Elastic Deflection of Wood-Fiberglass Composite Beams

Thirty 1- by 1- by 16-inch composite beams were made with cores of six wood species and faces of fiberglass and epoxy resin 0.05 inch in thickness. The 0.9-inch cores ranged in specific gravity from 0.08 for balsa (Ochroma lagopus) to 0.92 for hickory (Carya sp.). The modulus of elasticity of the wood cores ranged from 208,000 psi for balsa to 2,908,000 for hickory. The fiberglass faces had an MOE of 4,370,000. The beams were conditioned to approximately 10 percent moisture content. Load was applied to the center of a 14-inch span as prescribed in ASTM D 143. Loading was taken to failure which characteristically took the form of compression failures in the uppermost fibers of the wood core. A second series of matched all wood beams was machined to 0.65 by 0.65 by 16 inches. These beams were initially loaded to approximately one-half of proportional limit at a span-depth ratio of 22:1. They were then tested successively at span-depth ratios of 18:1, 14:1, 11:1, and 8:1. These data were plotted and average values for pure E and modulus of rigidity were determined. The modulus of rigidity of the composite beams was then predicted by two methods. The first neglected core shear and the second took shear into consideration. When compared with the actual results, the prediction by the first method was always high and was poorest for balsa core where the difference was ? percent. In the second method where shear was taken into account predicted values were generally slightly lower than the actual MOE and were also closer to the actual MOE. This method, which takes into account shear contributions to the deflection, should be equally applicable over a wide range of span-depth ratios and is preferred for predicting the MOE of wood-fiberglass composite beams.

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