Václav Sebera

Finite Element Analysis of Mode Stirrer Impact on Electric Field Uniformity in a Microwave Applicator

Nonuniformity of the electromagnetic field in microwave ovens is assumed to be one of the reasons for nonuniform drying of materials. There are several approaches that are partly able to eliminate such a negative phenomenon. One of them is a placement of mode stirrers in the microwave (MW) applicator. Therefore, the goal of this study was to numerically investigate the influence of a proposed fan-like mode stirrer on the uniformity of the electric field (EF) in an experimental MW device. This was done with the help of 3D harmonic high-frequency finite element (FE) simulation of the EF distribution in the MW applicator. Within the work, two FE models of the MW device were built, analyzed, and compared to each other. The first FE model does not incorporate the mode stirrers. The other one has two mode stirrers, each placed in front of two waveguide ports. In both models, the EF uniformity is studied in 10 height levels and in two mutually perpendicular directions. Change of the EF uniformity was analyzed in terms of coefficients of variation (CoVs) of the electric vector sum across the applicator. Results of simulations show that the mode stirrers decrease CoVs (EF uniformity) in 8 of 10 studied height levels ranging from 2 to 20% and increase them in two topmost levels ranging from 3 to 7.3%. With respect to the fact that the simulations did not consider the mode stirrers’ rotation, the calculated effect might be lower than the actual effect. However, harmonic FE analysis was shown to be an efficient way for investigating, to a degree, the mode stirrer influence on the EF uniformity in a microwave applicator.

Determination of local material properties of OSB sample by coupling advanced imaging techniques and morphology-based FEM simulation

Abstract The goal was to determine local mechanical properties inside of oriented strand board (OSB) based on a realistic morphology-based finite element (FE) model and data acquired from a physical test performed on the same material. The spatial information and local grayscale intensity from CT-scans obtained from small OSB sample was transformed into a 2D regular morphology-based FE mesh with corresponding material properties. The model was then used to simulate the actual compression test performed on the specimen using simplified boundary conditions. The simulated strain fields from the model were compared with the actual strain field measured on the specimen surface during the compression test by means of a full-field optical method, named digital image correlation (DIC). Finally, the original set of material properties was adjusted by an iterative procedure to minimize the difference between the simulated and the measured strain data. The results show that the developed procedure is useful to find local material properties as well as for morphological modeling without the need of segmentation of the image data. The achieved results serve as a prerequisite for full 3D analyses of the complex materials.

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.

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. .

New Design Method for Lap Scarf Joint Used for Reconstruction of Timber Structures

The article describes a development of a new carpentry join suitable for reconstructions of timber structures; it is possible to replace only damaged parts of wooden beam with help of the joint. The mechanical behavior of the joint was analyzed in details. The inclined faces transfer the direction of loading and use of less stiff wooden dowels allows better distribution of stresses. The design methodic has been created so that the maximum of joint potential is used safely.