12/4/2009
NEWS BRIEFS
Report: Fastener Joints May Boost Fire Risk on Composite Aircraft
As government agencies prepare to approve composite materials for aircraft makers Boeing and Airbus, questions have arisen over dangerous arching potential at the fastener joints in next generation jets, according to an editorial in the Fort Worth Star-Telegram.
The fastener joint issue arises from the difference in how aluminum aircraft handle routine lightning strikes compared with composites, writes Hans Van der Zanden, a materials engineer specializing in durability.
While aluminum-skinned aircraft have “built-in protection from the lightning strikes routinely encountered by all commercial aircraft,” Boeing reportedly has had to embed wire mesh in the composite skin of its 787 Dreamliner to help direct high-voltage strikes around critical components.
“Attempting to provide equivalent protection, Boeing has embedded wire mesh in the 787’s thin composite skin and engineered fine tolerances to avoid spark gaps in metal fasteners,” Van der Zanden writes in the Star-Telegram.
“Since this wire mesh must be connected across every joint and since thousands of fastener holes must be drilled through the composite, gaps overlooked or developed during operations can facilitate dangerous arcing that could spark in-flight fires.”
Composite aircraft makers face other problems with the revolutionary material.
On the 787, high loads at the ends of the stringers – long composite rods that stiffen the inside of the wing skin – have caused the jet’s composite material to delaminate at the stress points during structural testing. Boeing engineers have added fasteners to the stringers to repair the structural flaw.
According to Van der Zanden, composites can absorb moisture, and “expansion and contraction during freeze/thaw cycles trigger concealed delamination, degradation and loss of structural strength over time.”
“Unlike with aluminum, fatigue prediction is far more difficult with composites, which are brittle and subject to sudden and catastrophic failure,” he writes.
Van der Zanden calls for rigorous new standards for scheduled composite maintenance, repair and overhaul.
“Public safety demands that the FAA and its European counterpart deny certification of 787- and A350-type aircraft until it can be soundly demonstrated that such composite jetliners are safe enough to carry passengers.” ©2009 GlobalFastenerNews.com
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