The Effect of Moisture Content on the Micro-damage Processes of Spruce Wood, Investigated by Acoustic Emission Method and Electron Microscopy

Abstract

Spruce was investigated by acoustic emission during tensile tests. The examined moisture contents were 0%, 12%, and 26%. The fracture surfaces were photographed using electronmicroscopy. The experiments showed, that micro-damage processes start near the ultimate strength, regardless of moisture content. This indicates the brittle behaviour of wood. The analysis of detected acoustic events at different moisture contents indicated that the number and properties of events supposedly resulting from breaking do not change with increasing moisture content. Decrease occurs in the total number of events at higher moisture contents as a consequence of the increasing acoustic attenuation of wood and the elimination of friction type events. Electron microscopic analysis of fracture surfaces supports the result of acoustic emission experiments. The fracture surfaces showed characteristic brittle tension and shear across the cell wall of different cells. Based on both investigations we can say that wood has brittle fracture characteristics in the 0-30% moisture content range.