Variation of the tree ring micro-hardness demonstrated on spruce wood

Abstract

Micro-hardness was used for the study of wood structure (Norway spruce) in the line perpendicular to tree rings (radial surface). The steel indentor 0.25 mm in diameter with flat head was used for this purpose. The individual penetration tests were performed at constant velocity 0.0167 mm/s into a depth of 0.3 mm. Local wood strength was defined as the mean pressure on the indentor head at 0.02 mm penetration. The set of tests (∼ 320) gave information about stress variation in dependence on the location of the test place in the tested surface. The stress was understood as a parameter describing the growth properties of wood similarly like the density usually used in dendrochronology. The measured strength variation is in agreement with visually observed tree rings. The acquired data made it possible to determine the mean characteristic points of the tree ring as well as the development of the parameters in dependence on the weather variations.