Project

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Reliable timber and innovative wood products for structures: Adhesive Bonding of Structural Hardwood Elements

Applicant Niemz Peter
Number 140002
Funding scheme NRP 66 Resource Wood
Research institution Institut für Baustoffe ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Civil Engineering
Start/End 01.03.2012 - 31.05.2015
Approved amount 341'032.00
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All Disciplines (2)

Discipline
Civil Engineering
Material Sciences

Keywords (13)

Adhesive; Moisture resistance; Delamination; Beech; Moisture induced stresses; Mechanosorption; Fracture behaviour; Adhesion; Creep; Cohesion; Simulation; Micro mechanics; Hardwood

Lay Summary (German)

Lead
Klebverbindungen in Tragwerkselementen aus Laubholz Nach Umnutzungen von Gebäuden und auch zum Beginn der Heizperiode häufen sich Schäden in Leimholzbindern, indem sich nach Jahrzehnten spontan Holzbretter ablösen. In Produkten aus Laubholz ist durch dessen höhere Quellmasse und Steifigkeit die Gefahr noch grösser. Wie lassen sich Klebstoffe und Verfahren verbessern, um die Zuverlässigkeit von Klebverbindungen bei Laubholz über Jahrzehnte sicherzustellen?
Lay summary
Hintergrund
Eine Reihe neuer Entwicklungen für Anwendungen im Holzbau setzt auf die überlegenen mechanischen Eigenschaften von Laubholz. Zudem wird durch die Forstwirtschaft immer mehr Laubholz bereitgestellt. Beispiele sind Brettschichtholzträger aus Buche oder mit Buchenlamellen in den Decklagen. Da Buchenholz besonders stark quillt und eine hohe Steifigkeit hat (hoher Elastizitätsmodul), treten in Klebverbindungen bei Verwendung von Buche deutlich höhere Eigenspannungen auf im Vergleich zum Nadelholz, die den Nachweis der Klebfugengüte nach geltender Normung sehr erschweren. Dies liegt zum einen daran, dass die verfügbaren Klebstoffsysteme für Nadelhölzer entwickelt wurden und nicht auf die Eigenschaften von Laubhölzern abgestimmt sind. Zum anderen basiert die bestehende Normung auf Testverfahren für Nadelholz, das bislang im Holzbau dominiert.

Ziel
Ziel ist es, in Zusammenarbeit mit Schweizer Klebstoffherstellern einen an den Aufbau von Buchenholz angepassten Klebstoff zu entwickeln und zu prüfen. Mittels bruchmechanischer Ansätze und numerischer Nachweise bestimmen die Forschenden das Delaminationsverhalten der verklebten Brettschichtholzträger und entwickeln eine zuverlässige Nachweismethode. Ein Vorteil der bruchmechanischen Ansätze ist, dass diese – im Gegensatz zu derzeit verwendeten Spannungsnachweisen – auch den Einfluss der Elementgrösse berücksichtigen. Wie sich die geklebten Laubholzelemente bei langzeitlichen jahreszeitlichen Klimaschwankungen verhalten, erfassen die Forschenden in numerischen Simulationen und vergleichen diese mit Experimenten an Brettschichtholz aus Buche bei alternierender Luftfeuchte.

Bedeutung
Der Sicherheitsnachweis der Klebverbindungen ist die Grundvoraussetzung für eine wirt-schaftliche Nutzung von Laubholzprodukten in der Schweiz und im Ausland. Zudem würde ein spezieller Schweizer Klebstoff für Laubholz ein Marktsegment bedienen, das sich im In- und Ausland mit einer rasanten Geschwindigkeit entwickelt.

Direct link to Lay Summary Last update: 25.01.2013

Lay Summary (French)

Lead
Assemblage par collage d’éléments de structures porteuses en bois de feuillus Les dommages partiels constatés au niveau des fermes en bois lamellé-collé apparaissent fréquemment suite à un changement d’affectation de l’ouvrage ou au début de la période de chauffage. Au bout de plusieurs décennies, il se produit un détachement spontané des planches en bois. Ce risque est accru pour les produits à base de feuillus, en raison de l’ampleur supérieure de leur gonflement et de leur rigidité.
Lay summary

Contexte
Une série d’innovations, destinées à des applications dans la construction en bois, mise sur la supériorité des propriétés mécaniques du bois de feuillus, que le secteur sylvicole fournit en quantités croissantes. Les poutres en bois de hêtre lamellé, ou avec une couche de lamelle de hêtre en parement, en sont des exemples. Comme le hêtre gonfle très fortement et présente une rigidité élevée, les assemblages par collage du bois de hêtre sont soumis à des contraintes internes nettement supérieures à celles du bois de résineux, de sorte qu’il est difficile de prouver la qualité du joint de collage selon les normes en vigueur. Ceci est en partie dû au fait que les systèmes de colles disponibles ont été conçus pour des résineux et sont donc inadaptés aux propriétés des bois de feuillus. Par ailleurs, la normalisation actuelle repose sur des essais pour les résineux, essences qui dominent à ce jour la construction en bois.

