Moisture-Related Distortion of Timber Boards of Radiata Pine: Comparison With Norway Spruce

Based on material data obtained by several researchers at Forest Research in New Zealand, with respect to variations in the main material properties from pith to bark, the distortion model developed earlier for Norway spruce has been further modified for radiata pine. Numerical simulations were performed for both pine and spruce to investigate how different sawn pattern options affect the shape stability of individual boards. Results for spruce presented earlier have shown clearly that warping of the timber products is strongly influenced by the annual ring patterns within the individual boards. Comparisons between the two species were performed to study how the radial variations in the basic properties such as shrinkage parameters, stiffness parameters, and spiral grain have influence on the warping. Generally, the intrinsic patterns of variation in wood properties within stems were similar, and both species show a tendency to distort with changing moisture environment. There are strong indications that intelligent re-combination of material in glued products may overcome many of the inherent problems in using biological material with predictable variation in material properties

Finite element modelling of moisture related and visco-elastic deformations in inhomogeneous timber beams

Wood is a hygro-mechanical, non-isotropic and inhomogeneous material concerning both modulus of elasticity (MOE) and shrinkage properties. In stress calculations associated with ordinary timber design, these matters are often not dealt with properly. The main reason for this is that stress distributions in inhomogeneous glued laminated members (glulam) and in composite beams exposed to combined mechanical action and variable climate conditions are extremely difficult to predict by hand. Several experimental studies of Norway spruce have shown that the longitudinal modulus of elasticity and the longitudinal shrinkage coefficient vary considerably from pith to bark. The question is how much these variations affect the stress distribution in wooden structures exposed to variable moisture climate. The paper presents a finite element implementation of a beam element with the aim of studying how wooden composites behave during both mechanical and environmental load action. The beam element is exposed to both axial and lateral deformation. The material model employed concerns the elastic, shrinkage, mechano-sorption and visco-elastic behaviour of the wood material. It is used here to simulate the behaviour of several simply-supported and continuous composite beams subjected to both mechanical and environmental loading to illustrate the advantages this can provide. The results indicate clearly both the inhomogeneity of the material and the variable moisture action occurring to have had a significant effect on the stress distribution within the cross-section of the products that were studie

Wood construction under cold climate:Part one: Impact of cold temperatures on the shear strength of different adhesives glued wood joints of Norway spruce and Scots pine

As wood constructions increasingly use engineered wood products worldwide, concerns arise about the integrity of the wood and adhesives system. The glueline stability is a crucial issue for engineered wood application, especially under cold climate. In this study, Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) joints (150mm x 20mm x 10mm) were bonded with seven commercially available resins (PUR, PVAc, EPI, MF, MUF1, PRF and MUF2) and tested at six temperatures (20, -20, -30, -40, -50 and -60 °C), respectively. Generally, for both species, temperature changes significantly affected shear strength of wood joints. As temperature decreased, the shear strength decreased. PUR resin resulted in the strongest shear strength at all temperatures tested. MF resin responded to temperature changes in a similar ways as the PUR resin. The shear strength of wood joints with EPI resins was sensitive to temperature change. MUF, PRF and PVAc resins demonstrated different characters with Norway spruce and Scot pine. At room temperature, all types of adhesive showed relative stability, in terms of shear strength variation. While at low temperature, the shear strength varied considerably. More specimens need to be tested in further work to more completely present the issue. The EN 301 and EN 302 may need to be specified based on wood species.Godkänd; 2014; 20141112 (aliwan

