DEVELOPMENT OF THE 5-POINT BENDING TEST TO DETERMINE SHEAR MODULI OF WOOD COMPOSITES

Current American Society for Testing and Materials (ASTM) methods of d etermining interlaminar shear moduli of structural wood composites are laborious and prohibitively expensive. Consequently, limited informat ion about these properties exists, and virtually no quality control mo nitoring 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 mod ulus (E). A 1-1/8-inch-thick oriented strandboard was tested over nume rous spans in both flatwise and edgewise orientations. This enabled ev aluation of the two principal interlaminar Gs, the in-plane (edgewise) G, and corresponding Es. Superposition and Timoshenko beam theories a re 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 loadi ng- pure-moment deflection with respect to the load head and total def lection - are compared as methods of evaluating E. Results indicate th at E and G are generally independent of span length. For accurate meas urements of G, however, a test span should be chosen such that at leas t 40 percent of the five-point midspan deflection is caused by shear d eformation. Superposition theory was the preferred method of calculati ng E and G because it gave accurate results more simply. Additionally, total midspan deflection of quarter-point loading provided better det ermination of E than did pure-moment deflection. Comparison between fi ve-point results and those of ASTM standards for inter-laminar shear a re also presented.