Kent Persson

NONLINEAR MECHANICAL BEHAVIOUR AND ANALYSIS OF WOOD AND FIBRE MATERIALS

Abstract The mechanical behaviour of wood was studied from a micro up to a macro level. Wood is a cellular material possessing a high degree of anisotropy. Like other cellular solids, it often exhibits a highly nonlinear stress–strain behaviour. In the present study the mechanical properties of the cellular structure of wood are characterized and modelled, the irregular cell shape, the anisotropic layered structure of the cell walls and the periodic variations in density being taken into account. The continuum properties were derived by use of a homogenization procedure and the finite element method. Stiffness and shrinkage properties determined by this procedure are presented and are compared with measured data. The constitutive properties thus determined at various structural levels can be used in numerical simulations of the behaviour of wood in different industrially related areas. One such area is that of the refining process in mechanical pulp manufacture. Simulations of the deformation and fracturing of wood specimens loaded under conditions similar to those found in the refining process are presented. The numerical and experimental results obtained are compared.

Resistance of flight feathers to mechanical fatigue covaries with moult strategy in two warbler species

Flight feather moult in small passerines is realized in several ways. Some species moult once after breeding or once on their wintering grounds; others even moult twice. The adaptive significance of this diversity is still largely unknown. We compared the resistance to mechanical fatigue of flight feathers from the chiffchaff Phylloscopus collybita, a migratory species moulting once on its breeding grounds, with feathers from the willow warbler Phylloscopus trochilus, a migratory species moulting in both its breeding and wintering grounds. We found that flight feathers of willow warblers, which have a shaft with a comparatively large diameter, become fatigued much faster than feathers of chiffchaffs under an artificial cyclic bending regime. We propose that willow warblers may strengthen their flight feathers by increasing the diameter of the shaft, which may lead to a more rapid accumulation of damage in willow warblers than in chiffchaffs.

Experimental and numerical determination of the hygroscopic warping of cross-laminated solid wood panels

Abstract The moisture-induced warping of three-layered cross-laminated solid wood panels made of Norway spruce was studied. The panels were exposed to different climate conditions at 65% and 100% relative humidity at the two panel faces. The results showed increasing cup deformation with an increasing relative thickness of the outer layers. The annual growth ring orientation was found to have a significant influence on the magnitude of the cup deformation. Measurements and numerical simulations of the moisture distribution within the panel were made in order to provide data for numerical simulations of the warping. A distinctive moisture profile with a conspicuous influence of the adhesive bond lines was found. The coefficient of diffusion of the adhesive bond lines was determined from the measurements and simulations. The mechanical material model used for the warping simulations takes into account elastic strain, moisture-induced swelling, and mechano-sorptive strain. The simulations showed good agreement with the warping test results. The most important material parameters for the cup deformation, which were identified in a parametric study of a panel with vertically oriented annual rings, are the moduli of elasticity and the swelling coefficients in the longitudinal and radial direction. Furthermore, the mechano-sorptive coefficient in radial direction was found to have a significant influence on warp.

Computational Methods for Laminated Glass

AbstractAn existing, recently developed solid-shell finite element is proposed for efficient and accurate modeling of laminated glass structures. The element is applied to one test example treating a thin laminated glass structure subjected to biaxial bending and the performance concerning accuracy and efficiency is compared with standard three-dimensional solid elements. Further examples illustrate how the element could be applied in the modeling of laminated glass structures with bolted and adhesive joints. For these examples, experimental data for relevant quantities are provided as a comparison. It is concluded that the element is an excellent candidate for the modeling of laminated glass.

Elastic layer model for application to crack propagation problems in timber engineering

A fracture mechanics model for analysis of crack initiation and propagation in wood is defined and applied. The model has the advantage of being simple, yet it enables reasonably general and accurate analysis commonly associated with more complex models. The present applied calculations are made by means of the finite element method and relate to progressive cleavage fracture along grain. The calculations concern a tapered double cantilever beam specimen and an end-notched beam. Comparisons are made of experimental test results. The fracture properties of the wood are modelled by means of a very thin linear elastic layer located along the crack propagation path. The properties of the layer are such that the strength and fracture energy of the wood are represented correctly. This makes a single linear elastic calculation sufficient for strength prediction. Both crack development and pre-existing cracks can be analyzed. Both material strength and fracture energy and stiffness are taken into account, their relative influence on structural strength being different for different elements. The fracture layer is in the finite element context represented by joint elements. Propagation of a crack can be analyzed either by a series of elastic calculations corresponding to different crack lengths or by use of a finite element code for non-linear analysis. The computational results include sensitivity analysis with respect to the influence of the various material parameters on structural strength.

