Aerocomposite material certification: To coupon or not to coupon?

There is much discussion in the composites industry about the potential to migrate away from the expensive and voluminous physical testing required to certify a composite material for use in aerospace applications. In particular, there is hope that the industry might dispense with coupon testing, which requires the fabrication and assessment of thousands of test plaques. Therefore, it was standing room only at CAMX 2015 (Oct. 26-29, Dallas, TX, US) for D. Scott Norwood’s presentation titled, “Composite Structures Development and Certification for Modern Military Aircraft.”

Norwood, a senior staff engineer at Lockheed Martin Aeronautics Co. (Ft. Worth, TX, US), talked primarily about the company’s efforts to understand the cost and value of composites testing certification programs. Lockheed, he reported, has looked at several years of legacy composites manufacturing programs and developed a database to char- acterize the depth and breadth of composites testing done during that time. Norwood presented the resulting data in several slices, looking at composite product form (coupon vs. component, for example), test objectives, material (composites vs. metals) and test type (static vs. fatigue). Of particular interest, Lockheed assessed the cost and time required by the “building block” approach typically used for

composites certification, starting with coupon testing and evolving through element, sub-component, component and airframe testing. In general, Norwood said, coupon testing (developing allowables) consumes the most time, but at the least cost, while airframe testing is the most expensive, but can be done relatively quickly. Notably, composites are tested more than non-composite materials, mainly because legacy test data is more readily available for the latter. And, static testing is, by far, the most common. Norwood also was careful to emphasize the value of physical airframe assessment, including testing for crack spread, interlaminar shear strength and non-normal operating conditions.

The upshot? Norwood said “computational strategies” for certification are currently ill-suited to airframe design, but that they could help “reduce the building block work scope” by way of “more focused testing,” primarily in the element/ sub-component/component stages. “We can’t go into elaborate testing matrices. We have to become much more focused and efficient.”

As for coupon testing, Norwood said, “We still see that as pretty important work that has to be done. Coupon data is too valuable to the rest of the test protocols. That’s basic homework that has to be done and makes us all feel better.”