Certifying Composite Designs for Aerospace and Defense
Wherever you find newer and particularly larger aircraft these days, you're seeing the use of composite materials. The latest commercial planes, as well as innovative defense prototypes, demonstrate the awareness of aerospace OEMs regarding the value of composites for stiffness and structural strength exceeding metals, plus weight savings and decreased fuel consumption. The military may have taken an early lead in pushing the use of composites, but now both sectors are fully committed to advancing the technology.
Although employed as long ago as the 1950s for small aircraft components, composites had a rather bumpy ride for a while, generating much buzz in the 1980s, then falling somewhat out of favor in the 90s. In the earlier days, engineers were not fully aware of the damage tolerance issues, and the material systems at the time had lower through-thickness properties so they weren't as durable or impact resistant. Composites were also more expensive than metals, making trial-and-error methodologies costly.
However, over the last decade-and-a-half, new resins were developed to toughen composites and increase their damage tolerance. Material systems overall improved, along with the chemistry, as did the expertise of suppliers and the industry's knowledge of how to produce more robust and cost efficient designs.
From Fairings to Fuselages
At first, flight safety led to the use of composites for aerospace toward non-load-carrying, non-primary structures like fairings. As confidence in the material rose and was supported with real-world performance benefits, load-bearing structures began to be considered for metal-to-composite material tradeoffs.
In smaller aircraft with less highly loaded fuselages—but minimum-gauge requirements dictated by environmental influences like hail damage—the economics of a switch from metal to composites might not always pay off. But for a larger-fuselage plane, or for wings and empennages of any size plane, such a change could bring significant weight savings; both the Boeing 787 and the Airbus 380 are made up of more than 50% composites, a move that cut some 20% in weight over previous designs.
Despite these advantages, the challenge of designing weight-bearing structures out of composites remains significant. Multi-material, multi-ply composites remain much more mathematically complex to model and design compared to metals.
Fortunately, computing tools have improved dramatically over the last 10-15 years, allowing composites analysis, simulation and optimization to be carried out more quickly and accurately. The ability to design for optimum weight reduction, as well as to refine manufacturing processes on the shop floor, all in less time, have also benefitted from the more sophisticated digital tools now available, leading to greater acceptance of the deployment of composites and enabling the production of the bigger, lighter aircraft we see today.
It's All About Speed to Certification
No matter what the aircraft or the materials used to make it, manufacturers have a common goal of reducing the total project schedule, from kickoff to FAA certification. Adding composites into the product development equation can introduce greater complexity because you are bringing so many processes together—design, analysis, testing, curing of the laminate, the robotic application of the fiber on the tool, etc. The technologies involved need to be in communication because they impact each other; how you design or optimize a laminate affects every other downstream function, so passing the data more efficiently between disciplines is critical. As design iterations progress, a tight feedback loop is essential to achieve a fully optimized composite design.
OEMs working with composites are becoming increasingly cognizant of how early design decisions affect the downstream efforts of the manufacturing team to produce a final, certifiable part. The technology continues to mature, but we are already identifying the sweet spots where processes can be automated and improved upon. While composites are still in the relatively early stage of adoption in aerospace, we're now at the point where we are able to take lessons learned and apply them to process improvement with measurable results.
NASA, obviously attuned to the significance of speeding certification of air and space vehicles, has taken note of this progress, creating the Advanced Composite Consortium (ACC) in 2015 to bring stronger composite material analysis, design, and manufacturing into practice. The stated goal is to help maintain American leadership in aviation manufacturing; the project aims to reduce product development and certification timelines by 30 percent for composite aircraft.
Demonstrating Certifiability to the FAA
Analysis traceability and visibility are highly important for certification so, on the pathway to achieving it, an OEM must prove to the FAA that it has done its due diligence. Every aircraft manufacturer has to prove flightworthiness of each aircraft configuration being proposed so there can be no ambiguity in the structural analysis process, which has to be fully traceable and repeatable. It's not acceptable to use a “black box” of computational tools.
Strong support for an OEM's submission for certification can be provided by finite element analysis (FEA) output files of computed internal loads with, for example, NASTRAN or Abaqus—along with test data that validates methods and allowables. Collier's software tool automates the processes that provide these, giving the design engineer the ability to trace through their analysis, visualize results and understand the response of a composite structure, thereby confirming that the software and material input data are producing the correct answers.
Templates of a wing skin, a rib, or a fuselage are available to guide the user through the analysis. The software reports to the engineer all the analysis details, including input and intermediate data results, with stress methods fully documented by references to the published literature. The test-data correlation capability allows the user to store such data for later demonstration of the agreement between analytical prediction and test.
HyperSizer essentially automates the setup of a design-feedback loop based on the internal-load data from whatever number of FEA load cases (these can be in the thousands) are needed to reach an optimized design—and then computes the failure margins of safety. At this point the final deliverable from the software is an updated design to pass to the user's CAD software (such as CATIA), updated model properties to pass to the global loads model, and stress reports, both detailed and summarized, including:
- Tables of dimensions, minimum margins of safety, controlling load case and failure mode for each structural component
- Complete worked-through examples of stress analysis methods starting from FE load extraction and summation, section properties, reference stress to margin calculations.
- Section cuts of structural components with dimensions.
Safer, Lighter – and Manufacturable
The attractiveness of a lighter aircraft is of course the major reason why OEMs are increasingly turning to composites, but this obviously must go hand-in-hand with meeting all safety criteria. Collier's tool not only builds a positive margin of safety into every load case run, the composite design is optimized for the minimum weight that meets all applicable failure criteria with positive margins. Typical weight savings run between 20 and 40 percent.
As product development moves from design to manufacturing, digital tools continue to play an important role. Laminate designs are automatically incorporated into composite layup simulations in CATIA, for example. In preparation for this handoff, HyperSizer identifies the optimized ply schedules for a part, then sequences these further to account for layup producibility requirements. The ply schedules are then passed to CATIA for ply staggering and generation of part drawings.
This is the sequence being employed by some major aircraft makers today. However, smaller- to medium-sized companies are still spending a great deal of time performing their stress analysis and sizing in error-prone spreadsheets or custom scripts instead of commercial software. Collier's software can replace those scripts with automated stress analysis/sizing that quickly updates the FEA model and CAD layup schedules with every change to the design. Reduction in the design schedule can be considerable: from many months to a few weeks through this kind of software automation. Long term, the ability to call up and repeat a digital analysis on an aircraft with a 30- to 50-year lifespan—that can extend beyond any individual engineer's career—is of obvious benefit.
Regardless of their size, every manufacturer will have their own additional design criteria, such as proprietary material allowables and unique fabrication processes. HyperSizer can be customized to accommodate these, and is flexible to merge and adapt to each OEM's unique analysis methods for certification. Companies that do not have preexisting stress manuals use the software right out of the box because it's based on standard, traditional aerospace analysis. Established manufacturers often plug their legacy analysis methods directly into the tool.
Going forward, the use of composites in aircraft will increasingly depend on automating production on the manufacturing floor. Processes like curing and automated fiber placement (AFP) will largely be accomplished through robotics. Software makers are working to couple the power of digital analysis and simulation to the production technology, with the goal of establishing data communications from the stress analyst all the way to the AFP machine. The timeline from early design thought to finished, certified aircraft is on its way to becoming as streamlined and cost-effective as possible.
This article was written by Craig Collier, President, Collier Research Corporation (Newport News, VA). For more information, Click Here .