Friday, December 1, 2006
Pamela Kay Strong,
AM: What are the milestone projects in your career related to the use of composite materials?
Strong: I was introduced to composites at GE and made the first aircraft engine guide vane out of carbon fiber composites, saving almost 400 pounds/engine in weight and substantially increasing the engine efficiency. This led to specifying and developing the manufacturing process to cure consistent high-temperature polyimide (PMR-15) composites, which were used in the F-15 engine housing, in primary structure on Northrup’s B-2 Bomber, and to protect the Mars rover aeroshield from overheating as it passed through that planet’s atmosphere. Boeing’s Delta all-aluminum rocket fairings were replaced by composite structures, which saved enough weight to allow delivery of substantial additional payload.
In terms of MRO, how do composites fare compared to metals?
Composites are much more complex than metals, because the same resin system can be used with many different reinforcements. The manufacturing process of composite materials into components also plays an important role in final performance properties, due to the final consolidation/void content achieved. Repair of composites is vitally dependent on cure method and this consolidation/void content. Each part must be thoroughly evaluated, and even though standard repair manuals (SRMs) have been written, tried, and proven, repair is still an art, dependent upon the operators’ training, skills and techniques. However, if optimally designed, composite structures can be five times stronger than steel, and exhibit none of the corrosion or fatigue mechanisms seen in metals.
Some aircraft are flying longer than original design life. How are composites handling this legacy demand?
Within the aerospace industry, various programs in aging composites are getting ‘very good to excellent’ marks. A key issue, however, involves industry consolidation throughout the supply chain that sometimes makes it impossible to obtain originally qualified materials, in terms of the fiber and resin, from the originally qualified source for an aircraft built 30 years ago. So alternative materials must be explored, and material qualifications performed to assess substitution viability.
Composites do not experience fatigue failure the way metals do, so long as maximum fatigue value of the composite is not exceeded. The B-1B bomber, for example, was designed in the late 1970’s when the use of composites on aircraft was a novel concept. Manufacturing of 100 B-1B planes ended in 1986, and the composite parts have aged very well. They are repaired or replaced only when damaged. B-1B SRMs are updated on a regular basis with the latest repair methods, sometimes based on lessons learned on other programs. This fleet is continually upgraded with the latest equipment and materials so it will remain in top operational form until at least 2045.
Boeing is an innovator in using composites, and including maintenance and repair from day one for new-build aircraft. Why is that so important for an OEM?
Boeing is actively employing composites in their programs to help save weight and add strength and life to aircraft parts. Composites are wonderfully adaptable materials, but must be designed and manufactured within the confines and restrictions that suit these materials. Education in the particular material and performance properties of composites is an absolute must for anyone designing, building, handling or repairing composites. That is why Boeing requires extensive classes in composite certification for all of its technicians and engineers.
How could composite repairability be improved?
Composites obviously are contributing benefits to both new and aging aircraft, and have improved over time in availability as well as price. What would help in both production and repair of composites would be a industry-wide central database that could reduce initial material qualification costs and allow for more uniformity in design and repair, as well as overall acceptance of composite materials.