Numerical analysis of Moisture-Related Distortions in Sawn Timber

In timber exposed to moisture variations such as in wood drying, shape distortions are often a serious problem since it can make the wood products obtained unsuitable for construction purposes. Two characteristics of wood are that its behaviour is strongly orthotropic and that it is very sensitive to variation in moisture. In addition, wood is characterized by variation in its properties from pith to bark. A further important property of wood affecting its behaviour is its spiral grain. In addition, in timber containing much compression wood, drying distortion is highly dependent upon where the compression wood is located in the board. In the present thesis, a finite element method is used to simulate deformations and stresses in wood during drying. A three-dimensional theory for the numerical simulation of deformations and stresses in wood during moisture variation is described. The constitutive model employed assumes the total strain rate to be the sum of the elastic strain rate, the moisture-induced strain rate, the mechano-sorption strain rate and the creep strain rate. Wood is assumed to be an orthotropic material with large differences in properties between the longitudinal, radial and tangential directions regarding the stiffness parameters as well as the moisture shrinkage and mechano-sorption parameters. The influence of moisture content and temperature on the material parameters is likewise taken into account. In addition, the effect of inhomogeneity in the internal structure of the wood material is considered. The influence of the growth rings, the spiral grain and the conical shape of the log on the orthotropic directions in the wood is also taken account of in the model. Variations in the wood properties with distance from the pith are considered as well. The three-dimensional theory used for analysing the shape stability of sawn timber was implemented in a finite element program. To illustrate the types of results that can be obtained, the behaviour of boards during drying was simulated. These simulations yield information on unfavourable deformations and stresses that can develop during the drying process. Boards without external constraint during drying are analysed, as well as boards from different positions in the timber log, boards with deviations in the pith and boards with external constraints. The finite element model is also used to clarify how the material properties and the internal structure affect stiffness properties in sawn timber. The influence of the stiffness parameters, the spiral grain and the annual ring orientation are of particular interest. To investigate factors that influence drying deformations, a parameter study was performed in which the influence of different constitutive models and different material parameters was examined. Numerical simulations were performed to investigate how the annual ring orientation, the cross sectional size and the drying profiles affect the shape stability of sawn timber. The influence of radial variations in the basic properties, the spiral grain and the conical angle was analysed as well. The study involves an experimental investigation of density, grain angles, shrinkage parameters and the longitudinal elastic modulus of a number of spruce boards containing compression wood. On the basis of the data obtained, numerical simulations are carried out to determine deformations that develop in the boards during changes in moisture. Shape stability of sawn timber can often be improved by gluing pieces of wood together. To study how the internal location and orientation of the pieces influence the drying deformations of glued products, numerical simulations for different products were performed. The knowledge obtained can contribute to more effective use of the raw material through allowing boards with properties resulting in poor shape stability or poor stiffness to be sorted out. Possibilities for improving the shape stability through gluing pieces of wood together are examined as well

Numerical analysis of Moisture-Related Distortions in Sawn Timber

In timber exposed to moisture variations such as in wood drying, shape distortions are often a serious problem since it can make the wood products obtained unsuitable for construction purposes. Two characteristics of wood are that its behaviour is strongly orthotropic and that it is very sensitive to variation in moisture. In addition, wood is characterized by variation in its properties from pith to bark. A further important property of wood affecting its behaviour is its spiral grain. In addition, in timber containing much compression wood, drying distortion is highly dependent upon where the compression wood is located in the board. In the present thesis, a finite element method is used to simulate deformations and stresses in wood during drying. A three-dimensional theory for the numerical simulation of deformations and stresses in wood during moisture variation is described. The constitutive model employed assumes the total strain rate to be the sum of the elastic strain rate, the moisture-induced strain rate, the mechano-sorption strain rate and the creep strain rate. Wood is assumed to be an orthotropic material with large differences in properties between the longitudinal, radial and tangential directions regarding the stiffness parameters as well as the moisture shrinkage and mechano-sorption parameters. The influence of moisture content and temperature on the material parameters is likewise taken into account. In addition, the effect of inhomogeneity in the internal structure of the wood material is considered. The influence of the growth rings, the spiral grain and the conical shape of the log on the orthotropic directions in the wood is also taken account of in the model. Variations in the wood properties with distance from the pith are considered as well. The three-dimensional theory used for analysing the shape stability of sawn timber was implemented in a finite element program. To illustrate the types of results that can be obtained, the behaviour of boards during drying was simulated. These simulations yield information on unfavourable deformations and stresses that can develop during the drying process. Boards without external constraint during drying are analysed, as well as boards from different positions in the timber log, boards with deviations in the pith and boards with external constraints. The finite element model is also used to clarify how the material properties and the internal structure affect stiffness properties in sawn timber. The influence of the stiffness parameters, the spiral grain and the annual ring orientation are of particular interest. To investigate factors that influence drying deformations, a parameter study was performed in which the influence of different constitutive models and different material parameters was examined. Numerical simulations were performed to investigate how the annual ring orientation, the cross sectional size and the drying profiles affect the shape stability of sawn timber. The influence of radial variations in the basic properties, the spiral grain and the conical angle was analysed as well. The study involves an experimental investigation of density, grain angles, shrinkage parameters and the longitudinal elastic modulus of a number of spruce boards containing compression wood. On the basis of the data obtained, numerical simulations are carried out to determine deformations that develop in the boards during changes in moisture. Shape stability of sawn timber can often be improved by gluing pieces of wood together. To study how the internal location and orientation of the pieces influence the drying deformations of glued products, numerical simulations for different products were performed. The knowledge obtained can contribute to more effective use of the raw material through allowing boards with properties resulting in poor shape stability or poor stiffness to be sorted out. Possibilities for improving the shape stability through gluing pieces of wood together are examined as well