But
L’objectif est de mettre au point et de tester, en collaboration avec des fabricants suisses, une colle adaptée à la construction en bois de hêtre. En s’appuyant sur la mécanique de la rupture et sur des évidences numériques, les chercheurs déterminent le comportement de décollement des poutres en lamellé-collé et développent une méthode de détection fiable. Contrairement aux rapports de tension sur lesquelles on se base actuellement, cette approche présente l’avantage de tenir compte de l’influence de la taille des éléments. Avec des simulations numériques, les chercheurs étudient le comportement des éléments collés en bois de feuillus sous l’angle de l’effet à long terme des changements climatiques au fil des saisons et comparent ces données à des expériences réalisées sur le bois lamellé de hêtre exposé à des degrés humidité variables de l’air.

Portée
Prouver la sécurité des assemblages par collage est la condition indispensable à une exploitation économique des produits en bois de feuillus, en Suisse et à l’étranger.

Direct link to Lay Summary Last update: 25.01.2013

Lay Summary (English)

Lead
Adhesive bonding in structural elements made of hardwood When buildings are used in a new way or when the heating period begins, there is often damage to the laminated wooden beams. This can involve wood planks suddenly coming loose after decades. This threat is even greater in hardwood because it is more rigid and prone to swelling. How can adhesives and procedures be improved so that adhesive bonding of hardwood is more reliable for decades to come?
Lay summary

Background
A number of new developments for applications in timber construction rely on the superior mechanical properties of hardwood. In addition, more and more of it is being made available through forestry. Examples are glued laminated beams of beech or beech segments in surface layers. As beech has a strong tendency to swell and is very rigid (high modulus of elasticity), there is a lot more tension within adhesive joints of beech as compared to conifer. This makes it difficult to prove its suitability as glue joints according to current norms. On the one hand, this is due to the fact that the available adhesive systems were developed for conifers and not adjusted to the properties of hardwood. On the other hand, the existing norms are based on tests done on conifers, which have so far dominated timber technology.

Aim
The aim of the project is to develop and test an adhesive that is suitable for beech wood. By means of fracture mechanics approaches and numerical detection, the researchers will determine the delamination behaviour of the glued laminated wooden beams and develop a reliable detection method. One advantage of the fracture mechanics approaches is that—in contrast to the currently used tension detection—they also take into account the role played by the size of the element. In numerical simulations, the researchers will establish how the glued hardwood elements behave under seasonal climate changes in the long term. These simulations will be compared with experiments with glued laminated beech wood where alternating humidity was used.

Significance
Proving that adhesive joints are secure is essential to any commercial use of hardwood products in Switzerland and abroad. In addition, a special Swiss adhesive for hardwood would cater to a market segment that is growing rapidly both in Switzerland and abroad.

Direct link to Lay Summary Last update: 25.01.2013

Responsible applicant and co-applicants

Employees

Publications

Publication
Mixed-mode fracture toughness of bond lines of PRF and PUR adhesives in European beech wood
Samuel Ammann, Peter Niemz (2015), Mixed-mode fracture toughness of bond lines of PRF and PUR adhesives in European beech wood, in Holzforschung, 69(4), 415-420.
Rheological model for wood
Mohammad Masoud Hassani, Falk K. Wittel, Stefan Hering, Hans J. Herrmann (2015), Rheological model for wood, in Computer Methods in Applied Mechanics and Engineering, 283, 1032-1060.
Specific fracture energy at glue joints in European beech wood
Samuel Ammann, Peter Niemz (2015), Specific fracture energy at glue joints in European beech wood, in International Journal of Adhesion & Adhesives, 60, 47-53.
Comparative adhesion analysis at glue joints in European beech and Norway spruce wood by means of nano-indentation
Ammann Samuel, Obersriebing Michael, Connerth Johannes, Gindel-Altmutter Wolfgang, Niemz Peter (2014), Comparative adhesion analysis at glue joints in European beech and Norway spruce wood by means of nano-indentation, in International Journal of Adhesion & Adhesives, 50, 45-50.
Fiber and adhesive bridging at glue joints in European Beech
Samuel Ammann, Peter Niemz (2014), Fiber and adhesive bridging at glue joints in European Beech, in Wood Research, 59(2), 303-311.
Fracture properties of adhesive joints under mechanical stresses
Watson Pierre, Clauss Sebastian, Ammann Samuel, Niemz Peter (2013), Fracture properties of adhesive joints under mechanical stresses, in Wood research, 58, 43-56.

