Jan Tippner

The influence of wood density on longitudinal wave velocity determined by the ultrasound method in comparison to the resonance longitudinal method

The paper deals with the relationship between density and the velocity of sound wave propagation in wood. The wave velocity was measured using the heartwood of doussie (Afzelia bipindensis), merbau (Intsia bijuga), panga panga (Miletia stuhlmannii Taub.), tigerwood (Astronium graveolens Jacq.) and zebrano (Microberlinia brazzavillensis) by the two methods: ultrasound and resonance longitudinal. The velocity measured by the ultrasound method was always higher than the values gained by the resonance longitudinal method. However, there is a high correlation between the results of both methods. A change in sound wave velocity due to a change in density was not confirmed for any of the species. A similar conclusion was reached in the comparison among species.

Standard and non-standard deformation behaviour of European beech and Norway spruce during compression

Abstract The goal of the study is to investigate the non-standard deformation behaviour of wood loaded by compression parallel to the grain. This is represented as a negative increment of strain in the range of plastic deformations when the load continues to increase. The objectives of this study are to point out this problem and to provide its description based on the deformation fields that have been analysed using three approaches: a) full-field optical technique based on digital image correlation (DIC); b) “clip on” extensometer and its virtual analogy, and c) crosshead displacement method. Further, the negative strain phenomenon was studied depending on the sample length. The samples were made from the European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L. Karst.). Based on the strain analysis, it can be concluded that the deformation field consists of three sub-regions exhibiting different stiffness values (three-spring model). The failure of less stiff zones near the compression plates during the “non-standard” compression behaviour causes almost zero compression deformation of the stiffer middle zone or even leads to its expansion. The three-zone heterogeneity of deformation field induces a deviation of the displacement and strain measured by the proposed approaches. This phenomenon substantially influences the resulting longitudinal Young’s modulus and, therefore, should be of concern when measuring wood in such mode.

Utilization of digital image correlation in determining of both longitudinal shear moduli of wood at single torsion test

A sophisticated approach for the precise determination of both longitudinal shear moduli of wood at single test is introduced. The method is based on the combination of the torsion test inducing pure shear stresses in sample and an optical method providing the full-field strain data of such stress state. The proposed procedure of the longitudinal shear moduli determination consists of two main steps. In the first step, the apparent longitudinal shear modulus following the standardized procedure (EN 408+A1) was determined. Secondly, both longitudinal shear moduli were derived based on the apparent longitudinal shear modulus and the shear strain distribution on the radial and tangential sample surfaces. The wood of European beech (Fagus sylvatica L.) was used as material for the experiments. The exploratory analysis revealed the increasing difference between the longitudinal shear moduli determined in the longitudinal–radial plane and in the longitudinal–tangential plane as the total torsion angle increased as well as with the increase in the average torsion stiffness. Further, the longitudinal shear moduli and the torsional longitudinal shear strength did not correlate well. Therefore, they cannot be used in order to predict each other. Although such findings need more detailed studies, they should be taken into account when designing wood structures.

Verification of the elastic material characteristics of Norway spruce and European beech in the field of shear behaviour by means of digital image correlation (DIC) for finite element analysis (FEA)

Abstract Norway spruce (Picea abies L. Karst.) and European beech (Fagus sylvatica L.) samples were loaded in shear mode aimed at testing their elastic material characteristics applicable in finite element analysis (FEA). More precisely, experimental and numerical analyses of uniaxial tensile test parallel to grain in longitudinal-radial (LR) or longitudinal-tangential (LT) shear of plane are described. The elastic material models in the FEA are based on own experimental data and those of the literature. The verification of material characteristics was performed by 3D numerical models with the same parameters as for the experimental tests. The fully orthotropic elastic material model was applied in the uniaxial tensile tests. The digital image correlation (DIC) method served for verification of the numerical models with proposed elastic material characteristics. Good correlation was found between numerically predicted and experimentally measured data. The minor differences between the two data sets could be mainly attributed to certain natural wood characteristics, which were neglected in the proposed models, i.e. especially variation of earlywood and latewood density. The proposed elastic material models offer general data sets for the evaluation of mechanical response of timber structures and especially in timber connexions.

Determination of the elasto-plastic material characteristics of Norway spruce and European beech wood by experimental and numerical analyses

Abstract Experimental and numerical analyses are presented concerning of compression tests parallel and perpendicular to the grain, three-point bending, and double-shear joints in compliance with the relevant test standards (ASTM D2395, BS 373, EN 383 and EN 26891). Woods of Norway spruce (Picea abies L. Karst.) and European beech (Fagus sylvatica L.) were tested to describe their non-linear behavior. Elasto-plastic material models were the basis for the finite-element (FE) analyses with the input of own experimental data and those of the literature. The elasto-plastic material model with non-linear isotropic hardening was applied based on the Hill yield criterion in regions of uniaxial compression. The material characteristics were first optimized and validated by means of basic 3D FE models under the same conditions as applied for the experiments. Afterwards, the validated material models were implemented into the solver with more complex numerical analyses of wooden dowel joints. Concurrently, the digital image correlation (DIC) served for verification of the numerical wooden joint models. A good agreement (with a relative error up to 16%) was found between numerically predicted and experimentally measured data. The differences may be mainly attributed to some natural characteristics of wood which were not considered in the proposed material models. The proposed elasto-plastic material models are capable of predicting the wood’s ultimate strength, and therefore could contribute to a more reliable design of wood structures and their performance.

