The aerospace industry faces an ongoing challenge of manufacturing materials that are both stronger and lighter. Advanced materials have been used to achieve this goal. With the advent of nanotechnology, however, it may be possible to create almost perfect materials that can both save money and increase passenger safety. —
Aircraft parts are subject to varying loads during flight, which can develop cracks in high stress areas. Due to the fail-safe and crack arrest design feature of aircraft, the possibility of catastrophic failure is remote. If structural parts are not inspected and repaired on a regular basis, however, cracks will increase in size and cause structural failure and loss of life.
Aircraft inspection and subsequent repairs are costly to airlines, but this can be reduced significantly by introducing materials that have superior mechanical and fracture resistant properties. Material with enhanced mechanical and fracture properties will directly contribute to lighter structure; hence lower fuel consumption and less environmental impact.
Both high fuel prices and international efforts on climate change have brought attention to the need for greater fuel efficiency. In the aerospace industry, this has meant an increased use of advanced composite materials to reduce the weight of structures. Composites are considered to have superior properties compared to aluminum alloys and are increasingly used in aircraft and space structural components. In spite of the beneficial properties of composites, such as their light weight, their behavior in the presence of defects is not well understood. These defects can be introduced in the composite part due to impact or improper handling. Other issues of concern are damage by lightning (since the material has poor electrical conductivity), degradation due to exposure to the sun’s ultraviolet rays, and delamination caused by out-of-plane load, impact, or moisture.
Another avenue for developing better materials is nanotechnology. More and more scientists are working in this field, where material behavior must be looked at using the bottom-up approach. The ultimate goal is to gain the ability to know how to link all events that will take place when materials are subjected to external forces, from the bottom of length scale (the nanoscale) to the continuum level.
Researchers working with nanoparticles such as carbon nanotubes have discovered that the tiny particles have superior mechanical properties compared to current aerospace materials. Increasingly, a concerted effort has been directed toward the possibility of their application in aerospace components. Government agencies and research institutions have directed funds towards research to incorporate nanoscale materials into existing aerospace materials to utilize their superior mechanical properties.
For instance, engineers are optimistic that a hybrid material formed by the dispersion of nanoparticles into the matrix of a polymer will have superior fracture and fatigue properties. Embedding nanoparticles into the matrix of a polymer is an art, and the key is providing an adequate amount of strength at the interface between nanoparticles and the matrix. The strength of bonds at the interface is directly related to the ultimate performance of the nanocomposite material. The integrity of the interface bond is connected to proper chemistry and the proper distribution of nanoparticles in the matrix of the parent material.
Establishing strong bonds at the interface between nanoparticles and the matrix is possible through the trial and error laboratory approach. Trial and error is costly and time-consuming, and at the end of the day it may not yield useful results.
On the other hand, a multiscale modeling technique also can establish a link between nanoparticles interfacing with the polymer matrix, and will provide the proper chemistry at the interface through quantum mechanics. Results can be relayed to higher levels via molecular dynamics and finite element methods. Thus, characterization of the interface and correlation to properties from nano- to micro- to macroscale regions is critical. Experimental characterization of the structure and mechanical properties at the interface region through the atomic force microscope (AFM) would validate the model and further strengthen our fundamental understanding of interfacial phenomena.
The AFM has another potential use for producing aerospace materials with exceptional strength and mechanical properties. It is possible to use an AFM to position the arrangement of individual atoms.
Viewing the microstructure of a typical aerospace aluminum alloy through an electron microscope reveals that the arrangement of atoms is far from perfect. These imperfections, such as dislocations, grain boundaries, and voids, can all contribute to weak mechanical properties. Indeed, it has been analytically shown that the theoretical strength (strength of a material free from defects) of a typical aluminum alloy can be 100 times larger than the actual value measured in a mechanical testing lab.
Fabricating aluminum alloys so that they are free from these defects, then, could reduce aerospace industry fuel consumption and carbon emissions significantly. One possible means of using nanotechnology to create these perfect alloys would begin with an AFM or similar nanoscale manipulator removing individual atoms from a reservoir. The manipulator would then place the individual atoms one by one on a sheet. The arrangement of the atoms would be such as to eliminate voids, displacements, and other defects. Controlling and manipulating atoms in this way may be difficult but it is possible by using the scanning tunneling microscope tips to move atoms around in order to assemble unit cells in all directions.
Over time, these perfect sheets would be layered to produce defect-free, three-dimensional objects with strength far exceeding that of objects made via conventional means.
This technique may sound far-fetched, but it is merely an extrapolation of actual experimental results. As far back as 1989, researchers working 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. I am optimistic that the method I outlined above can be implemented in the near future for aerospace materials.
That such materials are possible is not enough to warrant their use. They must also be cost effective to employ.
To provide a sense of how expensive these materials can be and still be economically viable, consider this simple cost analysis for the fuel consumption of a typical commercial aircraft for a non-stop 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. Assuming there is a 20 percent reduction in weight as a result of the new material, it will be possible to calculate the total monetary savings during the life of the aircraft. Based on the figures provided above, the gallons-per-pound ratio for this aircraft is 40,000/500,000, or 0.08 gallon/lb.
The total savings over the life of an aircraft built from nanoscale-assembled alloys is then calculated this way:
[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 number of flights in the life of the plane (about 60,000)]
The savings is an astonishing $2,400,000,000 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. The calculated saving is significant for airlines and, more importantly, lowers fuel consumption and decreases an undesirable environmental impact.
To be sure, it is not simple to capture and link all events that take place along every length and time scale. Understanding the interaction of atoms at the nanoscale when a material is subjected to flight load, and then trying to connect it to the next level is a vexing problem. For example, using a multiscale model to simulate the interaction of only a few neighboring atoms that have been displaced requires considering many-body interactions. The simulation must correctly capture both the displacement and time events where atoms are vibrating as fast as a trillion times per second. Applying Newtonian mechanics to estimate the force of interaction between displaced atoms is limited because of the lack of raw computational power. It isn't an impossible problem, however, and scientists both in academia and the private sector could well solve it if they have sufficient funding.
Today's aerospace industry is constrained by a growing number of problems that, when solved, will ultimately shape its direction. While international competition requires the rapid, low-cost production of reliable, efficient and easy-to-maintain aircraft, ongoing growth in air transport calls for the development of new aircraft which can meet demanding operating criteria in terms of load and range. At the same time, society has imposed and will continue to impose increasingly stringent environmental and safety requirements on the industry.
It is thus both possible and necessary, I believe, to utilize nanotechnology to fabricate stronger and lighter material for aerospace components. By achieving this noble goal, the aerospace industry is helping itself by saving money, and helping the environment by using cleaner technology.
Bahram Farahmand is Chief Scientist at Technical Horizon Inc.Bahram Farahmand is Chief Scientist at Technical Horizon Inc.