Collaboration

Group / person Country
Types of collaboration
Empa, OU: Wood, Dr. Zimmermann, Prof. Burgert Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Research Infrastructure
ETH Zurich, IBK, Prof. Fontana, Prof. Frangi Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Research Infrastructure
Bern University of Applied Science, Dr. Pichelin, Dr. Rehm Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Research Infrastructure
ETH Zurich, IfB, Prof. Burgert Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Research Infrastructure
Holzforschung München, Technische Universität München, Prof. Richter, Dr. Windeisen Germany (Europe)
- Research Infrastructure
- Exchange of personnel

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
Montagskolloquien für die Praxis: Holzverklebung Arbeiten in der Schweiz Talk given at a conference Modellierung der Verklebung von Holzverbindungen 24.11.2014 Zürich, Switzerland Hassani Mohammad; Wittel Falk;
Montagskolloquien für die Praxis: Holzverklebung Arbeiten in der Schweiz Talk given at a conference Bruchenergie von Buchenholz und verschiedenen Holzverklebungen bei variabler Feuchte 24.11.2014 Zürich, Switzerland Ammann Samuel; Niemz Peter;
3rd International Conference on Processing Technologies for the Forest and Bio-based Products Industries Talk given at a conference Influence of mechanical and chemical surface preparation on wooden shear strength of glued wood 26.09.2014 Salzburg, Austria Ammann Samuel; Niemz Peter;
International Conference on Wood Science and Engineering Talk given at a conference Prediction of delamination and fracture in adhesive bondlines 26.09.2014 London, Great Britain and Northern Ireland Wittel Falk; Herrmann Hans; Hassani Mohammad;
Smart functional materials for shaping our future Talk given at a conference Smart as wood – New perspectives on a natural smart material 19.09.2014 Debrecen, Hungary Wittel Falk; Hassani Mohammad;
68th International Convention Talk given at a conference Influence of mechanical and chemical surface preparation on wooden shear strength of glued wood 10.08.2014 Toronto, Canada Ammann Samuel; Niemz Peter;
Montagskolloquien für die Praxis - Eigenschaften und Verwendung von Laubholz Talk given at a conference Holzverbund – Möglichkeiten und Grenzen der Modellierung 09.12.2013 Zürich, Switzerland Hassani Mohammad; Herrmann Hans; Wittel Falk;
International Conference on Wood Adhesives Talk given at a conference Delamination Behaviour of Adhesively Bonded Structural Hardwood Elements 09.10.2013 Toronto, Canada Ammann Samuel; Hassani Mohammad; Niemz Peter; Wittel Falk;
RILEM Conference on Materials and Joints in Timber Structures – recent Advancement of Tech-nology Talk given at a conference Investigation of delamination process in adhesively bonded hardwood elements under changing environmental condition 08.10.2013 Stuttgart, Germany Niemz Peter; Herrmann Hans; Ammann Samuel; Wittel Falk; Hassani Mohammad;
International IUFRO Conference MeMoWood Poster Investigation of delamination process in adhesively bonded hardwood elements under changing environmental condition 01.10.2013 Nancy, France Ammann Samuel; Niemz Peter; Hassani Mohammad; Herrmann Hans; Wittel Falk;
International IUFRO Conference MeMoWood Poster Adhesive bonding of structural hardwood elements 01.10.2013 Nancy, France Wittel Falk; Niemz Peter; Herrmann Hans; Hassani Mohammad; Ammann Samuel;
Complas XII International Conference on Computational Plasticity Talk given at a conference Rheological Material Model for Wood 03.09.2013 Barcelona, Spain Ammann Samuel; Herrmann Hans; Hassani Mohammad; Niemz Peter; Wittel Falk;
26-th International Scientific Conference of Wood Technology Faculty „WOOD – MATERIAL OF THE XXI-st CENTURY”), Talk given at a conference Effect of moisture content on the tensile, compressive and bending properties of European Beech wood 20.11.2012 Rogow, Poland, Poland Niemz Peter;
Workshop on Micro-characterization of wood materials and properties Talk given at a conference Failure Mechanism in Adhesive Bonds 26.10.2012 Edinburgh, Scotland, Great Britain and Northern Ireland Niemz Peter; Wittel Falk;
Workshop on Micro-characterization of wood materials and properties Talk given at a conference Failure Mechanism in Adhesive Bonds 24.10.2012 Edinburgh, Scotland, Great Britain and Northern Ireland Wittel Falk; Niemz Peter;
ECCOMAS Talk given at a conference Physical-mechanical properties of some European Hardwoods 10.09.2012 Wien, Österreich, Austria Niemz Peter;
The 5th Conference on Hardwood Research and Utilization in Europe Talk given at a conference Physical-mechanical properties of sycamore maple and beech 10.09.2012 Sopron, Ungarn, Hungary Niemz Peter;
IUFRO – All Division 5 Conference Talk given at a conference Physical-mechanical properties and state of art in the investigation of European hardwood 09.07.2012 Lisboa Portugal, Portugal Niemz Peter;
15. Holztechnisches Kolloquium Talk given at a conference Einfluss verschiedener mechanischer Holzbearbeitungsverfahren auf die Verklebungsgüte von Buchenholz 29.03.2012 TU Dresden, Germany Niemz Peter;


