Ductile Adhesively Bonded Timber Joints<br />Simon Hehl, Research Assistant Thomas Tannert, Research FellowTill Vallée, Pr...
Outline<br />Ductility in the context of bonded timber joints<br />Experimental investigations<br />Numerical modeling<br ...
Ductility in the context of bonded timber joints<br />Ductility holds a central status in civil engineering<br />Allows fo...
Ductility in timber structures<br />Timber is not ductile<br />If not considering some plastic behavior in compression<br ...
Experimental investigationsSeries 1<br />Tests on adhesively bonded double lap joints (DLJ)<br />Timber: Spruce<br />Consi...
Experimental investigations<br />~4550 MPa<br />~450 MPa<br />~150 MPa<br />
Experimental investigationsSeries 2<br />Tests performed on DLJ involving graded adhesive layers<br />Soft adhesive in the...
For all adhesives<br />No significant differences in the load-displ’ curves <br />Brittle failure in the timber<br />No ob...
Experimental investigations: Series 1<br />
Experimental investigations: Series 2<br />
Experimental investigations<br />Neither did the use of ductile adhesives enhance ductility nor increase the ultimate stre...
Numerical modeling<br />All configurations were numerically modeled<br />Using Ansys v11<br />Material properties gathered...
Numerical modeling<br />
Numerical modeling<br />
Numerical modeling<br />
Numerical modeling<br />FEA confirms that ductile adhesives indeed lead to a reduction of stress magnitudes<br />Correspon...
Numerical modeling<br />Each element has a different volume, Vi, compared to the specimen on which strength was determined...
Numerical modeling<br />
Numerical modeling<br />
Discussion<br />The use of ductile adhesives leads to a reduction in stress magnitudes, but also to increasing the volumes...
Numerical modeling<br />CHAMFERED DLJ<br />Stresses reduced<br />Till Vallée, Juan Murcia, Thomas Tannert and David Quinn,...
Beyond adhesives and/or timber<br />T. Vallée, J. R. Correia, and T. Keller, „Probabilistic strength prediction for double...
Conclusions<br />The lack of ductility of timber, and structures made thereof is a serious issue<br />To overcome this iss...
Thank you for your attention<br />I’m happy to answer your questions<br />till.vallee@fibreworks.org<br />
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Ductile Adhesively Bonded Timber Joints

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Ductile Adhesively Bonded Timber Joints

  1. 1. Ductile Adhesively Bonded Timber Joints<br />Simon Hehl, Research Assistant Thomas Tannert, Research FellowTill Vallée, ProfessorTimber & Composites LaboratoryBern University of Applied Sciences<br />
  2. 2. Outline<br />Ductility in the context of bonded timber joints<br />Experimental investigations<br />Numerical modeling<br />Discussion<br />Conclusions<br />
  3. 3. Ductility in the context of bonded timber joints<br />Ductility holds a central status in civil engineering<br />Allows for load redistribution, announces failure<br />Ductility is usually achieved through plastic behavior<br />Yielding of steel<br />Plastic behavior of concrete in compression<br />Timber has to be considered as being brittle<br />Especially when it comes to tensile and shear<br />
  4. 4. Ductility in timber structures<br />Timber is not ductile<br />If not considering some plastic behavior in compression<br />Ductility in timber structures can be found in the connections<br />In the context of adhesives, it boils down to consider the use of plastic adhesives<br />Preliminary work done for Glulam<br />Similar research performed of G/C-FRP<br />
  5. 5. Experimental investigationsSeries 1<br />Tests on adhesively bonded double lap joints (DLJ)<br />Timber: Spruce<br />Consideration of three different adhesives<br />A commonly used stiff epoxy<br />Two softer plastic adhesives<br />Three overlap lengths<br />
  6. 6. Experimental investigations<br />~4550 MPa<br />~450 MPa<br />~150 MPa<br />
  7. 7. Experimental investigationsSeries 2<br />Tests performed on DLJ involving graded adhesive layers<br />Soft adhesive in the middle portion vs. stiff outside<br />Variation of the grading level, i.e. proportion of soft vs. stiff<br />
  8. 8. For all adhesives<br />No significant differences in the load-displ’ curves <br />Brittle failure in the timber<br />No objective signsof ductility!<br />No influence on strength<br />Experimental investigations<br />
  9. 9. Experimental investigations: Series 1<br />
  10. 10. Experimental investigations: Series 2<br />
  11. 11. Experimental investigations<br />Neither did the use of ductile adhesives enhance ductility nor increase the ultimate strength of adhesively bonded joints<br />Since the use of softer, especially plastic, adhesives leads to a reduction of stresses, this results sounds contradictory<br />
  12. 12. Numerical modeling<br />All configurations were numerically modeled<br />Using Ansys v11<br />Material properties gathered for this purpose <br />including the orthotropic properties of timber<br />Including the plastic properties of the adhesives<br />Determination of all stress components that trigger failure<br />σx,σy, τxy<br />
  13. 13. Numerical modeling<br />
  14. 14. Numerical modeling<br />
  15. 15. Numerical modeling<br />
  16. 16. Numerical modeling<br />FEA confirms that ductile adhesives indeed lead to a reduction of stress magnitudes<br />Correspondingly an increase of joint strength would be expected<br />This leads to the conclusion that joint strength is not exclusively driven by a stress based approach<br />Actually, the problem is best described using a probabilistic method<br />
  17. 17. Numerical modeling<br />Each element has a different volume, Vi, compared to the specimen on which strength was determined, V0<br />Its strength is thus different…<br />Each of the elements i has a probability of survival<br />…determined using Weibull<br />…Ps of joint = ΣPs,i<br />Size effects<br />Weibull<br />probabilistic<br />
  18. 18. Numerical modeling<br />
  19. 19. Numerical modeling<br />
  20. 20. Discussion<br />The use of ductile adhesives leads to a reduction in stress magnitudes, but also to increasing the volumes over which these stresses act<br />There is a lack of correlation between stress magnitude and joint strength<br />Probabilistic methods can be used to explain the apparent contradiction raised by this study<br />
  21. 21. Numerical modeling<br />CHAMFERED DLJ<br />Stresses reduced<br />Till Vallée, Juan Murcia, Thomas Tannert and David Quinn, Influence of stress reduction methods on the strength of adhesively bonded composed of brittle adherends, subm. To Int. JourAdh&dh.<br />
  22. 22. Beyond adhesives and/or timber<br />T. Vallée, J. R. Correia, and T. Keller, „Probabilistic strength prediction for double lap joints composed of pultruded GFRP profiles, part II: Strength prediction“, Composites Science and Technology, vol. 66, no. 13, pp. 1915-1930, 2006.<br />
  23. 23. Conclusions<br />The lack of ductility of timber, and structures made thereof is a serious issue<br />To overcome this issue, several researchers point towards the use of ductile adhesives<br />…their claim is based on a correlation between structural response and adhesive ductility, i.e. stress reductions<br />This study, amongst others, gives indications that this approach is highly hypothetical<br />…and offers an explanation why the lack of the expected pay-off using ductile adhesives was predictable, and never clearly demonstrated experimentally<br />
  24. 24. Thank you for your attention<br />I’m happy to answer your questions<br />till.vallee@fibreworks.org<br />

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