Honeycomb Core and the Myths of Moisture Ingression
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Honeycomb Core and the Myths of Moisture Ingression John H. Fogarty
Received: 8 October 2009 / Accepted: 2 December 2009 / Published online: 15 December 2009 # Springer Science+Business Media B.V. 2009
Abstract While it is true that honeycomb core can trap moisture, it is a myth that widespread moisture ingression is an inevitable outcome when honeycomb sandwich structures are exposed to real world environments. It is also a myth that when moisture ingression occurs, progressive weight gain and strength loss are inevitable outcomes. Using a rebuttal of a 2004 honeycomb-critical paper as the focal point, this paper summarizes multiple sources which indicate both that moisture ingression is preventable and, even if it does occur, that proper material choices can prevent severe consequences. These claims are further supported by results from in-plane compression testing of typical (0.064 g/cm3 aramid-paper/phenolic core with 0.116 cm carbon/epoxy faces) composite honeycomb sandwich specimens after impacting, water submersion, and extensive thermal cycling. Keywords Honeycomb . Sandwich structures . Composites . Moisture ingression . Water trapping . Environmental exposure
1 Introduction Honeycomb sandwich structure, while popular due to its high bending stiffness to weight ratio, is saddled with a reputation for proneness to moisture ingression and for susceptibility to a host of consequent problems. There is a grain of truth in this belief. When a structure has faces sometimes no thicker than a sheet of paper, and a core that is sometimes literally made from intricately folded paper, then in some respects it will, indeed, be fragile. If a large-acreage structure made from thin-face honeycomb sandwich is exposed to the rough and tumble of real world handling, the odds are high that there will be occasional breaches that allow local moisture ingression. It is a myth, however, that widespread moisture ingression is inevitable. It is also a myth that, if moisture ingression occurs, it is inevitably and severely damaging to structural performance. These myths are engendered by real events, but only because those events are misunderstood. J. H. Fogarty (*) The Boeing Company, P.O. Box 516, St. Louis, MO 63166, USA e-mail: [email protected]
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Appl Compos Mater (2010) 17:293–307
In May of 2004, a paper titled The Case Against Honeycomb Core [1] was published. It summarizes the unpleasant experiences of an engineer with over thirty years of service in the aerospace industry and, in doing so, accurately represents the opinions of not just one, but many engineers. As part of its arguments, however, the paper ascribes negatives to honeycomb sandwich structure that have more to do with poor design practice or material choice than with any intrinsic property of honeycomb core. It also contains statements such as “it is just a matter of time before water will find its way into the core of most honeycomb designs and initiate the damage process,” the sort of broad and frightening claim that, even when couched as opinio
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