Knowledge transfer events

Active participation

Title Type of contribution Date Place Persons involved
13. Kolloquium Klebtechnik Forschungsnetzwerk Mittelstand 26.02.2013 Frankfurt a.M., Germany Niemz Peter; Ammann Samuel;
Fachtagung Kleben von Holz und Holzwerkstoffen, Bayern Innovativ 19.06.2012 Würzburg, Germany Herrmann Hans; Niemz Peter; Ammann Samuel; Wittel Falk; Hassani Mohammad;
SAH Statusseminar 23.04.2012 Bern, Switzerland Niemz Peter;


Self-organised

Title Date Place
Montagskolloquien für die Praxis - Holzverklebung Arbeiten in der Schweiz 24.11.2014 Zürich, Switzerland
Montagskolloquien für die Praxis - Eigenschaften und Verwendung von Laubholz 09.12.2013 Zürich, Switzerland
Montagskolloquien für die Praxis - Eigenschaften und Verwendung von Laubholz 26.11.2012 Zürich, Switzerland

Awards

Title Year
Election to the Board of International Academy of Wood Science 2012
Wilhelm Klauditz Preis 2012 des Internationalen Vereins für Technische Holzfragen e.V. 2012

Abstract

Structural glulam elements generally made from coniferous wood are well established in timber engineering with vast experience in construction, design and joining. Only by composing smaller, selected wood parts with bondings, could engineered structural wood elements outperform solid timber and compete with steel or reinforced concrete. Therefore reliable adhesive joints build the fundament of modern wood structures and contribute towards the success of timber engineering in general. Unfortunately cutting and re-joining wood in different order creates problems due to the natural orthotropy in mechanical and hygric properties that lead to residual stresses in adhesive bond lines. Today this uncertainty, among others like processing, drying, sorting, and grading defects, is dealt with by enforcing rather high safety factors of about 2-3 [C10] and also by restricting adhesives approved for construction. In order for an adhesive to be approved, a number of delamination tests have to be performed without significant visible delamination, such as extreme water immersion and drying cycles to address long term durability (DIN EN 301 / 302-2). Additionally, short term shear tests have to be performed, for example on block shear specimens (DIN EN 392). A high mean shear strength and required wood failure percentage are believed to indicate a “good” bonding. However, what should be kept in mind is that these codes were originally developed for adhesive bonding of softwood, and today’s adhesive systems as a consequence were developed to fulfill them.Nowadays, a number of innovative developments of wood products for structural applications are based upon the application of either pure hardwoods, or hardwood in combination with softwood. Examples are highly loaded glue-laminated beech beams, that cannot be realized with softwood, hybrid beech/spruce beams, veneer based products out of hardwood like load pins or strengthened nodes made of beech or ash that are typical weak points in wood constructions out of softwood [A0]. All these ideas generate higher requirements for suitable adhesive bondings, in particular with respect to changing climatic conditions. Adhesive systems can only be successful on the international market if the durability and strength of adhesive bondings can be assured to satisfaction. Unfortunately until now, not one formaldehyde free adhesive system has been approved for adhesive bonding of hardwood for structural purposes [A2]. One reason is the fact that adhesive systems are mainly confectioned for bonding spruce, which has quite distinct pore-space characteristics from hardwood. Another reason lies in the application of the codes described above also for hardwood. However, due to the much more excessive hygric behavior of hardwood, and beech in particular, residual stresses arise that are much higher than those in softwood bond lines.It is the goal of this project to address structural bondings of hardwoods with hardwood or spruce components, to work out the foundation for new, reliable structural timber products that make local use of hardwood, wherever it is advantageous. To overcome the demanding standards, more knowledge on the physical processes happening inside components at the bond line is essential, along with reliable testing methods that focus on mechanical properties, their assessment and transferability to the structural scale. We tackle the problem from three perspectives: (i) the micro-mechanisms of failure of glued zones in wood and hard-/softwood adhesive joints, (ii) the macroscopic perspective of assessing reliable failure properties for bond lines and (iii) the multi-field modeling and transferability of failure of bond lines onto the structural scale. Each approach will be the thesis of one of three, strongly interacting PhD students. It is our aim to advance adhesive bonding with hardwood. Such advancements are advantageous for the Swiss forest, timber, and adhesive industry since they resemble key technology for reliable adhesive bonding of timber constructions in new applications, wood based materials, and innovative wood products.
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