Erik Serrano

Contact-free measurement and non-linear finite element analyses of strain distribution along wood adhesive bonds

Abstract The strain distribution along wood adhesive bonds was studied using a contact-free measurement system based on a white-light digital image correlation (DIC) technique. Two different specimen geometries and three different adhesives were investigated. The specimen geometries were according to the standards EN302-1 and ASTM D905. The adhesives tested were a phenolic resorcinol (PRF), a one-component polyurethane (PUR) and an epoxy (EPX). In addition to the experimental investigation, a finite element study using a non-linear fracture mechanics model for the adhesive bond line was carried out, aimed at investigating whether deformation measurements could predict differences in the mechanical behaviour of the adhesives. The measurement technique was found to be capable of distinguishing, in terms of their strain distributions at a given load, adhesives that differed markedly from one another. For example, the brittle PRF adhesive showed more localised strains than the more ductile EPX and PUR adhesives did at the same load level. Another conclusion from this study is that the measurement technique used is applicable to situations in which large strains occur. Thus, the technique used here is of great interest for use in the calibration of finite element models and constitutive theories and for the design of test set-ups.

A numerical study of the shear-strength-predicting capabilities of test specimens for wood-adhesive bonds

The sensitivity to geometrical imperfections of various test methods for wood-adhesive bonds was investigated using nonlinear finite element analysis. The adhesive layers in the specimens were modelled using a nonlinear softening, fracture mechanics model. The specimen types corresponded to the standards according to ASTM-D4680, ASTM-D905, EN-302-1 and a modified version of the ASTM-D3535 specimen. The results show that the prediction of bond line strength is highly dependent on both the specimen type used and adhesive properties such as strength, fracture energy and the shape of the stress-slip behaviour of the adhesive layer. For example, it was shown that the ASTM-D905 specimen was not able to rank the adhesives in terms of local bond line strength in the correct order. Another finding is that the sensitivity to geometrical imperfections and erroneous load application in the test methods is also highly dependent on specimen and adhesive type

Glued-in rods for timber structures — a 3D model and finite element parameter studies

Abstract A nonlinear 3D finite element model and a theoretical parameter study in relation to glued-in rods for timber structures are presented. A strain-softening crack band model was used to characterise the behaviour of the adhesive layer between the rod and the wood. The model is general in the sense that it bridges the gap between the theory of an ideal plastic bondline and the theory of linear elastic fracture mechanics. Two parameter studies were made. One in relation to fracture energy and geometrical parameters and the second in relation to loading conditions. The results show that the fracture energy is of major importance for the pull-out load capacity, that the present model can be used to predict such phenomenon like the size effect and that the loading in pull–compression results in lower load-bearing capacities than the loading in pull–pull.

Influence of bondline brittleness and defects on the strength of timber finger-joints

Numerical finite element results on the mechanical behaviour of finger-joints for laminated timber products are presented. Simulations are based on a nonlinear fracture mechanics description of the bondlines in the finger-joints. It is shown that, for a commonly used resorcinol–phenol adhesive, there is potential of increasing substantially the finger-joint strength by increasing the fracture energy of the bond. However, even a single, small glueline void can have a considerable influence on reducing the joint strength.

Glued-in rods for timber structures—An experimental study of softening behaviour

A test method to obtain the strength and the fracture characteristics of the bond of glued-in rods for timber structures is presented. Test results from a series of tests using the proposed method are also given. The test method makes it possible to record the complete stress-displacement response of small test specimens. This response includes the softening behaviour after peak stress,i.e. the decreasing stress at increasing deformation. The test series include three adhesives, two rod-materials, four load-to-grain angles and two timber qualities with different densities. A method to evaluate the fracture-softening behaviour is also proposed. The fracture softening is evaluated by using only a part of the stress-displacement curve. The part used in the evaluation is determination in terms of the slope of the descending part of the stress-displacement curve.RésuméL’article traite d’une nouvelle méthode d’essai destinée à déterminer la résistance et les caractéristiques de rupture du joint de colle de barres collées, utilisées dans les constructions en bois. Les résultats d’une série d’essais effectués avec la méthode proposée sont également indiqués. La méthode d’essai permet de déterminer l’ensemble de la relation contrainte-déformation pour les petits échantillons. Cette relation comprend le comportement isoadoucissant après contrainte maximale, c’est-à-dire la réduction de la contrainte pendant l’augmentation de la déformation. La série d’essais porte sur trois types de colle, deux matériaux de barres, quatre différents angles entre charge et orientation des fibres, ainsi que deux qualités de bois différentes, de densités différentes. Une nouvelle méthode d’évaluation du comportement iso-adoucissant est également indiquée en utilisant seulement une partie de la courbe contrainte-déformation. La partie utilisée pour l’évaluation est déterminée par la pente de la partie descendante de la courbe contrainte-déformation.

