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The scanning electron microscope (SEM) was used in determining the causes of fracture which occurred in an adhesively bonded aluminum honeycomb sandwich panel. The panel fractured during structural tests conducted while it was submerged in water. Similar panels structurally tested in air did not fracture indicating that applied loads were not the major cause of fracture. A thorough examination of the fractured panel was conducted, and the following factors were identified as the causes of fracture : ( 1 ) Water entered the panel through the adhesive bond on the edge close-out members and then migrated from cell to cell around the edges of the honeycomb core. Porosity in the supported adhesive film and capillaries through the film, which were caused by incomplete bonding to the adhesive carrier, provided paths for water migration. (2) Water caused corrosion both on the honeycomb cell walls and at the edges of the core which were against the adhesive carrier. (3) Adhesive bonding to the core was either nonexistent or destroyed in the presence of water.
The unique features of the SEM provided a new approach to fracture analysis of specimens with complex topographical surfaces. The advantages of directly viewing the specimen in three dimensions at high magnifications are illustrated in SEM photomicrographs.
scanning, electron microscopes, fractography, fractures (materials), honeycomb structures, sandwich panels, adhesive bonding, corrosion, aluminum, loads (forces), porosity, capillary water, water influx, evaluation
Acting group engineer, Chemical Materials Development Laboratory, Lockheed-Georgia Company, Marietta, Ga.