Grzegorz Musielak

The identification of fracture in dried wood based on theoretical modelling and acoustic emission

Physical interpretation of the three characteristic groups of acoustic signals emitted during convective drying of wood is the main subject of this paper. The acoustic emission (AE) was to evidence the fracture intensity during drying of a birch wood sample of cylindrical shape. To explain the three characteristic groups of acoustic signals, and particularly the last one, a mechanistic model of drying was applied to analyse the drying induced stresses in the tested sample. One can conclude from this analysis that the third group of acoustic signals arises when the surface stop to shrink and the wet core (initially in compression) begins to dry. The shrinkage of the core causes compression of the boundary layer and tension of the core. Thus, the reverse of the stress signs in the cylinder cross-section takes place and this possibly involves the destruction of wood structure in the tensed core.

Experimental Validation of the Heat and Mass Transfer Model for Convective Drying

The aim of this article is to present a self-consistent mathematical model describing the heat and mass transfer phenomena during the convective drying both in the constant and in the falling drying rate periods. This general model is developed on the basis of the theory of mixtures and the thermodynamics of irreversible processes. The boundary conditions are formulated and the numerical algorithm enabling calculation of the temperature and the drying curves in the two mentioned periods of drying is constructed. In this paper much effort is devoted to the experimental validation of the model. The convective drying of a cylindrical sample made of kaolin was examined both experimentally and numerically for comparison and the distribution of temperature and the drying curves were determined. A very good agreement of the experimental and theoretical results is stated.

Permanent Strains in Clay-Like Material During Drying

One of the characteristic features of clay-like materials is the change of their mechanical properties during drying as the moisture content is changed. At high moisture contents they are viscoelastic and then become elastic-plastic and finally brittle-elastic or even stiff. Elastic strains are reversible but viscous and plastic strains are not reversible. The permanent strains are usually undesirable in dried products because they cause warping. In brittle materials, on the other hand, microcracks and cracks are generated. The number of cracks depends on the material properties, sample shape, and drying conditions. The aim of this work is to examine experimentally the permanent strains during drying. The samples used in experiments were made of different materials (two types of clay and kaolin) and were dried convectively. Constant but nonsymmetric drying conditions were applied. The strains of the samples were measured online using a video camera.

POSSIBILITY OF CLAY DAMAGE DURING DRYING

The convective drying of plate made of clay is considered in the paper. The model of drying with material coefficients dependent on the moisture content and the temperature is presented. The material is assumed to be brittle-elastic (linear elasticity with moisture dependent material constants). The strength of the material estimated by bending test, is moisture dependent. Numerical simulation of drying processes is done. Through the comparison of the stresses arising during drying with the strength of the material the possibility of dried body damage is shown. The range of constant drying conditions in which damage is expected is specified.

INFLUENCE OF THE DRYING MEDIUM PARAMETERS ON DRYING INDUCED STRESSES

ABSTRACT A thermomechanical model of drying of capillary-porous materials whose material constants depend on moisture content and temperature is presented in the paper. The finite element method is used for the solution of two-dimensional problem of convective drying of a prismatic bar. The moisture distributions, temperature distributions, drying induced strains and stresses for various drying medium parameters are determined. The effect of these parameters on moisture distribution and in particular on drying induced stresses is discussed.

Fracturing of Clay During Drying: Modelling and Numerical Simulation

Non-uniform distribution of moisture contents inside of the porous materials during drying results in compressional stresses inside of the material and tensional ones close to the surface. The tensional stresses together with brittleness of dry material are the reasons of fracturing of the material. The proposed model consists of two parts. The mass transfer is described by simple diffusion equation. The mechanical behaviour of the material is modelled with the network model in which the material is modelled as the set of small particles interconnected elastically via springs. The spring constants and strengths depend on Young modulus and the material tensional strength. The dependences of these material parameters on the moisture content are determined. Then two 2D initial-boundary problems are solved. The results show possible way of cracks initiation and generation by drying.

Deformations and Stresses in Dried Wood

The deformations and the evolution of drying-induced stresses in wood are studied based on a model which takes into account the alteration of mechanical properties of wood in the course of drying. A two-dimensional initial-boundary value problem is solved with the help of the finite element method. An influence of wood anisotropy on the deformation and the stress distributions and evolution of maximal stresses is analysed.

Influence of Varying Microwave Power during Microwave–Vacuum Drying on the Drying Time and Quality of Beetroot and Carrot Slices

The aim of this work was to study the influence of microwave power variation during microwave–vacuum drying on both drying time and dried material quality. Red beetroot and carrot were chosen as the material. The quality of the dried material was described by its color change and β-carotene loss (for carrot) due to drying. The first series of experiments consisted of drying under constant microwave power. Only the lowest microwave power gave an undamaged product. During the second series of experiments, the microwave power was changed in time intervals. However, the use of interval conditions either had a small influence on the process time or did not provide satisfactory quality of the dried samples. During the third series of experiments, initial drying was performed with higher microwave power and then the process was continued with the lowest applied power. This method shortened the process time and gave the best quality of material.

Stresses and Strains in Elastic, Viscoelastic, and Plastic Materials during Drying

The mechanistic theory of drying is developed for the purpose of stress and strain analysis in wet clay-like materials subjected to convective drying. The aim of these studies is to show the difference in stress and strain patterns for cases when the material is considered to be elastic, viscoelastic, or elastic-plastic. Such an analysis is justified by the fact that many materials under drying, e.g., clay-like or woody materials, reveal different mechanical properties and safer, mostly inelastic deformations. In addition, inelastic models are able to describe some additional phenomena as, for example, stress reverse that often appears in dried materials. It is displayed through comparison of the experimental measurements with the numerical results obtained on the basis of inelastic constitutive equations applied to drying materials. These results show that drying models that include inelastic deformations are more realistic and present more natural stress and strain patterns than the purely elastic model. The analysis is presented on an example of kaolin-clay plate subjected to convective drying. The numerical results concerning inelastic deformations are visualized on plots and photographs taken from the samples tested experimentally.

Non-linear Model for Wood Saturation

The paper presents a non-linear model of saturation with fluid of anisotropic capillary porous bodies and the results of experimental investigations of wood saturation with methacrylate. The obtained experimental curves illustrating the distribution of methacrylate in wood samples allow the estimation of material coefficients and verification of the theoretical model. The theoretical model is developed based on the balance equations of mass, momentum, and energy, and the thermodynamics of irreversible processes. The non-linear differential equation, describing the distribution of methacrylate content in wood and its evolution during the saturation is solved numerically. The theoretical curves obtained on the basis of both linear and non-linear models are compared with the experimental data and better agreement between them for the non-linear model is stated.