Study_of_Cracks_in_the_Cutting_Zone

3.2.2 Study of Cracks in the Cutting Zone

The process of cutting is connected with formation of cracks. In this connection, the influence of cracks on mechanical wood properties must be mentioned. It depends on their relative dimensions, directions with respect to the strain and critical cross-section and also to mechanical strain. The cracks reduce also the strength of wood by destroying its integrity.

The presence of a micro-crack is not a sufficient condition for total wood deformation. To come to wood deformation, the micro-cracks must propagate along the fibres in either radial or tangential directions of the machined wood. Each micro-crack "blunts" and plastic yielding is formed around it, whereas the local concentration of tensions is negligible when compared to the tool loading. To come to deformation, the micro crack must propagate until the material integrity is destroyed, i.e. the material is divided into parts, or until other propagating cracks that started to form in similar conditions meet.

If we proceed from the front surface in radial planes then the fibrous wood is the most cleavable. The cleavability is lower in tangential plane and higher in radial plane. The influence of medullary rays on the strength of the wood depends not only on their proportion (coniferous species approximately 6 %, broadleaved species approximately 15 %) within the total volume, but also on their dimensions, shape, height, etc. During compression but also tensile stress across the grain in radial direction, higher strength and rigidity are manifested in the case of those species with strong rays. In this direction the medullary rays (spring part of annual growth) is reinforced by the load. In the tangential load the medullary rays have opposite influence, they lower the compactness especially of the summer wood of the annual growth. Irregular course of grain direction, twisting, knottiness, knurly grain and other defects disturbing wood homogeneity deteriorate its cleavability (Vanin, 1955).

During the cleaving, the cutting wedge is pressed among the wood fibres at the face of the log in longitudinal direction. In this direction the fibres show the least resistance against separating them from one another. The cleaving compresses the fibres first and only later penetrates between them and enlarges the existing crack while tearing the fibres from one another in the plane of the wedge compression action (Mikleš, 1993), (Dibala, Koniar, 2006).

If we want to quantitatively analyse the monitored area for wood and wood materials we must start not only from their physical and mechanical characteristics, from refraction mechanics, but also from the theory of the cutting process (Irwin, 1960).