杉木目测等级规格材抗拉强度长度尺寸效应

Size Effect of Length on Tensile Strength of Visually-Graded Chinese Fir Dimension Lumber

以人工林杉木规格材作为研究对象,研究了不同目测等级下规格材测试长度与其抗拉强度之间的相互关系;通过斜率法和形状参数法求出了高、低目测等级组试件抗拉强度5%分位值和50%分位值的长度尺寸效应系数.结果表明:用非参数法求得的高、低目测等级组试件抗拉强度5%分位值的长度尺寸效应系数分别为0.12,0.25,50%分位值的长度尺寸效应系数分别为0.17,0.29,低目测等级组试件的长度尺寸效应系数大于高目测等级组试件;该长度尺寸效应系数可以在人工林杉木规格材的实际应用中使用;利用两参数威布尔分布拟合所测强度时,除高目测等级组试件抗拉强度5%分位值外,用形状参数法估计的参数与用非参数方法求得的参数的倒数近似.

Length size effect is one of important factors to adjust tensile strength of dimension lumber to an allowable property. The relationship between the tensile strength and the length of Chinese fir visuallygraded dimension lumber was investigated. 412 dimension lumbers were divided into two groups.high visual grades（H） and low visual grades（L）, and then sorted into three matched test sub-groups for each visually-graded group by dynamical modulus of elasticity. Three test spans（150 cm, 200 cm and 250 cm） were selected and the tensile strength property was tested. The length size effects of 5th and 50th percentiles of tensile strength were estimated by the slope method and the parametric method. Using nonparametric estimates, the estimated length effect parameters of H and L are 0.12 and 0.25 for the 5th percentile and 0.17 and 0.29 for the 50th percentile, respectively. The parameters of L are larger than those of H. The length size effect could be used when using the dimension lumber for practical purposes. When two-parameter Weibull distribution functions are fitted to the strength data, the estimated shape parameters of the Weibull distribution by the parametric method are almost identical to the inverse of nonparametric parameters except for the 5th percentiles for H.