Center for Forest Damage and Risk Management, Forestry and Forest Products Research Institute (FFPRI), Tsukuba, Ibaraki, Japan.
PLoS One. 2021 Jan 15;16(1):e0245631. doi: 10.1371/journal.pone.0245631. eCollection 2021.
The wind force acted on a tree constantly changes in magnitude, direction, and distribution. We developed a method to measure simultaneously the amount of force (F), centroid of the distributed force (C), and direction of force (D) on a tree trunk using four strain gauges. F and C were estimated from the difference in the bending moments at two different positions along the long axis of the stem. D was estimated using the difference in the sensor outputs at two different radial positions at the same height. In principle, the two strain gauges should be oriented precisely 90° apart; however, this is unrealistic on an actual tree trunk. To calculate D, we developed a new method to detect the radial position and modulus of elasticity of each strain gauge after attaching it. We conducted three types of experiment. First, we loaded a wood pole with weights arranged in 11 patterns to test the accuracies of F and C for a distributed load. Next, we applied tensile forces to the wood pole and an evergreen conifer sapling from eight directions to test the accuracy of D, F, and C. On average, estimation errors were < 2% for both the distributed load and circumferential tensile load. Our method can estimate F, C, and D precisely, even if the wood is uneven and the strain gauges are not aligned. This is a great advantage for field wind force measurements.
风力作用于树木的大小、方向和分布不断变化。我们开发了一种使用四个应变计同时测量树干上力的大小（F）、分布力的质心（C）和力的方向（D）的方法。F 和 C 是通过沿茎的长轴在两个不同位置的弯曲力矩的差异来估计的。D 是通过在同一高度的两个不同径向位置的传感器输出的差异来估计的。原则上，两个应变计应该精确地定向 90°分开；然而，这在实际的树干上是不现实的。为了计算 D，我们开发了一种在将应变计连接后检测每个应变计的径向位置和弹性模量的新方法。我们进行了三种类型的实验。首先，我们在木杆上加载了以 11 种模式排列的重量，以测试分布载荷下 F 和 C 的精度。接下来，我们从八个方向对木杆和常绿针叶树苗施加拉伸力，以测试 D、F 和 C 的精度。平均而言，对于分布载荷和周向拉伸载荷，估计误差均<2%。即使木材不均匀且应变计未对准，我们的方法也可以精确估计 F、C 和 D。这对于现场风力测量是一个很大的优势。
