Increasingly, aircraft designers have been turning to composites to help make their vehicles lighter, more fuel-efficient and more comfortable for passengers.
Half of the Boeing 787 and the Airbus A350, for example, are constructed of composite materials, and other manufacturers, like Bombardier, are adopting composites for a variety of aircraft sections.
Composites present a number of substantial advantages to aircraft designers -- as well as potential problems.
Pros and Cons of Composites
Composites do have some attractive material properties.
First of all, composites offer a very high stiffness-to-weight ratio. Very stiff fibers (usually carbon or glass) are embedded in a matrix (usually some sort of plastic).
The fibers provide the stiffness, and the matrix provides the glue to produce a stiff structure that is very light. Plastics and the fibers generally are less dense than metals, but the fibers have greater stiffness, providing for a larger stiffness-to-weight ratio.
Since composites are composed of a matrix reinforced with a fiber, it's rare for large cracks to develop in them.
Small cracks ordinarily stop when they run into a stiff, neighboring fiber. When extreme forces are applied to the structure, composites indeed may crack, but the energy required for complete fracture is significant.
Metals are susceptible to both fatigue and corrosion -- each of which has resulted in high-visibility calamities over the years. The famous Aloha Airlines disaster in the 1980s was the result of fatigue.
Aircraft operate in very corrosive environments, and inspections for corrosion damage are carried out often. Composites don't corrode, which is a plus, and they are also not subject to fatigue damage to the extent of metal structures.
Because of this, new aircraft with composite fuselages, such as the Boeing 787, can provide some additional passenger comfort amenities not available on a metal aircraft.
For example, the pressure in the cabin in flight can be higher, producing less ear popping on landing. This is possible since the pressurization differences between the inside cabin and the outside air can be higher for a composite fuselage.
This is due to its superior cyclic load capabilities, which is the primary cause of fatigue damage. The humidity levels in the cabin also can be higher due to the corrosion resistance of composites, which will produce fewer headaches and dry mouths after a long flight.
Another passenger benefit to composites is that due to their very stiff material properties, windows can be larger.
From an engineering perspective, composites offer some additional advantages. For one, their stiffness properties can be tailored since they are stiffer in the direction of the reinforcing fibers.
Composites are usually built up with laminates where unidirectional fabric layers are stacked on top of each other in different orientations to give the structure maximum stiffness where it is needed.
Also composites can be tailored to slightly change shape in designed ways with an applied load, which allows designers to create more aerodynamically efficient wing structures. With composites, engineers also are more easily able to embed sensors into the aircraft's skin to allow pilots to watch for any damage. That capability can significantly reduce the likelihood of a small problem growing into a dangerous one.
Despite these benefits, aircraft manufacturers justifiably have been very cautious in transitioning to composites. One reason is that since composites are often constructed of different ply layers into a laminate structure, they can "delaminate" between layers where they are weaker.
Out-of-plane loads perpendicular to the layers are one cause of delamination, so designers have to be aware of all of the potential loads paths in the structure to avoid this. Similarly, any loads that try to compress the length of the fibers can cause delamination.
Much like pushing on the ends of a deck of cards, the entire stack can come apart. The internal load distribution in a composite can be very complex, which can cause layers to separate in certain combinations of loads.
This definitely poses some design issues to be addressed that don't often come into play with metallic structures.
Because of the threat of delamination, engineers who design composite structures take special care to make certain that loads placed on the composites are primarily in-plane -- where the fibers are strong -- and that buckling does not occur.
Compounding the difficulties confronting engineers, composites cannot be inspected for weakness or internal damage in the same way that metals can. Delamination and cracks in the composite matrix are usually internal to the composite and will not be visible from the surface.
Techniques are available to find such faults -- such as the embedded sensors previously mentioned -- but they require a different methodology than that to which the industry is accustomed.
A final issue revolves around the joining of composite components to metal structures.
The composite is stiffer than the metal, so from an engineering standpoint, the composite carries most of the loads. To compensate for this additional stress, manufacturers must build up the joint with more material, a process that adds weight to the aircraft.
Moreover, the metal will expand and contract much more than the composites to which it is joined, an imbalance that can cause joint failure.
The solution seems simple enough: Use more composites. Joining composite to composite eliminates some of the issues but they can cause others.
For example, joining of composites is often done with an adhesive layer that is prone to delamination under certain types of loads. Using fasteners to join composites presents other difficulties due to the stress concentrations from drilled holes and thermal expansion mismatch between the composites and the fasteners.
Ideally, designers are looking to create more integral composite structures that do not require joining.
The Importance of Simulation
Because so many technical variables can impact the use of composites, manufacturers are relying more than ever on computer simulation to design and test composite structures virtually before constructing the actual aircraft.
However, simulation becomes more complex than in the past because, rather than analyzing a uniform metal component, the system must deal with layers of fibers that have a directionality that can influence the response of the structure. This heterogeneous material has its own particular properties that must be considered.
The number of design variables to consider is multiplied exponentially, which often results in very conservative designs that do not take full advantage of the many unique properties of composites.
Advanced simulation software is enabling engineers to account for these variables in a more reliable and systematic fashion. For instance, Altair ProductDesign, with its optimization algorithm that has been developed especially for composite design, is able to precisely calculate where designers should incorporate fewer plies or more of them, which ply angles should be used where in the structure, and how to best stack the plies.
These solutions offer up the calculations that engineers need to fabricate the right composite structure for the particular application. This process takes advantage of the computer that can cycle through the variables in a very efficient manner to come up with a better design.
Composites are likely to assume an ever more crucial position in the aviation industry, and with composites playing a larger role, the industry can expect a lift that takes it and its customers to new heights of safety, comfort and efficiency.
Robert Yancey is senior director-global aerospace and marine, for Altair, a Troy, Mich.-based provider of product-design services, advanced engineering software, on-demand computing technologies and enterprise analytics solutions.
