Longitudinal Stress Waves for Lumber Stress Grading: Factors Affecting Applications: State of the Art

Longitudinal stress waves have the potential for widespread use in stress-grading lumber but more research is necessary to enhance its application. This state-of-the-art paper highlights factors that affect stress-wave transit in lumber and suggests some additional study. One advantage of stress waves is speed; they travel at 10,000 to 20,000 feet per second in wood parallel to the grain. But stress waves are affected by several factors. Stress-wave speed is moderately slower at higher MC or temperature. As a rough approximation, speed is affected by about 1 percent per percent MC change in the hygroscopic range and by about 0.05 percent per degree Fahrenheit. Grain angle has a greater effect on speed: over 1 percent loss per degree increase in grain angle up to about 30?. While stress-wave speed is slowed through knots and the curved grain around knots, a knot does not have much effect on the overall speed of a stress wave in lumber when there is substantial straight grained wood beside the knot. In lumber with cross grain and knots, the stress wave does not propagate with a normal wave front as supposed by the long slender rod theory but has a wave front that leads in the direction of the grain and lags across the grain or through knots. This nonnormal wave front causes some problems in timing stress waves, particularly over shorter spans that may be desirable for stress grading. More research on stress-wave evaluation of short lumber spans, emphasizing lumber strength prediction, should enhance the use of stress wave for lumber stress grading.

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