Fundamental Research on the Development of the Fracture Analysis of Wood Using the Piezoelectric Effect.

本项目的目标是对木材在断裂过程中的压电效应及其在断裂分析中的应用进行基础研究。针对电子电荷测量中常用的电荷放大器不具有静态灵敏度的问题，采用液压控制的循环疲劳试验机，采用静态平均载荷和小循环载荷相结合的方法来测量压电体的电荷。这一改进使得在非常高的应力水平下进行压电测量直到断裂载荷。用圆板试件测量了压电效应的各向异性。检测到了纤维方向加载和垂直于纤维方向加载的压电性，尽管通常观察不清楚。随着应力循环频率的降低，压电荷量减少，特别是在10赫兹以下的减少尤为显著，这可能是由于木材中的湿度导致了电流的泄漏。随着应力水平的增加，压电效应(单位应力的压电电压)略有减小。在断裂起始载荷下显著降低，在接近极限应力时突然增大。测试了冲击载荷(弯曲模式、平行于纤维方向的剪切模式、平行于纤维的单轴模式、径向、切向和LT-45、ﾟ方向和解理模式)下的压电效应。在上述加载方式下均证实了材料的压电性。尤其是在弯曲模式下，在断裂过程中检测到了非常复杂的信号。然而，木材的压电性效应的起源还不够清楚。应进行结晶学和结构分析方面的陈述。

The object of this project was directed to fundamental research on the piezoelectric effect of wood during fracture and its application to the fracture analysis.1. A hydraulically controlled cyclic fatigue testing machine was used to measure the piezoelectric charge by combining a static mean load and small cyclic load, because the charge amplifier which is commonly used for the measurement of electronic charge does not have static sensibility. This improvement enabled the piezoelectric measurement at very high stress level until fracture load.2. The anisotropy of the piezoelectric effect was measured by using circular plate samples. The piezoelectricity in the fiber-direction loading and in the perpendicular to fiber direction loading was detected although usually they had not been clearly observed. Piezoelectric charge decreased as the frequency of the stress cycle decreased, especially the decrease below 10 Hz was remarkable, probably because of the leakage of current by humidity in the wood.3. The piezoelectric effect (piezoelectric voltage per unit stress) decreased slightly with the increase of the stress level. It decreased remarkably at fracture initiation load and increased suddenly at near ultimate stress.4. The piezoelectric effect under impact loadings (bending mode, shear mode parallel to the fiber direction, uni-axial mode parallel to the fiber-, radial-, tangntial-, and LT-45^ﾟ-directions, and cleavage mode) was examined. The piezoelectric effect was confirmed in above all loading modes. Especially in bending mode, a very complex signal was detected during the fracture process.5. However, the origin of the piezoelectric effect of wood has not been enough clarified. A representation on crystallographic and structural analysis should be done.