# Indentation Hardness and Elastic Recovery of Some Hardwood Species
## 4 Discussion
### Self-Re-Deformation  
The ability to self-shallow the imprint after removing the measuring force (self-re-deformation) seems to depend on the density of the wood. A graphical representation of the self-re-deformation ability in the three main cross-sections of wood is presented in Figures 13-15. The wood species with the highest density exhibited the highest ability of self-re-deformation. This is in line with our previous research [8] and reports from the literature [23]. In the case of the clear sapwood of kiln-dried Scots pine, the ability of side elastic self-re-deformation ranged from 45% (sphere-shaped intender, 1000 N) for densified material to 91% (cylinder-shaped intender, 2500 N) [24]. As shown in Figures 13-15, the ability to self-decrease in the depth of the imprint after removing the load in all main cross-sections (R, T and L) slightly increased with increasing wood densities. This tendency was observed for both measuring forces, 30 kG and 100 kG. In the R and T cross-sections, after the load is removed, the self-re-deformation ability was greater for the measuring force of 100 kG and less for the measuring force of 30 kG. In the L cross-section, the ability of self-re-deformation in the tested range depends only on the density of the wood (it does not depend on the value of the measuring force). These results show that the ability of self-re-deformation depends both on measuring force and the wood density in the R and T cross-sections; however, in the L cross-section, the ability of self-re-deformation depends on the wood density only. Overall, these results suggest the different progressions of cell-structure deformation in the R and T cross-sections compared to the L cross-section [25]. Cells are strongly elongated in the L direction; during compression in the R or T directions, they occur in the following sequence: (a) the linear-elastic bending of the cell walls, (b) the plastic buckling of the cell walls and reduction of void volume, and (d) cell walls are visco-elastically compressed (densification on a macroscopic level). The linear-elastic \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ratio**} & \multirow{2}{*}{**Equation**} & \multicolumn{6}{c}{**Wood Specie**} \\ \cline{3-8}  & & **Alder** & **Ash** & **Beech** & **Birch** & **Iroko** & **Linden** \\ \hline Hardness depending & HB(L 100 kG)/HB(L 30 kG) & 128\% & 136\% & 138\% & 134\% & 130\% & 118\% \\ on the measuring & HB(R 100 kG)/HB(R 30 kG) & 157\% & 145\% & 136\% & 150\% & 159\% & 107\% \\ force value & HB(T 100 kG)/HB(T 30 kG) & 143\% & 125\% & 155\% & 157\% & 142\% & 119\% \\ \hline Hardness depending & HB(R 30 kG)/HB(L 30 kG) & 26\% & 45\% & 41\% & 31\% & 36\% & 30\% \\  & HB(R 100 kG)/HB(L 100 kG) & 32\% & 47\% & 40\% & 35\% & 44\% & 28\% \\ on the cross-section & HB(R 30 kG)/HB(T 30 kG) & 123\% & 104\% & 113\% & 109\% & 124\% & 109\% \\  & HB(R 100 kG)/HB(T 100 kG) & 104\% & 89\% & 129\% & 114\% & 111\% & 122\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: HB values ratios. Three main cross-sections of wood (R, T, and L) and two measuring forces. bending of the cell walls is almost fully reversible, and the densification of cells is partially reversible. When the indenter is pressed in the R and T directions, the wood cells bend and collapse after reaching their plastic collapse load. Compression in the L direction (axially) causes the kinking of elongated cell walls in the L direction [26]. Kink (failed yield) occurs by local plastic buckling [27] or by the fracture of the cells' ends [28]. Local plastic buckling usually begins at points where the cells bend to make space for a ray [25]. [PERSON] and [PERSON] described a similar deformation process of balsa wood cells under longitudinal compression. They related the course of deformation to wood density, stating that it is by the initial elastic and then plastic buckling of cell walls in low-density specimens, while kink band formation and end-cap collapse dominate in higher-density specimens [29].