Fractal Features and Acoustic Emission Characteristics of Wood Fracture

Abstract

In this chapter, the fractal theory is applied to study the fractal feature of the fracture surfaces of five types of woods along the grain and the relationship between the fractal dimension of fracture surface and fracture toughness of wood is established. The results show that fracture toughness parallel to the grains of various woods is different because of their textural diversity and such differences are also shown in the morphology of fracture surfaces. Furthermore, there is an evident and direct proportional relation between the fractal dimension and fracture toughness along grains. Then the evolution characteristics of microstructure during the bending failure of the clear samples and samples with the transverse crack of four types of woods are studied by acoustic emission (AE) technique, and the initiation and expansion of different types of damage of wood component are identified with AE characteristic parameters. The results showed that: (1) 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 in peak load or fracture stage for the standard sample. (2) The initiation and expansion of crack tip could be monitored efficiently by AE technique in the whole process of wood three-point bending test for the notched sample. (3) The AE signals were related to different damage patterns/modes. The AE characteristics of cell wall fracture were high amplitude, high energy, and long duration time 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 time AE events. (4) Kaiser effect appeared at low loading and Felicity effect at high loading under repeated wood bending loading. The Felicity ratio could better indicate the damage degree of wood structure.