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Composite materials are now, after three decades, firmly established in the structures of certified military and civil aircraft and space vehicles. Unlike the situation with metallic materials however, the availability of basic material property data with statistical validity remains one of the major obstacles to the introduction of new composite materials and to the more widespread usage of composites in general.
(Published on August-September 2005 – JEC Magazine #19)
BY CHRIS RIDGARD, GROUP AEROSPACE MARKET SECTOR MANAGER
This is a particular problem for smaller aircraft manufacturers who have neither the budget to develop the required databases, nor access to those generated by larger organizations, often in support of a major aircraft programme. Even for the larger organizations, however, the situation is far from satisfactory. The current practice is that essentially the same materials are tested over and over, with minor variations in resin content or test methods, with the resulting data then being regarded as proprietary and never shared or released into the public domain. A result of this wasteful practice has been an unnecessarily high cost of the materials and the stagnation of materials development and, indeed, many aircraft programmes no longer contain any budget for material characterization.
The AGATE initiative
In the United States, the absurdity of this situation was recognized in the mid 1990s by the FAA and NASA, who funded an initiative known as AGATE (Advanced General Aviation Transport Experiments), aimed at supporting the needs of the general aviation industry. AGATE consisted of a consortium of the FAA, NASA, Wichita State University and several prepreg suppliers. Its basic principle was to shift the responsibility of generating basic material property data from the multitude of end users to the one material supplier producing the product. The objective was to generate data once and once only and to share the resulting database with multiple end users. In order to achieve this objective, it was necessary to select one specification for each material (i.e. no more than 34% resin content for one customer, 35% for another, etc., or a 193 gsm fabric on one specification, 200 gsm on another) and to standardize on the test matrices and test methods to be used.
Once the database had been generated, the pass/fail criteria for the various material properties were derived statistically from the database. Such pass/fail criteria were embodied in a material specification and were used both for batch acceptance testing and for “equivalency” testing. Equivalency testing is used to qualify a new manufacturing site by means of a small single batch test programme, to verify that the new end user’s process produces performance equivalent to that achieved in the original multi-batch allowables test programme. If equivalency is demonstrated, there is no need to repeat the original qualification testing and the new end user is free to use the shared allowables database.
A fundamental strategic asset
The Advanced Composites Group recognized that the shared databas principle was the most obvious and costeffective way to make their composite materials available for use on aircraft structures, and like the other suppliers who participated in the original AGATE programme before them, have adopted this approach as a fundamental aspect of their strategy. Shared databases have either been produced or are in the process of being produced on three different resin systems and several product forms, including unidirectional and woven carbon fibre prepregs, woven glass prepregs and woven quartz prepregs. The resin systems include a 180°C autoclave cured toughened epoxy, HTM®45, an oven vacuum bag variable cure cycle epoxy with 120°C wet service capability, MTM®45, and a lower cost oven vacuum bag curing epoxy with 80°C wet service capability called MTM®46.
FAA approval of the testing and the resulting data is a fundamental part of the programme and the data will ultimately be submitted for publication in Mil Handbook 17 and the associated proposed AMS P17 specifications. A recent development is the NASA funded National Center for Advanced Material Performance, based at the National Institute for Aviation Research, Wichita State University Kansas, which has as one of its aims the establishment of a trusted and robust source of material databases in conjunction with Mil Handbook 17. ACG intends to work closely with NCAMP to help achieve this objective. It should be noted that NCAMP will not only provide a forum for the presentation of data but will also fund additional data generation. This illustrates an important principle of shared databases which is often misunderstood. A material supplier is but one possible producer of shared data – a university, government institution or an airframe manufacturer can do the same. In short, it is irrelevant who produces the data; what matters is how the data are produced and how they are made available to others.
Turning composites into commodity products
The shared database principle is in fact nothing new. Metallic materials have been treated in this way and can be procured to standardized national specifications in Europe, the United States and elsewhere. What this offers the potential to do is to turn composite materials into commodity products, readily available and affordable. It has to be acknowledged that there is some way to go before the shared database practice becomes truly established worldwide. There are many issues to be addressed, such as ensuring the accountability of the material supplier in the absence of an independently controlled material specification, and reaching agreement on the test matrices and test standards used to generate the data, to name but two. The FAA workshops made significant progress in addressing these and other issues, however it will be necessary for a similar consultation and dialogue to begin in Europe and elsewhere. There is no reason why practices at the detailed level, for example which test standard is preferred, cannot be different in different parts of the world as is the case now even with metallic materials, while still adopting and benefiting from the basic shared database principles.In the end, it may well be that non-technical issues, such as the concern over ceding control of the material qualification process to a third party such as a material supplier, may prove to be the biggest obstacle, however it seems certain that the obvious benefits of the shared database approach will eventually become clear to all. When this does happen, the benefits of affordable composite materials with trustworthy supporting data will be available to all users of composites, not just those within the aerospace industry, and will inevitably facilitate the wider use of these materials.