The work concerns a three-point bending test of beams made of plywood, high density fibre boards, cardboard, and wood-epoxy mass. The goal of the investigation was to determine the effect of thickness and type of wood-based facings on stiffness, strength, ability to absorb, and dissipate the energy of sandwich beams with an auxetic core. The cognitive goal of the work was to demonstrate the possibility of using recycled materials for facings and cores instead of popular wood composites. Experimental studies and numerical calculations were performed on correctly calibrated models. Experimental studies have shown that the beams with HDF facings (E = 1528 MPa, MOR = 12.61 MPa) and plywood facings (E = 1248–1395 MPa, MOR = 8.34–10.40 MPa) have the most favourable mechanical properties. Beams with plywood facings also have a good ability to absorb energy (1.380–1.746 J), but, in this respect, the beams manufactured of HDF (2.223 J) exhibited better capacity. The use of an auxetic core and facings of plywood and cardboard significantly reduces the amount of dissipated energy (0.0093 J, 0.0067 J). Therefore, this type of structures can be used for modeling beams carrying high deflections.

The research in materials with auxetic properties [1] (i.e., exhibiting negative Poisson's ratio [2] at least in some directions) has been growing for about 30 years. The manufacturing of the first man-made auxetic foam by Lakes [3] as well as the formulation and solution of the first mechanical [4,5] and thermodynamic models [6,7] explaining their unusual mechanical properties sparked an ongoing interest and intense studies of these interesting novel materials. Over these past 30 years, the studies on auxetics diverge into different aspects. Among others, the search for auxetic properties in new materials, [8,9] the theoretical studies of various models exhibiting auxetic properties, [10-46] or the creation of auxetic composites [47,48] to enhance mechanical properties of materials. The latter is of particular importance if one considers practical applications of negative Poisson's ratio materials. [49-55] In this respect, auxetic structures [24,54,56-66] are interesting (among others) in terms of improving stiffness and durability of materials. They tend to form synclastic, sphere-like shapes instead of anticlastic curvature which is characteristic for common materials with positive Poisson's ratio, subjected to bending. [67-69] Nowadays in wood industry, furniture which were designed with the use of synclastic shapes, are typically made from bent steel frame (usually upholstered), plastics, or bent plywood. Technology of bent plywood is strictly connected with thermophysical treatment, mold casting, and temperature-pressure bending process. The process is both time-and resourceconsuming. Furthermore, with the development of each new shape, expensive procedure of a new mold preparation and implementation into technological process is required. This situation could be significantly improved by implementation of auxetic structures into the industry. Auxetic structures improve several of mechanical properties, especially shear, impact, and bending resistance. They also improve ability to absorb impact energy as well as improve overall stiffness of structure. [65,66] Despite promising results of mechanical characteristics in the work by Smardzewski, [70] concerning examination of multilayered sandwich panels, particularly honeycomb panels with auxetic core manufactured from wood-based materials, to the date, the literature lacks research on the use of auxetic structures in wood-based composites. International research is focused on innovative, light composites, that could reduce the cost of product manufacturing by reducing material usage and overall product weight. Thus, in effect reducing also costs related to logistics. [70,71] Such composites could replace typically used wood-based materials such as particleboard, low, medium, and high-density fiberboard (LDF, MDF, and HDF) or plywood. Many studies [67,72-78] deal with honeycomb sandwich panels and their usage in wood industry for the purpose of door filling, vertical, and horizontal elements of furniture body, package filling, etc. Sandwich panels should have ...

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The literature lacks comparisons of analytical and numerical calculations that have been verified experimentally for elastic constants of auxetic cells in cores manufactured from wood materials. The aim of this study was to determine the effect of auxetic cell geometry and the type of material used in their manufacture on elastic properties of the honeycomb panel core. This paper describes properties of the materials, from which core cells were modeled and presents mathematical models of cell properties. The method of numerical optimization of cell shape was specified, and the numerical calculations concerning modeled cells are given along with the course and results of experimental tests. Additionally, the results of analytical, numerical, and experimental tests were compared. Cell geometry had a considerable effect on elastic properties of honeycomb panel cores, particularly the angle of the cell wall. Moreover, geometric imperfections had a significant effect on the results of analytical calculations. Based on numerical calculations, satisfactory consistency between these results and experimental tests was obtained.