初冬时期树枝弯曲性能的试验

以初冬时期北京常见的7个树种树枝为试验材料,将树枝平均分为两份,分别在新鲜状态和自然干燥6个月后进行三点弯曲试验,计算出抗弯模量、抗弯强度和应变,试验结束后分别测出各树种树枝的含水率、基本密度和气干密度。结果表明,初冬时期各树种新鲜树枝的抗弯弹性模量在1 368~4 327 MPa,干树枝的抗弯弹性模量在2 827~6 174 MPa,干树枝的抗弯模量是新鲜树枝抗弯模量的1.29~2.58倍;新鲜树枝的抗弯强度在35.3~65.3MPa,干树枝的抗弯强度在55.3~178.5 MPa,干树枝的抗弯强度提高了53.1%~73.3%;新鲜树枝的应变因树种差异较大,最大可达6.40%,最小为0.54%,干树枝的应变在0.24%~3.36%,各树种干树枝的应变较新鲜树枝均有减小。统计结果表明,初冬时期树枝的弯曲性能与其含水率密切相关,即随着含水率的增加,抗弯模量与抗弯强度降低,而弯曲变形增加。在自然环境载荷作用下,新鲜树枝将发生较大弯曲变形,而干树枝可能被直接折断。

Experiment on Bending Properties of Branches during Early Winter

We studied seven common branches collected at early winter in Beijing.We performed three point bending test for the branches when they were fresh or air-dried after six months, respectively.We calculated the bending modulus of elasticity ( MOE) , modulus of rupture ( MOR) and strain of these branches, and tested the water content, basic density and air-dried density of the fresh and dry branches after the bending tests.MOE of the fresh branches were in 1 368-4 327 MPa, while those of the dry ones were in 2 827-6 174 MPa, about 1.29 to 2.58 times compared with those of the fresh branches. MOR of the fresh branches were in 35.3-65.3 MPa, while those of the dry ones were in 55.3-178.5 MPa, increased by from 53.1%to 73.3%compared with those of fresh branches.The strains of the fresh branches were significantly different between different tree species.The maximum strain was 6.40%, and the minimum strain was 0.54%.The strains of the dry branches were in 0.24%-3.36%.The strains of dry branches were smaller than those of fresh ones.The statistic results in-dicated that flexural properties of the branches during early winter period were closely related to the water content.MOR and MOE decreased while the bending deflection increased with the increasing of the water content.Under the attack of natural environment, the fresh branches would bend easily, while the dry ones would break directly.