应力波在落叶松活立木中传播影响因素数值模拟

Numerical Simulation of Influence Factors on Stress Wave Propagation in Larch Standing Trees

探究应力波在落叶松活立木中传播的影响因素,有助于研究应力波在人工林活立木中的传播机理。依据固体介质中的应力波传播理论和弹性力学理论,将活立木看作只由心材层和边材层组成的两层结构材料,基于活立木的正交各向异性假定,利用COMSOL Multiphysics多物理场仿真软件对应力波在活立木中的传播进行了模拟计算,并研究了敲击载荷频率、活立木胸径和心材比对应力波传播的影响。结果表明,应力波波速随着载荷脉冲频率的增大而减小;对于胸径为10cm的活立木模型,当传播距离达到1.2m时,应力波波阵面已经转换为一维平面波,而对于胸径超过30cm的活立木模型,应力波在0~1.2m传播距离内是以三维膨胀波的形式传播;活立木胸径对应力波的传播速度有影响,当胸径小于10cm时,波速较小且基本没有发生变化,当胸径从10cm增加到40cm时,应力波的波速随着活立木胸径的增加而增加,而当胸径超过40cm时,波速略微增加后保持相对稳定;应力波在活立木中的波速随着心材比的增大而减小。胸径对应力波在活立木中的传播形式以及波阵面形状有影响,而敲击载荷频率和心材比对应力波在活立木中的传播形式以及波阵面形状没有影响,但三者都会对应力波的传播速度产生影响,数值模拟最佳的敲击载荷频率为2.5 kHz,应力波在活立木中的传播速度不只取决于边材的力学性能,而是受到心材和边材的共同影响。

The primary purpose was to investigate the influence factors on stress wave propagation in larch standing trees and pave a foundation for further study on the propagation mechanism of stress wave in standing trees.Standing trees were considered to be a two-layers material only consisted of heartwood and sapwood,and the propagation of stress wave in standing trees was simulated.The effect of loading impulse frequency,diameter at breast height(DBH)and ratio of heartwood on stress wave propagation in standing trees were studied by using COMSOL Multiphysics finite element analysis software based on stress wave propagation theory in the solid medium and orthotropic assumption of standing trees.It was found that the velocity of stress wave in standing trees was decreased with the increase of impulse frequency;for a tree model with 10cm DBH,the wave front of stress wave was changed into one-dimensional plane wave as the propagation distance was increased to 1.2m,however,the stress wave still propagated as three-dimensional dilatational wave for a tree model with DBH over 30cm though the propagation distance was increased to 1.2m;DBH had an influence on the propagation speed of stress wave,wave velocity was firstly small and almost no change as DBH was less than 10cm,and then it was increased when DBH was changed from 10cm to 40 cm,finally,slightly increased and remained relatively stable as DBH was over 40cm;the velocity of stress wave in standing trees was decreased with the increase of ratio of heartwood.DBH had an impact on the propagation patterns and shapes of stress waves in standing trees,however,the impulse frequency and the ratio of heartwood had no effect on the propagation patterns and shapes of stress.But all of them had an influence on wave propagation velocity.The optimal loading impulse frequency was 2.5kHz.The propagation velocity of stress wave in standing trees was not only depended on the mechanical properties of sapwood,but in fact relied on both heartwood and sapwood.