Needs Change, Materials Change, Composites Grow

An example of the increase in the use of composites is shown by more than 50% of the Airbus A350 consisting of composite structures.

Composites, of course, weigh considerably less than conventional aluminum and titanium materials, and they offer a number of other benefits as well.

Carbon fiber reinforced plastic (CFRP) is inherently very stiff; therefore, when it is used to build a fuselage, the aircraft’s core structure is stiffer than with other materials. As a result, cabin pressure can be at a higher level when flying, more closely approximating air pressure near the ground. With higher pressure in the cabin, studies show that passengers experience less fatigue while flying, and there will be less ear popping when landing. In addition, windows can be larger with a stiffer fuselage, so passengers will not have to duck down to see out the window.

Of great importance, composites generally suffer less fatigue than metals. For metallic substances, after repeated cycles of cabin pressurization and depressurization, any small flaw will grow and form cracks over time. Fatigue failure has been documented in a number of high-profile incidents with older aircraft that have developed holes or lost entire fuselage sections from metal fatigue.

Because composites are made of two materials, any cracks that develop in the plastic matrix usually travel only a very short distance before encountering a fiber, which, being much stiffer, will arrest the crack. Multiple matrix cracks can link up to cause a problem or, in some cases, fibers can crack as well. Such cracks are usually the result of extreme load cases, which are very uncommon, while fatigue in metal structures can occur from normal operating conditions.

Another advantage of composites is that they enable sensors to be embedded easily so that crews can monitor any damage in the structure, making it unlikely that damage will progress to the point of structural failure.

In addition to monitoring for cracking, the sensors can detect degradation of material performance from moisture absorption or solar radiation. These issues can usually be controlled with proper coatings.

Composites are advantageous as well because they do not corrode. In addition to fatigue, corrosion damage is the other major cause of failure for metallic structures. The corrosion issue with metallic airplanes is the main reason that the air cabin is so dry when flying. This low humidity, coupled with the low pressure, can cause fatigue and discomfort for passengers. With a composite fuselage, the air cabin will have higher humidity levels, further improving the passenger experience.

Considering all these factors, the aircraft industry – by nature, and by necessity, a conservative group – has tended to take the more conservative approach to materials.

Potential Drawbacks
With all of the previous said, composites are subject to their own distinct problems, the most prominent of which is delamination. CRFPs are made of fabric layers, called plies, that are stacked up with fiber directions in pre-defined orientations to create a laminate. Laminates are very strong in-plane, or in the direction of the fibers. In the direction perpendicular to the stack direction, the composite is inherently weak, because no fibers are running in this direction. The composite strength in this direction is dependent on the plastic material properties, which are much weaker than the fibers. Therefore, any loading perpendicular to the stack direction can cause delamination where the plies begin to separate.

Compressive loads also can cause delamination, much like a stack of papers will separate if you push on the ends. Even in-plane loads can cause out-of-plane forces at the edges of a composite laminate, which can result in delamination. Engineers must be careful in designing composite laminate structures to make sure that loading is primarily in-plane, that buckling loads are controlled and that edge effects are considered.

An additional challenge is the difficulty that composites present when attempting to inspect for damage. Both delamination and matrix cracking occur in composites and, unlike metal, the damage cannot be detected by a visual inspection, since many of these cracks are sub-surface. Various nondestructive evaluation techniques have been developed to find this type of damage, but these techniques are often different from and more difficult than the common methods used to inspect metallic structures. The detection methods for composites include the previously mentioned embedded sensors.

A particularly difficult issue with composites involves joining methods. Whenever a composite component must be joined to metal, great care must be excercised. Since the composite is stiffer than the metal, the composite tends to carry most of the load and therefore must be built up, and that extra material adds weight instead of reducing it. In addition, metals expand and contract much more than composites do, and this thermal mismatch causes issues that designers must address. The solution, again, has been the need to add weight back in. If, however, one composite structure is joined to another composite component, the differential disappears; so the more that composites are used, the less that designers need to worry about composites meeting metals. Also, if the composite structure can be manufactured as an integral structure, traditional joining methods (e.g., bonding or riveting) can be avoided. As an example, the fuselage section of the 787 is a completely integral structure.

Aircraft manufacturers are increasingly relying on computer simulation to design composite structures. The increased number of variables one must consider with composite structures (e.g., number of plies and ply orientation) makes design more difficult, so computer simulation becomes more necessary. In addition, the increased stiffness of the material often requires non-linear methods for the analysis to handle the large deformations. Traditional metallic airframes frequently relied on linear finite element analysis, such as NASTRAN, which is often not adequate for composite structures.
 

