The composite Dreamliner barrel exceeded expectations during a lengthy, recent round of strength tests. Boeing engineers proved the composite barrel design by taking it to “limit load,” which simulates the most extreme conditions the 787 could conceivably face.
Test engineers then took the test section to 150 percent of limit load — a condition called "ultimate load” that is required for certification. The barrel was then pushed well beyond ultimate load to a destruct-condition that is beyond two and a half times the force of gravity.
There were “audible” indications of damage, but not to the extent expected. More static testing will be performed on the full plane before its first flight.
Positive results to date on Dreamliner testing are leading to a flood of new composite aircraft projects. One expert says demand for carbon fiber in the aerospace industry will rise from 5,210 metric tons in 2006 to 10,700 metric tons in 2010.
One of the keys to the strength of composite aircraft sections in general has been development of new grades of adhesives.
“Bonding aircraft with composites versus metals bring some significant challenges,” says Tim Dietz, 3M Aerospace senior technical manager. For example, composites can absorb up to three percent of their weight in moisture from the environment. “We’ve developed a formulation to tolerate those levels of moisture during cure,” he says.
The newly developed Scotch-Weld Structural Adhesive Film AF 555 is a thermosetting, modified epoxy adhesive film that is used with a honeycomb or part of a monolithic composite structure. It can be cured from 300F to 355F. Another version, AF 500 is designed for use in 250F curing applications.
The materials are applied at room temperature via either manual lay-up or automated systems. Typically adhesive systems are kept frozen to avoid any premature reactions. The amount of time the adhesives are out of the freezer (“out time”) is significantly longer for composite aircraft because of the size of the sections. The AF 555 has an out time of one year, a step change for adhesives. Typical out times are in the five to 45-day range.
At the request of a customer, 3M also developed a version with lightweight conductive screens to dissipate lightning strikes on composite aircraft surfaces.