Contact-free measurements and numerical analyses of the strain distribution in the joint area of steel-to-timber dowel joints

Two different dowel-type joints of differing size were loaded in tension parallel to the grain. The strain distribution in the joint area was studied using a contact-free measurement system. The results were compared with those of numerical analysis. The measurement system was found to be a valuable complement to traditional measurement techniques and also to numerical analyses performed in parametric studies. The measurements obtained were shown to be particularly useful for studying the highly nonlinear behavior of timber that is of special importance in characterizing timber dowel joints.Several observations of considerable interest were made on the basis of numerical results in combination with results of the contact-free measurement system. A non-uniform strain distribution in the joint area was detected, for example, and shear strains and tensile strains, both parallel and perpendicular to the grain were found to be concentrated in the areas most likely to influence the failure mode of the joint. A general observation made was that the larger size joint failed in brittle manner, an observation supported by previous research. ZusammenfassungZwei verschiedene Stabdübelverbindungen unterschiedlicher Grösse wurden auf Zug in Faserrichtung belastet. Die Dehnungsverteilung im Bereich der Verbindung wurde mittels eines berührungslosen Messsystems untersucht und mit den Ergebnissen aus einer numerischen Analyse verglichen. Das Messsystem erwies sich als wertvolle Ergänzung zu herkömmlichen Messtechniken sowie zu den Ergebnissen von Parameterstudien. Die Messungen erwiesen sich als besonders hilfreich zur Untersuchung des stark nicht-linearen Verhaltens von Holz, das für die Charakterisierung von Holzdübelverbindungen grosse Bedeutung hat.Die numerischen Ergebnisse führten zusammen mit den berührungslosen Messungen zu einigen interessanten Erkenntnissen. So wurde zum Beispiel eine ungleichmässige Dehnungsverteilung im Verbindungsbereich festgestellt. Darüber hinaus fand man heraus, dass Schubdehnungen und Zugdehnungen parallel und senkrecht zur Faser konzentriert in den für den Bruch massgebenden Bereichen vorkamen. Generell wurde in Übereinstimmung mit früheren Untersuchungen festgestellt, dass bei den grösseren Verbindungen ein Sprödbruch auftrat.

Effective stiffness prediction of GLT beams based on stiffness distributions of individual lamellas

Abstract The mechanical properties of structural timber—particularly in terms of stiffness and strength—are subject to high variability, which also affects the properties of timber products made from structural timber, e.g., glued laminated timber (GLT). In this paper, the influence of the longitudinal stiffness variability of wooden lamellas on the effective stiffness of GLT is investigated. In a first step, the local fiber orientation on the surfaces of 350 lamellas of Norway spruce was determined by an optical scanning device. This fiber angle information in combination with a micromechanical model for wood was used for the generation of a longitudinal stiffness profile of each lamella. Recording the position and orientation of each lamella, a total number of 50 GLT beams were assembled (with 4, 7, and 10 laminations) and tested under four-point bending. Knowing the stiffness profile of each board and its location within the GLT beam allowed for an accurate numerical finite element model, which is able to predict the effective GLT stiffness with high accuracy. Interesting insights into the relation between the stiffness of lamellas and the resulting GLT beams could be gained, and finally, a numerical simulation tool which is able to reproduce the experimental results appropriately was obtained.

Dowel deformations in multi-dowel LVL-connections under moment loading

Abstract The aim of the experimental study presented herein is the assessment and quantification of the behavior of individual dowels in multi-dowel connections loaded by a bending moment. For this purpose, double-shear, steel-to-timber connections with nine steel dowels arranged in different patterns and with different dowel diameters were tested in four-point bending. In order to achieve a ductile behavior with up to 7° relative rotation, the connections were partly reinforced with self-tapping screws. The reinforcement did not influence the global load–deformation behavior, neither for dowel diameters of 12 mm nor for 20 mm, as long as cracking was not decisive. The deformation of the individual dowels was studied by means of a non-contact deformation measurement system. Thus, the crushing deformation, that is, the deformation at the steel plate, and the bending deformation of the dowels could be quantified. In the case of 12 mm dowels, the bending deformation was larger than the crushing deformation, while it was smaller in the case of 20 mm dowels. Moreover, dowels loaded parallel to the grain showed larger bending deformations than dowels loaded perpendicular to the grain. This indicates that the loading of the individual dowels in the connection differs depending on their location.

An experimental study of the effects of moisture variations and gradients in the joint area in steel-timber dowel joints

Abstract This experimental study examines the influence of moisture variations on the load-bearing capacity of steel-timber dowel joints. The glulam specimens used to manufacture the joints were first exposed to controlled climate changes. After being stored in the climate chambers, holes were drilled and dowels were inserted. Then, the joints were loaded to failure. The main aim of this study was to investigate how and to what extent moisture variations and gradients in the joint area affects the load-bearing capacity in tension parallel to the grain. The load-bearing capacity was found to be reduced under these conditions when compared to reference joints. Moreover, the brittleness of the joints increased with the time the joints had been exposed to drying. Moisture-induced stresses and cracks in the joint area were found to be a possible explanation of the results. A similar interpretation was given in other studies where other structural timber elements have been studied. This raises an important question of how such moisture effects should be considered in design codes. One possibility is to include the influence of moisture-induced stresses in the k mod factor used in Eurocode 5. Another way could be to consider this type of moisture effect as an equivalent mechanical load case.

A numerical study of the effects of stresses induced by moisture gradients in steel-timber dowel joints

This experimental study examines the influence of moisture variations on the load-bearing capacity of steel-timber dowel joints. The glulam specimens used to manufacture the joints were first exposed to controlled climate changes. After being stored in the climate chambers, holes were drilled and dowels were inserted. Then, the joints were loaded to failure. The main aim of this study was to investigate how and to what extent moisture variations and gradients in the joint area affects the load-bearing capacity in tension parallel to the grain. The load-bearing capacity was found to be reduced under these conditions when compared to reference joints. Moreover, the brittleness of the joints increased with the time the joints had been exposed to drying. Moisture-induced stresses and cracks in the joint area were found to be a possible explanation of the results. A similar interpretation was given in other studies where other structural timber elements have been studied. This raises an important question of how such moisture effects should be considered in design codes. One possibility is to include the influence of moisture-induced stresses in the k mod factor used in Eurocode 5. Another way could be to consider this type of moisture effect as an equivalent mechanical load case. (Less)