Long-term fracture toughness of wood

Long-term fracture toughness of spruce, pine and Kerto LVL material has experimentally been determined. In mode II loading (forward shearing mode) an end-cracked beam specimen was used, while the compact tension specimen was used in mode I loading (opening mode). The specimens were exposed to a constant load, while the relative humidity was cyclically changed between 40 and 90 %. The length of a whole cycle was two weeks. In mode II loading the cyclical humidity conditions reduced the time to failure drastically in contrast to constant humidity conditions. For an end-cracked spruce beam loaded to 75 % of its short-term capacity the life time was reduced from 66 to 4 days. The duration of load effect is similar for pine material under cyclic humidity conditions. For Kerto LVL material the effect is more severe. In mode I loading no general conclusions could be drawn, as so few specimens were broken during the loading time of seven months.