Composites Momentum
With more reliable simulation options and advancements in the science of composites, the aviation industry has been incorporating composites into aircraft designs at an accelerating pace. More than 50% of the Boeing 787 and Airbus A350 consist of composite structures. While such extensive use of composites in aircraft is a recent development, CFRPs have been used for the last few decades in some primary structures, such as tail sections, and such non-primary structure as cargo bays.

Weighing the Issues
Considering all these factors, the aircraft industry – by nature, and by necessity, a conservative group – has tended to take the more conservative approach to materials. The industry has more than 80 years of solid data on metal structures, but very little on the performance of composites. Aerospace companies are enjoying the weight-saving characteristics of composites, but because they are not as familiar with these materials and their testing, they have remained conservative in design. Computer analysis methods are playing an important role in the design of composite structures to furnish a better understanding of the performance and to better target the test data that is needed.

An industry-related issue regarding composites is that their manufacturing process is very different from that of metals. As a result, plants must completely change their equipment, a necessity that creates an infrastructure problem. For example, composites are laid up with fabric layers rather than machined from a block of metals. Additionally, the tools required to drill a hole or trim an edge are quite different because of the very stiff fibers, which would quickly damage a traditional metal tool bit. The aviation industry will need to install entirely new types of tooling equipment to produce composite structures.

A quarter century ago, the auto industry faced a similar situation. Composite and molded body panels are becoming more standard in the automotive arena, and automakers have modified their manufacturing methods and tooling to account for this change. In fact, given the volume requirements for the automotive industry, as they adopt more composites to meet fuel efficiency requirements, they will likely have a significant influence in improving the efficiency of manufacturing methods. With new aerospace composite structures, automation is already playing an important role, but improvements are continually needed.
 

As an example of composite usage, the fuselage section of the 787 is a completely integral structure.

Resolving the Challenges
Just as composites are very different from metals in the complexities related to potential damage and to manufacturing methods, increased complexity also enters into computer simulation as applied to composites. Rather than analyzing metal structures with uniform material properties, simulating composite structures requires the analysis of layers with directionality that are bonded together – a heterogeneous material with its own distinctive properties and variations.

This complexity, however, makes simulation even more critical to successful production. Quality inspection cannot rely just on visually examining the part; engineers need to review the mathematics of the structure to understand its nature, and simulation is the only way to attain that type of data. While many metal parts on an aircraft do not undergo simulation, all structures made from composites must go through it.

In calculating the complexity of materials and design variables of composites, Altair ProductDesign applies its optimization techniques to guide the manufacture of composite structures, determining where plies should be reduced or built up and how they should be oriented. These kinds of parameters cannot be varied automatically in the simulation as with conventional analysis tools that simply analyze the structure but do not provide insight into how the structure should be designed for ultimate performance. Most composites are laminated structures, but determining how to lay up the plies remains an art, rather than a science. Simulation can provide the necessary calculations on which the fabrication artist can build the structure. A number of years ago, Altair developed an optimization algorithm specifically for designing composites – a capability that other software providers have not yet achieved.

In the optimization process, simulation tools also reveal weight-reduction opportunities and excellent insights into where the composite structure is likely to experience critical loads. This data translates into shapes and dimensions that capitalize on the physics of the system to provide the best structure with the lightest weight and greatest sustainability.

The Future of Composites
The 787 and A350 already are designed with more than 50% composite materials. Taking off from that experience, Bombardier is creating a composite wing skin and fuselage for its C series aircraft, which will compete in the same market as the 737 and A320.

In some cases, for smaller aircraft with 90 seats or so, composites may not be the best choice, because the cost/benefit ratio is different from that of larger planes. The benefit in fuel efficiency may be outweighed by the higher costs of using composites. That said, some very small aircraft – seating four to nine people – are being built with 90% composite materials in instances where the large fuel savings are significant to the aircraft owners.

The industry trend continues to move toward composites, but some doubt remains as to whether they make sense for all classes of airplanes today. As manufacturing technology improves, however, nearly all aircraft are likely to benefit from these new materials, just as the auto industry has done in using composites for body panels and in applying its own manufacturing breakthroughs.

Composites hold considerable promise for the aviation industry. As they are proven on the large 787 and A350 and more data is compiled on their performance, more aircraft builders are likely to begin investing in the technology. Along the way, simulation tools for finite-element analysis and optimization will help reduce the turbulence in aircraft development en route to more fuel-efficient aircraft.

Altair Engineering
Troy, MI
altair.com