Mode I fracture properties of thermally-modified spruce wood (Picea abies) at different moisture contents

The influence of moisture content and density on fracture properties of unmodified and thermally modified spruce wood (Picea abies) has been investigated. Thermal modification was done at 180 & DEG;C, 200 & DEG;C, and 230 & DEG;C, and samples were conditioned at 20%, 44%, 76%,% and 88% relative humidity. Double cantilever beam specimens with TL and RL orientation were used for testing. The crack length was determined via digital image correlation, and the compliance of the specimen was determined via multiple loading-unloading cycles of the specimen. The strain energy release rate was calculated, which decreased with an increasing degree of modification and increased with increasing wood moisture content. The analysis showed that the strain energy release rate increased more with wood moisture content for unmodified samples, while the dependence of the release rate on moisture was less obvious for modified samples. The strain energy release rate values are higher in the TL orientation than in the RL orientation, while the crack length has no significant influence. The highest strain energy release rate was 0.256 N/mm for the unmodified TL specimens exposed to 88% relative humidity, and the lowest was 0.071 N/mm for the RL orientation of the thermally modified specimens at 230 & DEG;C and then exposed to 88% relative humidity. From all measurements, functions were fitted to calculate the strain energy release rate as a function of orientation, degree of modification, moisture content, and density of the spruce wood, which can be used to calculate the strain energy release rate for any combination of the factors studied.