Friday, May 1, 2009
The rising demand for new materials with higher strength to weight ratios has created a dramatic growth in sandwich composite technology. Sandwich construction employs a lightweight core that has a flexural strength and flexural modulus far exceeding that of the skin laminates alone. The most common type of core material is honeycomb, which is used primarily in the aerospace industry.
The main disadvantage of honeycomb type cores used in the Aircraft and Aerospace industries is that of delamination which can cause a catastrophic failure of the vehicle. This is caused primarily by the failure of the epoxy adhesives to maintain a bond between the facing skins and the core because of the very small bonding area that honeycomb cores edges offer.
This is further exacerbated by the fact that honeycomb type cores create pockets of trapped air within the closed cells of the core when the skins are attached. The air pressure experienced at high altitudes is much lower than the trapped air within the cells with the result that the skin is pushed away from the inner core by the air pressure. Ingress of water into already partially delaminated cells at high altitude freeze into ice particles which expand and force the skin to separate from the core. Eventually after many cyclic operations the skin will delaminate. Additionally, lightning strikes can cause entrapped moisture within the core to immediately turn to steam with catastrophic results to the integral strength of the panels.
The Aerospace Industry remains the greatest consumer of honeycomb materials, whether for civil aircraft, military jets, helicopters, aero-engines or the newer space satellite and launchers.
The Director of NASA’s Marshall Space Flight Center once stated that “our (composite) technology has not yet advanced to the point that we can successfully develop a new reusable launch vehicle that substantially improves safety, reliability and affordability.” He was referring to the failure of the composite fuel tank panels of the NASA X-33 Reusable Space Vehicle due in part to honeycomb delamination.