Numerical modeling in timber engineering – moisture transport and quasi-brittle failure

With the rising popularity of timber structures and the increasing complexity of timber engineering projects, the need for numerical simulation tools specific to this building material is gaining rapidly in importance. in particular, moisture transport processes and the quasi-brittle failure behavior, both difficult to describe, present major challenges and are of great relevance in practical construction. For these reasons, this article presents numerical modeling concepts for predicting moisture gradients, estimating effective stiffness and strength, and numerically identifying potential cracking mechanisms in wooden components. These concepts are validated through experimental test programs, and the associated challenges are addressed. selected results ultimately demonstrate the capabilities and relevance of such methods for timber engineering

Numerical simulation of wooden boards considering morphological characteristics and complex failure processes

Abweichender Titel laut Übersetzung der Verfasserin/des VerfassersZsfassung in dt. SpracheDer natürlich gewachsene Baustoff Holz weist ein komplexes Materialverhalten auf. Hauptursache dafür ist die wuchsinduzierte Orthotropie des astfreien Materials, zusätzlich jedoch wird dieser Effekt durch das Vorhandensein von Ästen und den daraus resultierenden Faserabweichungen verstärkt. Eine zuverlässige Vorhersage des mechanischen Verhaltens von Holzbrettern und Holzprodukten mittels numerischer Simulationstools ist daher nicht einfach umzusetzen. Allerdings vergrößert der Wunsch nach einer besseren Ausnutzung bzw. einer effizienteren Verwendung dieses wertvollen Baustoffs den Bedarf für solche Vorhersagemodelle laufend. Dadurch motiviert wurden in dieser Arbeit Simulationstools für Holzbretter entwickelt, die die Berücksichtigung von realistischen dreidimensionalen Faserverläufen rund um Äste und des komplexen Materialverhaltens von astfreiem Material ermöglichen. Die resultierenden Spannungs- und Verzerrungsfelder im elastischen Bereich und die Genauigkeit der berechneten Faserrichtungen werden mittels vollflächigen Verformungsmessungen an 4-Punkt-Biegeversuchen validiert. Weiters wurde ein Kriterium zur Bestimmung des Zeitpunkts des Strukturversagens entwickelt. Dabei werden die qualitativen Spannungsänderungen in vorher definierten Volumen rund um die Äste analysiert. Diese Herangehensweise basiert auf der Entstehung von Versagenszonen in der Nähe der Äste, welche hauptsächlich durch Zugspannungen quer zur Faserrichtung verursacht werden. Dies wurde durch Vergleiche mit 4-Punkt-Biege- und Zugversuchen von Holzbrettern mit unterschiedlichen Querschnittsabmessungen bestätigt. Im nächsten Schritt wurden die Versagensmechanismen selbst beschrieben und mit der Zellstruktur von Holz in Verbindung gebracht, weil sowohl die Rissinitiierung als auch die globale Richtung der Risse auf der Makroskala von den Struktureigenschaften der kleineren Längenskalen bestimmt werden. Um dieses Verhalten korrekt abbilden zu können, wurden alle möglichen Versagensmechanismen von zwei sich wiederholenden Einheitszellen ermittelt, welche Früh- bzw. Spätholz abbilden. Dabei wurde die sogenannte -eXtended Finite Element Method- angewandt. Mittels eines Stichprobenverfahrens wurde eine Vielzahl möglicher Belastungskombinationen erstellt und die beiden Einheitszelltypen damit entsprechend belastet. Für alle Simulationen wurden die entstandenen Versagenszustände ausgewertet, klassifiziert und globalen Rissrichtungen zugeordnet. Die daraus folgende Festlegung von zwei Mehrflächen-Versagenskriterien mit zugeordneten Rissrichtungen für die beiden Zelltypen erlaubt eine Implementierung in Subroutinen von kommerzieller Finite-Elemente-Software. Die Anwendung dieser Methode auf Simulationen von Zugversuchen auf der Jahrringskala zeigte, dass damit die entscheidenden Versagensmechanismen in Holz korrekt abgebildet werden können. Um eine direkte Anwendbarkeit zu ermöglichen, werden in einem letzten Schritt herkömmliche Sortierkriterien für die Festigkeitssortierung von Holzbrettern evaluiert und neue vorgestellt. Während herkömmliche Kriterien die 3D-Lage und Orientierung von Ästen kaum berücksichtigen, würden neuartige Scantechnologien eine weitaus höhere Ausbeute im Sortierprozess erlauben. Deshalb werden Algorithmen für die manuelle und auch die automatische 3D-Rekonstruktion von Holzbrettern vorgestellt. Basierend auf den neuen Astinformationen werden neuartige Sortierkriterien entwickelt, welche die Berücksichtigung von Ästen, resultierenden Faserabweichungen und, vor allem für den Fall einer Biegebelastung, Astlagen ermöglichen. Eine statistische Auswertung von herkömmlichen und neuartigen Sortierkriterien zeigt, dass durch den Gebrauch dieser zusätzlichen Informationen signifikante Verbesserungen erreicht werden können.The naturally grown material wood exhibits a rather complex mechanical behavior. This is mainly caused by the growth-induced orthotropy of the clear wood material, and further increased by knots and the resulting fiber deviations in their vicinities. A reliable prediction of the mechanical behavior of wooden boards and wood products by means of numerical simulation tools is not easy to accomplish. Nevertheless, for a better utilization and a more efficient use of this valuable building material, the demand for such prediction tools increases continuously. Motivated by this need, within this work, simulation tools for wooden boards were developed, enabling the consideration of realistic three-dimensional fiber deviations around knots and taking the complex mechanical behavior of clear wood into account. The resulting stress and strain fields within the elastic regime and the accuracy of the calculated fiber directions are validated by means of four-point bending tests, which are accompanied by full-field deformation measurements based on digital image correlation techniques. Furthermore, a criterion, which indicates the point of structural failure, was developed. In the process, the effective strength values are determined by examining the qualitative stress changes in predefined volumes around knots. This approach is based on the formation of failure zones predominantly caused by perpendicular-to-grain tension in the vicinity of knots, and is confirmed by comparisons to four-point bending and tensile tests of wooden boards with various cross-sectional dimensions. In a next step, the failure processes themselves were described and linked to the cell structure of wood, because the initiation and also the global direction of cracks on the macroscale are governed by structural features on the lower length scales. To depict this behavior accurately, all possible failure mechanisms for two repetitive units, representing late- and earlywood, respectively, are identified by an approach based on the extented Finite Element method. By using sampling techniques, a wide range of possible loading combinations were generated and applied to the two cell types. For all simulations, the obtained failure modes were evaluated, classified and global crack directions assigned. The following definition of two multisurface failure criteria with predefined global crack directions for the two cell types individually allows the implementation into subroutines of commercial Finite Element software. Through application of this approach by simulating tensile tests on the annual year ring scale its capability to reproduce essential failure mechanisms in wood correctly could be shown. Finally, intended for direct practical application, the evaluation of common and the introduction of new indicating properties (IPs) for the strength grading of wooden boards are presented. While commonly used IPs hardly consider the 3D position and orientation of knots within wooden boards, new scanning techniques would allow highly improved yields within the wood grading process. Thus, algorithms for the manual and also automatic 3D reconstruction of wooden boards are presented. Based on this new knot information, novel IPs were developed, which allow for the consideration of knots, resulting fiber deviations and the knot location, which plays an important role under bending load. A statistical evaluation of common and novel IPs shows that by using these additional information significant improvements can be reached.12

