Selection of Low-Strength Pieces By Nondestructive Testing: Douglas-Fir and Hem-Fir 2 By 4’s in Flexure

Several nondestructive methods for predicting modulus of rupture (MOR) were applied to four randomly selected samples of 2- by 4-inch dimension lumber. Each sample included 100 pieces of one grade and one species. Standard and No. 1 and better grades of Douglas-fir, plus standard and select structural grades of hem-fir were included. The objective was to determine which nondestructive method was best, and how efficient each method was, in identifying from each grade the five (5 percent) pieces lowest in MOR. Then, just a part of a population would need to be tested to destruction to establish the 5-percent exclusion value for purposes of design. Two nondestructive methods requiring loading pieces on edge were better than those in which pieces were loaded flatwise over different spans and those in which MOE was measured by a stress-wave timer. One method of edge-loading was to load the specimens at third-points of the span with the worst edge down (in tension); the other method was to load the piece over a 3-foot span with a single center load oriented at the critical (weakest) cross section and with the worst edge of the piece down. Both methods of loading on edge indicated that about the weaker one-third of the sample, as indicated by measurement of MOE, included the 5 percent of the pieces lowest in MOR. With some of the other nondestructive methods, as much as the weaker three-fifths of the sample (as indicated by MOE) was needed to include the 5 percent lowest in MOR.

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