Effect of Thermal Treatment on Fracture Toughness of Wood Specimen Dried by Modeled High-Temperature Setting Method

Abstract

The high-temperature setting method may be put to practical use in Japanese sawmills for drying square timbers including pith. In order to prevent splits and cracks, green timber is softened by heating and then rapidly dried at high temperature in low humidity air, so as to generate compression stress on the surface. In this study, we perform wedge splitting tests to obtain the fracture energy, and discuss how thermal treatment affects the timber's fracture toughness.Spruce specimens were curved to perform the wedge splitting test. The curved air-dried specimens impregnated with sufficient distilled water to imitate green wood. Two sets of timber specimens were used : the first set, to test the degradation caused by the initial heating process, and the second set, to examine the further degradation effects attributable to the so-called drying set process. The wet specimens were heated for 12 hours, at one of four temperatures (105°C, 85°C, 75°C and 65°C) in saturated humidity air. For the drying set process, those specimens initially heated at 85°C were dried for 12 hours in four temperature conditions (120°C, 90°C, 75°C and 55°C) in low humidity air (33% of relative humidity).As a result, the fracture toughness of almost all specimens was less than that of the control specimen. In particular, that of heated wet specimen was lower with increasing process temperature. This also seems to be based on the thermal decomposition of wet hemicelluloses, since wet hemicelluloses are thought to decompose at 80-90°C. The specimen dried at 55°C displayed the least toughness of all. As this glass transition temperature of wet lignin is approximately 60°C, this specimen was probably insufficiently softened to prevent splits and cracks from occurring on the surface.