Heat and mass transfer model for wood including free water transport

Conference logo shows incorrect conference date "2020".Knowledge about wood moisture conditions in a timber component is essential to predict its mechanical behavior. Not only stiffness and strength properties are highly dependent on wood moisture content but also diffusion coefficients, density, specific heat capacity and the thermal conductivity. Therefore, modern prediction tools, which are able to describe these effects, can benefit the development of new wood-based products. Especially, if they exhibit complex geometries and are made of materials with different moisture characteristics, as different and direction-dependent coefficients of expansion may lead to critical stresses. Transport mechanisms below the fiber saturation point were developed by [1-2]. Three coupled differential equations describe bound water, water vapor and energy conservation. Free water exists above the fiber saturation point with the corresponding transport mechanisms described in [3]. Values of the free water content can be much higher than those of bound water and water vapor. Thus, within the areas, where the switch from the transport mechanisms below the fiber saturation point to those above occur, high gradients can exist. To deal with these within the finite element method different procedures, like upstreaming and mass lumping [4], were used. A three-dimensional Abaqus User-Element Subroutine was developed to describe these coupled equations.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)BM für Land- und Forstwirtschaft, Umwelt und Wasserwirtschaft (bm:lfuw); European Commission17

Simulating failure mechanisms in wooden boards with knots by means of a microstructure-based multisurface failure criterion

(VLID)279048

A numerical approach to describe failure of wood - From the wood cell level up to wood-based products

For the description of the failure processes in clear-wood, a multiscale approach, based on the Finite Element (FE) method, was performed. In a previous work, failure mechanisms at the single wood cell level were identified by using a unit cell approach in combination with the eXtended Finite Element Method (XFEM). Finally, a multisurface failure criterion was obtained. Within this work, these results were combined in another unit cell at the annual year ring level, where late-(LW) and earlywood (EW) cells form a layered structure. Subsequently, a single multisurface failure criterion with predefined global crack directions at the clear-wood level could be won, which will be implemented into the commercial FE software Abaqus through a subroutine. In combination with a previously developed FE simulation tool, which allows the 3D virtual reconstruction of different wood-based products, including knots and the surrounding fiber deviations, the main failure mechanisms in such products can now be captured realistically. Thus, the influences of knot configurations on several effective properties, like modulus of elasticity or bending strength, can be determined. Moreover, the resulting effective stiffness properties are used to study strengthening and load-transfer effects between lamellae in Glulam and CLT elements

Experimental study on glued laminated timber beams with well-known knot morphology

