Effect of closely spaced knots on the failure mechanism of timber beams

Khaled A. Saad, A. Lengyel
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Abstract

linear This research presents a parametric three-dimensional finite element study on the effects of closely spaced knots and related fibre deviations on the flexural failure mechanism of wood beams. The model considers the effects of the position of the knots along the beam's longitudinal and vertical axis. The numerical models were validated by bending tests performed on six timber beams. The actual three-dimensional geometry of knots and related fibre deviations were determined accurately based on an algorithm proposed previously by the authors. The elastic-plastic constitutive law of Nordic Spruce wood was considered based on the Hill anisotropic model. The failures were numerically predicted with the help of the Tsai-Wu failure criterion. The validated numerical models can also be used based on visual inspections. The user needs only to define the position and size of the knots and the space between them. Moreover, the model allows defining different fibre patterns in the knot vicinity. The model considers a fixed knot located in the tension zone at the mid-span of the beam and a moving knot adjusted at horizontal and vertical centre-to-centre distances d and v from the fixed one. Results revealed that regardless of the distance d (where v = 0), the failure will initiate at the same load levels for both knots. However, moving the adjacent knot diagonally ( v not equal to 0) causes shear failure between the knots. The part of the clear wood between the knots is ineffective if the knots' centre-to-centre distance is less than three times the knot diameter.
密距节对木梁破坏机理的影响
本文采用参数化三维有限元方法研究了密节及相关纤维偏差对木梁受弯破坏机制的影响。该模型考虑了节点沿梁的纵轴和纵轴位置的影响。通过对6根木梁进行弯曲试验,验证了数值模型的有效性。根据作者先前提出的算法,准确地确定了结的实际三维几何形状和相关纤维偏差。基于Hill各向异性模型,研究了北欧云杉木材的弹塑性本构规律。利用Tsai-Wu破坏准则对其进行了数值预测。经验证的数值模型也可用于目视检测。用户只需要定义结的位置和大小以及它们之间的空间。此外,该模型允许在结附近定义不同的纤维模式。该模型考虑位于梁跨中张力区的一个固定结和一个移动结,移动结与固定结之间的水平和垂直中心距离分别为d和v。结果表明,无论距离d(其中v = 0)如何,两个节的故障将在相同的负载水平上启动。然而,沿对角线移动相邻的结(v不等于0)会导致结之间的剪切破坏。如果结的中心到中心的距离小于结直径的三倍,那么结之间的透明木部分是无效的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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