Numerical Analysis of Wood Chipping

Normally, in pulp wood chipping, the chips desired are thinner and have less fiber damage than those currently being produced. However, experiments to develop chippers are rather laborious, so there is a need for a mathematical model to complement the experiments. A two-dimensional finite element model was developed in order to be able to predict chip thickness and chip damage, the model incorporated a fictitious crack model and elasto-plastic material properties. The sensitivity of the model to changes in material data was controlled, and the results of the calculations were compared to experimental observations reported in the literature. Finally, the influences of the geometry of the cutting knife and of the friction between knife and wood were calculated. The general impression of the modelled chip formation was that the results of the finite element model were in reasonable agreement with experimental observations and that the model yields qualitative results that are trustworthy. The calculations indicate that there is a strong interaction - which greatly influences chip formation - between the knife angle and the coefficient of friction between the knife and the wood. The results suggest that a chipper knife should have a small knife angle complemented with a bevel, and should have a small coefficient of friction to wood.

Designing Bolt Fixed Laminated Glass with Stress Concentration Factors

Abstract A general method for determining stress con centration factors for laminated glass balustrades with two plus two bolt fixings with variable positions is developed. It is demonstrated how the stress concentration factors can be presented graphically in design charts and representative charts are displayed for the case of a more specific bolt-fixed balustrade type. In general, the use of simple formulas and design charts allows the maximum principal stresses of the balustrade to be determined for any relevant combination of variable geometry parameters involved.

The Hygroscopic Warping of Cross-Laminated Timber

This chapter focuses on moisture-induced deformations in three-layered cross-laminated timber with a symmetrical build-up, where the fibre direction of the middle layer is oriented perpendicular to that of the outer layers. Dimensional stability, i.e. the ability to resist warping, is of main interest for the application of such wood panels. The cross lamination of the layers is advantageous to warping. The moisture-induced expansion/contraction of each single layer is partly restrained by the adjacent layers. The free swelling and shrinkage of adjacent layers differ approximately by a factor of 10 (radial/longitudinal) to 20 (tangential/longitudinal). As a consequence of this difference, stresses and even cracks may occur. In large-scale panels warping was observed. This reduces the serviceability in the practice. Due to a climate gradient considerable distortions (warp) in the form of cup and twist may occur. Models to calculate the moisture and stress fields are provided and validated to experimental tests.

Numerical Investigation of Vibration Reduction in Multi-storey Lightweight Buildings

In order to reduce the vibration transmission in multi-storey wood buildings, it is common to insert viscoelastic elastomer materials between parts of the buildings. The studies presented here investigate to which extent different design choices for the elastomer layers affect the isolation of low-frequency vibrations (0–100 Hz). A finite element model of two storeys of a multi-storey wood building, involving blocks of elastomer material in between the storeys, was used to perform numerical investigations. Parametric studies were carried out, considering different properties of the elastomer material and different placements of the elastomer blocks. Considering the transmission from the floor of the upper storey to the underlying ceiling, the material properties of the elastomer material were found to affect the vibration levels appreciably. A too stiff elastomer material can result in an amplification of the vibration levels in the ceiling for certain frequencies, whilst a less stiff material, in general, reduces the vibration transmission. The placement of the elastomer blocks was varied by shifting the position of the blocks while maintaining their centre-to-centre distance, resulting in a small effect on the vibration levels.

Numerical and Experimental Studies on Scale Models of Lightweight Building Structures

Lightweight buildings are sensitive to low-frequency vibrations, making it difficult to construct them in such a way that noise and disturbing vibrations are kept at an acceptable level. In the design of vibration reduction measures, it is desirable to have computational models for predicting the effects of structural modifications. Validations of the models to experimental data have to be performed to ensure reliable predictions. The experimental studies are simplified if full-scale models can be scaled down in size. In the paper, methods for designing scaled experimental models of building structures are discussed. An example, the scaling of a wooden building structure, is presented.