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Adhesive Joints in Timber Engineering. Modelling and Testing of Fracture Properties

Serrano, Erik LU (2000) In Report TVSM
Abstract (Swedish)
Popular Abstract in Swedish

Trälimfogar är en väsentlig del av modern träkonstruktionsteknik. För att höja förädlingsvärdet hos träråvaran, tillverkas idag flera olika sorters produkter vilka är beroende av limfogar. Exempel på sådana produkter är limträ och fanerträ (LVL). Ett annat exempel där limfogar används är vid inlimning av skruvar eller stänger för sammanfogning eller förstärkning av limträ. Förband av inlimmade skruvar är ett effektivt sätt att åstadkomma ett momentstyvt och hållfast förband utan att behöva använda stora stålbeslag.



För att förstå och kunna modellera exempelvis limträbalkars mekaniska beteende, måste man även kunna beskriva limfogarnas inverkan. Även om limfogar ofta utgör en... (More)
Popular Abstract in Swedish

Trälimfogar är en väsentlig del av modern träkonstruktionsteknik. För att höja förädlingsvärdet hos träråvaran, tillverkas idag flera olika sorters produkter vilka är beroende av limfogar. Exempel på sådana produkter är limträ och fanerträ (LVL). Ett annat exempel där limfogar används är vid inlimning av skruvar eller stänger för sammanfogning eller förstärkning av limträ. Förband av inlimmade skruvar är ett effektivt sätt att åstadkomma ett momentstyvt och hållfast förband utan att behöva använda stora stålbeslag.



För att förstå och kunna modellera exempelvis limträbalkars mekaniska beteende, måste man även kunna beskriva limfogarnas inverkan. Även om limfogar ofta utgör en geometriskt liten del av en konstruktion är deras beteende ofta avgörande för hela konstruktionens bärförmåga.



Avhandlingen behandlar experimentella och numeriska metoder för att beskriva trälimfogars mekaniska beteende. Speciell vikt har lagts vid limfogarnas brottmekaniska beteende. Med brottmekaniskt beteende menas det som händer i limfogen när limfogens lokala styrka har uppnåtts och en spricka utvecklats. Avhandlingen är en sammanläggningsavhandling och består av en inledande introduktion och sammanfattning samt sju artiklar. De sju artiklarna beskriver dels experimentella och dels numeriska undersökningar av trälimfogar.



De experimentella metoderna omfattar studier av trälimfogars och inlimmade skruvars brottmekaniska beteende. De egenskaper som bestämts är styvhet, styrka, brottenergi samt spännings-förskjutnings kurvors utseende. Vid bestämning av spännings-förskjutningssambanden har särskild vikt lagts vid att registrera den nedåtgående delen efter maxspänning. Undersökningarna visar att trälimfogars beteende karakteriseras av en sådan nedåtgående del och att detta brottmjuknande kan mätas under stabila förhållanden.



De numeriska undersökningarna syftar till att beskriva limfogens inverkan på konstruktionselements mekaniska respons. De konstruktionselement som har behandlats är fingerskarvar, limträbalkar och inlimmade skruvar. Resultaten av de numeriska studierna visar att limfogens inverkan kan vara avgörande för dessa konstruktionselements bärförmåga. Det visar sig också att inte bara fogens lokala styrka (hållfasthet) är en viktig parameter. I många fall är limfogens brottenergi och de ingående materialens styvheter de helt avgörande parametrarna. (Less)
Abstract
This thesis, which is a compilation of seven papers, concerns the mechanical testing, numerical analysis and constitutive modelling of wood-adhesive bonds in timber engineering. Applications such as finger-joints, glued-laminated timber and glued-in rods are considered. The experimental studies include the testing of the fracture characteristics of wood-adhesive bonds, including both wood-to-wood bonds and glued-in rods of either steel or glass fibre reinforced polyester. The numerical studies relate to the strength of finger-joints, laminated beams and glued-in rods for timber structures.



In the experimental studies, the complete stress-displacement response of small specimens, particularly their fracture softening... (More)
This thesis, which is a compilation of seven papers, concerns the mechanical testing, numerical analysis and constitutive modelling of wood-adhesive bonds in timber engineering. Applications such as finger-joints, glued-laminated timber and glued-in rods are considered. The experimental studies include the testing of the fracture characteristics of wood-adhesive bonds, including both wood-to-wood bonds and glued-in rods of either steel or glass fibre reinforced polyester. The numerical studies relate to the strength of finger-joints, laminated beams and glued-in rods for timber structures.



In the experimental studies, the complete stress-displacement response of small specimens, particularly their fracture softening behaviour beyond peak stress, was recorded. A major outcome from the experiments is that wood-adhesive bonds can behave in a fracture-softening manner, and that it is possible to record this under stable conditions.



In one of the numerical studies the finite element method was employed to analyse the stress distribution around zones of low stiffness in a laminated beam. A fracture mechanics analysis was also performed of the delamination of a laminated beam. The results show that the often made assumption of a stress redistribution taking place around weak zones is not necessarily true. Another finding is that the delamination of an initially cracked glulam beam tends to be increasingly dominated by mode II failure as the lamination thickness decreases.



In another study, also related to finger-joints and laminated beams, the finger-joint failure in a glulam beam was simulated using a nonlinear fictitious crack model with stochastic properties. The results show the proposed approach to be able to account for such phenomena as the size effect and the laminating effect. Another observation is that finger-joint fracture energy, i.e. the ductility, has a major influence on lamination and beam strength. The influence of bondline defects on the tensile strength of a finger-joint was also investigated. It was demonstrated that even a small defect in the form of a glueline void, can have a relatively strong influence on the tensile strength. It was also demonstrated that the strength of finger-joints is largely influenced by the outermost finger.



