Summary
The effect of hydrostatic pressure on the density, the ultrasonic velocities and the microstructure of
spruce and cherry wood has been studied. Generally speaking, under hydrostatic pressure wood becomes
less heterogeneous and less anisotropic than natural wood. In spruce, crushing and buckling of the thin-walled
cells in the earlywood takes place. This also has the effect of disrupting the medullary rays, which
assume a zig-zag path through the structure. Cherry has a much more homogeneous structure, and the
main effect of the hydrostatic pressure is compaction of the vessels by buckling of the walls. The fibres
are scarcely affected by the treatment. The width of the earlywood zone decreased after the application
of pressure by 26% in spruce, and by 11% in cherry. The average density was increased by the hydrostatic
pressure by 26% for spruce and by 46% for cherry. The densitometric profile of spruce demonstrates
significant changes following the pressure treatment, with the minimum density DMin increasing
and the maximum density DMax decreasing. For cherry, the densitometric profile is shifted rather uniformly
towards higher densities, and the annual ring profile is spatially slightly compacted but otherwise
similar to that of untreated wood. The anisotropy of wood (expressed by the ratio of acoustic invariants)
decreased by 56% for spruce and by 33% for cherry. The structural damage in spruce is predominantly
found in the radial (R) direction, and this corresponds to a reduction of 73% in the velocity of the longitudinal
ultrasonic waves in the radial direction, VRR. In cherry, the structural damage is mainly in the
transverse, T direction. The velocity of the longitudinal ultrasonic waves in the transverse direction, VTT
is reduced by 44%. The medullary rays in cherry seem to be the most important anatomical feature influencing
the propagation of ultrasonic waves.
Water Immersion Technique for Measuring Attenuation and Phase Velocity of Longitudinal Ultrasonic Waves in Plastics