Effect of knots on stress waves in lumber

Abstract

An impact stress wave was induced in the end of 2 by 6 lumber containing knots. Rather than a normal, perpendicular-to-the-axis profile in transiting by a knot, the stress wave tended to Iead in zones of clear wood in the direction of the slope of grain or slope of the annual rings and to lag behind the knot. Of three methods evaluated to time the stress wave, the "average" timing method was more consistent than other methods in flagging the 6-inch length containing a knot from adjacent clear wood 6-inch lengths. Stress wave modulus of elasticity (ESW) calculaled from the data tended to be lowest with the average timing method. AIl ESW's were higher than ESB (static bending modulus of elasticity); however, the two types of moduli tended to approach each other as knots were removed by ripping away successive 3/4-inch strips down to a specimen width of 1-3/4- inches. The results of this study should provide guidance in establishing stress wave methods for machine grading of lumber.

Citation

Gerhards, C.C. 1982. Effect of knots on stress waves in lumber. Res. Pap. FPL-384. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 1982: 28 pages