Some Variables Affecting Microtensile Strength of Douglas-Fir

Tension parallel to the grain tests were conducted on green specimens of Douglas-fir 0.003 inch by 0.06 inch by 1.75 inches. Strength ranged from 3165 psi to 9243 psi with a mean of 5599 psi. Specific gravity based on green volume was found to average 9.28 for springwood and 0.70 for summerwood. The only highly significant correlation coefficient when springwood and summerwood specimens were considered separately, was between tracheid length and cellulose content. Multiple correlation indicated that 78 percent of tensile strength variation was accounted for by cellulose content, percent cell wall, tracheid length, and fibril angle. Springwood and summerwood specimens failed differently with springwood failing in tension across cell walls and summerwood failing partly in shear. The heritability of special attributes of extremely good trees should be determined

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