pomelo fell in the forest, would anyone hear it splat? Nope. There is no splat to be heard. Even though that pomelo plunged to earth from as much as 50 feet up, the delicate citrus fruit remains intact, protected by a complex, spongy peel that is one of nature’s marvels.
What does this have to do with the U.S. Navy? Well, the nation’s seaborne fighting force has plenty of vessels it would like to protect from impacts, and it’s awarded a $300,000 grant to a Boston University research team led by Associate Professor J. Gregory McDaniel (ME, MSE) to take a page from the pomelo’s playbook.
Over the next three years, McDaniel and colleagues will combine biology, materials science, and computational mechanics to engineer novel, lightweight materials that replicate this unique fruit’s energy absorption mechanisms, on a larger scale. Eventually, the technology might be applied to cell phone cases, packaging for sensitive equipment, and other civilian uses.
“I’ve always been intrigued by bio-inspired engineering,” says McDaniel. “Nature keeps building things and testing them all the time, right in front of our eyes.”
The Naval Engineering Education Consortium (NEEC) grant is aimed at developing materials that mitigate not only impact but shock and blast as well. “Those are three different things, but they all happen on a very short time scale of high stress or force on something that might break,” says McDaniel.
Searching for solutions in the biology literature, McDaniel found a host of papers praising the pomelo. “It’s regarded as having this kind of magical impact resistance,” he says. “There seem to be a lot of design features in the pomelo fruit that are there for a reason, and are not random.”
A sort of grapefruit native to southeast Asia, the pomelo sprouts upon the branches of the pomelo tree, which can grow up to 50 feet tall. Every season, ripe pomelos plummet to the forest floor, traveling at 30 miles per hour. But instead of bursting upon impact and then laying around rotting, the fruit survives the fall and keeps its enticing appearance so that wandering animals will gobble it up and leave its seeds far away, planting new pomelo trees.
The key lies in the peel’s three-layered construction: Inner and outer membranes encase a spongy, foam-like layer, itself consisting of up to 18 mini-layers and pockmarked throughout with pores, or voids.
“It’s what’s called visco-elastic—kind of a rubbery elastic,” says McDaniel. “If you squish it, it kind of deforms itself to absorb the shock, then slowly rebounds,” not unlike a memory-foam mattress.
Working with Aidan Jimenez, the new PhD student that the Navy grant has enabled him to hire, McDaniel’s task here is to figure out precisely what the pomelo is doing right, and translate that to a protective covering for a torpedo or a ship’s hull.
“There are interesting questions we need to answer,” says McDaniel. “What should be the size of the voids in the foam in the inner layer? What should be the thicknesses and stiffnesses of the membranes? We’ll create a model of the pomelo peel that is parameterized—i.e., completely described by a finite number of parameters—and then use computational mechanics to predict the performance as we change those parameters.”
The team will ultimately fabricate and test a first-of-its-kind peel, made of engineered rubbers and polymers, that the Navy can use.
“We’re absolutely not the pioneers in identifying the qualities of the pomelo fruit,” says McDaniel, “but what we are trying to do is go a little further in finding the optimal parameters that will allow us to steal this strategy, and take this all the way to a coating or layer that actually goes on a structure and works to absorb impact.”
An aspect of the NEEC grant that McDaniel especially appreciates is its emphasis on the PhD student’s career development. This includes an annual ten-week summer internship at the Naval Undersea Warfare Center in Newport, Rhode Island, as well as year-round mentoring and networking. “Getting job offers after this is to be expected,” McDaniel says. “It’s an amazing opportunity.”