## 1 Introduction
Reliable and accurate measurement of these properties is crucial to determining the durability of lumber in working conditions and selecting the appropriate type of wood for different purposes. The elastic behaviour of wooden elements loaded in bending is affected by many factors, including temperature, density and moisture content of wood, the geometry of the sample, and the loading rate. These influential factors have been the subject of many experimental studies [19]. The dimensions of knots are also a significant factor because the knot size negatively affects the mechanical strength of wood [20]. Various static and dynamic methods have been used for measuring elastic deformations and determining elastic modulus. Static methods are usually based on bending [21; 22; 23; 24; 25; 26; 27] or tensile testing [28], while dynamic methods include the resonance flexure method [29; 30; 31], ultrasonic methods [32; 33; 34; 35], or the resonant (vibration) method [36]. Bending is often a preferred static method used to measure the elastic modulus [37; 38], and it can be performed on a simple device with an accuracy of 2.6%, according to [PERSON] et al. [39]. The hardness of the material is usually determined using the indentation technique. However, the hardness values depend on which tool is used and which parameters are measured. For wood, the anisotropy, heterogeneity, and hygroscopicity also affect the hardness measurement. [PERSON] and [PERSON] [40] described the advantages and limitations of existing methods for testing wood hardness. They pointed out that the first group of test methods, comprising standardised hardness test procedures based on static macro-indentation, such as Janka, Brinell, and Monnin, are not specially designed to estimate the hardness of the lumber. In contrast, a non-destructive method Piazza and [PERSON], as a modification of the Janka method, is designed specifically for the mechanical characterisation of wood elements. Depending on the chosen measurement method, wood often exhibits qualitatively different behaviour. In order to mitigate the effect of visual measurements on the consistency and accuracy of the results, [PERSON] et al. [41] proposed a modification to the Brinell method for determining the wood hardness. Instead of visual measurement of indentation diameter, they relied on the ability of modern testing devices to measure the indentation depth accurately. As a result, the obtained hardness values were in stronger correlation with the density of the material. The advantage of the modified Brinell method is that the reliability of the measured hardness values is less affected by the presence of coatings on the wood surface than the results of testing based on the original Brinell method. [PERSON] et al. [42] point out that rounded indenters, such as a ball ([PERSON], Janka, Krippel, and Meyer tests) and a cylinder (Monnin test), are much better suited for wood. Due to the specific structure of wood, the hardness values at the longitudinal section are significantly lower than at the cross-section. Using beech samples, the authors tried to unify the methods of wood hardness testing according to [PERSON], [PERSON], and Monnin, based on [PERSON]'s law. The surface quality is one of the essential properties of wood that affects the strength characteristics and the output quality of manufacturing processes such as joining and bonding. The main characteristic of surface quality is its roughness. [PERSON] and [PERSON] [43], in their review paper, gave specific recommendations regarding the selection of instruments for roughness measurement, the measuring length, the measurement resolution, the elimination of form errors (with or without prior removal of wood pores), the use of a Gaussian regression filter, the appropriate cut-off length, the most useful roughness parameters for describing the processing roughness, and a method to separate the processing roughness from anatomical roughness. [PERSON] et al.