Engineering students at the Massachusetts Institute of Technology (MIT), led by postdoc Joenyoon Lee, are working on a new method to develop aerospace-grade composite materials – without the need for the massive heat and pressure vessels used in current processes.
It’s big news for the aviation industry, considering that current processes are extremely expensive and consume a lot of energy. Should the new method become successful, the cost of making aircraft wings and fuselages would come down drastically. Also, it would speed up the process significantly.
Then and Now; A Brief Background
To appreciate what makes the MIT breakthrough so significant, it’s crucial to understand how aerospace-grade composite materials are made.
The MIT team describes a modern aircraft’s fuselage, for example, as made from multiple sheets of different composite materials. When building the fuselage, these sheets are stacked and molded into the required shape. Then, the structure is wheeled into high heat and pressure vessels to fuse it into a single amalgam.
Given the size of some of the large aircraft, you can see why this process isn’t easy. For the giant airplanes, you’d easily need a vessel the size of a three-story building for the fuselage to fit in. Building and operating such a large heat or pressure structure is both complex and expensive.
To reduce costs, in 2015, research, supported in part by Airbus and Lockheed Martin, and led by Lee, was initiated to look into ways to build aerospace-grade composites without the need for an oven or autoclave. The team’s first major breakthrough came recently in the form of an Out-of-Oven (OoO) heating technique.
Key to the OoO heating process are razor-thin sheets of carbon nanotubes. When the time comes to heat and fuse the sheets together, the parts are wrapped in a shroud of nanotubes. Then, an electrical current is passed to induce conductive curing.
According to the MIT team, the process produces composites as strong as the materials made in traditional aerospace-grade manufacturing ovens – using just 1% of the energy!
Out-of-Autoclave Pressurization Next on the List
Pressurization vessels help to press composite materials together, squeeze out any voids, and eliminate any air pockets.
“Each ply of material has a microscopic surface roughness such that when you put two plies together, air is trapped in between the surfaces,” says Brian Wardle, Professor of Aeronautics and Astronautics at MIT. “This is the primary source of voids and weaknesses in composite materials. The purpose of an autoclave is to push those voids to the edges and ultimately out of the composites.”
Lee’s team is now working on Out-of-Autoclave (OoA) methods to induce the necessary pressure without the need for traditional autoclaves. And, there’s already significant progress in this area too, though MIT says that most of the OoA techniques they’ve developed so far generate composites where up to 1% of the material contains voids. Composite materials made in a modern autoclave, by comparison, are so high in quality that any voids present are typically negligible and often not measurable.
The team hopes to employ carbon nanotubes, but this time as nanoporous networks (NPNs), to achieve the same standards. They theorize that placing NPN films between composite layers would help draw composite layers together, consequently forcing out voids during the curing process.
Still a Long Way to Go
Up to now, the researches have only experimented with samples that are up to a few centimeters wide. So, it could be a while before the OoO and OoA techniques arrive in aviation. What’s indisputable, however, is the potential impact of these two new technologies. It could be revolutionary for composite manufacturing in the aviation industry.