Stress Wave Speed and MOE of Sweetgum Ranging from 150 to 15 Percent MC

Stress-wave techniques, considered as alternate means to machine grade lumber, have been studied to some extent with dry lumber but not with green lumber. A set of clear sweetgum sapwood 2 by 4’s with initial moisture contents of about 150 percent were used to evaluate the effect of moisture contents above fiber saturation on speed of stress waves and modulus of elasticity (E) calculated from basic stress-wave formulas. Speeds of longitudinal and flexural stress waves, density, and static flexural stiffness were measured on the 2 by 4’s at the original condition and after several stages of drying down to 15 percent moisture content. Several different measures of elasticity were calculated from the data for comparison. The speed of both types of stress waves increased curvilinearly but at different rates as moisture content decreased. E calculated from the basic longitudinal stress-wave formula was strongly dependent on moisture content over the whole range studied, attaining a very high value at 150 percent. E calculated from the basic flexural stress-wave formula, however, did not change much at moisture contents above about 50 percent. Longitudinal stress wave E was considerably higher than either the flexural stress wave E or the center point loading E from static bending. Although the differences could not be justified by correction for shear or rotary inertia, the various E’s followed similar trends with decreasing moisture content below about 40 percent. Based on model calculations, the results of this study have implications concerning stress grading of lumber. The longitudinal stress-wave technique may overestimate the static E substantially for lumber much above 50 percent moisture content. The E of kiln-dried lumber containing wet pockets may also be overestimated by an amount that is a function of moisture content and size of wet pocket. When applied to short spans of lumber, the longitudinal stress-wave technique may downgrade “dry” lumber containing wet pockets. The estimation of E from flexural stress waves may also be affected by wet pockets, depending on their position as well as their size.

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