This invention relates generally to structural laminates, and more particularly to a laminate formed by thin facing sheets bonded to a core panel of reticulated end-grain balsa wood to provide a sandwich laminate of exceptionally light weight and high strength.
Balsa has outstanding properties unique in the field of lumber, for on the average it weighs less than nine pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength that is superior to any composite or synthetic material of equal or higher density. Balsa is dimensionally stable and may be processed by standard woodworking techniques.
It is known that end grain balsa wood is capable of supporting far greater loads than flat-grain material of the same density and that low-density balsa in the end-grain direction will support greater loads than flat grained material of higher density. The cellular structure of balsa is such that the number of cells per cubic foot is extremely high, the wall thickness of each cell being quite thin. The cells are effectively independent of each other, each cell being comparable to an independent column or fiber. The fibers are substantially parallel to each other.
Structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end grain balsa, the resultant laminates having a remarkably high strength-to-weight ratio as well as excellent thermal insulation properties.
End-grain balsa-core sandwich laminates are widely used in transportation and handling equipment, such as for floors of railroad cars, shipping containers, cargo pallets, bulkheads, doors and reefer bodies, as well as in a variety of other applications. These laminates are also employed for structural insulation in aircraft applications, in housing and in boating.
There are certain applications where the weight of existing types of laminates having a solid balsa core still exceeds optimal requirements despite the fact that balsa is inherently of light weight. Thus in aircraft flooring or in radar domes constructed of balsa-wood sandwich laminates, it is desirable that the weight of the laminate be reduced without materially impairing the strength of the laminate.