ESTIMATION OF TENSILE STRENGTH PARALLEL TO GRAIN OF WOOD SPECIES

ABSTRACT According to the ABNT NBR 7190: 1997 standard, tensile strength of wood samples shall be defined as the ratio between the ultimate strength from a stress test and the specimen cross-sectional area (350 mm2). However, due to the complex wood anatomy and experimental difficulties, the fractured surface in tested samples are not perpendicular to the loading direction; therefore, such fractured surface becomes larger than the sample cross section, overestimating tensile strength by this method. Using an alternative approach, we aimed to determine the approximate fracture surface of wood specimens subjected to tensile test. For this purpose, we used the least square method to estimate tensile strength of four hardwood species, which were equally divided into strength classes. Then, we determined the relationship between the cross-sectional areas of intact and fractured specimens. The results showed that the approximate area of a fractured surface was 2.14 higher than that of an intact sample. As a result, tensile strength estimates were 47% lower than those currently estimated, which is unfavorable for the safety of construction structures.

摘要 按照巴西国家标准ABNT NBR 7190:1997的规定，木材试样的抗拉强度应定义为应力试验测得的极限强度与试样横截面积（350 mm²）的比值。然而，由于木材解剖结构复杂且试验存在诸多难点，受试试样的断裂面并不与加载方向垂直，因此断裂面面积大于试样原始横截面积，导致该方法测得的抗拉强度被高估。本研究采用替代试验方案，旨在测定受拉木材试样的近似断裂面面积。为此，我们选取四种阔叶材树种的试样，按强度等级均匀分组，并采用最小二乘法对其抗拉强度开展估算；随后建立了完好试样与断裂试样的横截面积之间的关联关系。结果表明，断裂面的近似面积较完好试样横截面积高出2.14倍。据此，修正后的抗拉强度估算值较现行方法所得结果低47%，这对建筑结构的安全性而言是不利的。