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

本课题的研究目标是对木材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.针对常用的电荷测量放大器不具有静电灵敏度的问题，采用液控循环疲劳试验机，通过静态平均载荷和小循环载荷相结合的方法测量压电材料的电荷。这种改进使得压电测量能够在非常高的应力水平下进行，直到断裂载荷。用圆板样品测量了压电效应的各向异性。在纤维方向加载和在垂直于纤维方向加载的压电检测，虽然通常他们没有被清楚地观察到。随着应力循环频率的降低，压电荷降低，特别是在10 Hz以下的频率下，压电荷的降低更为显著，这可能是由于木材中的湿度引起的电流泄漏.随着应力水平的增加，压电效应（每单位应力的压电电压）略有下降。在起裂载荷时，应力应变率显著下降，在接近极限应力时，应力应变率突然上升.研究了冲击载荷（弯曲模式、平行于纤维方向的剪切模式、平行于纤维方向、径向方向、切向方向和LT-45方向的单轴模式以及劈裂模式）下的压电效应。在上述所有加载模式下，均证实了压电效应。特别是在弯曲模式下，在断裂过程中检测到非常复杂的信号.然而，木材压电效应的起源还没有得到足够的澄清。应进行晶体学和结构分析。

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.