A nonlinear 3D finite element model was employed in a parameter study of glued-in rods in timber structures, a strain-softening model being used to characterise the adhesive layer. Parameter studies in relation both to the fracture energy and the geometrical parameters and to loading conditions were performed. The results show that the fracture energy is of major importance for the pull-out load capacity, that the model in question can be used to predict size effects and that loading in a pull-compression manner results in lower load-bearing capacities than loading in a pull-pull manner.



Finally, an interface model based on damage mechanics is suggested for the modelling of wood-adhesive interfaces. This model accounts for joint dilatation and post-cracking friction. Also, a homogenisation scheme is presented for combining the proposed model with ordinary plasticity models for the adhesive bulk. This homogenisation procedure is based on assumptions regarding the stress and strain gradients typical of thin bondlines. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Dr Castera, Patrick, Scientific director, Laboratoire de Rheologie du Bois de Bordeaux (LRBB)
organization
publishing date
type
Thesis
publication status
published
subject
keywords
wood, test method, tensile strength, stress distribution, size effect, numerical simulation, laminating effect, laminated beams, joint, glued-laminated timber, glued-in rod, fracture mechanics, finite element method, finger-joint, experiment, damage, constitutive modelling, adhesive, bending strength, Environmental technology, pollution control, Miljöteknik, kontroll av utsläpp, Construction technology, Anläggningsteknik, Material technology, Materiallära, materialteknik
in
Report TVSM
pages
173 pages
publisher
Division of Structural Mechanics, Lund University, PO Box 118, SE-221 00, LUND, Sweden,
defense location
V-building, Lund Institute of Technology, John Ericssons Väg 1, Lund.
defense date
2000-12-08 10:15
external identifiers
  • other:ISRN: LUTVDG/(TVSM-1012)/1-173/2000
ISSN
0281-6679
ISBN
91-7874-095-9
language
English
LU publication?
yes
id
2729e7bd-b2e9-4e82-aca3-5fadf4d86d9e (old id 41135)
date added to LUP
2007-08-01 11:39:45
date last changed
2016-09-19 08:44:59
@phdthesis{2729e7bd-b2e9-4e82-aca3-5fadf4d86d9e,
  abstract     = {This thesis, which is a compilation of seven papers, concerns the mechanical testing, numerical analysis and constitutive modelling of wood-adhesive bonds in timber engineering. Applications such as finger-joints, glued-laminated timber and glued-in rods are considered. The experimental studies include the testing of the fracture characteristics of wood-adhesive bonds, including both wood-to-wood bonds and glued-in rods of either steel or glass fibre reinforced polyester. The numerical studies relate to the strength of finger-joints, laminated beams and glued-in rods for timber structures.<br/><br>
<br/><br>
In the experimental studies, the complete stress-displacement response of small specimens, particularly their fracture softening behaviour beyond peak stress, was recorded. A major outcome from the experiments is that wood-adhesive bonds can behave in a fracture-softening manner, and that it is possible to record this under stable conditions.<br/><br>
<br/><br>
In one of the numerical studies the finite element method was employed to analyse the stress distribution around zones of low stiffness in a laminated beam. A fracture mechanics analysis was also performed of the delamination of a laminated beam. The results show that the often made assumption of a stress redistribution taking place around weak zones is not necessarily true. Another finding is that the delamination of an initially cracked glulam beam tends to be increasingly dominated by mode II failure as the lamination thickness decreases.<br/><br>
<br/><br>
In another study, also related to finger-joints and laminated beams, the finger-joint failure in a glulam beam was simulated using a nonlinear fictitious crack model with stochastic properties. The results show the proposed approach to be able to account for such phenomena as the size effect and the laminating effect. Another observation is that finger-joint fracture energy, i.e. the ductility, has a major influence on lamination and beam strength. The influence of bondline defects on the tensile strength of a finger-joint was also investigated. It was demonstrated that even a small defect in the form of a glueline void, can have a relatively strong influence on the tensile strength. It was also demonstrated that the strength of finger-joints is largely influenced by the outermost finger.<br/><br>
<br/><br>
A nonlinear 3D finite element model was employed in a parameter study of glued-in rods in timber structures, a strain-softening model being used to characterise the adhesive layer. Parameter studies in relation both to the fracture energy and the geometrical parameters and to loading conditions were performed. The results show that the fracture energy is of major importance for the pull-out load capacity, that the model in question can be used to predict size effects and that loading in a pull-compression manner results in lower load-bearing capacities than loading in a pull-pull manner.<br/><br>
<br/><br>
Finally, an interface model based on damage mechanics is suggested for the modelling of wood-adhesive interfaces. This model accounts for joint dilatation and post-cracking friction. Also, a homogenisation scheme is presented for combining the proposed model with ordinary plasticity models for the adhesive bulk. This homogenisation procedure is based on assumptions regarding the stress and strain gradients typical of thin bondlines.},
  author       = {Serrano, Erik},
  isbn         = {91-7874-095-9},
  issn         = {0281-6679},
  keyword      = {wood,test method,tensile strength,stress distribution,size effect,numerical simulation,laminating effect,laminated beams,joint,glued-laminated timber,glued-in rod,fracture mechanics,finite element method,finger-joint,experiment,damage,constitutive modelling,adhesive,bending strength,Environmental technology,pollution control,Miljöteknik,kontroll av utsläpp,Construction technology,Anläggningsteknik,Material technology,Materiallära,materialteknik},
  language     = {eng},
  pages        = {173},
  publisher    = {Division of Structural Mechanics, Lund University, PO Box 118, SE-221 00, LUND, Sweden,},
  school       = {Lund University},
  series       = {Report TVSM},
  title        = {Adhesive Joints in Timber Engineering. Modelling and Testing of Fracture Properties},
  year         = {2000},
}