Comparative shear strength of seven types of wood composite panels at high and medium relative humidity conditions

Two shear properties, edgewise shear and interlaminar (or rolling) shear, were evaluated for seven types of composite panels at 85 percent and 50 percent relative humidity (RH), with temperature held constant at 72?F. Composite panels tested were: medium density fiberboard (MDF), hardboard, waferboard of two thicknesses, oriented strandboard (OSB), particleboard, and plywood. Average density of panels ranged from 30 to 64 pounds pcf, while average equilibrium moisture content ranged from 13.8 percent to 15.0 percent for the 85 percent RH and 16.2 percent to 8.1 percent for the 50 percent RH. Over 900 samples were tested in edgewise shear, and 160 in interlaminar shear. From the samples tested, it was found that hardboard had the highest edgewise shear strength and particleboard the lowest at both humidity conditions. The particleboard tested was the only board type that had an edgewise shear strength lower than plywood. In interlaminar shear, MDF was found to have the highest shear strength. Hardboard and thick waferboard had the lowest interlaminar shear strengths at 85 percent and 50 percent RH, respectively. At 85 percent RH, hardboard and waferboard had lower interlaminar shear strength than plywood. Among the boards tested at two RH conditions, MDF and OSB, exhibited equal or greater shear strength than plywood. As expected, all panel types displayed a lower shear strength at the higher humidity condition. From 50 percent RH to 85 percent RH, waferboard, particleboard, and hardboard exhibited a reduction of over 30 percent in edgewise shear strength; the greatest reduction among the seven types of boards tested. In interlaminar shear, hardboard and waferboard had the greatest strength reduction (25% to 34%) due to humidity changes.

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