Tensile Strength Behavior as a Function of Cellulose in Wood

Test material was obtained from freshly cut Douglas-fir. Tangential microtome sections of 100 microns thick were cut from three adjacent growth increments. The cellulose fraction in the sections were degraded through random scission of chains by means of 0.1, 1.0, 10.0 and 15.0 megarad integral doses of gamma irradiation. Through irradiation the essential change induced was in the degree of polymerization (DP) of cellulose and hemicelluloses. Tension test specimens 2.5 by 100 milli meters, taken from the same relative position of each growth increment were prepared by punching the selected microtome sections, using a specially machined cutting die fixed on a 1/2-ton converted arbor press. The tensile tests were conducted on a table model Instron tensile testing instrument. The total area under the load-elongation curve was measured using a planimeter. Specimen thickness was measured by a precision dial indicator and width determined with the aid of microscope. Each specimen was placed in a plastic bag before test. Temperature at time of test was kept under close control. Moisture-free, air-dry, and water saturated conditions were chosen for the tests to cover the widest possible range of strength variation in relation to moisture content. Six early- and latewood samples were tested at each temperature, moisture content, and cellulose chain length combination. A total of 540 samples were used in the tensile tests. Each series of residual sections from specimens used for mechanical test was used for cellulose DP determination. It was divided into early- and latewood zones. From each of the three increments, two DP measurements were made at each of five cellulose DP levels. Cellulose DP was estimated from results of intrinsic viscosity measurements on dilute solutions of cellulose nitrate in acetone. Intrinsic viscosity values were calculated from viscosity data using Schulz-Balschke equation and Davision’s relationship. Test results indicate that slippage mechanism is an important contributor to development of ultimate tensile strength in wood. Tensile strength properties of latewood are not only distinctly higher than those of earlywood, but also the response of the two growth zones to changes in cellulose DP, moisture content, and temperature is different. Decrease in cellulose DP reduces strength and ultimate strain more in the low than in the high DP regions. It is assumed that latewood tracheids with their thick secondary wall may be stronger in tension than the cementing material in its resistance to longitudinal shear stresses. Tests also showed that elastic properties in tension are little affected by random depolymerization of cellulose, and that tensile strength of wood with low DP cellulose is more sensitive to moisture changes than that of wood having cellulose of long-chain structure.

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