Nowadays, the impact of knots on the failure behaviour of glued laminated timber (GLT) beams is considered by subjecting the single lamellas to a strength grading process, where, i.a., tracheid effect-based laser scanning is used to obtain information about knot properties. This approach single-handedly defines the beam’s final strength properties according to current standards. At the same time, advanced production processes of such beams would allow an easy tracking of a scanned board’s location, but, at this point, previously obtained detailed information is already disregarded. Therefore, the scanning data is used to virtually reconstruct knot geometries and group them into sections within GLT beams. For this study, a sample of 50 GLT beams of five different configuration types was produced and tested under static four-point-bending until failure. As for each assembled lamella the orientation and position within the corresponding GLT beam is known, several parameters derived from the reconstructed knots can be correlated to effective GLT properties. Furthermore, the crack patterns of the tested beams are manually recorded and used to obtain measures of cracks. A detailed analysis of the generated data and their statistical evaluation show that, in the future, dedicated mechanical models for such timber elements must be developed to realistically predict their strength properties. A potential approach, using fluctuating section-wise effective material properties, is proposed.Vienna Business Agenc

A numerical simulation tool for wood grading: model validation and parameter studies

Growth irregularities such as knots always result in a pronounced reduction in strength in wooden boards. The presented paper deals with the experimental validation of a newly developed numerical simulation tool (Hackspiel et al., Wood Sci Techol, submitted, 2013) which allows the investigation of such defects by means of physically based numerical simulations. Thereby, advanced models for the description of the three-dimensional fiber course and for the mechanical material behavior are used. The stepwise validation covers the validation of the model for the elastic behavior covering the model for the grain course in the first place. For the validation of the model’s strength prediction capabilities, a total number of 52 boards were tested in tension and bending. The corresponding strength predicted by the simulation tool showed good agreement with the test results. The validated tool was then used to perform parameter studies in which the influence of various knot-related parameters on strength was investigated. The observed trends help to identify decisive knot parameters for board strength, which should receive particular attention in the grading process

A numerical simulation tool for wood grading model development

Wood is a naturally grown material that has growth irregularities, especially knots and site-related defects. The former result in a pronounced reduction in stiffness and strength of boards. The lack of knowledge regarding the effects of growth irregularities on the mechanical behavior of boards motivated the investigation of such defects by means of physically-based numerical simulations. A model which is based on the combination of the finite element method with sophisticated descriptions of the fiber course and of the material behavior has been developed. Comparisons of model predictions with corresponding experimental results in terms of relations between tensile strength and knot area ratio-values show good agreement and underline the predictive quality of the simulation model. The enhanced insight into the mechanical functionality of a stem–branch junction gained by the newly developed numerical multiscale model might contribute to improve the grading criteria and, thus, to achieve an economic benefit for the wood-processing industry

Computational mechanics concepts for wood-based products and timber structural elements

Conference logo shows incorrect conference date "2020".The mechanical behavior of wood products highly depends on structural features on several length scales. This leads to a high amount of random fluctuation in mechanical properties at the structural level. In practice, homogeneous material behavior within timber elements is assumed and uncertainties in loading and load-bearing capacity are considered by using partial safety factors. Those are not directly linked to the mechanical behavior of the considered elements. Therefore, a stochastic framework opening up the possibility to establish such links by combining structural analysis and probabilistic descriptions of wood could be an important step in timber design. For example, in GLT beams the mechanical behavior mostly depends on the tensile properties of individual boards. To describe their fluctuations, simulations on this level are performed. The reconstruction of knots and the implementation of new fracture mechanical methods allows the prediction of stiffness and strength properties for knot sections. Condensation of those results into stiffness and strength profiles permits the development of probabilistic models and the random generation of such profiles, and their use in wood product simulations. This can be used for sensitivity analyses of timber engineering designs or to obtain probabilistic descriptions of the uncertainties at the level of timber elements.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)BM für Land- und Forstwirtschaft, Umwelt und Wasserwirtschaft (bm:lfuw); European Commission17

A 3D model for knots and related fiber deviations in sawn timber for prediction of mechanical properties of boards

Increased use of wood has led to complex timber constructions and new types of engineered wood products. In simulations, however, mainly simplified models are used to describe this material with its strongly varying properties. Therefore, reliable prediction tools for mechanical properties of wooden boards are needed. Those varying properties mainly originate from knots and fiber deviations. Thus, we use fiber directions on board surfaces to reconstruct knots within boards. Combined with a fiber deviation model we assess our model with experiments on different levels: fiber directions on surfaces, strain fields and bending stiffness profiles. This model now better describes fiber patterns near knots and knot clusters. Also, we showed that accurate modeling of the pith is important to avoid large regions of incorrect fiber deviations. Furthermore, modified knot stiffness properties were successfully used to consider pre-cracked knots. Finally, we obtained multiple bending stiffness profiles, where we showed that even local effects can be simulated accurately. We anticipate our tool to be a starting point for improving strength grading models, where effects of knot configurations can be studied more easily than with experiments alone. Furthermore, the presented improvements will render the simulation of realistic failure mechanisms in wooden boards more likely