Development of the five-point bending test to determine shear moduli of wood composites

Current American Society for Testing and Materials (ASTM) methods of determining interlaminar shear moduli of structural wood composites are laborious and prohibitively expensive. Consequently, limited information about these properties exists, and virtually no quality control monitoring of them is performed. This paper investigates the feasibility of using quarter-point and five-point loading of the same specimen to simultaneously determine shear modulus (G) and shear-free Young’s modulus (E). A 1-1/8-inch-thick oriented strandboard was tested over numerous spans in both flatwise and edgewise orientations. This enabled evaluation of the two principal interlaminar Gs, the in-plane (edgewise) G, and corresponding Es. Superposition and Timoshenko beam theories are used to characterize midspan deflection of five-point loading, and the subsequent effects on calculation of G are compared. In addition, two techniques for measuring midspan deflection of quarter-point loading — pure-moment deflection with respect to the load head and total deflection — are compared as methods of evaluating E. Results indicate that E and G are generally independent of span length. For accurate measurements of G, however, a test span should be chosen such that at least 40 percent of the five-point midspan deflection is caused by shear deformation. Superposition theory was the preferred method of calculating E and G because it gave accurate results more simply. Additionally, total midspan deflection of quarter-point loading provided better determination of E than did pure- moment deflection. Comparison between five-point results and those of ASTM standards for interlaminar shear are also presented.

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