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Relationship of some coniferous wood strength properties to specific gravity variations within growth increments Homoky, Stephen George John

Abstract

Tensile and compression strength properties of six coniferous woods were studied at the tissue level. Relationships of these properties to specific gravity variations in three adjacent growth increments of each species were explored. Pacific yew was excluded from tensile strength analyses, since the material available did not lend itself to micro-tensile testing. The main purpose of the investigation was to examine, at the tissue level, in what manner specific gravity influences tension parallel and compression perpendicular to grain strengths. Wood strength—specific gravity relationships for gross wood based on earlier studied, were compared to tissue relationships. Distributions of specific gravity and stresses within growth increments of woods having gradual transition from earlywood to latewood, as represented by western white pine, and of woods having abrupt transition, as Douglas fir, were also compared. Feasibility of radial micro-compression test methods established previously for Douglas fir were re-examined and extended to all six species. Experimental material, from freshly felled trees was never dried before physical testing, except western red cedar. Specimens for tensile and compression tests were cut from each increment studied. Micro-specific gravity determinations, based on green volume and oven-dry weight, were performed on broken tensile test specimens after extraction with standard solvents. Physical tests were carried out by established techniques. Regression analysis was employed to establish equations and curves best describing relationships of maximum micro-tensile and micro-compression stresses to specific gravity. Test results revealed highly significant relationship between maximum micro-tensile stress and specific gravity, and between maximum micro-compression stress and specific gravity. The latter relationship is curvilinear, expressed by an exponential curve fitting five of the six species studied. Pacific yew, also significantly correlated to specific gravity at 95 per cent probability, was described by the same basic equation applied to the grouping of the other five woods, but with different constants. This suggests that specific gravity influences maximum micro-compression stress variations in species of greatly different physical and anatomical, characteristics in varying degrees. Comparing tensile and compression stress—specific gravity variations of gross wood with those of wood tissue, it was found that in both properties specific gravity caused greater stress increase of gross wood than of tissue, as illustrated by respective regression lines. No definite trend of specific micro-compression stress within growth increments was found. Specific micro-tensile stress distributions showed a peak-value close to or at the initiation of latewood. Specific gravity, maximum micro-tensile stress and maximum micro-compression stress in woods having gradual transition from earlywood to latewood vary gradually across the increment, suggesting trends of a second degree parabola. In woods where transition is abrupt, the increase of these properties is abrupt at or close to the initiation of latewood. If in such woods the latewood zone is wide the distribution curve is sigmoid. Methods for testing wood tissue in radial compression, as well as theories related to the analysis of stress-deformation curves, have been verified. Ultimate load is recorded at the inflection point on the curve, beyond the proportional limit. At this phase of compression ultimate compressibility of the tracheids is achieved.

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