In recent years, the pace to create a standard database for composite materials, making them easier to use in structural design, has accelerated. An excellent example of this was the Advanced General Aviation Technology Experiments, better known as AGATE, a program jointly sponsored by NASA, FAA and the composites industry. The program, which ended in November 2001, demonstrated the economics and the technological feasibility of developing a common material physical and mechanical property database.
Under the program, a number of composite material manufacturers agreed to produce materials and then worked with regulators and parts manufacturers to establish a shared database of commonly used material properties. The FAA has supported that goal through the AGATE program, additional R&D programs sponsored jointly with NASA, and programs at the FAA Technical Center. The FAA contribution focused on safety and certification issues. Progress was made in developing policy and methodology for acceptance criteria, equivalency sampling practices, and material and process controls for qualification databases. The AGATE methodology has been used to create material property data for a number of aircraft programs, such as PAC Lancair and Cirrus Design.
The development of off-the-shelf composite materials is contingent on three factors: the development of an accessible database for agreed upon “commonly used” materials; a standard material procurement specification, which includes key material properties parameters to assure products consistent with the original data; and a standard processing definition to assure that test sample material properties are comparable when testing to establish equivalency. For many years, this triad of factors has assured that the material property values of metals are consistent and repeatable. Every material that is entered into MIL-HDBK-5 is required to have a material procurement specification. Every manufacturer that wishes to heat-treat a metallic material must use the standard industry process for the particular alloy. Proposals, currently before the MIL-HDBK-17 Coordination Committee, are addressing the questions of population size and specification (pedigree) issues for composite material.
While the AGATE program provided a base for developing a shared property database, additional enhancement is needed. The process for material qualification as well as the process methodology required to prove equivalency must be standardized. The AGATE process can be greatly enhanced by achieving a higher degree of robust data. As design engineers know, the more robust the database — the greater number of replicates it includes — the higher the confidence level that the database values truly represent the material property population. Knockdown factors associated with the use of smaller data populations can be reduced. Several attempts at demonstrating equivalency have failed due to the small population of the datasets developed for the AGATE program. In addition to the associated design confidence and safety benefits, the common database development would provide an economic incentive to material producers, as well, by enlarging the end user potential.
Both material producers and the user community must be able to demonstrate product equivalency to the database, regardless of batch or date of manufacture; if the product is not consistent within prescribed limits over time, the ability of a third party to validate equivalency is jeopardized. Several instances of this have been encountered with attempts to show equivalency to MIL-HDBK-17 historical datasets. A common database would allow materials manufacturers to monitor the process over time to identify and control deviations from the established property database before they become problematic.
In industry discussions about this issue, many are skeptical about our ability to produce shared databases and common materials, but the desire to have them is enormous. The AGATE approach has proven to many in the composites industry that it can be done while actually increasing the level of safety in structural designs. More robust datasets and specification control will enhance the current process. As shared databases become familiar to the industry, the concept’s usefulness will create the need to add shared databases at higher building block levels (e.g., laminate, notched effects and joints). The next few years show great promise for the development of a new and better way for designers to utilize composite materials; one free of the need to show large benefit ratios to justify material allowable development programs.
The work to establish common materials and develop the AGATE methodology for general industry is an ongoing effort. The FAA through collaboration with MIL-HDBK-17 has taken a lead in the effort.
To learn more about shared property databases and common specifications attend the next MIL-HDBK-17 Coordination Meeting or contact Peter Shyprykevich or the author at the FAA William J. Hughes Technical Center in Atlantic City, N.J.