A Simple Method for Selecting Balsa (Ochroma Lagopus Sw) of High Tensile Strength

Balsa has a unique combination, of low specific gravity and high strength parallel to the grain which makes it extremely useful as core material in aircraft. Due to a large degree of natural variability in strength the wood must be sorted, because only wood with uniform high tensile strength along the grain can be used. This study devised a simple method to identify high-strength balsa and accurately define its limits. Material used was “AA” grade (aircraft) stock which had failed to pass the scribe test, meaning the presence of “corcho.” Strength variation zones were evident when a cross section was out with a 13-inch circular groovier saw. Fibers were broken, not cut, showing gradations of failure from very fibrous to a brash type with no protruding fibers. To test the accuracy of the saw separation, measurements were made of the tensile strength of each zone. Results gave tensile strengths of 3,958, 2,064, and 1,305 psi, the greatest value corresponding to the most fibrous saw cut, which also had the highest specific gravity. Microscopic examination of the sections revealed that wood of the two lowest strength values had less fiber and, more parenchyma than the strongest wood. These high proportions of parenchyma in relation to fiber were to a great extent responsible for the low tensile strength of so called “corcho” in balsa wood.

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