To study wood fracture on its cellular level, small tensile specimens of pine (Pinus sylvestris [L.])
were fractured in situ in tension inside the chamber of an ESEM (Environmental Scanning Electron
Microscope). Fractured surfaces of macroscopic tensile test specimens were also studied with
an ESEM. The same kind of fracture phenomena were observed in both small and large specimens.
The in situ tests proved to be reproducible and the results revealed typical fracture propagation0
directions and order in softwood under longitudinal tension. The gradual change of material
properties of wood in the radial direction was found to strongly influence the fracture process.
Tests under mode I and mode III loading were performed on side grooved Compact-Tension specimens
of larch and beech under steady state crack propagation to study the damage and fracture behaviour and
the influence of two fibre orientations. From the complete load-displacement diagram, all important
damage and fracture mechanical values (stiffness/compliance, microstructural damage, crack initiation
energy, specific fracture energy, etc.) have been determined. Crack initiation energy and specific fracture
energy are approximately ten times higher for mode III loading than for mode I loading in both wood
species. Crack initiation occurs in mode III under external mode III loading, crack propagation, however,
takes place under mode I, owing to crack surface interference. The influence of fibre orientation on
the (fracture) mechanical properties of beech and larch is different. This difference may be explained
mainly by the high number of rays in beech.
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.
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.