Effects of Size and Moisture on Stress Wave E-rating of Structural Lumber

摘要

This paper discusses the effects of size and moisture content on stress wave properties of wood and evaluates the feasibility of using stress waves to E-rate Douglas-fir dimension lumber in green condition. Six size groups of Douglas-fir lumber were tested using both stress wave and static bending methods at different moisture levels, ranging from green to dry conditions. Stress wave speed increased continuously as moisture content decreased through the whole moisture range studied. General speed-moisture relationships were developed to adjust stress wave speed values from one moisture condition to any other moisture conditions. It was found that stress wave based dynamic modulus of elasticity (MOE) is generally higher than static bending MOE. However, the difference tends to be constant when moisture content is above fiber saturation point. This suggests that green lumber can be sorted effectively based on dynamic MOE values even if there are moisture differences between individual pieces.