Jerzy Smardzewski

Computer simulations of auxetic foams in two dimensions

Two simple models of two-dimensional auxetic (i.e. negative Poissons ratio) foams are studied by computer simulations. In the first one, further referred to as a Y-model, the ribs forming the cells of the foam are connected at points corresponding to sites of a disordered honeycomb lattice. In the second one, coined a Δ-model, the connections of the ribs are not point-like but spatial. For simplicity, they are represented by triangles centered at the honeycomb lattice points. Three kinds of joints are considered for each model, soft, normal and hard, respectively corresponding to materials with Youngs modulus ten times smaller than, equal to and ten times larger than that of the ribs. The initial lattices are uniformly compressed, which decreases their linear dimensions by about 15%. The resulting structures are then used as reference structures with no internal stress. The Poissons ratios of these reference structures are determined by stretching them, in either the x or the y direction. The results obtained for finite meshes and finite samples are extrapolated to infinitely fine mesh and to the thermodynamic limit, respectively. The extrapolations indicate that meshes with as few as 13 nodes across a rib and samples as small as containing 16 × 16 cells approximate the Poissons ratios of systems of infinite size and infinite mesh resolution within the statistical accuracy of the experiments, i.e. a few per cent. The simulations show that by applying harder joints one can reach lower Poissons ratios, i.e. foams with more auxetic properties. It also follows from the simulations performed that the Δ-model gives lower Poissons ratios than the Y-model. Finally, the simulations using fine meshes for the samples are compared with the ones in which the ribs are approximated by Timoshenko beams. Taking into account simplifications in the latter model, the agreement is surprisingly good.

Detection of failures of adhesively bonded joints using the acoustic emission method

Abstract Wooden glued constructions require touch-free monitoring of destructive processes, especially in adhesive bonds that are most exposed to failure. The objective of the investigations was to describe failure processes in the adhesive bond of wood joints, in particular to determine their initiation, propagation, and destruction. The acoustic emission (AE) method was employed as the carrier of information about changes occurring in glued joints, whereas the numerical method was applied to determine values of distribution of tangential stresses generated in adhesive bonds. The acoustic phenomena examined were described using the AE cumulative counts. The authors analysed acoustic signals generated in loaded wooden and plastic overlap samples glued together using polyethyl methacrylate glue as well as in solid samples. On the basis of the acoustic emissions obtained, it was possible to establish characteristic places and stages of escalating structural defects generated from bonds of adhesive joints. This was utilised later on, in conjunction with results of numerical calculations, to determine correlations occurring between the AE cumulative counts and generated tangential stresses. Dependencies established in this way were used to determine characteristic points during the propagation of destructive phenomena of wood adhesive joints. The results obtained proved that it was possible to predict the development of the destruction of wood adhesive joints on the basis of observations of the increasing AE cumulative counts of acoustic signals in combination with tangential stresses determined using the finite elements method.

Elastic properties of cellular wood panels with hexagonal and auxetic cores

Abstract Light cellular wood panels have been gaining increasing interest among furniture manufacturers, but only a few articles can be found dealing with modeling of mechanical properties of cellular wood panels with a paper honeycomb core inside. The present paper intends to fill this gap, and thus, the elastic properties of cellular wood panels with paper honeycomb of hexagonal and auxetic cells were evaluated. Analytical models have been employed by comparison of experimental data with those obtained by numerical calculations. The cores of the examined cellular wood panels exhibited strong orthotropic properties. Results of numerical calculations of sandwich beam deflections corroborated their satisfactory conformity. The results of laboratory measurements proved the correctness of the determined elastic constants.

Sound absorption of wood-based materials

Abstract From modern buildings to public spaces are made of concrete, steel, and glass. These materials increase propagation of sound and the reverberation time. Therefore, furniture should be good sound absorbers in such places. The objective of this study was to ascertain acoustic properties of wood-based materials by determining normal acoustic impedance on the surface and sound absorption coefficients. Experiments were carried out on 17 types of wood-based materials commonly employed in furniture design and manufacture. Investigations were conducted based on the transfer-function method. It was demonstrated that for frequencies between 125 and 500 Hz, the highest capability of sound absorption was determined of low surface layer density and high porosity. Honeycomb panels with paper core absorbed better sounds in the range between 1 and 2 kHz. Panels of considerable external surface irregularities were characterized by the most favorable acoustic properties for the frequency of 4 kHz.

