Greener Aerospace with Nanotechnology

During flight, aircraft parts are subject to varying loads, and can develop cracks in high-stress areas. If structural parts are not regularly inspected and repaired, cracks could increase, eventually causing structural failure and loss of life.

But aircraft inspection and repairs are costly to airlines. Moreover, high fuel prices and international efforts on climate change have brought attention to the need for greater fuel efficiency. Increasing international competition favors the rapid, low-cost production of reliable, efficient, and easy-to-maintain aircraft capable of increased load and range. In short, the aerospace industry faces a challenge: to develop advanced materials that are simultaneously stronger, lighter, safer, fuel-efficient, and cost-effective.

With nanotechnology, it now may be possible to create almost perfect materials that can increase performance and passenger safety while saving significant money.

Aluminum alloys have long been materials of choice for aircraft fuselages. But viewing the microstructure of a typical aerospace aluminum alloy through an electron microscope reveals that the arrangement of atoms is far from perfect. Dislocations, grain boundaries, and voids all weaken an alloy.

Indeed, analysis reveals that the theoretical strength of a defect-free aluminum alloy can be 100 times greater than actual measurements in a mechanical testing lab. That suggests that fabricating defect-free aluminum alloys could allow structural parts of required strength to be made of less material, and thus be lighter weight.

Perfect alloys could be produced using an atomic force microscope or a scanning tunneling microscope to position the arrangement of individual atoms without voids, displacements, and other defects. Such capability was demonstrated as far back as 1989, when researchers at IBM's Almaden Research Center in San Jose were able to spell out their company's name in xenon atoms. More recently, researchers at the same lab were able to measure, down to the piconewton, how much force was required to move a cobalt atom across a copper surface.

Exploring Composites

Composite materials—those in which fibers, commonly of carbon, are embedded in a matrix of resin or other polymer-—are increasingly used for structural components in aircraft and space vehicles. Composites are exceptionally light and strong. But their behavior is not yet well understood in the presence of damage by lightning (composites have poor electrical conductivity), exposure to the sun’s ultraviolet rays, or delamination caused by out-of-plane load, impact, or moisture.

A composite in which nanoparticles are dispersed into the polymer matrix may be more resistant to fracture and fatigue. Distributing nanoparticles throughout a polymer matrix is quite difficult, however, and strong chemical bonding between the nanotubes and the matrix are essential to the ultimate performance of the nanocomposite material. Because experimental trial-and-error is costly and time-consuming, multiscale modeling may prove useful in establishing a link between the nanoscale chemistry and a material's macroscopic behavior when subjected to flight load.

The Bottom Line

That such advanced materials are possible is not enough to warrant their use. They must also be cost effective to employ. A back-of-the-envelope calculation reveals that advanced materials, even if quite expensive, are economically viable to research and develop.

Consider a simple cost analysis for the fuel consumption of a typical commercial aircraft for a nonstop flight from Los Angeles to New York. The total weight of a medium-range aircraft after takeoff is approximately 500,000 pounds, including the 40,000-gallon weight of fuel; that yields a gallons-per-pound ratio for this aircraft of 40,000/500,000, or 0.08 gallon/lb.

Assuming there is a 20 percent reduction in weight as a result of new nanoscale-assembled aluminum alloys or nanoparticle-reinforced composite materials, let us calculate the total monetary savings during the life of the aircraft:

[The gallon/lb. ratio (0.08)] x [The cost of jet fuel (typically $5 per gallon)] x [The weight savings (500,000 pounds times 20 percent, or 100,000 pounds)] x [The number of flights in the life of the plane (about 60,000)]

The savings is an astonishing $2.4 billion per plane. Furthermore, if we assume the total number of aircraft that will be fabricated with the new material is conservatively estimated to be 1,000, then the total monetary savings throughout the life of a 1,000-aircraft fleet will be almost $2.4 trillion.

I am optimistic that advanced aerospace materials for lighter-weight aircraft are worth the investment. The fuel savings would be significant for airlines, while increasing strength and safety.

[Adapted from “Can Nanotechnology Make for Greener Aerospace?” by Bahram Farahmand, for Mechanical Engineering, March 2010.]