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

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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.