# Indentation Hardness and Elastic Recovery of Some Hardwood Species
## 4 Discussion  
### Hardness  
The Brinell hardness of wood is associated with wood density [10], and it can be increased by increasing the density of the wood after pressing. [PERSON] confirmed this possibility in tests involving the pressing of beech, oak, and pine wood, during which, after increasing the density of the samples by approximately 30%, the hardness increased by about 100% [13]. The hardness can also be reduced by reducing the strength of the wood after thermal modification [14]. Poplar wood, with a reduced modulus of elasticity (after thermo-modification at a temperature of 190-210 \({}^{\circ}\)C), had reduced hardness by about 25% [15]. Thermal modification influences the reduction of hardness in every main cross-section of wood differently: in the longitudinal cross-section by 3%, the radial cross-section by 15%, and the tangential cross-section by 25% [16]. The hardness is a property of the surface layer of the material [17], so the hardness of wood strongly depends on its density profile. Surface-densified pinewood shows an increased hardness and a high variation in measured hardness values, regardless of which testing method was used [18].  
Our research confirms a well-known feature of wood mentioned in the Introduction: the hardness strongly depends on the cross-section of wood [9,10]. Our study obtained the highest hardness values in the longitudinal cross-section (L), perpendicular to the trunk axis (the end hardness of wood). The hardness was smaller in radial and tangential cross-sections (R and T--the side hardness of wood) (Figure 5). Those results are in line with the literature. For example, the radial hardness of Amboyna wood is 30-40% of the end hardness, and its tangential hardness is approximately 120-130% of the radial hardness (hardness R and T--a force of 20 kG was used; hardness L--a force of 50 kG was used) [19]. Our research confirms this regularity; hardness measured with 30 kG of all tested wood species was 26-45% and 104-124%, respectively, while hardness measured with 100 kG: 28-47% and 89-129%, respectively. In the case of beech wood, these values were 40 and 129% (force 100 kG). Similar hardness ratios were obtained in the experiment by [PERSON] et al.: 42% and 126%, respectively (force 1000 N, 10 mm ball intender) [16]. Table 2 shows the ratios of hardness measured in our tests in dependence of measuring force used and in dependence of directions to wood fibers.  
Figure 12: The L cross-section: the permanent imprint depth (h) shares of the total imprint depth (\(H\)).  
The measured HB values also depend on the measuring force used. We obtained the higher hardness values at a force of 100 kG. Similar observations were made by [PERSON] et al. [20], who described the results of wood hardness tests, among others, of beech wood. A potential explanation for the higher hardness values at 100 kG than at 30 kG is the strain hardening effect, which increases with a decreasing indentation of the measuring ball. This phenomenon was observed when measuring the hardness of metals [21]; in wood, the material's cellular structure additionally influences it. Only after increasing the load did the plastic buckling of the cell walls reduce the volume of the voids and density cell walls [22]. Based on the results of our research, the tested species can be classified into hard (ash, beech), medium-hard (alder, birch, iroko), and soft wood (linden) species. The influence of the measuring force value on the measured hardness was the lowest for soft species; it was (depending on the grain direction) from 118 to 107%. That influence ranged from 125 to 176% for the remaining wood species, as shown in Table 2. In the case of hardness measurements in the L cross-section, the influence of the force on hardness was the least diversified (118-138%); while in the R cross-section, this influence was the most diverse (107-176%).