In situ tensile tests parallel to the grain were carried out in an Environmental Scanning Electron
Microscope (ESEM) chamber on Norway spruce (Picea abies [L.] Karst.) samples. The ESEM-mode
combined with a cooling device allowed examination of the specimens at a moisture content
of 12% with unsputtered surfaces. By recording load-displacement curves and observing crack
propagation simultaneously, a detailed image of fracture progress and tissue interaction could be
described. Since these experiments required a sufficient specimen size and geometry, focus was
concentrated on the methodology.
The green wood of twelve deciduous tree species was investigated regarding its radial and tangential
moduli of elasticity measured in tension (ER and ET, respectively). In addition, the wood density and the
volume fraction of rays were determined. A strong positive correlation was found between structural and
stiffness properties. A simple two component model was derived for the relationship between the transverse
elastic anisotropy factor (i.e., ER/ET) of the green wood and the relative volume fraction of the axial
and ray tissues. In the radial direction of the wood, the modulus of elasticity is influenced by the wood
density and the volume fraction of rays; in tangential direction only the density seems to be important.
However, the comparison between the elastic anisotropy and the volume fraction of rays indicates that
the rays may have an indirect influence due to their shape.
Crack propagation in wood is strongly influenced by the microscopic structure of the material. The
relationship between structure and function with regard to damage and fracture behaviour can
only be understood with a sufficiently fine level of examination. An experimental approach to perform
micro-wedge splitting tests on spruce and beech inside the chamber of an Environmental
Scanning Electron Microscope and under atmospheric conditions is presented. The specimens are
loaded in mode I in the TR crack propagation system. Based on the load-displacement diagram,
the characteristic parameters of fracture energy, critical load and initial elasticity are determined.
The load and displacement data for the in situ experiments are related to the obtained ESEM images
and allow a discussion of the fracture process on the cellular level. Density was found to be an
important factor for fracture mode and several crack arresting phenomena depending on the variation
of elasticity could be identified.