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Influence of cellulose chain length on the mechanical behavior of Douglas fir wood in tension parallel to grain. Ifju, Geza

Abstract

The cellulose fraction in 100-micron thick microtome sections from three growth increments of a Douglas fir tree was sytematically degraded through random scission of chains by means of 0.1, 1.0, 10.0, and 15.0 megarad doses of gamma irradiation. Degree of cellulose polymerization (DP) was estimated from results of intrinsic viscosity measurements on dilute solutions of cellulose nitrate in acetcne. Control and irradiated samples were tested in tension parallel to grain by employing a micro-scale test method. Tests were done at 25, 50, and 70°C temperatures in combination with moisture-free, air-dry, and water-saturated conditions of test specimens. Ultimate tensile strength, an elastic constant, ultimate tensile strain, and work to maximum tensile load have been calculated from experimental data. Results were statistically analyzed in relation to cellulose chain length, temperature and moisture content. Regression equations based on experimental results have been constructed. These explained a large part of the variations in tensile strength properties and are reported as three-dimensional diagrams. It is shown that tensile strength behavior of Douglas fir earlywood and latewood are distinctly different. Strength properties of latewood are not only higher by a factor of approximately 2 to 8 than those of early-wood, but also the response of the two growth zones to changes in cellulose chain length, temperature, and moisture content at test is different. The above characteristics are due to different deformation mechanisms in tension parallel to grain of the two growth zones. It is suggested that deformation in earlywood is intra-cellular, whereas in latewood it is primarily an inter-tracheid, phenomenon. Decrease in cellulose DP reduced strength, ultimate strain, and work to maximum load more in the low than in the high DP regions. This is explained by the increasing importance of inter-chain and/or inter-fibrillar slippage with decreasing chain length. Elastic properties are but little affected by changes in cellulose DP if the crystalline-amorphous ratio of cellulose in wood is not altered significantly by the treatment applied, such as accompanies gamma irradiation. A change in wood moisture content at time of test from the moisture-free to the water-saturated condition reduced strength properties of Douglas fir by approximately 20 to 50 per cent. The reductions in late-wood strength were significantly higher than in earlywood. A convex upward curve configuration relating strength and elasticity to moisture content is suggested from the experimental data. Effect of temperature on strength properties of Douglas fir within the range of 25 to 70°C is minor in comparison with that of moisture content. The relationship is probably linear. Tensile strength characteristics of Douglas fir wood with degraded cellulose are more sensitive to changes in moisture content than are those of wood having cellulose of long-chain structure. This behavior of wood in tension is also explained by the slippage mechanism of deformation.

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