Stiffness and strength of beech, birch and spruce in compression perpendicular to the grain and rolling shear: Influence of moisture content, annual ring structure, and test method

For an efficient material utilization of hardwood in engineering applications, mechanical properties and their relationships to the structure of the material are required. Herein, rolling shear (RS) and compression perpendicular to grain (CPG) stiffness and strength of southern Swedish beech, birch, and spruce were experimentally investigated. The experimental testing champaign consisted of (i) tests in three different ambient climates to quantify the moisture content (MC) influence and (ii) tests on two different characteristic scales to quantify the difference between the testing methods, and thus the influence of the annual ring structure. Since the analysis of variance for the investigated groups of different ambient climates yielded low statistical significance, a board-wise quantification method was applied. The effects of variations in density and annual ring structure were minimized by comparing samples from the same board. RS properties were equally influenced by moisture content in hardwood and spruce, while CPG properties of hardwood were stronger influenced by moisture content than spruce properties. Comparing results from the two different specimen sizes showed that the cylindrical orthotropic structure in the cross-section of boards substantially increased the effective RS strength as compared to smaller scale tests, whereas a weaker opposite trend was observed for CPG strength. RS and CPG stiffness were quite similar on both scales. The influence of the annual ring inclination on the material scale was further compared to multiscale material models for hardwood and softwood. The models showed that hardwood properties are stronger influenced by the density and much less by the annual ring inclination than properties of spruce, in agreement with the experimental analysis.