Composites are flying high in the aircraft industry as manufacturers look for ways to reduce weight, improve performance and create a better customer experience. Composites, of course, weigh considerably less than conventional aluminum and titanium materials, and they offer a number of other benefits as well. These include high stiffness-to-weight ratios, better fatigue performance, non-corrosive characteristics, and less susceptibility to crack growth.
That said, composites are subject to their own distinct problems. The damage modes are far different from those of metals. Delaminations and matrix cracks are common damage modes, and both can occur internal to the structure with no visible damage on the surface. This poses a problem for inspection, so many aircraft manufacturers are embedding sensors into the structure to detect damage. The manufacturing processes also are much different for composites, necessitating different equipment and tooling. The joining methods for composites require different techniques and analysis methods to design and certify.
Aircraft manufacturers increasingly are relying on computer simulation to design composite structures. The increased number of variables one has to consider with composite structures (e.g. number of plies and ply orientation) makes design more difficult, so computer simulation becomes more necessary. Also, the increased stiffness of the material often requires non-linear methods for the analysis to handle the large deformations. Traditional metallic airframes frequently rely on linear finite element analysis, which is often not adequate for composite structures. With Altair’s RADIOSS, linear and non-linear analysis of composites structures can be carried out.
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 percent of the Boeing 787 and Airbus A350 consist of composite structures. While such extensive use of composites in aircraft is a recent development, carbon composites have been used for the last few decades in some primary structures, such as tail sections, and non-primary structures, such as cargo bays.
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 only about 30 years 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, the companies have remained conservative in design. Computer analysis methods are playing an important role in the design of composite structures so as to enable better understanding of the performance and better targeting of the test data that is needed.
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. 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 numerical simulation, all structures made from composites must go through it.
In calculating the complexity of materials and design variables of composites, Altair ProductDesign applies OptiStruct 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 easily varied in traditional analysis tools; but with OptiStruct, the process can quickly allow the engineers to provide insight into how the structure should be designed for ultimate performance. In the optimization process, simulation tools 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. Most composites are laminated structures, but determining how to lay up the plies remains an art, rather than a science. Simulation, and more important, optimization methods, can provide the necessary calculations on which the fabrication artist can build the structure.
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; and 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.
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