## 1 Introduction  
Wood, along with metals, plastics, and composites, is one of the most widely used industrial materials. In addition to a lower price, the simplicity of processing and construction is one of its main characteristics. As an organic material with a beautiful natural texture, wood exhibits variations in its chemical, physical and mechanical properties even within the same species. Furthermore, the three principal axes define the differences in wood's structure and behaviour, and it responds in various ways to local temperature, humidity, and other influences. Therefore, the selection of appropriate parameters, both variables, and constants, is crucial when investigating wood processing [1]. [PERSON] [2] points out that on a macroscopic scale (10-200 mm), pure wood (without knots and deviations in grain size or other structural characteristics) is usually considered a homogeneous and continuous material. On the cellular level, the geometry and the internal cell arrangement can be defined via three local axes of material symmetry, the so-called longitudinal (L,1), radial (R,2), and tangential (T,3) directions. High-quality lumber is expensive and not always available in large quantities. Modification processes offer the possibility to improve the strength properties of wood, extend the lifespan of wood products, and expand the list of usable wood species. As the number of faster-growing species with shorter fatigue life increases, while quality wood becomes more difficult to acquire, modification processesbecome even more important. In a review paper, [PERSON] et al. [3] define and describe three groups of modification processes that are most frequently used to improve the characteristics of wood: chemical processing (acetylation, furfurylation, resin impregnation, etc. ), thermo-hydro processing (thermal treatment) and thermo-hydro-mechanical processing (surface densification). The chemical, physical and mechanical properties of wood depend on various factors such as the wood type and climate [4; 5; 6], growth conditions [7; 8], and modification processes [9; 10]. Thermo-hydro-mechanical processing of wood has been investigated from various aspects. [PERSON] et al. [11] applied roller pressing to mechanically remove water from flat sawn veneer and analysed the moisture content distribution after the processing. They also reported negative effects of compression on the thickness change and flexural strength. [PERSON] et al. [12] developed a method based on roller pressing to treat wood with either aqueous or non-aqueous solutions. Specim length, thickness, compression rate, and feed speed were studied. Moisture contents, compression ratio, and the number of pressing cycles influenced the impregnation efficiency. Some researchers studied the surface densification of softwood by the use of a constant pressure in different thermo-hydro-mechanical (THM) treatments to improve the mechanical properties of wood. It is well known that the density of wood correlates with its mechanical properties. Furthermore, as a porous material, wood can be compressed until its density reaches the density of a cell wall, which is about 1.50 g/cm\({}^{3}\). That means that the density of the surface layer can be increased by up to 40% [13; 14; 15; 16]. When heated, the wood softens and can be compressed without rupturing the cell walls. [PERSON] et al. [17] used a continuous press for wood panel production to apply constant pressure on low-density poplar samples along with the THM treatment. That led to densification within a few millimetres thick sub-surface zone. Considering the complex structure and anisotropic nature of wood, as well as the limitations of finite element analysis (FEA), [PERSON] [18] simulated the compression of wood using the material point method (MPM). The presented numerical calculations of densified wood structures provided insight into the relationship between wood anatomy and compression. Elastic modulus and hardness are the two most important mechanical properties of wood.