Over the past 25 years, the use of advanced composite materials in aircraft primary structures has increased significantly. In 1994, with the Advanced General Aviation Transport Experiments program, the National Aeronautics and Space Administration and the Federal Aviation Administration revitalized the use of composites in general and commercial aviation. Driven by the demand for fuel-efficient, light-weight, and high stiffness structures that have fatigue durability and corrosion resistance, modern large commercial aircraft are designed with more than 50 percent composite materials. Although there are key differences between metal and composite damage mechanics and durability concerns, the certification philosophy for composites must meet the same structural integrity, safety, and durability requirements as that of metals. Despite the many advantages, composite structural certification becomes challenging due to the lack of experience in large-scale structures, complex interactive failure mechanisms, sensitivity to temperature and moisture, and scatter in the data, especially in fatigue.
The overall objective of this research was to provide guidance into structural substantiation of composite airframe structures under repeated loads through an efficient approach that weighs both the economic aspects of certification and the timeframe required for testing, while ensuring safety. The research methodology reported here consisted of combining existing certification approaches used by various aircraft manufacturers with protocols for applying these methodologies. This will permit extension of the methodologies to new material systems and construction techniques.
In the early 1970s, composite materials were introduced to airframe structures to increase the performance and life of the airframe. In 1977, the National Aeronautics and Space Administration (NASA) Advanced Composite Structures Program introduced the use of composites in primary structures in commercial aircraft, i.e., the Boeing 737 horizontal stabilizer. In 1994, the Advanced General Aviation Transport Experiments consortium, led by NASA and supported by the Federal Aviation Administration (FAA), industry, and academia, revitalized composite material product development in general aviation by developing cost-effective composite airframe structures. Modern improved composite materials and matured processes have encouraged commercial aircraft companies to increase the use of composites in primary and secondary structures. Driven by the demand for fuel-efficient, light-weight, and high-stiffness structures that have fatigue durability and corrosion resistance, the Boeing 787 Dreamliner is designed with more than 50 percent composite structure, marking a striking milestone in composite usage in commercial aviation. Meanwhile, the Airbus A350 commercial airplane is being designed with a similar percentage of composite materials in its structure. Figure 1 shows the use of composites in several commercial aircraft applications.