## 1 Introduction  
The main properties of wood vary between forest species, but also between the same wood species; thus, the properties of wood cannot be controlled during different processing stages. Moreover, there are differences between wood-based composite materials, which are made of technical veneer (plywood), fibers (wood fiber boards), and chips (wood chip boards--PB--and long oriented chip boards--OSB), from fine sawdust (WPC boards) and timber (GLULAM) to wood chips (boards with mineral binders). In Europe, the consumption of wood and wood-based composite materials has increased year by year. The demand for these materials is differentiated from country to country. Even if the increase in people's needs was not the same in different areas of the globe, there was a total increase in the level of the consumption of wood and wood-based composite materials. So, the consumption of composite materials recorded an increase up to the level of 2010, followed by a stagnation period for wood chipboards and plywood and an increase in oriented strand boards (OSB) and medium-density fiberboards (MDF) [1, 2, 3, 4, 5]. Wood-based composite materials are used for applications in the furniture industry, construction, aviation, and musical instruments, showing many advantages over other materials, such as high temperature resistance, stiffness, very good durability, strengths,and higher rigidity and adhesion, in relation to metals or plastics [2; 6]. Considering the quality coefficients (the ratio between strength and density), sometimes, wood-based composite materials come close to the strongest materials, such as steel, cast iron, and Kevlar [7; 8]. The main advantage of these composite materials exceeds the area of their superior properties, through the use of residues obtained from the exploitation of forests, vines, and orchards; from the processing of round wood in the form of logs and other wood materials; but also from the use of wood products taken out of use (cabins, wooden buildings, temporary civil constructions, furniture, crates, etc.). From this point of view, wood-based boards are renewable, by the fact that, at certain time intervals, a base of raw material is created, through the collected remains and wood waste. Aside from all these advantages, composite materials also have the disadvantage of the use of adhesives for gluing, which are non-ecological materials with serious impact on human health [9]. And yet, through using natural adhesives, the attractiveness of these composite products increases significantly, becoming essential to current human activities. The quality of composite materials can be evaluated through observing their physico-mechanical properties through following specific tests, as well as through micro- and macroscopic evaluations of fibers, chops, veneers, sawdust, chips, or timber [1]. Two of the most important wood-based composite materials are plywood and medium-density fiberboard. Plywood is a very durable composite material; it is waterproof and is heavier than MDF [4]. Plywood has very good strength in relation to its weight and presents superior rigidity on both of it sides [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. It can be used as a structural material, in which case its dimensions and properties must be precisely calculated [20]. The mechanical resistance of plywood decreases with increasing humidity and water [21; 22; 23; 24]. Dimensional changes in longitudinal and transverse direction are on average 0.015% for every 1% change in its humidity [13; 17]. Medium-density fiberboards are produced from a series of lignocellulosic fibers, following a thermal and mechanical process of defibration that uses the combined action of heat with mechanical energy to break the bonds between cells [6; 23]. Depending on the manufacturing process of MDF, fibers with a high degree of fibrilization are used for the wet process, and long and thin fibers are used for the dry process [9; 25; 26; 27; 28; 29; 30; 31].