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S. Tschegg, D. Keunecke, E. Tschegg:
"Fracture tolerance of reaction wood (yew and spruce wood in the TR crack propagation system";
Journal of the Mechanical Behavior of Biomedical Materials, 4 (2011), 688 - 698.

The fracture properties of spruce and yew were studied by in-situ loading in an
environmental scanning microscope (ESEM). Loading was performed with a microwedge
splitting device in the TR-crack propagation direction. The emphasis was laid on
investigating the main mechanisms responsible for a fracture tolerant behavior with a
focus on the reaction wood. The fracture mechanical results were correlated with the
features of the surface structure observed by the ESEM technique, which allows loading and
observation in a humid environment. Some important differences between the reaction
wood and normal wood were found for both investigated wood species (spruce and yew),
including the formation of cracks before loading (ascribed to residual stresses) and the
change of fracture mode during crack propagation in the reaction wood. The higher crack
propagation resistance was attributed mainly to the different cell (i.e. fiber) geometries
(shape, cell wall thickness) and fiber angle to the load axis of the reaction wood, as basic
structural features are responsible for more pronounced crack deflection and branching,
thus leading to crack growth retardation. Fiber bridging was recognized as another crack
growth retarding mechanism, which is effective in both wood species and especially
pronounced in yew wood.

Fracture tolerance, Reaction wood, Spruce, Structure-property relationship