Experimental study of wood acoustic absorption characteristics

Abstract The objective of this study was to determine normal impedance on the surface as well as sound absorption coefficients for several wood species from Europe and from the tropical zone. The mathematical models of Miki, Attenborough, and Allard – dealing with acoustic properties of porous materials – have also been compared. The air flow resistivity exhibits a distinct link between fiber dimensions and wood porosity. The highest sound absorption coefficient was found for oak, ash, sapeli, and pine woods at 2 kHz frequency. The Attenborough model provides results closest to laboratory measurements, although it still requires significant improvements. The Miki and Allard models have some drawbacks and should be applied with reservation for the determination of wood acoustic properties.

Comparison of Contact Stress Distribution for Foam Seat and Seat of Auxetic Spring Skeleton

The objective of this paper is to present and compare the results of numerical solutions of contact problem for two types of seats subjected to typical sitting loadings. The first seat is made of a typical hyperelastic foam, the other is designed with an auxetic polyamid spring skeleton. Computer simulations of the seat structure under a typical static loading exerted by a human body are performed by means of ABAQUS FEA. The model provides an insight into deformation modes and stress field in relation to geometric and material parameters of the seat structure.The other type of seat, due to the fact of global auxecity and progressive springs characteristics reduces contact stress concentrations, giving an advantegous distribution of pressure and provides the sensation of physical comfort. The proper seat skeleton shape leads to an improvement of ergonomic quality.

Decohesion of glue bonds in wood connections

There is little information on the mechanism involved in destruction of furniture profile-adhesive joints. Information is especially needed on the destruction mechanisms for glue bonds in wood tenon and dowel joints upon external forces. Hence we investigated destructive processes for profile-adhesive joints and analysed the acoustic signals generated during loading of selected furniture joints. Acoustic emission (AE) is a method that allows observation and registration of destructive changes occurring in a material at the moment of stress. It provides possibilities to follow destructive processes that take place in a joint, beginning with microscopic cracks and ending up in total failure. There are links between AE signals and the causes of their excitation (Moliński 1998). AE phenomena were investigated in the context of glue overlap joints, such as in construction nodes (Porter and El-Osta 1972; Kirylov and Kovalczuk 1987; Baltruszajtis and Kovalczuk 1988; Beyon et al. 1990, 1992; Smardzewski and Gozdecki 2002; Gozdecki and Smardzewski 2005). Furniture profile joints have not yet been investigated satisfactorily. The strength of these joints depends not only on the strength of the glue line, but also on the strength of all the elements in a joint, such as dowels and holes or tenon and mortise. The main goal of this study was to determine the destruction mechanism for glue bonds in tenon and dowel joints during the action of external forces. These construction elements are commonly applied as nodes in skeleton furniture (Figure 1). The main objective was to predict critical load values that might lead to the destruction of construction nodes in furniture.

Mathematical models and experimental data for HDF based sandwich panels with dual corrugated lightweight core

Abstract Light layer honeycomb panels could replace traditional wood materials, if their stiffness and strength properties could be improved. The aim of this research was to design and determine elastic properties of sandwich panels (SPs) based on a dual corrugated HDF core. Stiffness matrix values of elements were determined by a numerical method. The 3D calculation results were compared with those of the homogeneous model. The calculation results were collated with those of experimental investigations. It was demonstrated that the linear elasticity modulus as well as the modulus of rupture of the SPs were comparable with mechanical properties of a particle board with identical thickness, while the SP has a 1/3 lower density. The panel core exhibited significant orthotropic properties. In the xy plane it could be characterized as an auxetic structure. The homogeneous model leads to results similar to those achieved from the 3D model and observed in experimental tests.