Method for determination of the softening behaviour of wood and the applicability of a nonlinear fracture mechanics model
Method for determination of the softening behaviour of wood and the applicability of a nonlinear fracture mechanics model
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木材软化行为的测定方法和非线性断裂力学模型的适用性
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发表时间：
1992
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通讯作者：
L. Boström
中科院分区：
文献类型：
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作者：
L. Boström
本文讨论了木材在纯拉伸和纯剪切下峰后行为的测定。“虚拟裂纹模型”（FCM）是一种非线性断裂力学模型；最初是为了描述混凝土中的裂纹扩展而发展起来的，它被用来证明非线性断裂力学的优点。提出了一种可以确定纯拉伸和纯剪切作用下应力-位移曲线的试验方案。为了证明测量木材的完全应力-位移关系的可能性，进行了实验。这些测试是在一块苏格兰松材（Pinus sylvestris）的材料上进行的。在纯拉伸条件下，研究了最弱方向（即垂直于晶粒）的三种不同晶粒取向和四种不同的含水率。在纯剪切中，只检测一种晶粒取向和一种含水率。根据这项研究，在这类实验中遇到的主要困难是为标本找到合适的几何形状。试件的长度对于稳定性条件是至关重要的。此外，试件被粘接到试验布置中，因此粘接强度和粘接面积与韧带面积的比值是得到木材破坏的重要因素。除了确定软化行为外，还提出了两个使用FCM的例子。第一个例子是紧绷试样的研究。理论分析显示了材料性能对计算的荷载-位移曲线的影响。此外，还进行了一系列试验，并对荷载-位移曲线的计算值与实测值进行了比较。第二个例子是一个非常简化的分析，分析木材在干燥过程中产生的裂缝。计算结果表明，从理论上模拟干燥过程中裂纹的形成和发展是可行的。(少)
The thesis deals with determination of the post peak behaviour of wood in pure tension and pure shear. Furhermore, the "Fictitious Crack Model" (FCM), which is a nonlinear fracture mechanics model ; originally developed for description of crack propagation in concrete, is employed to demonstrate the advantages of non linear fracture mechanics. A testing arrangement is presented with which the complete stress-displacement curve can be determined, both in pure tension and in pure shear. Experiments were made in order to show the possibility of measuring the complete stress-displacement relation for wood. The tests have been conducted on material from one piece of Scots pine timber (Pinus sylvestris). Three different grain orientations in the weakest direction, i.e. perpendicular to the grain, and four different moisture contents have been investigated in pure tension. In pure shear, only one grain orientation and one moisture content have been examined. The main difficulty encountered in such experiments is, according to this study, to find a suitable geometry for the specimens. The length of the specimen is crucial with respect to the stability conditions. Furthermore, the specimens are glued into the testing arrangement, and hence strength of the glue and ratio between glued area and ligament area are important, in order to get wood failure. In addition to the determination of the softening behaviour, two examples are presented where FCM is used. The first example is an investigation of the Compact Tension specimen. The theoretical analysis displays the influence of the material properties on the computed load-displacement curve. In addition : an experimental series was made, and a comparison is conducted between calculated and measured load-displacement curves. The second example is a very simplified analysis of the formation of cracks in wood subjected to drying. The caIculation demonstrates the possibility to theoretically simulate the formation and growth of cracks during the drying process. (Less)