The Relation of Fibre Length and Ray Dimensions to Sound Propagation velocity in wood of selected Tropical Hardwoods

This study investigates the relation of fibre length and ray dimensions to the sound propagation velocity in four commercial tropical hardwoods. The species used in the study were Doussie (Afzelia bipindensis Harms), Merbau (Intsia bijuga (Colebr.) Kuntze), Muiracatiara (Astronium graveolens Jacq.) and Wenge (Millettia laurentii De Wild.). The sound propagation velocity was established by the frequency-resonance method. A positive correlation was obtained between fibre length and sound velocity within species. The correlation was stronger in species with a higher proportion of libriform fibres. A trend analogous to fibre length within species was observed for ray ratio but, in contrast to fibre length, the correlation was strong also between species. The samples with higher and at the same time narrower rays (higher ray ratio) showed a higher velocity of sound propagation along the grain.

Structural assessment of a lapped scarf joint applied to historical timber constructions in central Europe

ABSTRACT The article deals with assessment of suitable applications of all-wooden lapped scarf joints intended for use in retrofitting of existing historical timber structures in Central Europe. The first part of the article addresses the bearing capacity and stiffness of a lapped scarf joint. In the second part, four types of roof structures typical for the region were analyzed and potential application of the joints is discussed. The third part of the article describes the influence of the application of the lap joints on the force distribution of the repaired structure, since lower stiffness of one member induces forces elsewhere in the structure as it is transmitted by the other members to the rest of the structure. Extent and influence of the stiffness change on the force distribution is briefly discussed.

Lapped scarf joints for repairs of historical structures

Jiří Kunecký Hana Hasníková Michal Kloiber Václav Sebera Jan Tippner This paper presents a description of the repair of timber structures using a prosthesis scarf joint designed for the replacement of damaged parts of beams. This new scarf joint makes use of the strutting effect of inclined contact faces where the forces are transmitted through wooden coupling elements wooden pins or dowels. The scarf can be modified in four variants according to the relevant stress and is suitable for historically valuable timber structures. It meets both functional and aesthetic requirements. The designer structural engineer will learn in the methods the loading capacity or stiffness of the beam with the designed joint, its recommended dimensions and detailed geometry. The contractor will appreciate the description of the execution and maintenance of the joint. .

Wood anatomy and acoustic properties of selected tropical hardwoods

Selected anatomical features and ground tissue composition were studied in four tropical hardwoods (Afzelia sp. [Doussie], Intsia sp. [Merbau], Astronium sp. [Muiracatiara] and Millettia sp. [Wenge]). These woods can be applied in musical instrument production, especially for xylophone bars. The measured density, 1st bending natural frequency and the logarithmic decrement of damping were used to calculate other acoustic properties such as dynamic young modulus of elasticity (Erf ), specific modulus of elasticity (E´/ρ), internal friction (tan δ), and acoustic conversion efficiency (ACE).The correlations between anatomy and acoustic properties were determined. Despite difficulties in specifying general characteristics of hardwoods due to their complicated and variable structure, correlations valid between species were found for specific modulus of elasticity. Specific modulus of elasticity was negatively correlated with ray tissue volume and positively with ray height to width ratio and fiber length. The diversity in ground tissue composition was the main criterion for the species choice and probably should condition diverse correlations of anatomical features found for individual species. It seems that better acoustic properties (higher stiffness, ACE or specific modulus of elasticity) are performed by wood with longer fibers and slender rays, causing minimal deflection of adjoining fibers.

Structural and acoustic properties of African padouk (Pterocarpus soyauxii) wood for xylophones

The possibility of supplementing the subjective traditional assessment of wood quality for percussion instruments with reliable acoustic measurements was studied. Sawn boards of African padouk (Pterocarpus soyauxii Taub.) were selected in a timber yard and classified into five grades by using the dynamic combined specific modulus of elasticity. Then they were sawn into raw xylophone bars, which were afterwards traditionally graded by listening to the sound damping at flexural bending excitation and additionally by acoustic measurements. A high quality grade matching of xylophone bars and sawn boards was confirmed. Highly graded material was mostly quarter sawn with straight or moderately interlocked grain. The sound quality of raw xylophone bars was not influenced by the wood density. It negatively correlated with sound damping (tan δ) and positively with the specific modulus of elasticity and with absolute and relative acoustic conversion efficiency. The audial grading of xylophone bars could be improved or substituted by relative acoustic conversion efficiency.