关键词: 华北落叶松, 边缘木, 朝向, 物理力学性质, 裂缝

Abstract:

Objective: Through resolving the differences in the physical and mechanical properties of Larix gmelinii var. principis-rupprechtii (larch) timber with different north-south orientation, this study explored the mechanisms underlying crack development in wooden columns during outdoor exposure, aiming to provide scientific theoretical guidance for the rational utilization of larch logs and solid wood. Method: Based on the growth characteristics of the trees, the southern edge and the northern edge trees of larch on the shady slope were selected as research subjects to analyze the influence of north-south orientation on growth ring width, microstructure, and physical-mechanical properties of wood. Additionally, an outdoor exposure test of log columns covered with a roof was performed to explore the impact of orientation on cracking in log columns. The changes in the length and width of cracks, as well as the crack locations, were recorded at monthly intervals. Result: Influenced by the comprehensive environmental factors, both the south edge and the north edge trees of larch exhibited wider annual ring of the northward growth. At the same tree height of a larch tree, there were no significant differences in microscopic morphology between the north-facing and south-facing samples. The air-dried moisture content of the north-facing samples was higher than that of the south-facing samples, but the density and shrinkage in all orientation of the south-facing samples were higher than those of the north-facing samples. The variation pattern in mechanical strength was highly positively correlated with the density trend. The north-south difference in mechanical strength of the northern edge wood was the greatest, and the bending strength, bending modulus of elasticity, and parallel-to-grain compressive strength of the south-facing sapwood were 53.7%, 76.9%, and 21.4% higher than those of the north-facing sapwood, respectively. During the 10-month outdoor exposure test of the larch log columns, cracks appeared rapidly after the bark was removed due to the moisture gradient formed between the inside and outside of the log. The moisture content of the sapwood decreased rapidly from 110%?130% to 54%?70%, while the moisture content of the heartwood remained relatively stable at 30%?38%. At this point, cracks were primarily manifested by an increase in length, with the width remaining relatively unchanged. When the moisture content of the sapwood decreased to the fiber saturation point (15%?21%), the crack length remained stable, but some cracks continued to widen. As the overall moisture content of the log columns dropped to 13%?15%, the crack morphology tended to stabilize. The south edge larch samples were more prone to develop larger cracks compared to the north edge ones. Sunlight exposure was the key factor influencing the crack distribution in the upright log columns. The exposure to sunlight due to their orientation can be a dominant factor influencing the distribution of cracks in the upright log columns. Conclusion: Within the scope of this experiment, the width of growth rings is a determinative factor for the differences in the physical and mechanical properties of wood between the southern and northern edge trees of larch. By measuring the width of the growth rings within the same larch plant, the differences in wood properties at different positions can be predicted. Moreover, cracks in log columns are more likely to propagate on surfaces with narrower growth rings during the outdoor exposure process of wooden columns. In practical use, positioning the wider growth ring side of the log columns against the sun can effectively reduce the appearance of cracking.

Key words: Larix gmelinii var. principis-rupprechtii, border tree, orientation, physical and mechanical properties, crack