ACOUSTIC EMISSION CHARACTERISTICS AND FELICITY EFFECT OF WOOD FRACTURE PERPENDICULAR TO THE GRAIN

The processes of wood fracture perpendicular to the grain of standard and notched specimens of one softwood and one hardwood were studied for the timbers' acoustic emission (AE) characteristics. The fracture evolution and mechanisms observed from their microscopic structures are discussed. AE event counts developed slowly and most were the low amplitude AE events at the low strains and a large number of high amplitude AE events appeared at the peak load or fracture stage for standard specimens. The initiation and expansion of the crack tip could be monitored efficiently by the AE technique in the three-point bending test for notched specimens. The AE signals were related to different damage modes. The AE characteristics of cell-wall fracture were high amplitude, high energy and long-duration AE events but the AE characteristics of cell-wall damage and spallation, cell-wall buckling and collapse were low amplitude, low energy and short-duration AE events. The Kaiser effect appeared at low loading and the Felicity effect at high loading under repeated wood bending loading. The Felicity ratio could better indicate the degree of damage of the wood structure.